Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2
DOI 10.1186/s40645-014-0031-4

REVIEW

Open Access

The geospace response to variable inputs from thelower atmosphere: a review of the progress madeby Task Group 4 of CAWSES-II
Jens Oberheide1*, Kazuo Shiokawa2, Subramanian Gurubaran3, William E Ward4, Hitoshi Fujiwara5,
Michael J Kosch6,7,8, Jonathan J Makela9 and Hisao Takahashi10

Abstract
The advent of new satellite missions, ground-based instrumentation networks, and the development of wholeatmosphere models over the past decade resulted in a paradigm shift in understanding the variability of geospace,that is, the region of the atmosphere between the stratosphere and several thousand kilometers above ground where
atmosphere-ionosphere-magnetosphere interactions occur. It has now been realized that conditions in geospace
are linked strongly to terrestrial weather and climate below, contradicting previous textbook knowledge thatthe space weather of Earth's near space environment is driven by energy injections at high latitudes connectedwith magnetosphere-ionosphere coupling and solar radiation variation at extreme ultraviolet wavelengths alone.
The primary mechanism through which energy and momentum are transferred from the lower atmosphere isthrough the generation, propagation, and dissipation of atmospheric waves over a wide range of spatial andtemporal scales including electrodynamic coupling through dynamo processes and plasma bubble seeding. The
main task of Task Group 4 of SCOSTEP's CAWSES-II program, 2009 to 2013, was to study the geospace response
to waves generated by meteorological events, their interaction with the mean flow, and their impact on theionosphere and their relation to competing thermospheric disturbances generated by energy inputs fromabove, such as auroral processes at high latitudes. This paper reviews the progress made during the CAWSES-II
time period, emphasizing the role of gravity waves, planetary waves and tides, and their ionospheric impacts.
Specific campaign contributions from Task Group 4 are highlighted, and future research directions are discussed.
Keywords: Geospace; Thermosphere; Ionosphere; Tides; Planetary waves; Gravity waves; Traveling ionosphericdisturbances; Traveling atmospheric disturbances

Introduction
The space weather of Earth's upper neutral and ionized atmosphere is strongly influenced by energy injections athigh latitudes connected with magnetosphere-ionosphere
coupling and solar radiation variability at extreme ultraviolet wavelengths. A variety of new evidence obtained
over the past few years now demonstrates unequivocallythat the geospace environment also owes a substantial
amount of its variability to waves forced in the lower partsof Earth's atmosphere, that is, the troposphere and stratosphere. This paradigm shift in understanding the causes of* Correspondence: joberhe@clemson.edu
1
Department of Physics and Astronomy, Clemson University, Clemson,
SC 29634-0978, USA
Full list of author information is available at the end of the article

geospace variability was made possible by the advent ofsatellite missions like Imager for Magnetopause-to-Aurora
Global Exploration (IMAGE) (Burch 2000), Thermosphere
Ionosphere Mesosphere Energetics and Dynamics (TIMED)(Yee et al. 1999), CHAllenging Minisatellite Payload(CHAMP) (Reigber et al. 2006), Gravity Recovery andClimate Experiment (GRACE) (Tapley et al. 2004), andConstellation Observing System for Meteorology (COSMIC)(Anthes et al. 2008), advanced ground-based observing capabilities like Poker Flat Advanced Modular Incoherent
Scatter Radar (PFISR) (e.g., Nicolls and Heinselman 2007)
and airglow observing networks (e.g., Shiokawa et al.
2009; Makela et al. 2009), and the development of ‘wholeatmosphere’ general circulation models from the groundto the ionosphere such as Whole Atmosphere Community© 2015 Oberheide et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly credited.

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Climate Model - thermosphere extension (WACCM-X)(Liu et al. 2010a), Whole Atmosphere Model (WAM)/
Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics Model (CTIPe) (Fuller-Rowell et al. 2010), andground-to-topside model of Atmosphere and Ionospherefor Aeronomy (GAIA) (Jin et al. 2011). See the list of abbreviations at the end of the paper. It is by now without
dispute that geospace owes much of its longitudinal,seasonal-latitudinal, and day-to-day variability to meteorological weather processes in the troposphere andstratosphere (e.g., Immel et al. 2006; Forbes et al. 2006;
Oberheide et al. 2006b; Hagan et al. 2007; Goncharenko
et al. 2010; Fritts and Lund 2011; Maute et al. 2012; andreferences therein). The primary mechanism throughwhich energy and momentum are transferred from thelower atmosphere is through the propagation and dissipation of atmospheric waves, including electrodynamic
coupling through dynamo processes and instability seeding. Table 1 summarizes the basic characteristics of thethree most important classes of atmospheric waves: gravity waves (GWs), atmospheric tides, and planetary waves(PWs).
CAWSES-II Task Group 4 (TG4; What is the geospace
response to variable inputs from the lower atmosphere?)
was therefore charged to elucidate the dynamical couplingfrom the low and middle atmosphere to the geospace(i.e., the upper atmosphere, ionosphere, and magnetosphere), for various wave frequencies and scales, andfor equatorial, middle, and high latitudes. As meetingthe challenge clearly requires a systems approach involving experimentalists, data analysts, and modelers
from different communities, an essential part of TG4
was to encourage interactions between atmosphericscientists and plasma scientists on all occasions. To
distinguish geospace variability due to solar and magnetospheric driving from above from processes propagating from below, and in order to dissect the problem
into solvable pieces, four projects were formed to respectively address each of the following science questions: (1)How do atmospheric waves connect tropospheric weather
with ionosphere/thermosphere variability? (2) What is therelation between atmospheric waves and ionospheric instabilities? (3) How do the different types of waves interact
as they propagate through the stratosphere to the ionosphere? (4) How do thermospheric disturbances generated
by auroral processes interact with the neutral and ionizedatmosphere?

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Several observational campaigns were conducted withineach project, supported by regular workshops, conference
sessions, and business meetings. Results were not only disseminated through the peer-reviewed literature but also to
the broader CAWSES community through quarterly TG4
newsletters. This paper describes these activities and reviews the progress made over the CAWSES-II period
from 2009 to 2013. Each project and its scientific outcomes are described in a separate section. The manuscript