Nishitani et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0270-5(2019) 6:27

REVIEW

Progress in Earth andPlanetary Science

Open Access

Review of the accomplishments of midlatitude Super Dual Auroral Radar Network(SuperDARN) HF radars
Nozomu Nishitani1* , John Michael Ruohoniemi2, Mark Lester3, Joseph Benjamin Harold Baker2,
Alexandre Vasilyevich Koustov4, Simon G. Shepherd5, Gareth Chisham6, Tomoaki Hori1, Evan G. Thomas5,
Roman A. Makarevich7, Aurélie Marchaudon8, Pavlo Ponomarenko4, James A. Wild9, Stephen E. Milan3,
William A. Bristow7, John Devlin10, Ethan Miller11, Raymond A. Greenwald2, Tadahiko Ogawa12 and Takashi Kikuchi1

Abstract
The Super Dual Auroral Radar Network (SuperDARN) is a network of high-frequency (HF) radars located in the high- andmid-latitude regions of both hemispheres that is operated under international cooperation. The network was originallydesigned for monitoring the dynamics of the ionosphere and upper atmosphere in the high-latitude regions. However,
over the last approximately 15 years, SuperDARN has expanded into the mid-latitude regions. With radar coverage thatnow extends continuously from auroral to sub-auroral and mid-latitudes, a wide variety of new scientific findings havebeen obtained. In this paper, the background of mid-latitude SuperDARN is presented at first. Then, the accomplishments
made with mid-latitude SuperDARN radars are reviewed in five specified scientific and technical areas: convection,
ionospheric irregularities, HF propagation analysis, ion-neutral interactions, and magnetohydrodynamic (MHD) waves.
Finally, the present status of mid-latitude SuperDARN is updated and directions for future research are discussed.
Keywords: Mid-latitude SuperDARN, Ionosphere, Magnetosphere, Convection, Ionospheric irregularities, HF propagation
analysis, Ion-neutral interactions, MHD waves

Introduction
The Super Dual Auroral Radar Network (SuperDARN) is
a network of high-frequency (HF) radars located in thehigh- and mid-latitude regions of both hemispheres that isused to study the dynamics of the ionosphere and upper
atmosphere on global scales. As of 01 Jan 2018, there werea total of 36 SuperDARN radars, 23 in the Northern
Hemisphere and 13 in the Southern Hemisphere as shownin Fig. 1. The fields of view (FOVs) of the mid-latitude radars are shown in red. Table 1 shows a list of all theSuperDARN radars (details of geomagnetic coordinates
will be described in the “Meaning of geomagnetic coordinates” subsection). Figure 2 shows a photo of the Fort
Hays East (FHE) and West (FHW) antenna arrays and the
shelter that houses the electronic equipment for both radars. The main and interferometer antenna arrays of each
* Correspondence: nisitani@isee.nagoya-u.ac.jp1
Institute for Space-Earth Environmental Research, Nagoya University, Nagoya
464-8601, Japan
Full list of author information is available at the end of the article

radar are used for measuring the elevation angle, which isestimated from the phase difference between the signals
received on these two arrays.
SuperDARN achieved official status in 1995 when the
Principal Investigators’ Agreement was signed. Until about
2004, the SuperDARN radars were only able to monitor theregions of the ionosphere and upper atmosphere poleward
of about 60° geomagnetic latitude (MLAT). Ionosphericand upper atmospheric phenomena in the sub-auroral or
mid-latitude regions were largely inaccessible to the radars.
The first purpose-built “mid-latitude” SuperDARN radar,with a site located equatorward of 50° MLAT, began operating at the National Aeronautics and Space Administration (NASA) Wallops Flight Facility on Wallops Island,
Virginia (USA) (WAL) in 2005, shortly followed by thesecond mid-latitude radar in Hokkaido (Japan), Hokkaido
East radar (HOK) in 2006 (detailed discussion of the definition of the mid-latitude SuperDARN will be made in the“Definition of a mid-latitude radar” subsection). The success of these radars led to the construction of a second US© The Author(s). 2019, corrected publication [July/2019]. Open Access This article is distributed under the terms of the Creative
Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons license, and indicate if changes were made.

Nishitani et al. Progress in Earth and Planetary Science(2019) 6:27

aPage 2 of 57bFig. 1 Fields of view of the SuperDARN radars in the a Northern and b Southern Hemispheres

radar near Blackstone, Virginia (BKS) and finally to the deployment of a chain of radars that extends across North
America and the northern Pacific Ocean into eastern Asia.
As a result, there are now more than 10 mid-latitude
SuperDARN radars with additional radars under construction or planned, see the “Brief history of mid-latitude
SuperDARN” subsection and also the Additional file 1 titled “Historical Overview of Mid-latitude SuperDARN” forthe details of the mid-latitude SuperDARN history.
Over the past 15 years of operation of mid-latitude
SuperDARN radars, a number of scientific discoveries
have been made that encompass processes in the magnetosphere, ionosphere, thermosphere, and upper mesosphere, some with wide latitudinal and longitudinal
extents. It seems an appropriate time to review the newscientific results and critically assess these achievements.
There have been several previous papers that review theentire SuperDARN network (e.g., Greenwald et al. 1995;
Chisham et al. 2007) but none have focused exclusively on
mid-latitude studies. The purpose of this paper is to review
the accomplishments made with the mid-latitude SuperDARN radars. It is hoped this will enhance coordination between the SuperDARN groups in different countries andwill help make their accomplishments known to scientists inother research fields. In addition, by looking back on the scientific achievements, one can also look forward to the future. This review concludes with a discussion of several
aspects of the future directions of the mid-latitude SuperDARN network.

Basics of SuperDARN

SuperDARN radars observe scatter from a variety ofsources, including ionospheric irregularities, the ground/sea surface, meteor ionization trails, and possibly ice
crystals in the mesosphere, sometimes referred to aspolar mesosphere summer echoes (PMSE). Thus, theseradars are very versatile in terms of the science which
they can address. Here, a very brief discussion of the basics of the types of scatter, modes of HF propagation,and operating modes is provided, recognizing that thefirst two of these will be covered in more detail in termsof the mid-latitude radar observations in later sections.
SuperDARN radars are coherent scatter radars (Greenwald et al. 1995) where the signal received from the ionosphere is similar to Bragg scatter of X-rays in crystals.
Here, the signal is returned essentially along the same pathas the transmitted signal such that the radars receive direct backscatter, although it is possible for the transmitted
signal to be scattered in other directions. Due to the nature of Bragg scatter, the scale size of the irregularities
from which the signal is scattered is directly related to thehalf-wavelength of the transmitted signal. The irregularities in the ionosphere causing the backscatter can occur
at a range of different altitudes, including the D, E, and F
regions of the ionosphere (e.g., Milan et al. 1997a; Milanand Lester 1998; Milan et al. 2001). A more detailed
discussion of ionospheric irregularities is given in the “Ionospheric irregularities” section. There are two other potential
sources of returned signal from the ionosphere/upperNishitani et al. Progress in Earth and Planetary Science(2019) 6:27

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Table 1 List of SuperDARN radars. Magnetic latitudes/longitudes are in AACGM-v2 coordinates to one decimal point accuracy for 1
January 2018
Radar NameCountry

Geo. lat.

Geo. lon.

BoresiteMag. lat.

Mag. lon.

Year

Code

Northern Hemisphere
Adak Island East

USA

51.9

− 176.6

46.0

47.3

− 111.1

2012

ADE

Adak Island West

USA

51.9

− 176.6− 28.047.3− 111.1

2012

ADW

Blackstone

USA

37.1

− 78.0− 40.0

46.4

− 0.7

2008

BKS

Christmas Valley East

USA

43.3

− 120.4

54.0

48.7

− 56.2

2010

CVE

Christmas Valley West

USA

43.3

− 120.4− 20.048.7− 56.22010

CVW

Clyde River

Canada70.5− 68.5− 55.6

77.4

18.4

2012

CLY

Fort Hays East

USA

38.9

− 99.445.0

47.9− 30.12009

FHE

Fort Hays West

USA

38.9

− 99.4− 25.047.9− 30.12009

FHW

Goose Bay

USA

53.3

− 60.5

5.0

59.4

23.8

1983

GBR

Hankasalmi

UK

62.3

26.6

− 12.0

58.9

103.8

1995

HAN

Hokkaido East

Japan

43.5

143.6

30.0

36.9

− 143.2

2006

HOK

Hokkaido West

Japan

43.5

143.6

− 30.036.9− 143.2

2014

HKW

InuvikCanada68.4− 133.8

29.5

71.1

− 81.1

2008

INV

Kapuskasing

USA

49.4

− 82.3− 12.0

58.7

− 6.2

1993

KAP

King Salmon

Japan

58.7

− 156.7− 20.057.1− 96.6

2001

KSR

KodiakUSA

57.6− 152.2

30.0

56.8

− 92.32000

KOD

Longyearbyen

Norway

78.2

16.1

23.7

75.5

108.5

2016

LYR

Prince George

Canada54.0− 122.6

− 5.0

59.0

− 61.6

2000

PGR

Pykkvibaer

UK

63.8

− 20.5

30.0

63.5

66.0

1995

PYK

Rankin Inlet

Canada62.8− 92.1

5.7

71.5− 21.7

2007

RKN

Saskatoon

Canada52.2− 106.5

23.1

60.0

− 41.2

1993

SAS

Stokkseyri

UK

63.9

− 21.0− 59.0

63.7

65.7

1994

STO

Wallops Island

USA

37.9

− 75.5

35.9

46.9

2.7

2005

WAL

138.5

146.5

− 45.3− 145.2

2014

BPK

Southern Hemisphere
Buckland Park

Australia− 34.6

Dome C East

Italy

− 75.1

123.4

115.0

− 89.0

56.9

2012

DCE

Falkland IslandsUK− 51.8− 59.0178.3− 39.3

10.2

2010

FIR

HalleyUK− 75.5− 26.6165.0− 62.2

30.3

1988

HAL

KerguelenFrance− 49.4

70.3

168.0

− 58.5

124.5

2000

KER

McMurdoUSA− 77.9

166.7

263.4

− 79.9− 32.8

2010

MCM

SANAE

South Africa− 71.7

− 2.9

173.2

− 61.9

45.1

1997

SAN

South PoleUSA

− 90.0

118.3

75.7

− 74.3

19.2

2013

SPS

Syowa EastJapan

− 69.0

39.6

106.5

− 66.5

73.6

1997

SYE

Syowa SouthJapan− 69.0

39.6

165.0

− 66.5

73.6

1995

SYS

TIGER Bruny Island

Australia− 43.4

147.2

180.0

− 54.0− 132.3

1999

TIG

TIGER Unwin

Australia− 46.5

168.4

227.9

− 53.8− 105.2

2004

UNW

Zhongshan

China− 69.476.4

72.5− 74.9

99.0

2010

ZHO

atmosphere. The ionization that is created in the atmosphere by meteors can also lead to backscatteredsignal in SuperDARN data, but this typically occurs
in the near ranges as the meteor trail altitude range
is typically between 85 and 120 km (e.g., Thomas

et al. 1986; Hall et al. 1997; Chisham and Freeman2013). Finally, and perhaps more controversially, thereis the possibility that SuperDARN radars receive
scatter in near ranges which is similar to PMSE (e.g.,
Ogawa et al. 2002a; Ogawa et al. 2003; Hosokawa

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Fig. 2 Photo of the Fort Hays East and West radar antennas and radar equipment shelter

et al. 2005). There remains some level of controversy
over the validity of this interpretation which requires
further investigation (Ponomarenko et al. 2016).
SuperDARN radars are capable of operating in the
HF band of the radio spectrum between 8 and 20
MHz although most radars actually operate over anarrower range of frequencies, typically between 10
and 14 MHz. At these frequencies, radar signals canbe refracted by the ionosphere such that over thehorizon (OTH) propagation is possible. Although adetailed discussion of HF propagation is given in the“HF propagation analysis” section, a brief overview of
propagation is provided here and this is illustrated by
Fig. 3 which shows an example ray-tracing simulationfor 11 MHz transmissions on beam 12 of BKS at 1400
Universal Time (UT) on 18 November 2010 (deLarquier et al. 2013). The ray paths (colored in gray)Good aspect conditions(ionospheric scatter)span 5–55° in take-off elevation angle and propagate
through an International Reference Ionosphere (IRI)2011 ionosphere color coded by electron density according to the scale on the right. Several important
HF propagation modes can be identified:
1. The first is “penetrating rays” which have high enoughelevation angles to pass through the F2 peakdensity region without ionospheric refraction
bringing them to horizontal propagation. Theserays exit the topside ionosphere unless theyencounter ionospheric plasma irregularitiesaligned with the geomagnetic field direction (pinklines) that can produce ionospheric backscatter ina so-called “½-hop” mode. Ray segments with favorable aspect conditions for experiencing thisform of backscatter (i.e., ≤ 1° from orthogonality

Ground scatter

Fig. 3 Ray-tracing results for 11 MHz transmissions on beam 12 of the Blackstone radar propagating through the predicted IRI-2011 ionosphereon 18 November 2010 at 14:00 UT (see text for details). Reproduced from Fig. 3 of de Larquier et al. (2013)

Nishitani et al. Progress in Earth and Planetary Science(2019) 6:27

Fig. 4 A plot of Doppler velocity against radar beam and range
from CVE and CVW radars. This figure shows Doppler velocity
observations (positive toward the radar) for one scan of data from
CVW and CVE and indicates four separate regions of ionosphericscatter indicated by the number I—meteor scatter, II—low-velocity
ionospheric scatter from the sub auroral region, III—scatter from a
SAPS, and IV—low-velocity ionospheric scatter from the auroral
regions. Reproduced from Fig. 2 of Clausen et al. (2012)

with the geomagnetic field) are shaded black.
This ½-hop ionospheric backscatter can occur inboth the E or F regions with the E region ionospheric scatter dominating at nearer slant ranges.
2. Propagation of rays downward to the ground(including the sea surface) due to refraction by theionosphere is also illustrated here. These rays resultin “ground scatter” once roughness on the terrestrialaPage 5 of 57

surface is encountered and some power isbackscattered to the radar. The ground range to thenearest edge of the ground scatter is a little over1000 km while the outer edge extends to over 2000
km. The ground scatter is important for a variety ofstudies, including investigations of atmosphericgravity waves (see the “Ion-Neutral Interactions”
section) and magnetohydrodynamic (MHD) waves(see the “MHD Waves” section).
3. The signal can take multiple hops to even farther
distances, such that the radar can receive scatter fromthe ionosphere not just directly but from beyond thefirst location of ground scatter. Moreover, subsequent
refraction to downwards propagation can generatebands of ground scatter from more distant ranges.
The different types of ionospheric scatter are illustrated
by the data plotted in Fig. 4 (Clausen et al. 2012) where
single scans from the Christmas Valley East and West radars (CVE and CVW, respectively) are plotted. Four different types of scatter are identified in this figure. Scatter
type I is low-velocity scatter at very near ranges that has
been identified as signal returned from meteor ionization
trails (e.g., Hall et al. 1997). Scatter type II is in the western part of the CVW FOV and has velocities of the orderof 100 m s−1; this is ionospheric scatter from the F region
and is typical of the low-velocity backscatter routinely obtained at mid-latitudes on the nightside (e.g., GreenwaldbFig. 5 Schematic plots showing the a Northern and b Southern Hemisphere SuperDARN radar locations with respect to the auroral oval. The
auroral oval (yellow) for moderately disturbed conditions, as quantified by Holzworth and Meng (1975), is plotted as a function of AACGM
coordinates with the SuperDARN radar locations identified in green type and green closed circles (polar cap latitudes), blue type and blue closed
circles (auroral latitudes), and black type and red closed circles (mid-latitude radars)

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et al. 2006). At a somewhat higher latitude, closer to 60° N
MLAT, and labeled III, there is a narrow-in-latitude but
extended-in-local time (LT) region of high-velocity scatterthat is typical of a sub-auroral polarization stream (SAPS)
event (see the “Convection” section for more detailed discussion of this feature). Finally, the high-velocity scatter
type IV is more typical of the type of scatter observed in
the auroral zone by the high-latitude radars. Thus, Fig. 4
illustrates that even mid-latitude radars can observe
auroral scatter.
It is important to note that the latitude of a radar does
not specifically determine that it will always measure scatter from a specific region for two reasons. One is simply
that the equatorward edge of the auroral oval is not located at the same latitude for all LTs, being at higher latitudes at noon than at midnight (Fig. 5). Furthermore, thelevel of magnetic activity determines the relative locationof a radar with respect to auroral boundaries, and hence,
the type of scatter that is received. As magnetic activity increases and the polar cap area increases, the auroral oval
expands equatorward while under extremely quiet conditions the oval moves to higher latitudes as the polar cap
contracts (e.g., Milan et al. 2003).
SuperDARN radars have at least 16 different look directions along which they can sample over 70 or morerange gates. The normal, or common, operational modeof the radar provides a full azimuth scan every 1 or 2
min with integration times that vary between 3 and 7 s
per beam. In this mode, the scan is always synchronized
to start on the 1-min boundary. Special modes can also
be run on all the radars or a subset of radars. Here, themode of operation is the same among the radars participating in the special mode and could involve a scanning pattern very different from the standard common
mode. Finally, there are discretionary modes where individual radars run in a very specific way for specificscientific purposes. These modes are more likely to be run
on only one or two radars, often in support of regional
campaigns such as rocket launches or during heating campaigns. Special and discretionary modes can run beamscans in any sequence, with any time resolution and alsowith no synchronization. The allocation of time is madeby a working group of the community which allocates noless than 50% of time each month to common modes, up
to 20% of each month to special modes and up to 30% oftime to discretionary mode. The minimum time length ofany specific run is currently 6 h; although in the past, it
has been as long as 1 day. If there are no requests for either discretionary or special time, or radars are not running in these modes, then the operational mode defaults
to common mode.
The radars produce a variety of data products, the mostwidely used being the backscatter power (signal-to-noise
ratio), the line-of-sight (LOS) Doppler velocity, thespectral width, and the elevation angle of arrival. The radars transmit a sequence of 7 or 8 unevenly separated
pulses with an individual pulse length of 300 microseconds, which provides a range resolution of 45 km. A reduced pulse length of 100 microseconds (15 km range
resolution) has also been used on occasion (e.g., Lester et
al. 2004); at least some of the mid-latitude radars have thecapability of operating at this higher spatial resolution.
From combining pairs of different pulses in the sequence,different lags of an autocorrelation function (ACF) are
generated. Typically, 25–30 multi-pulse sequences are averaged in order to obtain a statistically meaningful ACF
against noise, which corresponds to ~ 3-s integration time
for a single beam. From the ACFs, the Doppler velocity is
estimated by fitting a linear function to the measured ACF
phase, while backscattered power and spectral width are
estimated from fitting a linear or a quadratic function to
the logarithm of the measured ACF power. Some radars
operate in slightly different modes from this but still create the basic data products described above.

Nishitani et al. Progress in Earth and Planetary Science

Definition of a mid-latitude radar

It is not so straightforward to characterize radars as belonging to the mid-latitude region. The auroral oval does provide
a scheme for identifying three zones as shown schematicallyin Fig. 5. The continuous band mapped out by auroral luminosity constitutes the auroral oval while the enclosed regionis the polar cap. Extending equatorward of the auroral oval
is the sub-auroral region. This figure suggests a natural assignment of radar identity based on which region dominatesin a radar FOV. However, the positions of the oval boundaries vary with time of day and also with geomagnetic activity
level. The diurnal variation is fairly predictable but the expansion and contraction with activity level is not. The oval
depicted in Fig. 5 is roughly appropriate to a Kp value of 2
which is fairly typical and corresponds to a slight geomagnetic disturbance. The positions of the radars in the Northern Hemisphere are indicated with dots and labeled with
their three-letter identifiers. The FOVs of the radars in theionosphere generally begin several hundred kilometers to
the north of the radar position. It can be seen that the original high-latitude radars follow the 60° N MLAT contour;
this geometry places them in excellent position to observe
across the oval on the nightside for typical conditions. The
radars at lower latitudes will then observe across a large (~10°) interval of sub-auroral latitudes, even deep on the nightside. This is used as our discriminator: radars sited equatorward of 55° MLAT are considered to be mid-latitude radars
while appreciating that during expansion of the oval thesemid-latitude radars will make extensive auroral observations.
Similarly, during periods of contracted oval, thehigh-latitude radars will observe across a substantial portionof the sub-auroral region. Even during relatively quiet
periods the high-latitude radars are capable of making

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sub-auroral observations over their nearer range intervals
and along more zonally oriented beam directions. This review incorporates findings on sub-auroral research topics
from both the mid-latitude and high-latitude radars and will
make some distinction as to whether radars were purposebuilt to be mid-latitude radars capable of making observations of the expanded auroral oval.

encounters an interesting complication: the magnetic field
is changing surprisingly rapidly (secular variation) with theresult that the geomagnetic coordinates of the radar sites
and their FOVs are changing. For example, the MLAT of
WAL has changed from 48.37° N to 46.96° N, a decrease of
1.4°, over the 15 years since its construction in 2005. Table 1
lists the AACGM-v2 coordinates of the radar sites to one
decimal point accuracy for 1 January 2018. The secularvariation should be borne in mind when comparing observations from different eras.

Nishitani et al. Progress in Earth and Planetary Science

Meaning of geomagnetic coordinates

The Earth’s magnetic field plays a dominant role in defining
the geometry of the auroral oval, and it is natural to map
auroral phenomena in a geomagnetic coordinate system.
The Altitude-Adjusted Corrected Geomagnetic (AACGM)
coordinate system was derived by Baker and Wing (1989)
in order to compare data from SuperDARN radars in opposite hemispheres and is based on tracing geomagnetic
field lines using the International Geomagnetic Reference
Field (IGRF) which is a mathematical description of the
Earth’s magnetic field updated at 5-year intervals (e.g., Barraclough 1987; Thébault et al. 2015). In AACGM coordinates, points along a given magnetic field line are given thesame coordinates and are thus a better reflection of magnetic conjugacy. The concept of AACGM was re-analyzed
by Shepherd (2014) which resulted in a procedure that
more accurately represents the underlying coordinate system. The new description, known as AACGM-v2, is the accepted basis for mapping SuperDARN data. Here, one

Brief history of mid-latitude SuperDARN

Figure 6 presents a timeline of radar operations with ordering by MLAT. The AACGM-v2 coordinates wereused. The first purpose-built mid-latitude SuperDARN
radar (WAL) began operation at Wallops in the springof 2005. This was followed by HOK, a radar in Hokkaido,
northern Japan, which started operation in 2006. Next, aradar was installed near Blackstone in 2008 (BKS). One
year later, as a part of the National Science Foundation(NSF) mid-sized infrastructure (MSI) program for “StormDARN,” the first radar pair was built near Hays, Kansas(Fort Hays) (FHE and FHW) in 2009, the second near
Christmas Valley, Oregon (CVE and CVW) in 2011, andthe third on Adak Island (ADE and ADW) in the Aleutians in 2012. A second SuperDARN radar in Hokkaido,
the Hokkaido West radar (HKW), was deployed in 2014.

ab

Fig. 6 Timeline of the SuperDARN radars in the a Northern and b Southern Hemispheres. The geomagnetic latitudes are in AACGM-v2
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In the Southern Hemisphere, a radar was briefly deployedon the Falkland Islands (FIR) from 2010 to 2011. This
radar resumed operations in November 2017. A new radar
was deployed at Buckland Park, Australia (BKP) in 2014.
For details of the history of the Mid-latitude SuperDARN,
please refer to the Additional file 1 titled “Historical Overview of Mid-latitude SuperDARN.”

because of their limited spatial coverage and radio wave absorption due to enhanced precipitation (“Convection” section). The network is also useful for monitoring theoccurrence of ionospheric plasma irregularities across thetransition from the auroral to the sub-auroral andmid-latitude zone. (“Ionospheric irregularities” section). An
important element of SuperDARN data capture is the role of
HF propagation analysis which provides clues to the disturbance processes in the ionosphere due to a variety of factors(“HF propagation analysis” section). In addition, SuperDARN
radars can also monitor ionospheric phenomena affected by
the exchange of energy and momentum between the charged
and neutral components of the Earth’s upper atmosphere(“Ion-neutral interactions” section). Finally, the radars area powerful tool for monitoring MHD waves over awide latitudinal range and studying their source
mechanisms (“MHD waves” section). After reviewing
these scientific topics, future directions of the network will be discussed from several points of view(“Conclusions and future directions” section).

Nishitani et al. Progress in Earth and Planetary Science

Structure of this paper

Figure 7 shows a schematic illustration of natural phenomena which can be studied by SuperDARN radars. It can beseen that SuperDARN observes a wide variety of phenomena, ranging from polar to mid-latitudes, and from themagnetosphere/ionosphere to the thermosphere/upper
mesosphere. Some topics have found new prominence inrecent years as a direct result of the development of the
mid-latitude SuperDARN radars. Specific examples include
sub-auroral and mid-latitude plasma flows, solar flare effects on the ionosphere, and earthquake-triggered ionospheric disturbances.
This paper consists of five sections, each of which examinesa specific scientific area, followed by a section on conclusions
and future directions. A major benefit of mid-latitude SuperDARN is its ability to monitor the expansion of convection tolower latitudes during geomagnetically active periods, which
was not possible with the high-latitude SuperDARN radars

Convection
Motivation for the first SuperDARN radar locatedequatorward of 50° MLAT, and the subsequent expansion at mid-latitudes, was largely due to the inability of the existing high-latitude network to

Fig. 7 Schematic illustration of natural phenomena which can be studied by SuperDARN radars

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measure the full latitudinal extent of ionospheric convection during geomagnetically active periods. During
times when the auroral region expands equatorwardof the lower latitude limit of the existing network (~60° MLAT), radars are no longer able to measure thecomplete extent of convection. In addition, absorption
due to enhanced precipitation can significantly attenuate radar signals at auroral latitudes, further reducing
measurements of the ionospheric convection electricfield.

Evidence of the need for measurements at lower latitudes
was provided by Ruohoniemi et al. (2001) who showed aprompt and widespread ionospheric response to a coronalmass ejection (CME)-driven shock. The high-latitude
SuperDARN radars used in this study showed a rapid increase in the convection velocity at all magnetic local times(MLTs), from dawn to dusk, an increase in the amount ofbackscatter observed and an equatorward expansion of thehigh-latitude convection region. The study focused on the
now-casting capability of SuperDARN during the shock

Nishitani et al. Progress in Earth and Planetary Science

a

e

b

f

c

g

d

h

Fig. 8 a–d GPS TEC maps depicting evolution of the polar tongue of ionization (TOI) with SuperDARN convection patterns overlaid. e–h SuperDARNLOS velocity measurements from radars located at mid-latitudes on the dayside and extending into the polar cap. Reproduced from Fig. 7 of Thomas
et al. (2013)

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arrival, but it also acknowledged that the auroral region
continues to expand equatorward of the existing SuperDARN FOVs and a reduction in the amount of backscatter occurs later in the event. Kane and Makarevich (2010)
showed this behavior explicitly in several examples of theionospheric convection response to storm sudden commencement (SSC) with a significant reduction in theamount of backscatter and magnitude of the velocities observed ~ 30 min after the SSC.
The success of expanding SuperDARN to the mid-latitude region and capturing the dynamics of storm-time convection is discussed in more detail in the “Storm-time
convection” subsection. Following the completion of thefirst mid-latitude radar (WAL), subsequent radars wereconstructed in rapid succession, extending the coverage of
mid-latitude phenomena by SuperDARN both in UT,
thereby increasing opportunities for observing transient features (discussed in the “Transients” subsection), and MLT,
allowing for the characteristics and dynamics of longitudinally extended features, such as SAPS and sub-auroral ion
drifts (SAIDs) to be determined (discussed in 10 subsections from “SAPS: Introduction” to “Implications for SAPS
formation scenarios”).

Goose Bay radar (GBR) can monitor the equatorward
progression of the auroral convection over a 6-h intervalfrom only 75–65° MLAT before observations are disrupted by enhanced precipitation and E region backscatter at nearer ranges. However, for the same period, themid-latitude radar (WAL) is able to track the convection
expansion from 70 to 55° MLAT, offering an additional
10° in latitudinal coverage and corresponding improvement in mapping the instantaneous global convection
pattern. Later studies of storm-time plasma circulationby Hosokawa et al. (2010) and Thomas et al. (2013) usedadditional mid-latitude radars to address the role of theconvection electric field in the transport of daysidestorm-enhanced density (SED) plasma through the cusp
and into the polar cap to form the polar tongue of
ionization (TOI). Figure 8 shows the evolution of a TOIin global positioning system (GPS), now a part of Global
Navigation Satellite System (GNSS), total electron content (TEC), and the LOS velocity from SuperDARN radars, including mid-latitude radars located on thedayside measuring the convection throat (Thomas et al.
2013). Previous efforts to determine the connection between mid-latitude SED and polar TOI were hamperedby the inability of the high-latitude SuperDARN radars
to observe the lower latitude sunward convecting regions
believed to be crucial in this global plasma circulation(Foster et al. 2005).
Baker et al. (2007) also showed the importance of
mid-latitude measurements for describing the morphology and strength of the large-scale convection electricfield pattern, particularly on the nightside during increased geomagnetic activity where convection extends

Nishitani et al. Progress in Earth and Planetary Science

Storm-time convection

During periods of enhanced geomagnetic activity triggered by a sustained southward interplanetary magnetic
field (IMF), the auroral electric fields associated withmagnetospheric convection are known to expand equatorward into the mid-latitude ionosphere (MLAT < 60°)
(e.g., Carbary 2005). Baker et al. (2007) demonstrated fora moderate geomagnetic storm how the high-latitudeFig. 9 Average patterns of Northern Hemisphere ionospheric convection calculated from SuperDARN data collected during periods of increased
geomagnetic activity (Kp ≥ 3) between June 2005 and April 2006. Contours of the global solution of electrostatic potential are shown as blacklines spaced every 4 kV. Adapted from Fig. 12 of Baker et al. (2007)

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to lower latitudes. Figure 9 shows statistical convection
patterns from Baker et al. (2007) both with and without
the mid-latitude WAL data during moderate to disturbed geomagnetic conditions (Kp ≥ 3), illustratingstronger overall convection (larger cross-polar cap potential) and more detail of the flows on the nightside associated with the Harang discontinuity.
A more recent study by Thomas and Shepherd (2018)used data from all available SuperDARN radars (including
10 mid-latitude radars) during the 7-year period spanning
2010–2016 to produce patterns of the high-latitude convection electric field using a technique similar to that usedby Ruohoniemi and Greenwald (1996); Ruohoniemi andGreenwald (2005); and Cousins and Shepherd (2010). The
statistical patterns produced by Thomas and Shepherd(2018) demonstrate that inclusion of mid-latitude data canincrease the cross-polar cap potential by as much as 40%
and provides a significantly improved specification of theplasma flows on the nightside during a wide range of geomagnetic activity.
Fiori et al. (2010), using the Spherical Cap Harmonic
Analysis (SCHA) technique, showed that convection canbe increased by moving the lower latitude limit, but caution should be taken with the amount of data going into
each pattern. Cousins and Shepherd (2010) demonstrated that the solutions to the statistical patterns wererelatively insensitive to the lower latitude boundary.
Under steady-state conditions a shielding electric field
formed at the inner edge of the plasma sheet and associated with the region 2 field-aligned currents (FACs)
counteracts the effects of the solar wind-driven magnetospheric convection at sub-auroral latitudes (Nishida1968). This shielding electric field is directed in the opposite sense to the dawn-dusk convection electric fieldand varies on time scales ≤ 1 h (Kelley et al. 1979). There
are two cases where significant electric fields may develop in the inner magnetosphere following IMF transitions which occur on timescales faster than the shielding
layer can respond. The first is the “under-shielding”
scenario where a sudden intensification in convection
causes the dawn-dusk electric field to penetrate to theinner magnetosphere and all the way to the equatorial
ionosphere. The second case occurs when the IMF Bz
component turns northward after prolonged southward
IMF conditions and the shielding electric field becomesdominant over the convection electric field, known as“over-shielding.”
Ebihara et al. (2008) presented the first mid-latitude
SuperDARN observations of the over-shielding electricfield using HOK. They examined two reverse flow periods during a moderate geomagnetic storm, the first ofwhich was attributed to over-shielding associated with anorthward IMF turning while the second occurred during southward IMF conditions and could not be

replicated in the ring current simulation. The second
one is probably associated with a substorm. When a substorm occurs, over-shielding is shown to appear at lowand mid-latitudes without northward turning of IMF by
global MHD simulation (Ebihara et al. 2014). A later
study by Kikuchi et al. (2010) also examined
over-shielding signatures during the same geomagnetic
storm, although in the context of equatorial DP2 fluctuations were attributed to alternating eastward and westward electrojets in the equatorial ionosphere. Using
mid-latitude SuperDARN contributions to the instantaneous global convection patterns, they suggested thedayside reverse flow vortices observed equatorward of
the larger two-cell convection correspond to the region
2 FACs responsible for over-shielding at the equator.
The study of over-/under-shielding phenomena with
mid-latitude SuperDARN radars remains an under-utilized capability and an area for future studies.
To summarize, storm-time convection electric fields
corresponding to a variety of geophysical drivers are observed by the mid-latitude SuperDARN radars. Lyons etal. (2016) presented a synthesis of ground- andspace-based observations characterizing these fields during the 17 March 2013 geomagnetic storm. They identified an inter-relationship between the expansion of theauroral oval, penetration electric fields, auroral stream activity, and SAPS, which is discussed in the next section.

Nishitani et al. Progress in Earth and Planetary Science

SAPS: Introduction

The sub-auroral region of geospace is where dramatic
plasma convection enhancements occur during geomagnetic storms and substorms. Initially identified with
low-orbit satellites as latitudinally-narrow regions of verystrong westward plasma flows up to 4 km s−1, they havebeen termed polarization jet or PJ (Galperin et al. 1973),
SAID (Spiro et al. 1979), and sub-auroral electric field or
SAEF (Karlsson et al. 1998). These narrow flow channels
were proposed to be driven by the poleward polarizationelectric field located equatorward of the electron precipitation region due to the radial separation between the
inner plasma sheet electrons and ions (Southwood andWolf 1978). A similar explanation has been proposed forbroader regions of enhanced plasma convection thatwere regularly observed during geomagnetic storms by
the Millstone Hill incoherent scatter radar (Yeh et al.
1991; Foster and Vo 2002). As a result, an encompassing
term of SAPS has been proposed to include both narrow PJ/SAID forms and broader regions (Foster and
Burke 2002).
Sub-auroral convection enhancements during substorms were also reported based on observations by very
high-frequency (VHF) coherent scatter radars (Unwin
and Cummack 1980; Freeman et al. 1992) and earlySuperDARN observations conducted with the most(2019) 6:27

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equatorward radars in the high-latitude network (Parkinson et al. 2003a; Koustov et al. 2006). These early radarstudies stopped short of directly associating the observed
signatures with SAPS. To describe their radar substorm
signatures, Unwin and Cummack (1980) used the term
drift spikes, while Freeman et al. (1992) called theirevents substorm-associated radar auroral surges or
SARAS. Similarly, Parkinson et al. (2003a) introduced
the term Auroral Westward Flow Channel (AWFC) to
describe their Tasman International Geospace Environment Radar (TIGER) Bruny Island (TIG) observations,while Koustov et al. (2006) used the term “SAPS-likeflows” in reference to strong convection regions seen by
King Salmon radar (KSR). The current definition of
SAPS does not involve any particular threshold in
plasma convection velocity (although there were attempts to estimate the slowest threshold of SAPS velocity, e.g., Nagano et al. 2015), and it only requires that
locally enhanced plasma flows should be linked to theextended dusk convection cell that extends equatorwardof the electron precipitation boundary (Huang et al.
2006). For this reason, most of these early radar observations should also be classified as SAPS.
The question of the nomenclature is ultimately related
to the question of physical origins and driving mechanisms. The radial charge separation and the associatedpolarization electric field in the magnetosphere arewidely accepted to be one of the two main drivers, withthe other one being positive feedback between the magnetospheric electric field and ionospheric conductance(e.g., Wolf et al. 2007). In this feedback model, themagnetosphere-ionosphere (MI) system is assumed toact as a current generator, with the total current being
conserved. In this case, the initial polarization electricfield drives ion convection in the ionospheric F region,
which increases heating and recombination rates, depleting ionospheric densities, and further strengthening
SAPS electric fields (Anderson et al. 1993). Despite ageneral consensus on the importance of these two processes for SAPS formation and evolution, there appears to
be a growing realization that these do not explain somecharacteristics of narrow SAID (Mishin and Puhl-Quinn
2007; Puhl-Quinn et al. 2007). Similarly, the discovery ofhighly dynamic and localized plasma flows within SAID/SAPS that are often referred to as the SAPS wave structure
or SAPSWS (Mishin et al. 2003; Mishin and Burke 2005)
has challenged the view of SAPS as a generally uniform
flow region with possibly one or more narrow SAID-likeflow channels (Erickson et al. 2002; Mishin et al. 2003; Foster et al. 2004; Mishin and Burke 2005).
SuperDARN observes SAPS as a multi-scale
phenomenon from large regions of sub-auroral westward
convection seen simultaneously by multiple radars tosmall-scale flow enhancements within only a few range
gates. Figures 4 and 10 present two examples of suchobservations. Figure 4 shows a snapshot of LOS velocities observed by the CVW/CVE pair on 09 April 2011 at
0840 UT. The SAPS scatter is observed near MLAT of
60°. The SAPS region extends across both FOVs and theflow velocities are well in excess of 500 m s−1. Figure 10
from Makarevich and Bristow (2014) presents the CVW
velocity on 17 March 2013, 0930–1430 UT versus timeand MLAT. During the period marked by the two vertical white lines, the westward flow is strong and the
MLAT range is narrow, which makes it consistent with
SAID. Small-scale structure is also clearly present near1215 UT; it takes the form of small enhancements thatare only 1–2 range gates in extent and that propagate
toward higher MLATs. Figures 4 and 10 thus illustrate
the three major advantages of SuperDARN in relation to
SAPS research: (1) global-scale spatial coverage thatenables SAPS observations over an extended MLT sector, (2) nearly continuous temporal coverage duringextended periods, and (3) relatively high spatial and temporal resolutions that enable investigations ofsmall-scale structure of SAPS.

Nishitani et al. Progress in Earth and Planetary ScienceFig. 10 CVW velocity in beam 4 on 17 March 2013, 0930–1430 UT. The green cells refer to the SAPS region. The two vertical white lines mark theperiod of narrow and intense SAID feature. The period of strong SAPS Wavy Structure (SAPSWS) activity is near 1215 UT. Reproduced from Fig. 2aof Makarevich and Bristow (2014)

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In the last decade, SuperDARN has provided numerous contributions to SAPS research that can be divided
into the following two groups, roughly corresponding tospatial and temporal features of SAPS. The first exploits
the advantage of global coverage of the sub-auroral andauroral ionosphere allowing SuperDARN investigations
to improve knowledge of global characteristics and external control of SAPS and, through that, achieve a better understanding of the relative importance of global/
external factors versus other drivers. The second exploits
the advantage of continuous coverage allowing SuperDARN investigations to improve knowledge of temporal
dynamics of SAPS including the importance of the MI
feedback mechanism. Studies that address these two categories of issues are respectively reviewed in the following subsections.
Global characteristics and control of SAPS

Study of the large-scale structure of SAPS has been
greatly advanced by the significantly expanded coverageof mid-latitude SuperDARN. In coordination with theoriginal high-latitude radars, the network of mid-latitude
SuperDARN radars is unrivaled in its ability to address
the spatial characteristics of SAPS on global scales.
Longitudinal extent and variation of SAPS

The longitudinal structure of SAPS in the sub-auroraland mid-latitude region has been investigated (Oksavik
et al. 2006; Koustov et al. 2006; Kataoka et al. 2007;
Clausen et al. 2012). In their study, Oksavik et al. (2006)
examined a SAPS flow channel equatorward of 60°
MLAT that was observed for several hours by WAL. It
was revealed that a fast westward flow appeared in the
pre-midnight sector, while an eastward flow wasco-located on the higher latitude side of the fast
westward flow in the post-midnight sector, forming aflow reversal as seen in Fig. 11 (Oksavik et al. 2006).
Kataoka et al. (2007) examined a similar flow reversal
with HOK and confirmed that the flow reversal is alsopresent in the post-midnight sector and is enhanced
during a magnetic storm.
In addition to these studies conducted with single radar,
the unique capability of SuperDARN for investigating
SAPS—the extensive longitudinal coverage provided by
combining several neighboring radars—is evident in more
recent studies. A remarkable example demonstrating thiscapability is given by Clausen et al. (2012), who studied
the spatiotemporal evolution of a SAPS flow channel, seenin the bottom/left panel of Fig. 12 as a narrow (few degrees wide) channel of westward flow extending over 6 hof MLT and through the FOVs of six mid-latitude SuperDARN radars. Comparison with the GPS TEC map in the
top/left panel of Fig. 12, reveals that the SAPS channel isassociated in latitude with the position of the ionosphericFig. 11 LOS Doppler velocity for a 1-min scan of WAL showing astrong sub-auroral westward flow channel. Overlaid is a swath of the135.6 nm aurora from TIMED GUVI, cross-track drift velocity from the
DMSP F15 spacecraft, and the track of the NOAA-18 spacecraft,which are used to determine the equatorward edge of electron
precipitation (shown as a thick dotted line). Adapted from Fig. 1 of
Oksavik et al. (2006)

trough over this wide range of MLT. In addition, Clausen
et al. (2012) determined from the distributed and near instantaneous measurements of the structure that the maximum flow speed within the SAPS increases exponentiallywith decreasing MLT. However, a more recent statistical
study suggests the MLT dependence is more commonly
quasi-linear (Kunduri et al. 2017).
Dependence of SAPS on solar wind and geomagnetic drivers

Since the deployment of TIG in 1999 and WAL in 2005
in the Southern and Northern Hemispheres, respectively,
continuous observations at sub-auroral latitudes havebeen carried out, enabling the correlation of SAPS characteristics with solar wind and geomagnetic conditions
to be examined. Several studies using SuperDARN data
have identified SAPS characteristics which are basicallyconsistent with those obtained with the Millstone Hill
incoherent scatter radar (ISR) (Foster and Vo 2002;
Erickson et al. 2011), namely, that SAPS tend to form
more often, with faster flow speeds, and at lower latitudes with increasing geomagnetic activity level (Parkinson et al. 2005, 2006; Kataoka et al. 2009; Grocott et al.
2011; Kunduri et al. 2012; Nagano et al. 2015; Kunduri
et al. 2017). These correlation characteristics strongly
suggest that SAPS are closely controlled by solar windconditions as well as by the ring current. A further
examination by Grocott et al. (2011) showed that thelatitudinal location of SAID varies on similar time scales
to those of the interplanetary magnetic field and auroral

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Fig. 12 Top/left: spatial distribution of GPS TEC with precipitating electron flux along a pass of NOAA/POES satellite. Bottom/left: 2-D map of LOS
velocities observed by mid-latitude SuperDARN radars over North America at 0840 UT on 9 April 2011. Right: Vector representation of the averagelarge-scale SAPS flow direction and the inferred SAPS speed identified by the three radar pairs. Time runs along the y axis, increasing toward thebottom. Adapted from Figs. 5 and 7 of Clausen et al. (2012)activity, while variations in its flow speed are moreclosely related to ring current dynamics. These results
are consistent with the idea that the poleward electricfield of SAPS/SAID is caused by the shielding effect of
the ring current coupled with the ionosphere throughthe Region 2 FAC system (Southwood and Wolf 1978).
Interhemispheric comparisons of SAPS

Several mid-latitude SuperDARN radars share conjugate
FOVs between the Northern and Southern Hemispheres
and, therefore, provide opportunities to examine interhemispheric conjugacy of the spatial and temporal characteristics of ionospheric convection. Conjugate characteristics ofSAPS were studied by Parkinson et al. (2005) with KSR and
TIG, whose FOVs were roughly conjugate to each other.
Besides some small asymmetry between the hemispheres,the major variations of SAPS flow are shown to be quite
similar, suggesting that the equipotential of geomagnetic
field lines holds to the zeroth order. The same conclusions
were drawn by similar studies but with a more closely conjugate set of radars (e.g., Kunduri et al. 2012). Grocott et al.
(2011) also show excellent agreement in the latitude of anarrow SAPS channel observed by BKS and FIR, as seen in
Fig. 13; however, differences in the width of the channel
were also noted. Interestingly, a detailed inspection of SAPS

characteristics by Kunduri et al. (2012) indicate that there isclear asymmetry in the ionospheric Pedersen current andelectric field between the conjugate SAPS flows, while theestimated cross-SAPS electric potential drop showed consistent variations. These results suggest that SAPS are quite
dynamic but controlled in such a way that the cross-SAPS
potential difference stays constant between the two hemispheres despite the occasional significant differences in thedetails of the electrodynamics between the conjugate
hemispheres.
Temporal dynamics of SAPS: Introduction

SuperDARN observations are characterized by high temporal resolution, the continuous nature of measurements,and large spatial coverage, which make them a formidable
asset to study the longevity and the temporal evolution of
SAPS-related phenomena at both large (up to 1000s of kilometer) and small (of the order of typical range-gate of theSuperDARN radar, i.e., 50 km). In the beginning, a recent
statistical study by Kunduri et al. (2017) of the occurrence of SAPS is introduced. Analyzing 4 years ofdata from the US mid-latitude radars, they found that
SAPS are observed at all levels of geomagnetic disturbance but with a dependence on disturbance level(characterized by Dst) such that quiet times have low

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Fig. 13 Conjugate radar observations of a SAPS flow channel for two different periods showing excellent agreement in the latitude of thechannel between BKS and FIR. Reproduced from Fig. 6 of Grocott et al. (2011)

occurrence rates (~ 10%) while storm times have very
high rates (approaching 100%). The SAPS feature isoften clearly defined against a background of lower
latitude, low-velocity sub-auroral scatter.
Lifetime/longevity and seasonal dependence of SAPS

From SuperDARN observations at mid- and sub-auroral
latitudes, a wide range of SAPS durations have been recorded, from a relatively short 30 min up to more than
6 h (Parkinson et al. 2003a, 2006; Koustov et al. 2006;
Oksavik et al. 2006; Grocott et al. 2011), in agreementwith studies based on other experimental facilities (e.g.,Anderson et al. 1991, 2001; Foster and Vo 2002). A large
fraction of these studies referred to substorm-time SAPS
in its narrow and short form of PJ/SAID, while longer
SAPS events were observed mostly by mid-latitude radars which are better suited for storm-time observations(Oksavik et al. 2006; Grocott et al. 2011).
In order to investigate possible seasonal trends in theoccurrence and properties of SAPS, a statistical study
was performed using observations of SAPS-like flows
from the KSR radar (Koustov et al. 2006). This study
shows that higher velocities occur over a larger MLT extent during winter months. The extent and magnitude offlows decrease through equinoctial and summer months.
These results agree with the interhemispheric studies of
SAPS by Kunduri et al. (2012) who show lower velocities
are observed in the summer Northern Hemisphere (BKSand WAL observations) than in the winter Southern
Hemisphere (FIR observations), but are in slight disagreement with another interhemispheric study by Parkinson et al. (2005), who show that summer Southern
Hemisphere velocities (TIG) are slightly higher than in
the winter Northern Hemisphere (KSR). To resolve theissue about seasonal dependence of SAPS strength andits controlling factors, a complementary statistical study

with one or several mid-latitude radars would clearly be
useful.
Evolution and variability of SAPS

Many SAPS events have been unambiguously related to
substorm activity (e.g., Anderson et al. 1993). At large
scales, a great deal of variability is observed during thecourse of SAPS lifetime, with latitudinal position, width,and flow velocity exhibiting significant variations with time(see Parkinson et al. 2006; Koustov et al. 2006; Oksavik et
al. 2006; Makarevich and Dyson 2007; Kataoka et al. 2007;
Ebihara et al. 2009; Makarevich et al. 2011; Clausen et al.
2012). Such variations are also clearly observed in the SAID
structure whether embedded in SAPS or not (Oksavik et al.
2006; Grocott et al. 2011; Kunduri et al. 2012), with the observed thinning of the SAID being possibly related to atransition between successive events (Grocott et al. 2011).
Moreover, Makarevich and Dyson (2007) and Clausen et al.
(2012), thanks to extensive spatial coverage provided byseveral radars, also show that velocity intensifications inside
SAPS are tremendously different with respect to longitude,
as illustrated in the right panel of Fig. 12. In addition, Clausen et al. (2012) show that large-scale variations of SAPS
speed are not accompanied by variations of SAPS flow direction which remain extremely stable throughout thecourse of the event at given longitudes.
SuperDARN observations of SAPS in conjunction
with global auroral observations have demonstrated
the important role played by auroral dynamics on thespatiotemporal evolution of SAPS. Makarevich et al.
(2009) studied the temporal evolution of SAPS in thecontext of simultaneous auroral dynamics provided by
the International Monitor for Auroral Geomagnetic
Effects (IMAGE) spacecraft and clearly show that auroral dynamics affect the latitudinal position of the
SAPS poleward boundary as well as the SAPS

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intensity, but not the SAID position. It is demonstrated that equatorward motion of the aurora can effectively compress the SAPS flow channel into its
narrow SAID form while simultaneously strengthening
it. Another effect of the auroral expansion and associated compression of the convection streamlines is an
eastward “mirror” flow channel on the poleward sideof the SAID. Furthermore, reconfiguration of the convection pattern during substorms results in the concurrent development of the Harang discontinuity andan intensification of SAPS, with strong coupling between them observed in most cases (Zou et al. 2009).
A significant fraction of SAPS events observed by SuperDARN have been associated with geomagnetic storms.
Kataoka et al. (2007) and Ebihara et al. (2009) explain the
dynamical character of the rapid westward flow observed in
SAPS as a direct manifestation of spatiotemporal variationsof plasma pressure in the inner magnetosphere and ring

current during storms. Their results are based on the comparison with the numerical modeling of the inner magnetosphere. Figure 14 shows a comparison between the
SuperDARN HOK observations of SAPS structure indicating temporal changes (top panel) with simulation resultsfrom the comprehensive ring current model (CRCM) undervarious boundary conditions (Ebihara et al. 2009). The middle panel shows the simulation result with the outer boundary condition determined by the energetic particle data of
the Los Alamos National Laboratory (LANL) geosynchronous spacecraft while the bottom panel shows the simulation
result with a fixed boundary condition (uniform and constant plasma density). These results clearly indicate the importance of ring current dynamics in controlling the SAPS
spatial structure.
High temporal resolution (1-min) of SuperDARN measurements also enables observations of small-scale variability inside SAPS. Several studies have demonstrated

Nishitani et al. Progress in Earth and Planetary Science

a

b

c

Fig. 14 Range-Time-Parameter plots of the Doppler velocities observed by beam 7 of the Hokkaido East radar (a) and calculated using the CRCM
numerical simulation with the plasma sheet boundary conditions determined by the LANL particle data (b) and with fixed boundary condition(c). Reproduced from Fig. 5 of Ebihara et al. (2009)

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the existence of strong flow pulsations during SAID periods (Parkinson et al. 2003a, 2005; Koustov et al. 2006;
Oksavik et al. 2006; Koustov et al. 2008; Makarevich andBristow 2014). These pulsations exist in the form of flow
channels of extremely high westward velocity (between 1
and 2 km s−1) that are localized in latitude (~ 50–100km) and have periods between 10 and 15 min (Parkinson
et al. 2003a, 2005; Koustov et al. 2006; Oksavik et al.
2006; Koustov et al. 2008). Lower periods, down to 1–2min, and hence higher frequencies (Makarevich andBristow 2014) and higher velocities up to 4 km s−1 (Parkinson et al. 2003a) have also been observed. These
structures appear close to the equatorial edge of SAPS
and the ionospheric trough and generally propagate toward higher MLAT (Koustov et al. 2006; Makarevich
and Bristow 2014). The cause of these pulsations has
been attributed to ultra-low frequency (ULF) wave activity, as shown by a correlation with spectral width (Parkinson et al. 2005) or to SAPSWS activity (Mishin et al.
2003; Makarevich and Bristow 2014).

generally show a two-stage evolution, with initiallyrelatively weak velocities followed by strong intensifications between 90 and 120 min after onset (Makarevich and Dyson 2007). This intensification phase starts, ingeneral, at the end of the substorm expansion phase or
during the recovery phase and ceases at the end of the recovery phase (Parkinson et al. 2003a, 2006; Koustov et al.
2006; Makarevich et al. 2009). The narrow SAID feature ismore likely to occur during this later stage of SAPS evolution (Oksavik et al. 2006; Parkinson et al. 2006; Makarevich and Dyson 2007). SAPS are also observed during themain phase of storms when several successive substorms
and associated auroral intensifications are observed (Grocott et al. 2011; Makarevich and Bristow 2014).

Nishitani et al. Progress in Earth and Planetary Science

Occurrence frequency of SAPS and their dependence on
substorm phase

As previously stated, SAPS are often closely related to
substorm processes. Parkinson et al. (2006) have estimatedthe percentage occurrence rate of strong westward flows
associated with SAPS-like feature of AWFC over 12months of measurements from TIG and found a rate ofabout 40%. Yet, with only one radar covering SAPS longitudes for only 25% of the time due to the Earth’s rotation,
they argued that the occurrence rate could be up to fourtimes larger. The implication is that SAPS could be occurring almost every night, making it a recurrent sub-auroral
process persisting for several substorm cycles (Makarevichand Dyson 2007; Koustov et al. 2008; Zou et al. 2009;
Makarevich et al. 2009), but not necessarily observed due
to a non-favorable radar location or propagation conditions. A comprehensive statistical study using the larger
longitudinal coverage of the existing mid-latitude SuperDARN radars will reduce the speculation in these results.
Great attention has been given to relating SAPS onset
and intensification to the different phases of the substorm
process. Koustov et al. (2006) and Makarevich and Dyson(2007) have shown that SAPS generally appear or undergoa marked increase, if already present, between − 5 and + 60
min after a substorm onset, with the negative delay being
due to a pre-existing substorm/SAPS event. Makarevichand Dyson (2007) examined several SAPS events seen simultaneously by TIG and TIGER Unwin radar (UNW) in
conjunction with a substorm onset as seen by IMAGE, andconclude that the time delay between substorm onset
and SAPS flow enhancement is likely due to the finite
propagation time from the substorm injection location
at substorm onset to the position of the radars. SAPS

Implications for SAPS formation scenarios

Observations from SuperDARN radars, particularly at
mid-latitudes, have been extremely beneficial in advancing our understanding of the underlying cause of SAPS.
As explained in the previous subsections, the MI feedback process that is believed to play an important role in
SAPS evolution implies that the sub-auroral electric field
intensification should be accompanied by a decrease in
the ionospheric electron density (e.g., Wolf et al. 2007).
Makarevich et al. (2011) and Makarevich and Bristow(2014) exploited the continuous nature of SuperDARN
observations by also examining simultaneous and continuous measurements of GPS TEC during a SAPS
event. A general decrease of electron content inside
SAPS during the first stage of weak SAPS evolution wasseen, suggesting that the long-term variation of SAPS
electron content is largely caused by a continuous depletion of ionospheric density and supporting the idea ofthe current generator scenario at large scales. On theother hand, an opposite trend was observed inside SAPS
with an overall TEC increase during the second stage of
SAPS evolution that is associated with SAID intensification and the appearance of SAPSWS (Fig. 15a from
Makarevich and Bristow 2014). This important result
was interpreted by Makarevich and Bristow (2014) interms of cross-scale coupling occurring within SAPS,
where net changes in ionospheric conductance and TEC
are positive when SAPSWS activity exists and negative inthe contrary. Finally, Kunduri et al. (2012) also showedthat variations in SAPS peak velocity exhibited a moderatecorrelation with the ASY-H index indicating that local
ionospheric conditions, such as the height-integrated Pedersen conductivity and thus electron density, may havean important influence on SAPS flow channel dynamics.
Figure 15 illustrates some of the important features ofthe plasma velocity—electron density relationship within
SAPS at short time scales (below 20 min) as presented
by Makarevich and Bristow (2014) utilizing close conjunctions between the CVW and a GPS receiver at

Nishitani et al. Progress in Earth and Planetary Science(2019) 6:27

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Fig. 15 a CVW velocity versus universal time (UT) and magnetic latitude (MLAT) on 17 March 2013 between 1150 and 1340 UT and slant ranges
1035–1665 km. The propagation of SAPS Wavy Structure (SAPSWS) 1–4 is shown by yellow lines. The range coincident with Ionospheric Pierce
Point (IPP) at 450 km between PRN 21 and Beaver Cove (BCOV) is shown by the pink line. b CVW velocity at IPP. The four periods with velocities< − 500 m s−1 are highlighted. c GPS TEC measurements at IPP. d Wavelet spectrum of CVW velocity at IPP. e Wavelet spectrum of GPS TEC. The
colored dots show the three highest peaks for each spectrum. Reproduced from Fig. 3 of Makarevich and Bristow (2014)

Beaver Cove (BCOV). Figure 15a illustrates a merging offlow channels with the SAID feature located westward
and poleward (Makarevich and Bristow 2014). Theyshow that each flow channel enhancement related to
SAPSWS is associated with a simultaneous decrease in
TEC, followed by a TEC increase outside of each flow
channel (see panels b and c of Fig. 15). Wavelet spectraof CVW velocities and GPS TEC also exhibited strikingly similar variations (see panels d and e of Fig. 15),
providing additional evidence for the ionospheric feedback instability scenario even at small scales.
In summary, continuous observations of SAPS by
SuperDARN show that the evolution of SAPS is strongly
controlled by the substorm onset location, prior time
history including previous substorm cycles, concurrent
auroral dynamics, MI, and cross-scale coupling processes. In the future, coordinated and distributed measurements will be central to further understanding theunderlying causes of SAPS and SAID. By its continued

expansion, the mid-latitude SuperDARN network will beideally situated to contribute to this effort.
Transients

The distributed and continuous nature of SuperDARN
observations are well suited for studying transient features observed in ionospheric convection. Even operating in the standard mode (stepping sequentially througheach beam to cover the whole FOVs in 1 min) SuperDARN radars are able to resolve many of the spatiotemporal ambiguities that are often encountered inobservations of transient events. Special experimental
modes have also been developed to increase the temporal or spatial resolution of observations in order tostudy particular aspects of transient phenomena.
In addition to storm-time phenomena, such as theprompt penetration electric field (PPEF) associated withover/under-shielding, several other types of transient
electric fields are observed by SuperDARN. These(2019) 6:27

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observations primarily occur in the high-latitude andpolar cap regions owing to their origins in the solar wind
and the favorable coupling on open magnetic field lines
at higher latitudes. Studies of these transients include
the response of large- and meso-scale convection toabrupt transitions in the IMF Bz (Ruohoniemi andGreenwald 1998; Shepherd et al. 1999; Milan et al. 2000;
Nishitani et al. 2002; Fiori et al. 2012) and IMF By (Chisham et al. 2000; Senior et al. 2002) magnetic field components, the solar wind dynamic pressure (Boudouridis
et al. 2007; Coco et al. 2008; Boudouridis et al. 2011; Gillies et al. 2012), and interplanetary shocks (e.g., Kaneand Makarevich 2010). These studies show generalagreement with a two-stage response consisting of aprompt initial perturbation in the convection velocity
seen nearly simultaneously at all MLTs, followed by aslower reconfiguration of the large-scale convection pattern that progresses from the dayside to the nightside.
A particular transient in the solar wind dynamic pressure, known as a sudden impulse (SI), occurs under various IMF conditions and has been studied extensively
with ground-based magnetometer networks. Observations show a large-scale twin-vortex and/or meso-scalepairs of traveling convection vortices (TCVs) associatedwith FACs resulting from a deformation of the magnetopause due to the abrupt change in the solar winddynamic pressure (Araki 1977; Friis-Christensen et al.
1988; Glassmeier et al. 1989). High-latitude SuperDARN
radars have observed TCVs (e.g., Thorolfsson et al.
2001) due to the contracted polar cap under northward
IMF. In one event, Hori et al. (2012) showed the convection electric field associated with a TCV negative SI (andpossible aftershocks) at high-latitude extending into the
sub-auroral region. Effects of the passage of TCVs arealso seen at mid-latitudes in ground scatter returns, as
shown by Kim et al. (2012). Meanwhile, Hori et al.
(2015) demonstrated using the high- and mid-latitude
SuperDARN data that there is a dawn-dusk asymmetryof flow vortices as shown in statistical convection velocity perturbations, caused by IMF By polarity, whichappears associated with negative SI events (dynamic
pressure decrease) that is not seen for positive SI events.
However, most of the observations were poleward of 60°
MLAT, and all measurements were considered not justthose associated with TCVs.
Other studies of transient convection phenomena include observations of bursty flows associated with dayside reconnection, such as flux-transfer events (FTEs)(Pinnock et al. 1993; Provan et al. 1998; Wild et al. 2001,
2003)—results mainly based on high-latitude SuperDARN observations—and on the nightside associatedwith substorm phenomena such as auroral beading, auroral streamers, and poleward boundary intensifications(PBIs) (Lyons et al. 2011; Gallardo-Lacourt et al. 2014a,

2014b; Lyons et al. 2015, 2016), with the latter studies
demonstrating a connection between enhanced nightsidepolar cap flows and the triggering of PBIs, auroral
streamers, and substorm onset, with associated SAPS
and SAID flows.

Nishitani et al. Progress in Earth and Planetary Science

Convection: Future directions

There are currently ten SuperDARN radars locatedequatorward of 50° MLAT in the Northern Hemisphere.
The expansion of SuperDARN to mid-latitudes has beena demonstrated success with previously unattainablemeasurements of the convection electric field equatorward of ~ 60° MLAT and associated contributions to avariety of phenomena occurring at mid-latitudes. Plans
are in place to build several more mid-latitude radars in
the European and Asian sectors. These radars will
provide critical measurements in these regions, enablingnear-continuous monitoring of convection in the
mid-latitude region and allowing several unansweredquestions to be addressed, such as the statistical natureof SAPS occurrence and the origins of SAPS and SAID.
The latter will benefit from more continuous observations at mid-latitudes but will also require complementary measurements of the electrodynamic parameterssuch as the electrical current and conductivity.
Another convection topic that mid-latitude SuperDARN will have a sizeable impact on is the specificationof statistical convection patterns or convection maps.
Several studies of the climatology of large-scale convection have been performed using SuperDARN observations. These studies bin the LOS radar data according tovarious combinations of the IMF magnitude, IMF clock
angle, solar wind speed, and dipole tilt angle in order toobtain a global solution of the electrostatic potential that“best fits” the radar observations in each bin. As moreradars became operational the statistics for any given bin
increased, thereby improving the quality of the solutions
and allowing for additional binning parameters or/and
finer bin resolution. Ruohoniemi and Greenwald (1996)
used data from only the GBR radar to construct the first
set of statistical convection maps. Ruohoniemi andGreenwald (2005) then performed a similar analysis
using the nine high-latitude SuperDARN radars available
at that time. Pettigrew et al. (2010) re-analyzed theNorthern Hemisphere, adding dipole tilt angle bins andseparate model patterns produced using the Southern
Hemisphere radars, in order to study the conjugacy between hemispheres. Finally, Cousins and Shepherd(2010) added solar wind velocity bins and the ability tointerpolate between discrete patterns in order to achievea more dynamical model of the large-scale convection inboth hemispheres.
In all of these studies, data from only the high-latitude
SuperDARN radars were used, resulting in limited data(2019) 6:27

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coverage in the polar cap and the mid-latitude region,
particularly during geomagnetically active periods. A
new statistical model has recently been developed thatuses measurements from the polar cap and mid-latitude
radars to complement the high-latitude radars (Thomas
and Shepherd 2018). The resulting patterns reinforce
what Baker et al. (2007) show in Fig. 9, namely that theaddition of data from the mid-latitude radars has a dramatic effect on the overall convection strength, increasing the cross-polar cap potential by as much as 25%(40% in Thomas and Shepherd 2018), and markedly
changing the potential contours on the nightside, wherea region of westward flow extending from dusk (18MLT) to past midnight (24 MLT) is observed equatorward of ~ 60° MLAT by the mid-latitude radars.

produce the ionization irregularities that represent viabletargets for backscattering with low-power HF coherent
scatter radars. Given the nearly continuous excitationthat is due to auroral processes, it is no surprise thationization irregularities are a nearly ubiquitous featureof the auroral ionosphere. The early SuperDARN radars
were sited approximately along an arc of 60° MLAT inorder to look into the auroral region and sense theseirregularities.
When the first radar specifically intended for making
observations in the mid-latitude ionosphere was proposed for the WAL site, the primary scientific motivation was to enable observations of the expansion ofauroral effects during geomagnetic storms, leading to
the term “StormDARN.” It was not apparent that the
sub-auroral ionosphere, defined as that region lying
equatorward of the electron precipitation boundary,would provide extensive irregularity targets. In fact, acategory of irregularities responsible for low-velocity
backscatter was found to populate the nighttime
sub-auroral ionosphere (Greenwald et al. 2006). Evidently, there are sources of plasma instability for generating irregularities with ample backscattering crosssections in an ionosphere that is markedly less structured and dynamic than in the auroral regions. The term
“StormDARN” has now been rendered obsolete by theattention given to the non-storm categories of
mid-latitude backscatter. Indeed, HOK, which is sited at
the lowest MLAT of any SuperDARN radar, observes irregularities that are so far removed from contact withauroral processes (equatorward of the electron precipitation boundary (EPB) by more than 10°) that it seemed
appropriate to categorize them separately as temperate
mid-latitude irregularities.
Table 2 summarizes the categories of mid-latitude irregularities for the purposes of this review. It should be
noted that the position of the electron precipitation
boundary (i.e., the auroral boundary) is key to these definitions as a fixed MLAT may at different times correspond to the auroral zone, the equatorward boundary
zone (with SAPS), or to the sub-auroral region, withpronounced differences for the physics of the ionosphereand the occurrence of irregularities.

Nishitani et al. Progress in Earth and Planetary Science

Ionospheric irregularities
The ionosphere is a weakly ionized plasma that is subject to structuring and plasma instability. The SuperDARN HF radar technique relies on the emergence of
decameter-scale irregularities in the plasma density for
generating coherent backscatter from the ionosphere. In
this section, the findings to date on the occurrence,
properties, and causes of the irregularities that are commonly observed at mid-latitudes by the SuperDARN radars are reviewed. As noted previously, the expansion of
the auroral oval during disturbances can cause the
mid-latitudes to be “auroral” in nature; hence,auroral-type irregularities fall within the scope of this review. Descending in latitude, irregularities are observed
to map out features near the edge of the auroral oval,e.g., SAPS, while at lower latitudes one encounters categories of irregularities which are distinctly sub-auroral.
Introduction of ionospheric irregularities

At auroral latitudes, the ionosphere is impacted by electric fields and particle precipitation of magnetosphericorigin. The resulting perturbations from the equilibrium
state of the plasma, observable as bulk particle motions,density gradients, temperature gradients, etc., represent
sources of free energy for triggering instability in theplasma (e.g., Fejer and Kelley 1980). Thermal fluctuations in electron density can be greatly amplified toTable 2 Categories of mid-latitude irregularities with respect to the Equatorward Precipitation Boundary (EPB)
Location in relation to the EPB

Occurrence/properties

Auroral irregularities

Poleward of the EPB

Observed at nearer ranges during storms, high-speed flow(~ 100s m s−1)

SAPS irregularities

On the equatorward side of the EPB andusually extending only a few degrees in latitude

Occurrence increases with disturbance level; westward,
high-speed flow (~ 100s m s−1) in the pre-midnight sector

Sub-auroral irregularities

Extending equatorward of the EPB indefinitely

Nightside, low-speed flow (~10s m s−1), predominantly
westward

Temperate mid-latitude irregularities

Equatorward of the EPB by more than 10°

Strong association with MSTIDs

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During quiet geomagnetic conditions, the equatorward
edge of the auroral oval reaches as low as 60° MLAT on
the nightside. The ground range to the edge of thequiet-time oval on the nightside is around 1200 km for
the US mid-latitude radars and around 2500 km for the
Hokkaido Pair of Radars (HOP). It is routine for the radars to generate ionospheric scatter at the shorter
ground range and relatively frequent at the longer
ground range. Consequently, the mid-latitude SuperDARN radars do observe backscatter from auroral irregularities even under non-storm conditions but any
activity observed within the oval is then likely due topropagation modes that are less favorable for analysis
than the half-hop mode. When there is significant geomagnetic disturbance and the oval expands equatorward,
the mid-latitude radars encounter the oval at nearer
ranges and, under the most disturbed conditions (Kp > 7
for the U.S. radars), the equatorward edge of the oval
reaches to the near-range boundary of the radar FOVs.
The radar observations are then dominated by backscatter from auroral irregularities. To date, there has notbeen an expansion event so extreme that the oval
boundary has reached to the near-range boundary of the
HOP radars, which are located at the lowest MLAT
among the SuperDARN radars. It is noteworthy that the
expansion that occurred for the historically extreme
event of March 1989 would have reached well beyond
the near-range boundary of the HOP radars (Rich and
Denig 1992).
The earliest work on the occurrence of irregularities
at mid-latitudes was reported with radars in theSouthern Hemisphere. With MLATs near 55° S TIG
and UNW reach into the auroral zone on the nightside at moderate ground ranges (~ 600 km), consequently, their observations routinely encompass boththe sub-auroral and auroral regions. Parkinson et al.

(2003b) produced maps of the occurrence of backscatter from TIG observed over 1-year near solar
cycle maximum and ordered by Kp. Comparing withmodel auroral boundaries, they determined that thebulk of the backscatter originated from within theoval although categories of both polar cap andsub-auroral backscatter were also identified, as shownin Fig. 16 (Parkinson et al. 2003b). In particular, they
noticed a high-velocity feature at the equatorward
edge of the oval in pre-midnight hours that would belater attributed to backscatter from SAPS. F region
echoes were found to be most frequent in autumn
while E region echo occurrence peaks in winter andsummer.
Subsequent work in the Southern Hemisphere using
TIG and UNW focused on E region backscatter usually
defined as having been collected from ranges lessthan about 600 km. Using 3 years of data, Carter and
Makarevich (2009) found a primary occurrence peak
for backscatter in the mid-morning sector (7–8 MLT)
and a secondary peak in the pre-midnight sector (20–23 MLT) for higher Kp values. There was evidence
that the occurrence of the evening echoes was moreconditioned by the magnetic aspect geometry owing
to limited refraction due to low E region densities
during disturbances. They reported two categories of
echoes, one with low velocities and no connection
between velocity magnitude and spectral width and asecond, more apparent during storms, with higher
velocities and a clear dependence of spectral width onvelocity magnitude. The difference in backscatter
properties is almost certainly due to the appearanceof auroral irregularities within the designated range
interval as the oval expands.
Carter and Makarevich (2010) extended the study of E
region backscatter to cover a total of 15 radars over thesame 3-year period. As the bulk of the radars operate at

Nishitani et al. Progress in Earth and Planetary Science

Auroral irregularities

Fig. 16 The occurrence rate (%) of ionospheric echoes from beam 4 of TIG detected during all seasons, and sorted according to geomagnetic
activity Kp > 1 to 2; Kp > 3 to 4 and Kp > 5 to 6. Model auroral oval boundaries are superimposed for AL = − 64 nT (Kp = 2−), AL = − 240 nT(Kp = 4−), and AL = − 458 nT (Kp = 6−). Adapted from Fig. 5 of Parkinson et al. (2003b)

(2019) 6:27

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high latitudes, the contrasts with TIG were illuminating.
TIG had peak occurrence at 7–8 MLT under quiet conditions while the high-latitude radars had a broad occurrence peak extending across the nightside with apreference for conditions of moderate disturbance. The
occurrence rate fell off steeply for TIG for Kp > 5. The E
region echoes observed by TIG at sub-auroral latitudes
showed a fundamental difference from the higher
latitude radars in terms of their diurnal variation and its
dependence on disturbance level. The implication is thatstorm-time irregularities at nominally sub-auroral latitudes differ significantly from high-latitude auroral
irregularities.
Ponomarenko et al. (2016) reported on verynear-range echoes (< 300 km) seen with mid-latitude radars. They identified meteor scatter, which peaks in theearly morning hours, and a new category of summer
daytime echoes hypothesized to be due to neutral turbulence in the lower E region. In addition, they identified
an evening E region echo associated with the auroral
region which they attributed to precipitating energetic
particles and strong electric fields. One notes that thisactivity must be associated with very high levels of disturbance in order to have auroral effects projected tosuch low latitudes. It was also speculated that thehigh-aspect angle region (HAIR) echoes (Milan et al.
2004), previously reported at sub-auroral latitudes(Carter and Makarevich 2009) and polar cap latitudes,
might contribute to a surprisingly wide variety ofnear-range echo types.
In summary, because of the long-range nature of HF
propagation, SuperDARN radars at mid-latitudes canobserve backscatter from auroral irregularities evenunder quiet geomagnetic conditions, albeit at ranges thatare less favorable for study than is the case forhigh-latitude radars. The more interesting applicationsof the mid-latitude radars are (i) to observe andcharacterize the storm-time auroral irregularities at
mid-latitudes and their impacts on regions that are usually sub-auroral and (ii) to compare the infrequent
mid-latitude auroral irregularities with the commonlyoccurring high-latitude auroral irregularities to see if differences in geomagnetic inclination, density distribution,
preconditioning of the ionosphere-thermosphere system,
etc. affect the conditions for ionospheric plasma instability and irregularity formation. The work to date has
demonstrated that the nature of the backscatteringchanges when the oval expands and auroral irregularities
begin to appear in the nearer ranges of the radars. Some
results on E region auroral irregularities have been obtained from the observations in the Southern Hemisphere. There is still clearly much to do, especially for F
region irregularities. An example of a promising direction is to relate the occurrence of storm-timeirregularities to scintillations on GNSS signal. Anotheris to conduct joint observations with mid-latitude incoherent scatter radar to obtain the necessary plasma
diagnostics to determine whether the irregularity
types and instability mechanisms that have been studied at high latitudes can account for the mid-latitude
irregularities.

Nishitani et al. Progress in Earth and Planetary Science

SAPS irregularities

Descending in latitude from the auroral zone, the electron precipitation boundary is encountered and sets thelocation of the equatorward auroral boundary. Within
the vicinity of this boundary, a distinctive convection
feature is often observed, the SAPS. SAPS is characterized by high westward velocity (hundreds to thousandsof meter per second), a narrow extent in latitude (a few
degrees), and extension in the east-west direction of
thousands of kilometers. It is believed to be associatedwith an equatorward displacement of the ion precipitation boundary from the electron precipitation boundary,whence it earns its designation as sub-auroral. However,
it is quite clearly associated with the auroral boundary
and its physical nature is so distinct from both higher
auroral latitudes and lower sub-auroral latitudes as to beconsidered a distinct category for the purpose of discussing irregularities and their properties. A detailed treatment of SAPS as observed by the mid-latitude
SuperDARN radars is given in the “Convection” section;
here, the implications for understanding this category of
mid-latitude irregularities are briefly reconsidered.
It appears that SAPS produce small-scale irregularities
and that propagation conditions under normal conditions do not seriously limit their detection, at least for
the US mid-latitude radars. The SAPS flow channel isassociated with the mid-latitude trough, as was seenclearly in the example presented by Clausen et al. (2012)using TEC data. Thus, both large electric fields andsteep density gradients are available during SAPS events
to destabilize the plasma and generate irregularities. An
obvious candidate mechanism is the gradient drift instability (GDI). At F region altitudes, this instability has
a positive growth rate if the plasma drift has a component in the direction of the density gradient. The nominal combination of westward SAPS flow and poleward
orientation of the poleward wall of the trough would notseem favorable. However, a careful examination of actual
events might reveal a small but sufficient degree ofalignment.
In summary, the SAPS phenomenon is critical for our
understandingofmagnetosphere-ionosphere-thermosphere coupling. It isa well-defined, repeatable, and somewhat predictablefeature of the ionosphere particularly under disturbed
conditions. The high visibility of SAPS to HF coherent(2019) 6:27

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scatter radar must be due to the self-generation ofsmall-scale irregularities. Determining the instability
mechanism responsible would represent a significant
advance for our understanding of plasma instability andattract the attention of a wider plasma physics community. More joint studies with systems that can provide
diagnostic measurements (Low Earth Orbit (LEO)
satellites, Incoherent Scatter Radars (ISRs)) should besought, and a challenge could be issued to ionosphericmodelers to reproduce the full spectrum of SAPS-related
effects, including the occurrence of small-scale irregularities in the mid-latitude SuperDARN radar observations.

sub-auroral ionosphere often generates backscatter(Greenwald et al. 2006). The activity persists for hours at
a time and is associated with low Doppler velocities. The
Greenwald et al. (2006) study utilized complementary
observations with the Millstone Hill ISR to argue thatthe irregularities could be due to, or enhanced by, thetemperature gradient instability (TGI) which requires
oppositely directed electron density and electron
temperature gradients. Figure 17 (Greenwald et al. 2006)shows the intensification of backscatter on a beam of
WAL that was directed toward Millstone Hill that
followed a reversal in the electron temperature gradientin the F region ionosphere at the time indicated. This
event was re-examined by de Larquier et al. (2014). By
taking account of the substantial tilt of the mid-latitude
geomagnetic field lines, they determined that the TGI
growth conditions were in fact satisfied in the F region
for the duration of the experiment. They attributed theweakness of the earlier activity to the shorting effect ofdaytime E region conductivity. A companion paper by
Eltrass et al. (2014) examined the source of the irregularities during this event in terms of first-principles plasma
physics. They developed the kinetic dispersion relations
for the TGI and the GDI in the frequency regime of theSuperDARN radars and solved for the growth rates inboth the meridional and perpendicular directions. They
determined that the growth rate for the TGI was muchhigher than that of the GDI throughout the event. It wassuggested that instability in the topside F region leads to

Nishitani et al. Progress in Earth and Planetary Science

Sub-auroral irregularities

The region of the ionosphere that extends equatorwardof the electron precipitation boundary is conjugate to
the inner magnetosphere and lacks obvious strong
sources of electric field. Studies with ISRs have revealed
the existence of 1–2 mV m−1 electric fields corresponding to drifts measured in tens of meters per second. No
significant particle precipitation occurs at these latitudes,
and so the ionospheric density distribution is far lesssubject to structuring and the formation of density gradients. These conditions would seem unpromising forthe onset of plasma instability leading to irregularities
especially at the decameter-scale lengths necessary for
generating HF coherent backscatter. Nonetheless, fromthe earliest observations with the first purpose-built
mid-latitude radar (WAL), it was clear that the nightside

Fig. 17 The association of quiet-time sub-auroral irregularities with gradients in ionospheric electron density and temperature. The top panel
shows time series of the electron density and electron temperature gradients measured by the Millstone Hill incoherent scatter radar. The pink
triangle marks the time that the gradient in electron temperature reversed from poleward to equatorward, establishing favorable conditions for
the TGI. The bottom panel shows a time series of backscatter power from one beam of the Wallops radar and the increase in backscatter powerthat followed the reversal. Adapted from Fig. 1 of Greenwald et al. (2006)

Nishitani et al. Progress in Earth and Planetary Science(2019) 6:27

growth of kilometer-scale irregularities that mapped via
their electric fields to lower altitudes and populated the
F region with smaller-scale irregularities through aprocess of cascading. This series of papers culminated ina study by Eltrass et al. (2016) that carried the earlier
analysis into the nonlinear (saturation) regime by
applying a gyrokinetic simulation model involving
particle-in-cell (PIC) calculations with Monte Carlo collisions. They found that conditions were favorable forlinear growth in the kilometer-scale regime and turbulent cascade to decameter-scale irregularities in the saturation stage. Analysis of GPS scintillation data revealed
a spectral index n, where the density perturbations δN
can be represented by a power law of wavenumber k−n,of about 2 for the mid-latitude irregularities with scale
sizes less than a few kilometers.
Sub-auroral ionospheric backscatter was initiallyconfused with ground scatter because of its low velocities and narrow spectral widths. The algorithm that
had been developed at high latitudes for distinguishing between the two types of backscatter in routine
work was found to be inadequate. Ribeiro et al.
(2011) developed a new technique based on a “depth
first search” method of identifying discrete events andtheir start and stop times and the locations of theirboundaries in range (see the “Propagation analysis:backscatter classification and geolocation” subsection
for details). These authors also coined the expression
sub-auroral ionospheric scatter (SAIS) to identify thistype of activity. Ribeiro et al. (2012) applied the technique to several years of observations with BKS andfound that SAIS is confined to local nighttime andoccurs on about 70% of nights. The absence of SAIS
during sunlight was attributed to the shorting effectof a conducting E layer.
de Larquier et al. (2013) performed a study of the altitude of the irregularities causing SAIS using well-calibrated elevation angle data from BKS. They found thatthe primary source region extends between 200 and 300
km in altitude, i.e., occupies the bottomside F region.
Figure 18 shows an example of the variation of the altitude of SAIS irregularities with range from a radar (deLarquier et al. 2013). It was determined that the spatial
distribution of SAIS backscatter within radar scans is
primarily a function of propagation conditions and magnetic aspect geometry implying that the irregularities arewidely distributed in the horizontal dimension.
In the Southern Hemisphere, the statistical study ofthe occurrence of backscatter with TIG reported byKane et al. (2012) is relevant to this discussion of
sub-auroral irregularities. They found a decrease inquiet-time F region backscatter when the Pedersen conductance modeled on the basis of IRI is high, as shownin Fig. 19. This finding is a convincing demonstration ofPage 24 of 57

Fig. 18 Observations of the altitude of the irregularities associatedwith SAIS from beam 12 of the Blackstone radar near local midnighton 18 November 2010. The two solid curves mark the region where
the magnetic aspect angle is within 1° of perpendicularity.
Reproduced from Fig. 7 of de Larquier et al. (2013)

the shorting effect of a conducting E layer on the occurrence of small-scale mid-latitude F region irregularities.
Ruohoniemi et al. (1988) described GBR radar observations of a sub-auroral backscatter feature that regularly occurs near dusk. Subsequently, the topic wastreated comprehensively over many radars and the backscatter named by Hosokawa et al. (2001) as dusk scatter
event (DUSE). Hosokawa and Nishitani (2010) applied
HOK and KSR data to determine that the center of
DUSE corresponds to the minimum of a modeled

Fig. 19 Daytime TIG echo occurrence versus Pedersen conductancefrom the IRI model. Each point is color coded according to thecorresponding value of the F region peak density NmF2, with the scale
shown on the right. Reproduced from Fig. 5 of Kane et al. (2012)(2019) 6:27

Page 25 of 57

trough, indicating an association with the poleward wallof the trough. Once again, the strong density gradient in
the poleward wall of the trough is thought to play a rolein generating irregularities.
In summary, the occurrence of coherent backscatter
using the mid-latitude radars from latitudes equatorwardof the equatorward edge of the auroral oval is now
well-established. One category, namely SAIS, is aprominent feature of the nightside observations. It ischaracterized by low Doppler velocities and small
spectral widths such that care must be exercised todiscriminate it from ground scatter. A number of observational results indicate that the conducting E layer
during the day suppresses the occurrence of F region
irregularities. Theoretical work has demonstrated that
the TGI and GDI combined with turbulent cascade
can account for SAIS. More varieties of sub-auroral
backscatter have been reported. It is noted that the
sub-auroral ionosphere has advantages over thehigh-latitude ionosphere for resolving the backgroundplasma parameters with sufficient confidence and precision to test ideas of plasma instability. One research
direction would be to model on first principles theshorting effect of E region conductivity. Anotherwould be to adapt the modeling work performed on
SAIS events to routinely account for the occurrenceor non-occurrence of SAIS.

Nishitani et al. Progress in Earth and Planetary Science

Temperate mid-latitude irregularities

The SuperDARN radars on Hokkaido are located at ~37° N MLAT which is more than 10° lower than the latitudes of the US mid-latitude radars. This difference iscomparable to that between the auroral and US radars.
Consequently, it seems appropriate to allow for the
possibility of significant variations between the “higher”
sub-auroral latitudes and the “lower” sub-auroral latitudes observed by the HOP radars. The irregularities
encountered routinely in HOK/HKW observations aredesignated as temperate mid-latitude in type.
Ogawa et al. (2009) described backscatter from the E
and F regions on the nightside that is associated withthe passage of medium-scale traveling ionospheric disturbances (MSTIDs) through the radar FOV. Figure 20
shows an example. The MSTIDs were found to be
propagating in southeast/southwest directions withphase speeds of 120–170 m s−1 and horizontalabcFig. 20 Keograms from the Hokkaido radar of a radar echo power on beam 15 and airglow intensity deviation and b Doppler velocity and c
airglow intensity deviation on beam 15 and 10 on 11 June 2007. Trace velocities indicated by red lines with numbers 1–4 and number 5 are 120
and 90 m s−1, respectively, and those indicated by black lines with numbers 1–4 and number 5 are 240 and 180 m s−1, respectively. Reproduced
from Fig. 12 of Ogawa et al. (2009)

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wavelengths of 300–600 km. Some activity showed aclear linkage between F region Doppler velocity variations and alternations in power visible as striations intime-range plots. The F region observations were interpreted in terms of E × B instability at the bottom of theF region where the background electric field is modulated by the polarization field generated through interaction of the MSTID-induced neutral wind with theionospheric plasma as explained in detail in Suzuki et al.
(2009). It was conjectured that the E region echoes could
be attributed to electrical coupling of the E and F regions. Suzuki et al. (2009) examined a similar event witha southwest propagating MSTID with a phase speed of
100 m s−1 and a horizontal wavelength of 300 km. The
alternations in Doppler velocity reached magnitudes of
50–60 m s−1. Airglow data were consistent with the radarobservations of striations, propagation, and speed. The
observations were interpreted in terms of the GDI acting
at the base of the F layer with an eastward componentof the electric field. A schematic for the instability isshown in Fig. 21 (Suzuki et al. 2009). The total electricfield consists of the F region dynamo field, E0, and theu × B component generated by the backgroundthermospheric winds. Modulation of u by the neutral
gas motions generated by MSTIDs generates
polarization electric fields parallel or antiparallel to the
propagation directions. The alternating polarization
fields, through E × B drifts, cause variations in the Doppler shifts with opposite signs for the enhanced/depleted
echo power, which also vary due to modulation of the
GDI.
Koustov et al. (2014) made a statistical study of the HF
radar signatures of nighttime MSTIDs at near ranges
corresponding to E region backscatter. They found a reversal in the sense of propagation through midnightfrom southward-southwestward (toward the radar) tonorthward-northeastward (away from the radar). The
average speeds for the two categories are 85 m s−1 and56 m s−1, respectively. The progression in phase fronts isapparent in both the velocity and backscattered power
data and amplitude and velocity are linearly related. The
MSTID motion is attributed to gravity waves propagating through the radar FOV. It is speculated that neutral
turbulence plays a role in generating the E region backscatter during the events.
Yakymenko et al. (2015) studied the statistical characteristics of short-range echoes observed by HOK
generally. Echo occurrence increases at night, andthere is a category of backscatter that occurs in summer in the morning-prenoon sector that can be associated with sporadic E layers. The irregularities are
attributed to the GDI. Echo occurrence does not depend
on Kp or Dst but is anti-correlated with the Ap index indicating that local factors and not high-latitude drivers are
responsible for the onset of irregularities. The authors
suggest that a significant number of these echoes are generated at rather high magnetic aspect angles of several degrees. They argue that non-GDI mechanisms such as
neutral turbulence must account for the appearance of irregularities that are not strongly magnetic field aligned.
The passage of MSTIDs and the onset of meridional
winds are also proposed as generators of ionosphericirregularities.
In summary, the most-studied type of temperate
mid-latitude irregularity is that associated with the passageof nighttime MSTIDs. Both Doppler velocity and backscattered power are well ordered by wave characteristics.
There has been some success in attributing the formationand properties of the F region irregularities to the GDI
with modification of the ionosphere by polarization fields
that are due to MSTID-induced neutral winds. A varietyof echo types is seen at near ranges and neutral winds, either through turbulence or passage of MSTIDs, arethought to play a significant role in the generation of E region irregularities. Two directions for future research are(i) comparison with observations of MSTIDs and their relation to irregularity formation at higher latitudes and, (ii)

Nishitani et al. Progress in Earth and Planetary Science

ent

on

em

anc

enh

leti

dep

v
v
B

J (northeast)u (southeast)(dip angle: 65°)
Fig. 21 Schematic interpretation of the optical and radar events
observed on 8 December 2007 by the Hokkaido radar. The dark grayand light gray shaded regions represent the depletions andenhancements, respectively in the 630-nm airglow intensity.
Reproduced from Fig. 4 of Suzuki et al. (2009)

Nishitani et al. Progress in Earth and Planetary Science(2019) 6:27

modeling of the ionospheric physics to account for thegeneration of irregularities and the properties of the backscatter during these events.

HF propagation analysis
One avenue of research that has matured significantly since
the advent of mid-latitude SuperDARN is the use of HF
propagation analysis for value-added research. These studies have built upon previous efforts at high latitudes whichused propagation analysis to investigate ionospheric irregularities (e.g., Milan and Lester 2001), meteor echoes (e.g.,
Hall et al. 1997), and traveling ionospheric disturbances(TIDs) (e.g., Bristow et al. 1996). Propagation analysis has
several distinct advantages at mid-latitudes, which are summarized in this section, along with some of the key research
results.

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Introduction of HF propagation analysis

HF propagation analysis can be broadly defined as any attempt to precisely determine the refractive pathways (or“modes”) connecting an HF radar to its backscatter targets. At relatively close-ranges, the propagation path tends
to be quasi-linear and thus specified completely by theslant range and angle of arrival measurements. However,as the range to the target increases, accurate specificationof the propagation mode becomes increasingly more ambiguous because the amount of ionospheric refraction depends on the electron density which is a highly variable
function of altitude, latitude, LT, and geomagnetic conditions. For this reason, propagation analysis at ranges beyond a few hundred kilometers is typically done using
simulations from an HF ray-tracing code (e.g., Jones andStephenson 1975) coupled with an empirical ionospheremodel such as IRI (e.g., Bilitza et al. 2014).

Fig. 22 Classification of LOS Doppler velocities from beam-7 of the Blackstone radar on 15 January 2010 according to a the traditional point-bypoint threshold method (top) and b the new clustering method (bottom). Ionospheric scatter is colored according to the scale at right while
ground scatter is shaded gray. Reproduced from Fig. 3 of Ribeiro et al. (2011)

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The over-arching purpose of propagation analysis
tends to fall into three broad categories: (1) backscatterclassification and geolocation, (2) ionospheric morphology and dynamics, and (3) radar performance assessment and calibration. The studies reviewed in thissection are therefore categorized under these three subsection headings. To minimize overlap with other sections, the focus of the discussion here is primarily on
how propagation analysis has been used as a tool to improve a methodology and/or scientific interpretation ofradar data.

these near-range echoes were found most likely to bemeteor backscatter, whereas those near local noon during midsummer were either PMSEs or produced bylower E region neutral turbulence. In some cases, particularly statistical studies, the propagation conditions
can sometimes be inferred from the context of the radarmeasurements themselves. For example, Parkinson et al.
(2003b) examined the occurrence of irregularities observed by TIG over a broad range of latitudes and found,among other things, that nightside echoes were typicallyobserved via 1.5-hop propagation near dusk and then via
0.5-hop propagation from pre-midnight to dawn.
Other studies have examined the extent to which radar
detection of ionospheric backscatter depends on the occurrence of irregularities versus HF propagation conditions. For example, Kumar et al. (2011) found therange-time evolution of F region ionospheric backscattermeasured by TIG during disturbed periods was partly
dependent on changes in propagation conditions associated with enhanced E and F region densities. Likewise,
Kane et al. (2012) found irregularity backscatter occurrence measured by TIG during geomagnetically quiet
times was most likely controlled directly by the ionospheric density during nighttime but by both densityand propagation conditions during the day.
Finally, using HF propagation analysis for accuratebackscatter classification and geolocation can be an important component of mid-latitude SuperDARN applications totally unrelated to space science. For example,
Greenwood et al. (2011) used data from TIG and UNW
to show SuperDARN radars can remotely monitor ocean
wind-wave directions from Bragg peaks in one or two
hop sea scatters. The dominant wind-wave direction isinferred by constraining a model of the directional sea
spectrum to the ratio of the two Bragg peaks. Goodagreement was found with the WAve Model (Benderand Leslie 1994), on average, but with the radar observations exhibiting far more spatial variability and structure.

Nishitani et al. Progress in Earth and Planetary Science

Propagation analysis: Backscatter classification andgeolocation

A central challenge of SuperDARN data analysis has always been how best to distinguish ground scatter fromionospheric scatter. Traditionally, ground scatter has
been identified by applying lower thresholds of a few
tens of meter per second on both the LOS velocity andspectral width (Blanchard et al. 2009). This simple approach has worked reasonably well at high latitudes because the ionospheric plasma tends to move at speeds ofa few hundreds of meter per second, and is thus moreeasily distinguishable from the ground, in most situations. At mid-latitudes, however, typical convection
speeds are much lower and comparable to the magnitude of Doppler velocity fluctuations expected forground scatter in the presence of dynamic ionosphericmodulations, such as TIDs. To overcome this ambiguity,
Ribeiro et al. (2011) presented a new algorithm for distinguishing ground and ionospheric scatter by identifying closed groupings of connected scatter points andassigning the propagation mode based on characteristics
of data clusters as a whole, rather than point-by-point.
Figure 22 compares the results of the traditional method(top) with the new method (bottom) for 24 h of BKS
data on 15–16 January 2010 (Ribeiro et al. 2011). Ionospheric scatter is colored according to the scale on theright while ground scatter is shaded gray. Local sunrise
is at ~ 13 UT so the left and right portions correspondto night and day, respectively. The lower panel shows
the new algorithm produces a much more sensibleclassification in which there is a clear separation between bands of low-velocity ionospheric scatter during
the night and ground scatter during the day. This slowmoving nighttime scatter is now called SAIS.
Elevation angle data are particularly important for accurate geolocation and distinguishing various sources ofbackscatter. Greenwald et al. (2017) found estimates ofground range and refractive index using virtual heightmodels are rarely consistent with results from ray tracing. Ponomarenko et al. (2016) used elevation angle
data to determine near-range echoes at mid-latitudes
measured by CVE and TIG. In the early morning sector,

Propagation analysis: Ionospheric morphology anddynamics

The second broad objective of HF propagation analysis
is to use radar measurements to identify perturbationsin the intervening ionospheric medium, such as seasonal
anomalies, storm-time effects, or TIDs. For example, deLarquier et al. (2011) used ray-tracing analysis to identify
an anomaly in mid-latitude evening ground scatter measurements such that ground scatter range decreasedaround sunset, rather than increased, as might be expected. Millstone Hill ISR data confirmed that theanomalous behavior measured by BKS on one particularnight was associated with elevated electron densities (seeFig. 23 from de Larquier et al. 2011). Statistical analysis

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abFig. 23 Observations of the mid-latitude evening anomaly on 13 August 2010 seen in a Blackstone radar beam-7 backscattered power and b
Millstone Hill ISR electron density versus altitude. Dashed lines identify solar noon and sunset. Reproduced from Fig. 5 of de Larquier et al. (2011)

showed the anomaly is most pronounced from April to
September (i.e., summer). Analysis with the Horizontal
Wind Model (HWM07) (Drob et al. 2008) suggests it isproduced by equatorward meridional winds pushing
lower layers of the ionosphere up geomagnetic field lines
to higher altitude where recombination is slower. A later
study by Milan et al. (2013) found a similar effect in FIR
radar ground scatter and associated it with the Weddell
Sea Anomaly (WSA) (Horvath and Essex 2003).
SuperDARN radars can also be used to monitor ionospheric conditions in real time, as “oblique ionosondes.”This capability was first demonstrated at high latitudes
by Hughes et al. (2002) by utilizing a special “sounding-mode” of radar operation to determine the maximum
usable frequency (MUF) and F2-layer critical frequency(f0F2) using variations in F region ground scatter skip
distance as a function of frequency. This result was
followed by Bland et al. (2014) who used TIG and UNW
data to show the technique is particularly well suited at
mid-latitudes because the higher ionospheric densities
mean that ground scatter echoes tend to dominate overionospheric echoes. Figure 24 shows the technique produces good agreement with the nearby ionosonde atMacquarie Island (Bland et al. 2014). However, propagation conditions play an important role because the technique was found to be much less effective in wintermonths and away from solar maximum when the 1-hop
F region ground scatter is less distinct and harder to isolate. These problems can be overcome by using elevation
angle measurements to better discriminate E and F region ground scatter, as well as identify contaminationfrom low-velocity ionospheric scatter (i.e., SAIS) andground scatter originating from the back lobe (Milan etal. 1997b).
Other studies have used perturbations in HF propagation conditions to infer the spatiotemporal response ofionospheric density to intense space weather disturbances. For example, Currie et al. (2016) used superposed epoch analysis to examine the range dependenceof backscatter observed by TIG and Kodiak radar (KOD)
during 25 intense geomagnetic storms and found a reduction in backscatter for middle to far ranges but anNishitani et al. Progress in Earth and Planetary Science(2019) 6:27

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Watanabe and Nishitani (2013) examined how theionosphere responds to solar flare events by analyzing
variations in the propagation conditions inferred from
HOK data. Most flares produced a sudden fade-out ofground scatter echoes, consistent with increased
ionization, but they also found positive Doppler shifts in
the ground scatter just before fade-out which were negatively correlated with elevation angle. This behavior isconsistent with increased D region electron density being
the most dominant factor during fade-out events, rather
than a lowering of reflection height due to increased F region density. The space weather impacts of solar flaresand SuperDARN were also studied using multiple radar
data in the recent literature (e.g., Berngardt et al. 2018;
Chakraborty et al. 2018; Fiori et al. 2018).
Propagation analysis: Radar performance assessment andcalibration

Fig. 24 Diurnal variation of foF2 for 12–13 September 2010 calculated
from Bruny Island radar data (black), Unwin radar data (red), and Macquarie
Island ionosonde (blue). Reproduced from Fig. 4 of Bland et al. (2014)

increase at close ranges. Peaks in absorption from anearby riometer showed no effect on the close-range
backscatter suggesting that this result cannot be explained by D region absorption. Instead, ray-tracing analysis suggested enhanced E region density canover-refract middle- to low-elevation rays so they do notreach the F region.

One reason why propagation analysis has been anunder-utilized tool within the SuperDARN community
is the elevation angle data collected at most radars is
not routinely calibrated and therefore tends to be unreliable. Proper calibration requires measuring thephase offset between signals returning to the mainand secondary antenna arrays, which is not trivial,
and its value tends to drift over time. However, Ponomarenko et al. (2015) developed an empirical method
for calibrating angle of arrival data using the contextual information provided within the data itself. Specifically, the phase offset is adjusted so that theelevation angles decrease with range in continuous
bands of ground scatter data, as expected (see Fig. 25

Fig. 25 Range-Time-Parameter plots of elevation angle for beam 7 of the HOK data at 06–12 UT on 23 February 2014 for: uncalibrated (top) andcalibrated data (bottom). Adapted from Fig. 6 of Ponomarenko et al. (2015)

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from Ponomarenko et al. 2015). Such calibration adjustments are necessary after each radar hardware/
software update but also desirable on a routine basis
because of gradual changes in the physical propertiesof antennas, cables, and electric circuitry.
Statistical comparisons between radar data and simulations using propagation analysis can also be usefulfor identifying deficiencies in standard SuperDARN
data processing. For example, Oinats et al. (2016b)
compared 1-hop ground scatter measurements from
HOK with simulated data to examine the accuracy ofSuperDARN echo geolocation. They found that assuming a fixed virtual height produces a systematic
error in geolocation which has a strong dependence
on LT, season, and solar activity level. They also
found contamination of backscatter from the backlobe can be as high as 35% and tends to be mostproblematic in winter and equinox.
Finally, ray tracing can also be a useful tool for demonstrating proof-of-concept for proposed new radar construction. For example, Nishitani and Ogawa (2005)
used the Jones-Stephenson ray-tracing code coupledwith the IRI and IGRF models to identify regions where
the then-proposed HOK would most likely measurebackscatter from ionospheric irregularities. Figure 26
from Nishitani and Ogawa (2005) shows the results fornighttime quiet geomagnetic conditions at 9, 11, and 13
MHz with X indicating potential irregularity backscatter
locations (i.e., ≤ 1° aspect angle). They were able toconclude that the proposed radar would collect morethan enough data to be useful as a facility for analyzing
mid-latitude ionospheric irregularities.

Nishitani et al. Progress in Earth and Planetary Science

Propagation analysis: Summary and future directions

In this subsection, it has been demonstrated how HF
propagation analysis can be used to conduct value-added
ionospheric research at mid-latitudes and assess radar performance. The mid-latitude ionosphere provides a much simpler propagation environment than at higher latitudes, so
convergence between measurements and simulated datausing ionospheric models is much more likely. For this reason, small model-data disagreements can often be interpreted
in terms of anomalous ionospheric behavior, deficiencies inmodel specification, or issues of radar performance.
Looking forward, there is scope for continued use of
propagation analysis to further refine our understanding of
mid-latitude ionospheric plasma morphology and irregularities. Two specific topics that deserve further examination
are solar cycle effects and interhemispheric influences.
However, such efforts will require reliable elevation angle
information. Shepherd (2017) described an algorithm fordetermining elevation angle data using interferometer arrays offset from the main array in three dimensions, rather
than one or two, thus providing more flexibility in the positioning of interferometer arrays for future radars.
Finally, it is worth noting that further development andrefinement of propagation analysis has the potential tovastly expand SuperDARN’s capabilities as an asset for

Fig. 26 Ray-tracing analysis (solid curves) and expected locations for ionospheric irregularity backscatter occurrence (asterisks) for HOK during
nighttime quiet geomagnetic conditions and transmission frequencies of (top to bottom) 9, 11, and 13 MHz, respectively. Adapted from Fig. 3 ofNishitani and Ogawa (2005)

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real-time space weather monitoring, beyond just specification of the ionospheric convection electric field. Specifically, a comprehensive multi-radar dataset of backscattered
power measurements combined with fully calibrated elevation angle data could conceivably be inverted to provide
hemispheric maps of ionospheric plasma density. Such adata product would be immensely useful to the space
physics community, as well as the society whose activity isaffected by space weather environment changes.

generation and propagation of TIDs, sudden onset of
acoustic-gravity waves (AGWs), mutual momentum exchange between the ion convection and neutral winds,
and generation of small-scale plasma irregularities. Thissection reviews mid-latitude SuperDARN radar contributions in understanding these phenomena.

Nishitani et al. Progress in Earth and Planetary Science

Ion-neutral interactions
Introduction of ion-neutral interactions

The exchange of energy and momentum between the
charged and neutral components of the Earth’s upper atmosphere are manifested in ionospheric phenomena observed by SuperDARN radars. Examples include

TIDs: Introduction

TIDs can be initiated by a number of sources andconsequently vary over a wide range of spatial scales
and propagation properties. Large-scale TIDs (LSTID)
are classified as perturbations with scale sizes greaterthan 1000 km and having periods longer than 30 min
while MSTIDs are perturbations of a smaller size anda shorter period (Hunsucker 1982). TIDs have been
extensively studied using various observationab

Fig. 27 a Model ray paths of HF radar propagation through an ionosphere modulated by a TID. Focusing and defocusing at the ground iscaused by the uneven contours of electron density (adapted from Samson et al. (1990)). b Example of FIR radar ground scatter data received
along a single beam, versus ground range and universal time, showing several signatures of TID moving toward the radar. Reproduced from
Fig. 2 of Grocott et al. (2013)

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techniques including ionosondes (Hocke and Schlegel
1996 and references therein; Crowley and Rodrigues
2012), incoherent scatter radars (Hocke and Schlegel
1996 and references therein), optical imagers (e.g.,
Shiokawa et al. 2002; Shiokawa et al. 2003), and GPS
receivers (Ding et al. 2008, 2011; Otsuka et al. 2011;
Tsugawa et al. 2003, 2004).
TIDs can be identified in SuperDARN radar data as
quasi-periodic range progressions of echoes in either ionospheric scatter (IS) or ground scatter (GS). An example of a
TID event is shown in Fig. 27 (Grocott et al. 2013). The top
panel uses ray-tracing analysis to demonstrate how
wave-like perturbations in ionospheric electron density can
produce selective focusing of the HF radar waves at specificranges (e.g., 1800 km) so that returned signals have stronger
backscattered power (Samson et al. 1990). The bottom panel
shows an example of how this behavior is manifested in FIR
GS data as regions of enhanced power moving toward theradar with a periodicity of ~ 30 min. At least 10 such excursions are visible. This signature is consistent with the previous model calculation results by Stocker et al. (2000).
SuperDARN signatures of LSTIDs

The number of LSTID events studied thus far with SuperDARN has been somewhat limited. Hayashi et al. (2010)
were the first authors to conclusively identify an LSTID
event, during a geomagnetic storm, using daytime HOK
GS observations and GPS Earth Observation Network
System (GEONET) network GPS data of TEC. In Fig. 28
from Hayashi et al. (2010), three large-scale density perturbations are clearly seen in the TEC data (bottom panel)with enhancements propagating equatorward (events 1
and 2) and one propagating poleward (event 3). The
speeds of the equatorward perturbations were 600 and
800 m s−1, respectively, with corresponding periods of 45
and 30 min. These values imply horizontal scales for theionospheric structures of 1500–1700 km. By contrast, thepropagation speed for the poleward-propagating perturbation was 600 m s−1 with a period of 75 min, implying ahorizontal scale of ~ 2700 km. Also, the TEC enhancements for the poleward-propagating perturbation were
found to be several times stronger (1 TEC unit (TECU)
versus 0.4 TECU, TECU = 1016 electrons m−2).
Turning now to the HOK data (top panel, Fig. 28), it
can be seen that the radar detected concurrent bands ofenhanced GS echoes, moving toward or away from theradar, with Doppler velocity enhancements up to 50 m s−1,
correlated with the TEC enhancements. Furthermore, themain body of the high Doppler velocity echoes showedpropagation speeds comparable to those inferred from theGPS data. Hayashi et al. (2010) suggested the equatorward
propagating LSTIDs (events 1 and 2) seen in both the
HOK and GPS data were signatures of AGWs generated
at high latitudes and propagating equatorward to

Fig. 28 LSTID signatures observed simultaneously by HOK and GEONETGPS receiver network on 15 December 2006. The upper panel shows
Doppler velocity of GS echoes measured by HOK, and the lower panel
shows GEONET-GPS total electron content (TEC) perturbations (sampled
along the direction of HOK beam 0 at lower latitudes) plotted as afunction of UT and geographic latitude. In the lower panel, the TEC
perturbation values were obtained by subtracting 60-min runningaverage. Three major disturbances are visible in each map, denoted as
events 1–3. Adapted from Fig. 5 of Hayashi et al. (2010)

mid-latitudes. Indeed, estimates of the expected density
perturbation amplitude based on linear AGW theory
showed reasonable agreement with observations forevents 1 and 2 but vastly underestimated the amplitude for the poleward-propagating signatures (event3). It was thus suggested that event 3 was instead an
LSTID initiated by sources in the Southern Hemisphere auroral zone. This interpretation is somewhat
anomalous because equatorward propagation is morecommonly observed for LSTIDs (Tsugawa et al. 2004;
Ding et al. 2008). A later study by Milan et al. (2013)using the FIR radar concluded that 1–2 h perturbations in nightside GS echoes were produced by equatorward propagating LSTIDs excited by geomagnetic
disturbances at auroral or sub-auroral latitudes.
SuperDARN signatures of MSTIDs: Introduction

It has long been anticipated that MSTIDs have different sources during daytime and nighttime and thus
different general characteristics (Kotake et al. 2006;
Ogawa et al. 2009; Kotake et al. 2007; Kelley 2011).

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Accordingly, after general characterization of MSTIDs,
daytime and nighttime observations are consideredseparately.

300 km spatial scales, and 100–150 m s−1 phase speeds.
A follow-up study (Frissell et al. 2016) considered amore extensive data set comprising six North American
mid-latitude radars and four auroral zone radars andconcluded that daytime MSTID parameters at high andmid-latitudes are generally comparable. One minor
difference was more frequent occurrence of shorterwavelength and faster moving MSTIDs at high latitudes,consistent with an earlier study by Ishida et al. (2008)
using a more limited data set. Other studies have shownthat nightside MSTIDs have typical parameters comparable to those during daytime (Oinats et al. 2015, 2016a;
Milan et al. 2013). Minor differences noted by Oinats etal. (2016a) are 20–30% longer periods and wavelengthsof nighttime MSTIDs although the differences are difficult to quantify because the data are presented as scatterplots. Data presented by Oinats et al. (2015) seem tosuggest that nighttime MSTIDs during summer have larger propagation speeds, by at least 30%.

Nishitani et al. Progress in Earth and Planetary Science

MSTID occurrence

The vast majority of SuperDARN publications on
mid-latitude MSTIDs are based on the analysis of GS
signals, but a few have been identified in ionosphericscatter (IS) F region echoes as well (e.g., Ogawa et al.
2009; Suzuki et al. 2009). Generally, the GS occurrencechanges dramatically versus season and time of day because of variations in the background ionospheric electron density. This tendency complicates assessment of
MSTID occurrence (e.g., Grocott et al. 2013; Frissell et
al. 2014). At mid-latitudes, GS is typically present during
daytime, centered around noon, and decreases dramatically toward winter as the electron density in the ionosphere becomes smaller (Grocott et al. 2013; Frissell et
al. 2014; Oinats et al. 2015). Thus, MSTID occurrence
rates should be normalized by the rate of GS occurrence,
which has only been done by Grocott et al. (2013).
One of the first SuperDARN publications on
mid-latitude MSTIDs examined TIG sea scatter measurements during the year 2000 (He et al. 2004) andfound more events occur during daytime (especially during postnoon hours) and in winter, with fewer eventsnear equinoxes and even fewer in summer. This tendency is consistent with Bristow et al. (1996) who also
reported more frequent auroral zone MSTID event detection in winter. Frissell et al. (2014) with the BKS data
and Oinats et al. (2015) with the HOK data both concluded the MSTID occurrence rate maximizes during
daytime in winter. By contrast, Grocott et al. (2013) examined normalized FIR MSTID occurrence for daytime
events and found the MSTID occurrence rate was higher
during equinoctial time (although the authors could not
fully conclude with respect to winter occurrence rates
because of insufficient statistics). To further complicate
matters, observations with other instruments are also
somewhat contradictory with respect to a preferential
season and time of day for MSTID occurrence (Ding etal. 2011; Kubota et al. 2011; Kotake et al. 2007). Clearly,
more work is needed.
Typical MSTID parameters

It has been established that the typical period, wavelength, and propagation speed of MSTIDs are very comparable across the MLAT of SuperDARN observations,and in both hemispheres (He et al. 2004; Ishida et al.
2008; Grocott et al. 2013; Frissell et al. 2014, 2016;
Oinats et al. 2015, 2016a). Frissell et al. (2014) presented
histogram distributions for these parameters based on
daytime BKS observations in the American sector. Typical values were found to be 30–40 min periods, 200–

Daytime MSTIDs: Introduction

Establishing the direction of MSTID propagation isfundamentally important for locating potential sources
and for understanding the mechanisms of MSTIDs
generation.
Daytime MSTIDs: Propagation direction

A number of SuperDARN studies have assessed the direction of daytime MSTID propagation observed by individual radars. For example, Ishida et al. (2008) analyzed
HOK and KSR data and found a great deal of similarity
between the propagation directions of MSTIDs in theauroral zone and at mid-latitudes, namely toward thesoutheast or southwest. Frissell et al. (2014) analyzed
BKS observations and identified two semi-distinct populations of daytime MSTIDs propagating over North
America: a major component propagating southeast(az =150°, where az is the angle between the propagation
direction and geographic north, positive clockwise) anda minor population propagating northwest (az = − 50°).
The dominant southeast propagation direction has beenfurther confirmed by Frissell et al. (2016) using datafrom 6 mid-latitude North American radars. Observations with FIR (Grocott et al. 2013) and TIG (He et al.
2004) are consistent with this result in the sense that the
majority of MSTIDs propagate toward the equator withsome eastward component. However, it is important toemphasize that numerous studies have reported minor
populations of MSTIDs propagating in seemingly“anomalous” directions which are likely associated withlocal factors. For example, Grocott et al. (2013) identified a persistence of MSTIDs propagating westward andrelated them to AGW generation by either the Andean
and Antarctic Peninsula mountains or the polar vortex.

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Ishida et al. (2008) also detected occasional MSTIDs
propagating westward and southeast which were thought
to be related to meteorological phenomena above thetroposphere. He et al. (2004) showed occasional occurrence of poleward-propagating MSTIDs.
Oinats et al. (2015) performed extensive analysis of
MSTID propagation directions by considering skip
distances of the HOK GS echoes. Four major directions were identified. Two directions were found to
dominate near noon: south and southwest in winterand northeast in summer (see their Fig. 2). In afollow-up study, Oinats et al. (2016a) found that thesecond cluster corresponds to MSTIDs at short radar
ranges reflecting disturbances in the E region (seetheir Fig. 2).
One working hypothesis for the preferential directionof observed MSTID propagation is a filtering effect ofneutral winds such that only those AGWs propagating
opposite to the neutral wind flow will be detected by theradars (e.g., Kotake et al. 2007; Oinats et al. 2016a). General agreement of SuperDARN-inferred directions withthis hypothesis has been invoked by Ogawa et al. (2009)
and Grocott et al. (2013). Oinats et al. (2016a) went further by modeling the expected directions of MSTID
propagation on the basis of the HWM07 neutral windmodel (Drob et al. 2008). Figure 29 shows the expected(modeled) occurrence of MSTIDs on the azimuth-LT
plane as gray-scale pixels and the actual MSTID occurrence by line contours with color signifying various
rates. Both echo detection through the E and F regions
are considered. For the dayside, one can see that theagreement is very good for the summer observations,slightly worse for the equinoctial observations, and muchworse for the winter conditions where shift to the southeast directions is as large as 30°–50°.

Nishitani et al. Progress in Earth and Planetary Science

All seasons

Summer

Winter
26

300

22

240

18

180

12

120

8

60

4

0

0
26

300

22

240

18

180

12

120

8

60

4

0

0

b
Azimuth, deg

Equinox

One classical scenario of MSTID onset is generation of
AGWs due to geomagnetic activity in the auroral zone
and subsequent propagation to mid-latitudes (Ogawa et

0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20
LT
LT
LT
LT

Occurrence Rate, %Azimuth, degaDaytime MSTID generation: Geomagnetic influences andpolar vortex activity

Fig. 29 a–b Modeled rates of anti-wind MSTID direction occurrence versus local time based on the HWM07 neutral wind model (Drob et al.
2008) applied to the center of the HOK FOV. Contours are MSTID occurrence rates according to the HOK observations of ground scatter from theE region. b The same as a but for ground scatter from the F region. Blue, red, and magenta contours are for the rates of 3, 5, and 10%,respectively. Adapted from Fig. 3 of Oinats et al. (2016a)

(2019) 6:27

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al. 2002b; Ishida et al. 2008). For the dayside, however,published papers have found mixed results regarding
this scenario.
Ishida et al. (2008) specifically targeted geomagnetic
influences through a statistical study of KSR, KOD,
and HOK GS signals in the auroral zone and at
mid-low latitudes during periods with relatively low
and high solar activity. For one quiet day event withnearly simultaneous monitoring of MSTIDs by all radars, the MSTID parameters, such as azimuth of
propagation, speed, and period, were very comparable.
On the other hand, for disturbed conditions, theMSTID parameters at mid- and high latitudes werequite different. By considering multiple events, withstatistics comprising 130 events in the auroral zone
and 40 events at mid-latitudes, the authors found agreat deal of similarity between the propagation directions of MSTIDs (although much more variability wasseen for the high-latitude MSTIDs) which seems tofavor a high-latitude source location.
Grocott et al. (2013) found almost no correlation of
MSTID occurrence with magnetic storm activity(SYM-H index, 0.3 correlation coefficient) but a more
pronounced correlation with auroral substorm activity(AE index, 0.57 correlation coefficient). Oinats et al.
(2016a) reported a minor increase in the speed and theamplitude of MSTID-related perturbations with the AE
index for HOK observations (although data in all LT
sectors were included in their analysis).
Finally, an extensive analysis of daytime MSTID
dependence on magnetic activity in the auroral zone
by Frissell et al. (2016) resulted in a firm negative
conclusion. These authors created a special index
quantifying MSTID activity according to the intensity of power modulations of the GS echo power foreach individual radar (10 radars were considered).
They found negligible correlation of the introduced
MSTID index with SYM-H and AE indices but highcorrelation with a newly defined Polar vortex index.
Figure 30 a from Frissell et al. (2016) shows variations of the MSTID index characterizing the level of
GS signal power fluctuations for quiet periods (dotted line and blue color of the background) and periods with strong MSTID activity (solid line andpink color of the background). Figure 30 b–d show
temporal variations of the polar vortex index (calculated from the variabilities of the geopotential differences at the 1 and 10 mb levels), AE, and SYM-H
indices, respectively. No visible correlation exists between enhanced MSTID activity and magnetic indices, while the polar vortex index shows generally thesame trends as the MSTID index (e.g., pronounced

Nishitani et al. Progress in Earth and Planetary ScienceFig. 30 Time series of North American SuperDARN MSTID, polar vortex, and geomagnetic activity indices for the 2012–2013 observational season.
a Continental MSTID index (gray and blue traces) and daily number of good sampling windows (dotted line). Background color indicates daily
MSTID score and classification, with MSTID-active days in reds and MSTID-quiet days in blues. b Polar Vortex Index. c Negative AE index. d SYM-H
index. In all panels, gray traces show raw data while blue traces show a 4-day running mean smoothing. Adapted from Fig. 9 of Frissell et al. (2016)

Nishitani et al. Progress in Earth and Planetary Science(2019) 6:27

decreases around 15 January and a monotonic decrease after March). Even some short-term excursions of the indices are consistent with each other.
On the basis of these data, Frissell et al. (2016) concluded that the southeast direction of MSTID propagation over North America is due to AGW
generation in the troposphere at the boundary regions of the active polar vortex area. They also demonstrated that the seasonal variation in MSTID
activity is in accord with stratospheric warmings,
pointing to the critical role of the lower atmospherein generation of daytime MSTIDs. It was assumed
that the neutral wind filtering effect controls the direction of the observed MSTIDs, in agreement withconclusions by other studies (e.g., Kotake et al. 2007;
Oinats et al. 2016a).
Polar vortex activity is likely not the only source ofdaytime mid-latitude MSTIDs. Indeed, in an earlier
paper, Frissell et al. (2014) discussed northwest propagating MSTIDs and related them to tropospheric convection processes over the Atlantic Ocean. Earlier,
Grocott et al. (2013) identified the possibility of
MSTID propagation from tropospheric processes overthe mountains in Antarctica. Finally, Oinats et al.
(2016a) reported that HOK often sees MSTIDs propagating northwest and attributed them to E region
ionospheric perturbations, also propagating in unusual
directions.

Nighttime MSTIDs: Introduction

Optical and GPS TEC observations have shown thatnighttime MSTIDs are a frequent phenomenon at
mid-latitudes (e.g., Shiokawa et al. 2003; Otsuka et al.
2011). However, detection of nighttime MSTIDs using
HF ground scatter signals is complicated by the factthat electron density decreases after sunset and theground scatter echo occurrence rates plummet (e.g.,
Frissell et al. 2014). This effect is especially profoundin winter. In addition, propagation conditions tend to
be more complex during nighttime making definitive
identification of quasi-periodic MSTID ground scatterpatterns/range bands much more difficult. In suchconditions, concurrent observations with other instruments such as optical imagers and/or GPS TEC receivers are extremely useful. For example, Shiokawa
et al. (2008) and Koustov et al. (2009) reported HOK
GS echo bands moving equatorward in unison with
optically identified MSTIDs. Finally, Oinats et al.
(2015) found nighttime mid-latitude MSTIDs are lessfrequent in winter; however, this may be due to a
general decrease in GS occurrence because optical observations show clear MSTID maxima at solstices(Duly et al. 2013 and references therein).

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Nighttime MSTIDs: Propagation direction

Ichihara et al. (2013) reported nighttime MSTIDs observed by HOK usually propagate southwest but sometimes north to northeast. These latter MSTIDs occur
more frequently after sunset, near the dusk terminator
or in the midnight sector. The authors also found northward propagating MSTIDs are not always accompanied
by similarly moving MSTIDs at lower latitudes (overcentral Japan), equatorward of the radar observational
area. These statistical results substantiate a report by
Shiokawa et al. (2008) who studied one event of northeast propagating MSTIDs during nighttime. MSTID motion seen in optical data during this event were related
to the descent of the ionospheric F region identified by aconcurrently operating ionosonde. HOK observed some
irregularly-propagating GS progressions that could havebeen identified as MSTIDs, but details were notinvestigated.
Oinats et al. (2015) indicate that the majority of nighttime MSTIDs occur during late evening to pre-midnight
hours (16–24 LT) and preferentially at equinox (seetheir Fig. 2). This finding is in disagreement with Duly etal. (2013). They found propagation to the southwest(az = 210°), similar to daytime MSTIDs over Japan, andpre-midnight MSTIDs occurrence consistent with thewind filtering effect (Fig. 29 from Oinats et al. 2016a).
Another cluster of events during morning hours, between 3 and 5 LT (see their Fig. 2) propagate to thenortheast (az = 30°). Surprisingly, these are not strong in
winter, probably because of weakening in the neutral
wind filtering effects, as predicted by Oinats et al.
(2016a) (see Fig. 29b).
Nighttime MSTIDs: High-latitude geomagnetic influences

One might generally expect better correlation between
nighttime MSTID occurrence and high-latitude geomagnetic activity because the auroral oval is located at much
lower magnetic and geographic latitudes on the nightside. So far, however, this expectation has not been fully
articulated. Oinats et al. (2016a) found increased amplitude of MSTIDs with AE magnetic index in all LT sectors but individual events studied by Shiokawa et al.
(2008) and Koustov et al. (2009, 2014) were during geomagnetically quiet conditions. Only one of the events
considered by Ogawa et al. (2009) can be related to geomagnetic disturbances at high latitudes while two others
occurred under very quiet conditions, according to theKp and AE indices. These mixed findings question theimportance of auroral zone sources for generating nighttime mid-latitude MSTIDs, similar to what has beenfound on the dayside.
Finally, it is noted that Ichihara et al. (2013) foundpreferential occurrence of northeast propagating
MSTIDs near dusk (see their Fig. 4) suggesting the solar(2019) 6:27

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terminator may also play an important role (e.g., Afraimovich et al. 2009). This hypothesis, however, has notbeen investigated with SuperDARN data yet.

al. 2009) and (2) polarity of the velocity is inconsistent withthat usually observed for MSTIDs over Japan with the middle and upper atmosphere (MU) VHF radar (although at
quite different MLAT).

Nishitani et al. Progress in Earth and Planetary Science

Nighttime MSTIDs: E region signatures and vertical
coupling

SuperDARN detection of seismically-activated TIDs

Electron density perturbations propagating at E region
heights can also be detected as MSTIDs in SuperDARN
GS and IS signals. Ogawa et al. (2009) were the first authors to report MSTID signatures in E region ionospheric echoes as quasi-periodical striations in HOK
ionospheric backscatter. Regions with enhanced echo
power were found to be magnetically connected to F region plasma depletions as inferred from 630-nm airglow
camera observations. Ogawa et al. (2009) suggested sucha coincidence is indicative of electrodynamic coupling
between these two regions in the form of a polarizationelectric field (Perkins 1973) which maps into the E region without attenuation, as envisioned theoretically(e.g., Yokoyama and Hysell 2010).
Koustov et al. (2014) performed a more comprehensive analysis of E region MSTID signatures in HOK
echoes and reported preferential occurrence of such
echoes in summer with a secondary maximum in winter.
This tendency is despite the fact that E region nighttime
HOK echoes are more frequent in winter (Yakymenko et
al. 2015). The authors found the E region MSTIDs
propagate toward the southeast or east during
pre-midnight hours and toward the southwest or west
during post-midnight hours with average speeds of 86 m
s−1 and 56 m s−1, respectively. Their typical spatial scaleis ~ 300 km. However, the Doppler velocity of the
MSTID striations were smaller than the speed of thepower enhancements implying HOK monitored extended plasma blobs moving across the FOV, presumably at the background wind velocity. Plasma motions
inside the blobs are assumed to be controlled by a combined effect of the electric field and neutral wind, as later
expanded by Yakymenko et al. (2015).
Several additional arguments supporting the notion ofstrong electrical coupling between the E and F regions during MSTID events have been identified. These include (1) F
region echo velocity is consistent with the expected electricfield direction within regions of depleted/enhanced electron
density (Suzuki et al. 2009), (2) E region plasma velocity iscomparable to that in the F region (Koustov et al. 2014), (3)polarity of the E region velocity changes consistently inneighboring regions with enhanced/depleted plasma density (Ogawa et al. 2009), and (4) the power of E region
echoes correlates with the measured Doppler velocity(Koustov et al. 2014). However, there are also several outstanding issues, including (1) sometimes, the region of depleted plasma correlates with a sheared plasma flow of
opposite polarities, instead of a single direction (Ogawa etWith extensive spatial coverage and reasonably good
temporal resolution, the SuperDARN radars can be a
powerful tool for investigating ionospheric disturbances generated by sources outside the atmosphere.
Only a few SuperDARN studies have been publishedthus far in this area, and the record will likely remain
short in the case of meteorites (Berngardt et al.
2015). However, HF radars have been proven to be
useful for tracking TIDs generated by intense earthquakes and tsunamis (Occhipinti et al. 2010) and theSuperDARN radars could become a powerful tool for
more systematic studies of such disturbances, especially over oceans where other ionospheric measurements are not possible.
Nishitani et al. (2011) reported the first SuperDARN
radar detection of ionospheric disturbances initiated bya major earthquake on 11 March 2011 off the Pacificcoast of Tohoku (magnitude, M = 9.0). During this event,
HOK was originally operating in a normal azimuth scanning mode but several minutes after the earthquake occurred it switched to a camping beam mode with asingle high-resolution 8-s beam. Data obtained on the
8-s beam identified five separate bands of ground scattermoving away from the radar with apparent speeds of 6.7,
6.2, 4.5, 3.9, and 3.5 km s−1 (see Fig. 31 from Ogawa et
al. 2012). The speeds of the first two GS bands were faster than any MSTIDs observed previously, by any observational means, while the speed of the three next
disturbances were found to be consistent with a scenario
of surface Rayleigh waves propagating into the observational area and generating acoustic (air) waves that generate electron density perturbations as they expand
upward to ionospheric heights (Ducic et al. 2003). Surprisingly, these kinds of waves with speed faster than 4
km−1 were not seen in the TEC data. It was concluded
that the radar observations were associated with
up-and-down motions of the ionospheric layers as fast
as 200 m s−1 (peak-to-peak values). Note that some ofthe disturbances, e.g., the one at 1512–1514 UT, were
due to aftershocks rather than the main shocks, making
it difficult to analyze the data in the later stage of theevent.
A follow-up study of the 11 March 2011 earthquake
event by Ogawa et al. (2012) examined a second class ofpoleward-propagating backscatter power enhancements
which arrived in the HOK observational area later thanthe Rayleigh wave perturbations and associated themwith slower-moving AGWs generated at the epicenter.

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Fig. 31 Doppler velocity of HOK echoes on beam 4 (echo power > 10 dB) versus Japanese Standard Time (JST) after an earthquake on 11 March
2011. Sloped lines mark several individual echo bands propagating northward away from the earthquake epicenter. Measurements at 900–1100
km range (dashed rectangles) are 2-hop signals while their 1-hop counterparts are identified by solid rectangles at 350–500 km range. Adaptedfrom Fig. 5 of Ogawa et al. (2012)

For these latter perturbations, no clear correlations between echo power and Doppler velocity were detected,
contrary to what is seen for typical nighttime MSTIDs(Ogawa et al. 2009). Another interesting phenomenon
was two bursts of quasi-periodic pulsations of echo LOS
velocity (2–4 min period) which occurred ~ 105 min and~ 180 min after the earthquake and lasted for about 30
min. It was suggested that these so-called “acoustic resonance” pulsations were produced by acoustic waves
trapped between the Earth’s surface and thermospheremodulating the ionospheric electric field and electron
density (Shinagawa et al. 2007). Why two separate resonance intensifications were observed, however, requires
further investigation.

Ion-neutral coupling and generation of plasma irregularities

The E × B drift of ionospheric plasma at F region
heights may alter the directional motion of neutral particles (ion drag effect) and neutral winds can affect thevelocity of charged particles. Several studies haveattempted to identify the efficiency of these interactions
and the time scales on which they operate.
Joshi et al. (2015) investigated mid-latitude ion-neutral
coupling during the geomagnetic storm of 2–3 October
2013 using co-located measurements of ionospheric convection from North American mid-latitude SuperDARN
radars and neutral winds from Fabry-Perot interferometers (FPIs). The time scales on which the coupling operates were analyzed using momentum exchange theory andFig. 32 SuperDARN ionospheric convection maps during the a, b main and c recovery phases of a geomagnetic storm on 2 October 2013, withfitted vectors in locations where measurements were obtained (color coded according to the scale at right) and superposed neutral windvelocity (black vector) measured at the Illinois (UAO) and Michigan (ANN) FPI sites. Locations of the mid-latitude SuperDARN radars operational atthis time are identified. Adapted from Figs. 4 and 7 of Joshi et al. (2015)

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time-lagged correlation analysis. During the main phase,they found the neutrals respond to the ion convection ona time scale of ~ 84 min. Figure 32 a and b illustrate thisresult. Prior to storm onset (0140 UT), the two wind velocity vectors can be seen to oppose the E × B drift ofplasma while several hours into the storm (0840 UT) thewind vectors have rotated into the direction of the plasma
drift. The time scale of this rotation is comparable to whatmight be observed at high latitudes using satellite observations (Killeen et al. 1984) but significantly faster than whatis expected from local ion-drag momentum forcing alone.
This tendency suggests that other storm-time influences
are important for driving the neutrals during the main
phase, such as Joule heating. During the late recovery
phase, however, the neutrals were observed to drive theions without any significant time delay (Fig. 32c from Joshi
et al. 2015) consistent with the so-called “neutral fly wheel
effect” or disturbance dynamo (Blanc and Richmond1980).
The importance of the disturbance dynamo effect at
mid-latitudes has also been confirmed earlier by Zou
and Nishitani (2014) who found that midnight flows
measured by HOK during winter are predominantly
westward at latitudes 40°–50° and their speed increases with the intensification of planetary magnetic
activity characterized by the Kp magnetic index. They
also found a ~ 12-h delay in achieving maximum
westward flow speed at mid-latitudes after a substorm
onset in the auroral zone. This time delay is significantly longer than the estimates by Joshi et al. (2015)
but still in qualitative agreement with the disturbance
dynamo model by Blanc and Richmond (1980), provided that one considers the much lower latitudes ofthe HOK observations.
Neutral winds can also play an important role
modulating mid-latitude F region electron density (deLarquier et al. 2011; Milan et al. 2013) and facilitating
the generation of small-scale plasma irregularities
through neutral wind turbulence (Gurevich et al.
1997). This process is particularly important at
mid-latitudes where magnetospheric electric fields are
weak. Yakymenko et al. (2015) found that the diurnal
variation of HOK echo velocity at short ranges cannot be
explained by variation in the E × B drift alone. However, byadding a neutral wind contribution to the irregularity velocity and assuming that the irregularities were not strictly
magnetic field-aligned, they were able to match the measurements with theoretical expectations. It was also noticed
that the short range HOK echoes are not confined to locations of zero aspect angle which suggests wind-induced irregularities are not strongly field-aligned. Another study by
Ponomarenko et al. (2016) concluded that a particular classof near-range echoes observed in the prenoon sector during
summer time are most likely from ionospheric irregularities

produced by the neutral wind turbulence in the lower E region, rather than meteor echoes as is commonly assumed,
because their velocity characteristics are similar to thewinds.
Finally, mid-latitude SuperDARN radars can be used
to monitor neutral processes in their own right, such asatmospheric tides and turbulization of icy particles atthe summer mesopause. Hibbins et al. (2011) used FIR
radar observations of meteor echoes to study atmospheric tidal motions in the mesosphere and found strong
semidiurnal tides in winter were much stronger in theSouthern Hemisphere compared to the Northern Hemisphere. They also noticed bursts of quasi 2-day activity insummer with amplitudes up to 60 m s−1. Ogawa et al.
(2013) reported two cases of mesosphere summer echoes(MSEs) detected at mid-latitudes by HOK with narrowspectral widths (< 10 m s−1), high power (> 20 dB), andrelatively fast Doppler velocities (up to 50 m s− 1). These
HF echoes were observed when medium frequency (MF)and VHF radars detected MSEs in a region several hundred kilometers westward of the HOK echo detection
zone. An interpretation in terms of MSEs was further supported by the fact that temporal variations of the HF echo
velocity correlated well with the wind velocity concurrently measured by the MF radar.

Nishitani et al. Progress in Earth and Planetary Science

Ion-neutral interactions: Summary and future directions

In this subsection, the contributions of mid-latitude
SuperDARN radars to studies of phenomena associatedwith ion-neutral interactions, predominantly TIDs, havebeen summarized. Looking forward, there is additional
need for coordinated studies in which radar observations
are analyzed in conjunction with other instruments, suchas GPS receivers, ionosondes, and airglow imagers. Suchstudies are most feasible in the East Asia and North
American sectors where the density of suitable instruments is particularly high, allowing for the tracking of
TID perturbations over extended ranges of latitude andlongitude. There is also further scope for examining theinterhemispheric conjugacy of TIDs using simultaneous
observations from the southern and northern radar networks, perhaps, in conjunction with in situ measurements obtained from satellites orbiting in both theionosphere (e.g., Swarm) and magnetosphere (e.g.,Arase).

MHD waves
Introduction of MHD waves

MHD waves are observed throughout the magnetosphere and therefore at a range of latitudes on the
ground, from the equator to polar latitudes. Thesewaves, often referred to as ULF (~ 1–1000 mHz) waves
or geomagnetic pulsations, have many different source
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or internal to the magnetosphere. External sources include the Kelvin-Helmholtz instability (KHI) on themagnetopause (e.g., Hughes 1994), solar wind dynamic
pressure variations (e.g., Araki et al. 1997), and waves
generated at the bow shock which directly propagate
through the magnetosphere (e.g., Fairfield 1969). Internal sources include waves related to substorm onset(e.g., Lester et al. 1983), and wave-particle interactions
where waves gain energy from particles in the magnetosphere (e.g., Chisham et al. 1992).
SuperDARN radars are particularly sensitive to MHD
waves as the latter generate periodic variations of plasma
drift observed as Doppler shift oscillations in the radar
returns. Critically, such waves can be observed either inionospheric scatter (Rae et al. 2005), in ground scatter(Ponomarenko et al. 2003), or in artificially stimulated
ionospheric scatter (Yeoman et al. 1997). The observations in ionospheric scatter provide direct measurementsof the E × B velocity which is imposed by the electricfield vector of the MHD wave. Care should be taken,
however, in discussing the electric field variations ofstanding waves and comparing them with magnetic fieldchanges because of the node-antinode relationship between them due to the presence of finite ionosphericconductivity, as pointed out by Sakaguchi et al. (2012).
On the other hand, in ground scatter, the observations
can effectively be considered to be associated with thebulk motion of the ionospheric plasma in the verticaldirection, although there are a number of processes involved here (e.g., Sutcliffe and Poole 1989). Furthermore,visualization of ULF waves in ground scatter can beproblematic since the scatter have low velocities. Artificially stimulated ionospheric scatter occurs from intensefield-aligned irregularities produced through the interaction of high-power HF radio waves with the ionospheric plasma below the waves’ reflection point.
However, this observational mechanism is not pertinent
to mid-latitude SuperDARN radars as there are nohigh-power radio waves transmitters, or heaters, in the
FOV of the existing mid-latitude radars. Finally, thestandard 16 beam scan with 3-s integration along eachbeam (1-min temporal resolution) is prohibitive for registering oscillations at frequencies above 8.3 mHz, andconsequently, discretionary or special modes have been
designed in order to expand the observed frequency
spectrum of MHD waves, thereby demonstrating theflexibility of the SuperDARN radars.
The mid-latitude SuperDARN radars are well placed toobserve the waves driven by these mechanisms. In particular, they are conveniently located to observe waves which
are driven by internal mechanisms such as substorms (Pi2)and wave-particle interactions (high-m Pc4–5). For substorms, the mid-latitude radars are not as badly affected byabsorption or changing in propagation conditions of the

HF signal due to particle precipitation into the D and E regions (e.g., Gauld et al. 2002) because the transmitted signals typically enter the ionosphere equatorward of theauroral regions. Furthermore, waves driven by
wave-particle interactions tend to map more to the fieldlines which thread the F region of the mid-latitude radars.
Finally, the presence of ground scatter tends to be morestable over a wider range of magnetic conditions for the
mid-latitude radars than for the high-latitude ones making
identification of ULF waves easier.
In this section, the observations of MHD waves by
mid-latitude SuperDARN radars are reviewed. Pi2 andPi1 pulsations are considered first, before discussing
Pc3/4, and Pc5 pulsations.

Nishitani et al. Progress in Earth and Planetary Science

Pi2 and Pi1 pulsations

Studies of Pi2 pulsations are typically event-based and related to data from other instruments, both ground andspace-born. In the first observations of sub-auroral Pi2 pulsations by a SuperDARN radar, at WAL, Gjerloev et al.
(2007) found that the LOS velocity variations at ~ 56o N
AACGM latitude were highly correlated with nearlyco-located magnetometer observations at Ottawa. Theseauthors used the relationship between the variations inmagnetic field and those in the LOS velocity to demonstrate that a predominantly shear Alfven mode wave can
explain the amplitude and phase characteristics. This event
occurred as part of a weak substorm, although observations
at geosynchronous orbit suggest that there were time differences between the ground and space signatures, with theground signatures leading those at geosynchronous orbit.
Frissell et al. (2011) presented observations of an individual Pi2 pulsation using data from BKS in a special high-time
resolution mode which was implemented to support theTime History of Events and Macroscale Interactions during
Substorms (THEMIS) mission. In this mode, each radar operated a scan with 3-s integration times along each beam,
but with a special beam, the camping beam, being sampled
after every other beam. Thus, the full scan would take 2 min
while there was a 6-s time resolution on the camping beam
which in this case for BKS was beam 7. The BKS observations of the Pi2 wave were between 54o and 58o N MLAT,
close to the latitude of the Remus magnetometer station
which observed temporal variations similar to those in the
Doppler velocity of the ionospheric scatter detected by theradar. A cross-phase analysis of magnetometer data over arange of latitudes indicates that these observations weremade very close to the ionospheric projection of the plasmapause. Fine spatial and temporal details in the signature of
the wave in the radar scatter suggest that there were periods
when all ranges would see the onset of an enhancement in
the velocity, while subsequently there would be evidence ofa dispersion in the signature across latitudes. The authorsinterpreted this in terms of field-line compressions which(2019) 6:27

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occur when there is coherence in the radar scatter while atthe time of the dispersion there is a relaxation of the field.
Furthermore, THEMIS observations of ion velocities byelectrostatic analyzers (McFadden et al. 2008) indicated two
bursty bulk flows (BBFs) separated by 135 s which occurred
about 1 min before the Pi2 event. Comparison of the LOS
velocity at a variety of ranges along beam 7 of BKS demonstrates an interesting effect in the profile of each of the firsttwo peaks of the Pi2 pulsation in the radar data (Fig. 33
from Frissell et al. 2011). In both cases, the waveforms
started coherently as they moved toward the maximum
value in the case of the second pulse and the whole first halfof the wave cycle in the case of the first pulse. On the subsequent parts of the wave, however, there was a more dispersed signature as the traces relaxed to the lowest velocity
on different time scales, which increased with increasing
range. Frissell et al. (2011) interpreted these observations as
the initial part of the wave pulse responding to the compression created by the passage of the BBF, while the lack of coherence was explained by the fact that the traces from thelonger field lines, farther ranges, took longer time due to thelonger field lines. This interpretation is indicative of the

response to the BBF compression as it passed over the fieldlines and then a relaxation after the BBF had passed.
By comparing the variations in the ionospheric velocity
measured by TIG and UNW with the magnetometer observations at MacQuarie Island (albeit at 65o S), Ponomarenko and Waters (2013) made the first direct
observations of Pi2 wave polarization transition between
ionosphere and ground. A special three-beam mode was
employed in this study and use of a Hilbert transformrevealed dynamics of the Pi2 evolution. The authors
found for this particular event, which has a frequency of~ 7.5 mHz, an approximate linear polarization of Pi2
transferred from the ionosphere to the ground with a
40o rotation of the polarization ellipse, rather than theconventional 90o suggested in earlier works (e.g., Southwood and Hughes 1983), which indicates that the conductivity profile is not as simple as originally modeled.
In a pair of papers, Teramoto et al. (2014, 2016) presented two further case studies of Pi2 pulsations using
data from the THEMIS mode. In the first (Teramoto et al.
2014), a Pi2 wave was observed by HOK in the ground/sea scatter, which was reflected via the ionospheric F region, as well as from a sporadic E layer. The observations

Nishitani et al. Progress in Earth and Planetary ScienceabFig. 33 a The BKS LOS velocity as a function of latitude and time. The BKS data along beam 7 are averaged over three range gates and plotted
as a function of UT for five sets of gates covering the latitude range of 53.9° N to 57.1° N. b The difference between the first part of each of thefirst two cycles and the second part. Reproduced from Fig. 3 of Frissell et al. (2011)

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in the two different data sets had the same period, 110 s,
and there appears to be no phase lag. The authors suggested that the concept of a pure cavity mode is not sufficient to explain the radar and coincident magnetometer
observations and that a contribution from an Alfven modemust be considered in order to explain the observations.
In a second study, Teramoto et al. (2016) presented
observations from three radars, HOK, TIG, and UNW,
together with ground magnetometer data and electricand magnetic field observations by the THEMIS A, D,
and E spacecraft. Note that, to date, this represents the
only study of MHD waves at any latitude which use
radar data from the two hemispheres. The interhemispheric radar observations were made near the plasmapause and are critical for the understanding of the event.
An analysis of the THEMIS spectra indicated that two
frequencies were present in the event under study, at

14 mHz and 28 mHz. Ground magnetometers do notappear to observe the higher frequency wave but spectral
maxima at both frequencies are seen in the radar observations. TIG is key here because it made observations
both inside and outside of the plasmapause, while UNW
and HOK observed only inside the plasmapause. TIG
observed frequencies inside the plasmapause (14 and28 mHz) which are different from those outside of it(14 mHz only), while inside the plasmapause HOK only
observed the lower spectral frequency and UNW measured the lower spectral frequency at lower latitudes andhigher spectral frequency further poleward. The estimations of Poynting flux at THEMIS indicate that thewaves were propagating earthward and duskward. The
authors concluded that the observed wave was a cavity
mode resonance which had been stimulated by

Nishitani et al. Progress in Earth and Planetary ScienceabcFig. 34 The FHE beam 10 LOS velocity as a function of time and results of a spectral analysis. a The FHE beam 10 LOS velocity as a function oftime for a number of different range gates. b The relative phase from the spectral analysis as a function of latitude while c shows the amplitude
from the spectral analysis. Note the red dashed line in b which gives the estimated position of the plasmapause. Reproduced from Fig. 12 of Shi
et al. (2017)(2019) 6:27

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compressional waves which had propagated duskwardfrom the source region in the midnight sector.
Shi et al. (2017) provided a careful and thorough analysis
of multiple data sets from THEMIS, ground magnetometer,and FHE, FHW, and BKS. Shi et al. (2017) attempted here
to separate a plasmaspheric virtual resonance (PVR) where
wave energy is primarily confined to the plasmasphere but
a small portion escapes beyond the plasmapause, from aplasmaspheric cavity resonance (PCR) where wave energy
should be confined to the plasmasphere. The radar data are
key as they provide high spatial resolution across theplasmapause as shown in Fig. 34 from Shi et al. (2017),
where the wave phase and power are taken across anumber of range gates of data from FHE. The peakin power occurs at 60.6° N which is just inside theestimated position of the plasmapause at 60.9° N,
while a minimum in power occurs at 59.60° N. The
main phase change occurs between 59.8° N and 59°
N. This tendency suggests that the wave is not a field
line resonance (FLR). The authors also concluded thatthe source mechanism is related to the downward
FAC in the substorm current wedge (SCW).
So far, the only observation of Pi1 pulsations by aSuperDARN radar were made at WAL while testing a
new high-resolution multi-pulse mode (Greenwald et al.
2008) when electric field pulsations with periods
between 13 and 20 s were observed during a substorm
expansion phase. Magnetometer data at Ottawa revealed
simultaneous magnetic pulsations with the same period(Fig. 35 from Greenwald et al. 2008). The top panel ofFig. 35 is a contour plot of the spectral power from awavelet analysis of the Ottawa X (geographic northsouth) component data which shows well-defined peaks
at 20 s at 0605 UT and between 25 and 30 s over 0608–
0609 UT. The middle panel shows the high time
resolution SuperDARN data which were available only
for parts of the analyzed interval due to the operationalmode. The most important segment lies between 0608
and 0609 UT, and it is zoomed on in the bottom panel.
During this segment, two oscillation cycles with periods

Nishitani et al. Progress in Earth and Planetary Science

a

b

c

Fig. 35 A comparison of magnetometer data from Ottawa with WAL LOS velocity. a The dynamic spectrum of the X component data from theOttawa magnetometer for the interval 0604 to 0611 UT on 1 August 2007. The contours represent different levels of power with the lighter
colors the higher power. b The high time resolution estimates of the LOS velocity from WAL for the same period. c The shorter interval is marked
by vertical dashed lines. Reproduced from Fig. 3 of Greenwald et al. (2008)

(2019) 6:27

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of 13–14 s were also seen by the Ottawa magnetometers, although in magnetic records the main spectral
peak was observed at 25–30 s. There is also evidence
for wave activity in the other high-resolution intervals
from Fig. 35.

believed to be generated at the Earth’s foreshock by
the ion-cyclotron instability in reflected proton beams.
These observations suggest that the Pc3/4 energy
propagates into and through the magnetosphere in
the isotropic fast mode and couples to field-line
guided mode Alfven waves at harmonics of the local
resonant frequency. Furthermore, Ponomarenko et al.
(2005) concluded that these waves were third harmonic poloidal mode FLRs.
In a follow-on study, Ponomarenko et al. (2010) reporta subsequent investigation of Pc3–4 waves in ionospheric scatter from TIG together with ground magnetometer data, but here the emphasis of the paper is on
waves observed on the nightside. The authors present
synchronous variations of Pc3–4 spectra near the
post-dawn cusp and the nightside plasmapause and plasmatrough with frequencies that closely match those of
upstream waves. They present direct evidence of acommon source for the day and nightside Pc3–4 waves
measured on the ground and in the ionosphere. The observed nightside Pc3–4 oscillations show a pronounced
dependence on IMF parameters that replicates the behavior of simultaneous, dayside data. They discuss a possible propagation scenario: (1) Upstream waves are
generated at the Earth’s foreshock. (2) These interact
with the magnetosphere to drive compressional waves
that travel throughout the magnetosphere. (3) These
couple to field-aligned Alfven waves that result in theionospheric and ground Pc3–4 signatures.
In a statistical analysis, Norouzi-Sedeh et al. (2015)
applied a manual event selection to over 300 days ofhigh-resolution data obtained in 2007–2009 by TIG
and UNW. The ULF wave signatures in Doppler velocity oscillations over a relatively wide frequency
range of ~ 0.5–50 mHz were identified by comparing
the event time series and Fourier spectra with thoseobtained from the underlying ground magnetometerdata at Macquarie Island. This procedure identified
194 events, most of which were detected between 15

Nishitani et al. Progress in Earth and Planetary Science

Pc3–4 and Pc5 pulsations

The first mid-latitude observations of MHD waves werereported by Ponomarenko et al. (2003) using TIG. This
work is generally a technical paper in which the authors
describe and illustrate a way to visualize ULF wave signatures in the range-time maps of the Doppler velocity
observed by SuperDARN radars. This visualization isachieved by applying median high-pass filtering andcompressed dynamic range of the gray-scale color map(Fig. 36 from Ponomarenko et al. 2005). This analysis revealed the presence of the ULF signatures for 4–5 h perday. A very important finding was that 60% of the MHD
waves were observed in the previously underused
ground scatter component. Four different types of MHD
waves were observed: (1) low wavenumber Pc5 waves,(2) high wavenumber Pc5 waves, (3) band-limited dayside Pc3–4 waves (thought to be generated via upstream
waves), and (4) narrowband nighttime Pc4 waves(thought to be local FLRs). Importantly, the authors
reported that 46% of the observed waves had no
ground magnetic signature. The methodology presented in this paper was then used as the primary
analysis method in three subsequent papers using
mid-latitude SuperDARN radars, two of which look at
Pc3–4 waves and one of which looks at Pc5 waves,which are discussed below.
Ponomarenko et al. (2005) investigated Pc3–4 waves
in sea scatter from TIG. These data were complemented by ground magnetometer data from Macquarie Island. The waves were observed between 8 and 12
MLT, and the authors concluded that the observed
waves were driven by the interaction of upstream
waves with the magnetosphere. Such waves are

Fig. 36 LOS velocity data from beam 4 of TIG as a function of range and UT. One hour of LOS velocity data is presented from 2000 to 2100 UT
on 28 September 2000. The gray scale, which saturates at + 50 m s−1 and − 50 m s−1, demonstrates the ability of this technique to visualize MHD
waves in the data. Note also the band of scatter between range gates 20 and 40 (which is probably ground scatter) shows no evidence of the
MHD wave on this occasion. Based on Fig. 4 of Ponomarenko et al. (2005)

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and 21 LT, which is consistent with the radar echo
occurrence (GS or IS) with a typical duration of ~ 1–
1.5 h. This peak in LT occurrence matches a surveyof radially polarized waves observed by the Active
Magnetospheric Particle Tracer Explorers Charge
Composition Explorer (AMPTE CCE) spacecraft (Anderson et al. 1990). A maximum entropy analysis wasthen applied to estimate the spectral peak locations
with higher precision than can be provided by thefast Fourier transform (FFT) analysis. The maximum
occurrence in frequency was observed at ~ 2 mHzwith a secondary maximum located near 4–6 mHz,
which matches the expected local FLR periods. Furthermore, Norouzi-Sedeh et al. (2015) suggested thepresence of discrete spectral peaks at 1.3, 1.6, 2.1,
and 2.8 mHz (Fig. 37), although these values vary
somewhat throughout the paper and, more importantly, the peaks’ magnitudes seem to be close to thestatistical variance level of the analysis. Similar
discrete frequencies were also identified over 1830–
2130 LT based exclusively on the ground magnetometer data, i.e., without pre-selection of the coincident
radar-magnetometer events. While the majority of theoscillations (80%) showed little phase change withrange, the rest showed zig-zag patterns which they
identified as the ionospheric signatures of FLRs andpostulated that they may be associated with the predicted location of the plasmapause or ionospherictrough region. The observed L values, however, aremuch higher than those predicted by models whichleaves an open question on the nature of these waves.
In an event study of a large-scale global mode oscillation at ~ 1.6 mHz which was triggered by a sudden commencement (SC) and observed across a wide range oflatitudes by ground magnetometers, Kawano et al.
(2016) interpreted the initial oscillation as a global magnetospheric cavity mode through comparison with magnetometer data. The authors argued that only a global
cavity mode can have a frequency of ~ 1 mHz in theplasmasphere, as the entire magnetospheric cavity radially oscillates at the same frequency. An interesting feature embedded within this interval is a wave whichdisplays the amplitude and phase pattern characteristicof a FLR, i.e., a single peak in amplitude at the resonant
latitude accompanied by a monotonic change in phaseacross the latitudinal peak. This amplitude and phasepattern is revealed by a detailed spectral analysis of thehigh resolution (15 km in group range) ground/sea scatter signatures observed by HOK near L = 2 which demonstrate that, for a 15-min part of the 1-h wave event, amaximum in spectral power and steady increase of thephase with latitude have been observed. If these features
are interpreted in terms of a poloidal FLR, as done by
the authors, then the values of the plasma density at theequatorial plane required to produce the 1.6 mHz eigen
oscillation would be an order of magnitude larger thanthose normally observed at these MLAT. Furthermore,
the characteristic “kink” in the latitudinal phase

Nishitani et al. Progress in Earth and Planetary ScienceFig. 37 A histogram of significant frequencies in the TIG data set. A three-point smoothing is applied to the data and is shown by the curve withthe slightly darker black line superimposed on the histogram. The vertical lines identify peaks of the smoothed curve showing certain frequencies
at 1.3, 1.6, 2.1, 2.9, and 3.3 mHz which appear to have higher spectral power than the neighboring frequencies. The frequency resolution is
0.1 mHz. Reproduced from Fig. 7 of Norouzi-Sedeh et al. (2015)

(2019) 6:27

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progression is effectively absent from the data. In order
to explain these discrepancies, the authors invoked (1) aquarter-wave type of FLR, (2) compression of theplasmasphere, (3) compression of the ionosphere, and(4) enhanced ion outflow caused by the main phase of
the magnetic storm, whose combined effect might haveproduced the observed oscillations.

ways in this respect. One is the integration time along
the beam and the other is the scanning from one beam
to the next. Greenwald et al. (2008) demonstrated thepotential of new ways of using the multi-pulse data to
improve the time resolution along a beam, and this potential has already been recognized within the community. Further, the potential afforded by imaging radars(for more detail see the “Technical developments” subsection) for ULF wave studies cannot be underestimated.
Finally, the development of analysis techniques which do
not require a continuous data set, such as theLomb-Scargle spectral analysis (Lomb 1976; Scargle1982), would benefit the analysis of SuperDARN, where
often there are data gaps.

Nishitani et al. Progress in Earth and Planetary Science

MHD waves: Future work

It is clear from the work that has been done on MHD
waves with the mid-latitude SuperDARN radars that
there is significant potential for further study. There areseveral future directions, both with existing data sets andradars, as well as potential new developments of radar
technology, as well as links to current and future space
missions. Two areas where the current data sets and radars could be used are identified before moving to new
developments.
The work that has been currently done on Pi2 pulsations has yet to result in any firm consensus regarding themodes that are present at mid-latitudes. This situation
perhaps is due to both the nature of the waves which are
relatively short lived and occur during the nighttime when
the presence of ground scatter is perhaps limited. Nevertheless, there are several directions which could be
followed here, notably in developing statistical analyses
which may enable us to separate out which modes aremost often present. Further studies which relate groundand space observations, notably with space missions suchas THEMIS, would also be helpful, in particular to determine the exact relationship between the wave signatures
and tail dynamics as exemplified by BBFs. Finally, the useof interhemispheric studies may help in determining thewave modes present as this would place additional constraints on the models.
Moving to Pc3–4, these tend to be smaller-scale waves
and so are more localized. Consequently, it is to be expected that future studies would focus on small sub-setsof radars, making use of the interhemispheric capability
that exists in the Japanese/Australian and the US/SouthAmerican sectors. Furthermore, future work is required todetermine the percentage of such waves which are related
to wave energy directly penetrating from the solar windcompared with internal sources of them such as
wave-particle interactions. Pc5 waves on the other hand
are more likely to be larger scale oscillations such that
more studies involving multiple radars, where possible,
would be sensible, especially as the network at
mid-latitudes continues to expand.
Finally, the development of new radar and analysis
techniques is considered. It is clear from the paper by
Greenwald et al. (2008) that higher time resolution observations will enable studies of higher frequency waves.
Currently, the SuperDARN technique is limited in two

Conclusions and future directions
Conclusions and science directions

In this paper, the scientific accomplishments of the
mid-latitude component of SuperDARN have beenreviewed in five specified areas, with some discussionof future science directions. Here is the briefsummary:
1. Convection: The expansion of SuperDARN to midlatitudes has been a demonstrated success with previously unattainable measurements of the convection electric field equatorward of ~ 60° MLAT now
possible, and the ability to study a variety of phenomena occurring at mid-latitudes. In addition to
the expected enhancement in the capability tomonitor plasma circulation during geomagnetically
active periods, the mid-latitude radars have significantly improved our understanding of SAPS by providing near-continuous measurements of theirtemporal and spatial variations, and their relationships with other measurements such as GPS TEC
and both ground and space-based imagers. Futurestudies will undoubtedly reveal the statistical natureof SAPS occurrence and the origins of SAPS andSAID.
2. Ionospheric irregularities: With the construction of
the first purpose-built mid-latitude SuperDARN radars at WAL and HOK, it is evident that additional
types of irregularities exist at mid-latitudes. These
include low-velocity irregularities in the sub-auroral
region, high-velocity irregularities at the edge of theauroral oval associated with SAPS, and irregularities
occurring during the passage of MSTIDs at temperate (lower) mid-latitudes. The relative simplicity of
the ionosphere at sub-auroral latitudes is a distinct
advantage for relating the irregularities to processesof plasma instability, and there has been impressive
progress in the modeling of irregularities in termsof the GDI, TGI, and coupling between the E and F

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regions. There is a good reason to think that the
mid-latitude radar observations are providing a critical testbed of measurements for developing comprehensive first-principle models and that this work
will lead to fresh insights into ionospheric plasma
physics of interest to a wide range of researchersand operators by encompassing the generation andgrowth mechanisms of several kinds of ionosphericirregularities and their relation to other phenomena(e.g., SAID / SAPS, MSTIDs, and E region shortenings due to enhanced conductivity) and to geophysical parameters (e.g., latitude and geomagnetic
activity).
3. HF propagation analysis: HF propagation analysis
can be utilized to conduct value-added ionosphericresearch at mid-latitudes and as a tool for assessing
radar performance. Looking forward, two scientifictopics that deserve further examination with propagation analysis are solar cycle dependencies and interhemispheric differences. Also, it is feasible thatcalibrated elevation angle data could be inverted toprovide ionospheric plasma density suitable for interhemispheric comparison.
4. Ionosphere-neutral interactions: The
contributions of mid-latitude SuperDARN to thestudy of phenomena associated with ion-neutralinteractions have been predominantly throughobservations TIDs (large scale and medium
scale). With mid-latitude SuperDARN, it is possible to study the characteristics of TIDs in the
sub-auroral and mid-latitude regions as a function of latitude, LT, and geomagnetic activity,and several important characteristics have beenclarified (e.g., the relationship to the intensity ofthe polar vortex). It has been demonstrated thatthe potential scope for study of ion-neutral
coupling extends to the ionospheric disturbancesthat are due to big earthquakes, tsunamis, volcanic eruptions, and tornadoes and to the timescales of flywheel effects and disturbance dynamo effects.
5. MHD waves: There has been a number of
discoveries made using the mid-latitude SuperDARN radars to study MHD waves. Statisticalstudies have been limited to Pc3–5 waves, andan important early finding was that 60% of
MHD waves occurred in the ground scatter received by the radars (Ponomarenko et al. 2003),
thus opening a new technique for analysis. Studies of nightside Pi2 pulsations have been mostly
limited to case studies involving multiple data
sets collected from the ground and from space.
The first estimate of the rotation of the Pi2
polarization ellipse was made with the results

indicating that the conductivity profile is not as
simple as originally modeled. In a serendipitous
observation when testing a new coding scheme,
Greenwald et al. (2008) made the first observations of Pi1 pulsations by a SuperDARN radar.
Further studies of MHD waves using extended
radar FOVs and new techniques are thus very
promising.

Nishitani et al. Progress in Earth and Planetary Science

Satellite conjunction

Mid-latitude SuperDARN radars have FOVs which
cover sub-auroral and auroral latitudes and are connected by magnetic field lines to the inner magnetosphere and near-earth plasma sheet. Comparison
between SuperDARN and satellite observations is crucial for understanding the dynamics of the coupled(inner) magnetosphere and ionosphere. Collaborative
studies with, for example, the THEMIS (Angelopoulos
2008) and the Van Allen Probes satellites (Mauk et al.
2013) have already proven to be capable of addressing
important science themes such as linkage of dynamicalaurora in the ionosphere and plasma evolution in themagnetosphere (e.g., Nishimura et al. 2010) and ULF
waves (e.g., Teramoto et al. 2016; Kim et al. 2017), andso forth. Recently the Exploration of energization andRadiation in Geospace (ERG) satellite (also known as“Arase” satellite) (Miyoshi et al. 2012) has joined such a
collaboration framework particularly for studies of theinner magnetosphere, after its successful launch at theend of 2016. The addition of ERG to the inner magnetospheric satellite fleet enables the global view ofionospheric convection obtained instantaneously bySuperDARN to be compared with simultaneous observations at multiple points in the magnetosphere.
Multi-spacecraft conjunction studies with SuperDARN
could be used to monitor the longitude/MLT evolutionof magnetosphere-ionosphere-coupled processes in an
instantaneous manner. Several such studies are expected in the near future. Finally, there are several
forthcoming spacecraft missions related to mid-latitude
SuperDARN, such as Solar-wind Magnetosphere Ionosphere Link Explorer (SMILE), Ionospheric Connection
Explorer (ICON), Global-scale Observations of the
Limb and Disk (GOLD), Demonstration and Science
Experiments (DSX), and Tool for the Analysis of RAdiations from lightNIngs and Sprites (TARANIS), as wellas ongoing missions such as Magnetospheric Multiscale
Mission (MMS).
Expansion of SuperDARN coverage

There are several plans to expand the reach of
mid-latitude SuperDARN in both longitude and latitude.
Some examples are the radars planned for several placesin Europe. Construction of an HF radar at Jiamusi,(2019) 6:27

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China, has recently been completed while constructionof an additional radar at Kunming in southern China isproceeding. There are also possibilities for the RussianSuperDARN-type radars to join the SuperDARN network in the future. Finally, there are plans to build aSuperDARN radar in the low-latitude and equatorial regions (possible candidates are southern Japan, Southeast
Asia, Pacific Ocean etc.) to study the horizontal structure of the ionospheric convection and irregularities associated with equatorial plasma bubbles over distancesof several thousand kilometers.

radar would also provide a continuous azimuthal
coverage for studying ULF waves with periods
smaller than 2 min (Pc4 and higher frequencies),
which currently can only be studied using two to
three beams in order to achieve a required samplingrate. Yet, another advantage will be the ability tosimultaneously sound at multiple frequencies (real“stereo” mode).
2. Software: The SuperDARN Data Analysis working
group periodically releases updated versions of the
Radar Software Toolkit (RST). The most recent
version is RST 4.2 (SuperDARN Data Analysis
Working Group 2018). The major recent
improvements are (a) full revision of the ACF
fitting package; (b) utilization of the latest statisticalmodels (Cousins and Shepherd 2010; Thomas andShepherd 2018) using multiple radar data in bothhemispheres and the inclusion of data from polar
and mid-latitudes versus the previous model (Ruohoniemi and Greenwald 1996) based on only one
radar (GBR); (c) the Chisham virtual height model(Chisham et al. 2008) was added as option in
addition to the original standard SuperDARN virtual height model, traditionally used to map radardata.
3. Algorithm: A task force, led by AurélieMarchaudon, has been formed to determine a
reliable way of calibrating interferometry (elevation
angle) data, including techniques utilizing lowelevation ground scatter (Ponomarenko et al. 2015),
near-range (“meteor”) echoes (Chisham and Freeman 2013) and artificial irregularities generated bypowerful radio waves (ionospheric “heating”) (Burrell et al. 2016).

Nishitani et al. Progress in Earth and Planetary Science

Interhemispheric conjugacy

Interhemispheric study of a variety of ionosphericphenomena such as SAPS is important for developing
improved understanding of processes in the magnetosphere, and the connection between the magnetosphere and ionosphere. Conjugacy studies are also
important for the study of ion-neutral coupling processes, e.g., to identify the dependence of TIDs on
local time, geomagnetic/geographic latitude and season. The present sets of radars which could be used
for such studies are (i) BKS (North America) and FIR(South America); (ii) HOK and HKW (Asia) and TIG,
UNW, and BKP (Oceania).
Technical developments

There are several prospects for further technical development in terms of (1) hardware, (2) software, and (3)algorithms.
1. Hardware: The BKP, which started operation in2014, is the first fully digital SuperDARN radar andis much more sensitive than the pre-existing radars
and much less susceptible to instrumental noise.
The same digital radar system developed for BKP is
now also running at the FIR site. Similarly, the University of Alaska, Fairbanks group has developed an“imaging” capability for the SuperDARN radar at
KOD (Parris and Bristow 2009). Likewise, University of Saskatchewan engineers are currently working on designing a digital “imaging” radar which
would replace consecutive sampling of the radar
beams by simultaneous multi-directional measurements. This “imaging” will be achieved through illuminating the whole radar’s FOV by widening themain antenna lobe during transmission and recording raw data at each antenna so that the beam
forming and data analysis are performed afterwards
in software. This approach would decrease the sampling rate for the circulation maps from 1 to 2 min
to several seconds and/or significantly improve theaccuracy in estimating the echo parameters by increasing the ACF integration time. The “imaging”

Cooperation with modeling activities

Cooperation with theoretical modeling and numericalsimulation activities is important for understanding
the magnetosphere-ionosphere-thermosphere coupledsystem. This is especially true for the mid-latitude
SuperDARN observations because during magnetic
storms the contribution from the ring current becomes dominant (internal forcing), and the convection
dynamics cannot be interpreted only in terms of the
expansion of the high-latitude convection. As alreadydiscussed in the “Convection” Section, Fig. 14 shows
one example of the coordinated study, a comparison
between the SuperDARN HOK observations of SAPS
structure indicating temporal changes (top panel) withsimulation results from the CRCM under various
boundary conditions (Ebihara et al. 2009). These results clearly indicate the importance of ring current
dynamics in controlling the SAPS spatial structure.
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needed to make further progress understanding thecoupled dynamics of the inner magnetosphere andsub-auroral ionosphere. Finally, efforts to use SuperDARN data to constrain first-principle models of
ionosphere, such as the Research Institute in Astrophysics and Planetology (IRAP) PlasmasphereIonosphere model (IPIM) (Marchaudon and Blelly2015), are currently in progress and will lead to consistent retrieval of ionospheric parameters over large
3D coverage regions.

kinds of observation techniques, including HF radar network (Tsuda et al. 2016).
The UK government has identified severe space
weather as a potential risk and has included this topic
in its National Risk Register of Civil Emergencies (seehttps://www.gov.uk/government/publications/national-risk-register-of-civil-emergencies-2017-edition). SuperDARN is contributing to both the science andunderstanding of space weather and could in the future potentially contribute to operational systems.
SuperDARN is also seen as a major contribution tonational capability providing long-term and large-scalemonitoring of the atmosphere through its Antarctic
SuperDARN program. Finally, the UK is a major contributor to the joint European Space Agency (ESA)and Chinese National Space Science Centre (NSSC)
space mission SMILE which is scheduled for launch
in 2021, and SuperDARN is seen as a major
ground-based contributor to that program. Other participating countries also have their national strategy
for the future of space science, and some of them are
related to the mid-latitude SuperDARN.

Nishitani et al. Progress in Earth and Planetary Science

National strategy

Funding of each mid-latitude SuperDARN radar isclosely related to the national strategy of the participant
country.
In the USA, the most recent “Decadal Survey” report commissioned by the National Research Council
identified four scientific goals for the space physics
research community over the next 10 years. SuperDARN radars at mid-latitudes are particularly relevantto the second goal: to determine the dynamics andcoupling of Earth’s magnetosphere, ionosphere, andatmosphere and their response to solar and terrestrial
inputs. To meet these goals, the report recommends
the creation of a new, integrated multiagency initiative—DRIVE—that will more effectively exploit NASAand NSF scientific assets to address the pressing
needs for improved space weather specification andforecasts. The five directives comprising the DRIVE
initiative are (1) diversify observing platforms withmicrosatellites and mid-scale ground-based assets, (2)realize scientific potential by sufficiently funding operations and data analysis, (3) integrate observing platforms and strengthen ties between agency disciplines,(4) venture forward with science centers and instrument and technology development, and (5) educate,empower, and inspire the next generation of space researchers. Much of the success of SuperDARN, todate, can be attributed to the fact that it was foundedon the very same collaborative principles that underpin these five directives and, as such, it is uniquely
positioned to play a key role in the development of
the DRIVE initiative as it moves forward.
In Japan, the government has selected master plan projects and drawn up a roadmap of scientific research every
few years, in order to identify future science directions.
The project “Study of Coupling Processes in the
Solar-Terrestrial System” was approved as a project of“Masterplan 2014” and “Masterplan 2017” by the Science
Council of Japan, and of “Roadmap 2014” (one of 11 new
approved projects) by the Ministry of Education, Culture,
Sports, Science and Technology (MEXT). SuperDARN isrelated to this project in that it studies the coupling processes in the solar-terrestrial environment using several

Training and public outreachConstruction and operation of mid-latitude SuperDARN radars is important not only for obtaining
new scientific results, but also for attracting young
students and training technical staff. Through thetraining activity, they can learn the engineering details of the radar system, characteristics of irregularities and HF wave propagation, and also the scienceof magnetosphere/ionosphere/upper atmosphere dynamics. Radars located where access is relatively easy
provide ideal opportunities for hands-on engineering
engagement. One example is that HOK and HKW in
Japan has been used for training members of Japanese Antarctic Research Expedition (JARE) every
year. Through the training activity, they can learn
the engineering details of the radar system in advance. However, even for those radars located in remote locations, the routine data analysis/processing
and software development activities provide excellent
opportunities for education and training.
For the continued operation and maintenance of the
mid-latitude SuperDARN radars, it is crucial that
relevant government agencies and the general public
understand the value they provide for basic research andspace weather applications (e.g., Jansen and Pirjola2004). It is also important that researchers in other fields
outside space science understand SuperDARN. For thispurpose, public releases of radar operations and newscientific findings in the most effective ways are necessary (Wild 2012).

Nishitani et al. Progress in Earth and Planetary Science(2019) 6:27

Additional file
Additional file 1: Historical overview of mid-latitude SuperDARN.
(PDF 212 kb)

Abbreviations
AACGM: Altitude-Adjusted Corrected Geomagnetic; ACF: Autocorrelation
function; AGW: Acoustic-gravity wave; AMPTE CCE: Active MagnetosphericParticle Tracer Explorers Charge Composition Explorer; AWFC: Auroral
Westward Flow Channel; BBFs: Bursty bulk flows; BCOV: Beaver Cove;
CME: Coronal mass ejection; CRCM: Comprehensive ring current model;
DSX: Demonstration and Science Experiments; DUSE: Dusk scatter event;
EKB: Ekaterinburg; EPB: Electron precipitation boundary; ERG: Exploration ofenergization and Radiation in Geospace; ESA: European Space Agency;
FAC: Field-aligned current; FFT: Fast Fourier transform; FLR: Field line resonance;
FOV: Field of view; FPI: Fabry-Perot interferometer; FTEs: Flux-transfer events;
GDI: Gradient drift instability; GEONET: GPS Earth Observation Network System;
GNSS: Global Navigation Satellite System; GOLD: Global-scale Observations of
the Limb and Disk; GPS: Global positioning system; GS: Ground scatter;
HAIR: High-aspect angle irregularity region; HF: High frequency; HOP: Hokkaido
Pair of Radars; HWM: Horizontal Wind Model; ICON: Ionospheric Connection
Explorer; IGRF: International Geomagnetic Reference Field; IMAGE: International
Monitor for Auroral Geomagnetic Effects; IMF: Interplanetary magnetic field;
IRAP: Research Institute in Astrophysics and Planetology; IRI: International
Reference Ionosphere; IS: Ionospheric scatter; ISEE: Institute for Space-Earth Environmental Research; ISR: Incoherent scatter radar; JARE: Japanese Antarctic
Research Expedition; JSPS: Japan Society for the Promotion of Science;
KHI: Kelvin-Helmholtz instability; LANL: Los Alamos National Laboratory;
LEO: Low Earth Orbit; LOS: Line-of-sight; LSTID: Large-scale traveling ionosphericdisturbance; LT: Local time; MEXT: Ministry of Education, Culture, Sports, Scienceand Technology; MF: Medium frequency; MHD: Magnetohydrodynamic;
MI: Magnetosphere-ionosphere; MLAT: Geomagnetic latitude; MLT: Magnetic
local time; MMS: Magnetospheric Multiscale Mission; MSEs: Mesospheresummer echoes; MSI: Mid-sized infrastructure; MSTID: Medium-scale travelingionospheric disturbance; MU: Middle and upper atmosphere; MUF: Maximum
usable frequency; NASA: National Aeronautics and Space Administration;
NSF: National Science Foundation; NSSC: National Space Science Centre;
OTH: Over the horizon; PBIs: Poleward boundary intensifications;
PCR: Plasmaspheric cavity resonance; PI: Principal investigator; PIC: Particle-incell; PJ: Polarization jet; PMSE: Polar mesosphere summer echoes; PPEF: Prompt
penetration electric field; PVR: Plasmaspheric virtual resonance; PWING: Study ofdynamical variation of Particles and Waves in the INner magnetosphere using
Ground-based network observations RST: Radar Software Toolkit; SAEF: Subauroral electric field; SAIDs: Sub-auroral ion drifts; SAIS: Sub-auroral ionosphericscatter; SAPS: Sub-auroral polarization streams; SAPSWS: SAPS wave structure;
SARAS: Substorm-associated radar auroral surges; SC: Sudden commencement;
SCW: Substorm current wedge; SED: Storm-enhanced density; SI: Sudden
impulse; SMILE: Solar-wind Magnetosphere Ionosphere Link Explorer;
SSC: Storm sudden commencement; SuperDARN: Super Dual Auroral Radar
Network; TARANIS: Tool for the Analysis of RAdiations from lightNIngs andSprites; TCVs: Traveling convection vortices; TEC: Total electron content;
TECU: TEC unit; TGI: Temperature gradient instability; THEMIS: Time History of
Events and Macroscale Interactions during Substorms; TIDs: Travelingionospheric disturbances; TIGER: Tasman International Geospace Environment
Radar; TOI: Tongue of ionization; TTFD: Twin Terminated folded Dipole;
ULF: Ultra-low frequency; UT: Universal Time; VHF: Very high frequency;
WSA: Weddell Sea Anomaly

Acknowledgements
This review paper originated from approximately one-week long workshops
in January 2017 and January 2018, during which we discussed the content
of the paper, its structure and future directions of the mid-latitude SuperDARN network. These workshops were carried out by the joint research program of the Institute for Space-Earth Environmental Research, Nagoya
University, and supported by funding agencies described in the ‘funding’section. Finally, we would like to thank everyone who contributed to thecompletion of the review paper.

Page 51 of 57

Funding
SuperDARN is a collection of radars funded by national scientific funding
agencies of Australia, Canada, China, France, Italy, Japan, Norway, South
Africa, United Kingdom and the United States of America. Organization of
the workshops for writing up the present review paper was supported by
Japan Society for the Promotion of Science (JSPS) (Grant-in-Aid for Specially
Promoted Research): Study of dynamical variation of particles and waves in
the inner magnetosphere using ground-based network observations(PWING), Project Number: 16H06286. Completion of the present review paper
was partly supported by JSPS (Grand-in-Aid for Scientific Research), Project
Number: 18KK0099. Part of the work of TH was done at the ERG-Science
Center operated by ISAS/JAXA and Institute for Space-Earth Environmental
Research (ISEE) /Nagoya University.
Availability of data and materials
The SuperDARN datasets shown in this paper are available in the Virginia
Tech website at: http://vt.superdarn.org.
The AACGM-v2 software for calculating geomagnetic coordinates of theSuperDARN radars is available in the Dartmouth College web server at:
http://superdarn.thayer.dartmouth.edu/aacgm.html.
Authors’ contributions
NN, JMR, and ML originally proposed the present review paper, attended theworkshops in Nagoya, Japan, and contributed to the writing of the paper.
JBHB, AVK, SGS, and GC attended the workshop(s) and contributed to thewriting of the paper as section heads. TH, EGT, RAM, AM, PP, and JW
attended the workshop(s) and contributed to the writing of the paper. SEM,
WAB, JD, and EM contributed to the content of the paper as mid-latitude
SuperDARN principal investigators and co-investigators. RAG, TA, and TK contributed to the content of the paper as the authors of the references cited in
the present paper and by commenting on the manuscript. All authors readand approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Institute for Space-Earth Environmental Research, Nagoya University, Nagoya
464-8601, Japan. 2Virginia Tech Department of Electrical and Computer
Engineering, Blacksburg, VA, USA. 3University of Leicester, Leicester, UK.
4
University of Saskatchewan, Saskatoon, SK, Canada. 5Thayer School ofEngineering, Dartmouth College, Hanover, NH, USA. 6British Antarctic Survey,
Cambridge, UK. 7University of Alaska Fairbanks, Fairbanks, AK, USA. 8Institut
de Recherche en Astrophysique et Planétologie, University of Toulouse,
CNRS, CNES, Toulouse, France. 9Lancaster University, Lancaster, UK. 10La
Trobe University, Melbourne, Australia. 11Applied Physics Laboratory, Johns
Hopkins University, Baltimore, MD, USA. 12National Institute of Information
and Communications Technology, Tokyo, Japan.
Received: 8 August 2018 Accepted: 5 February 2019

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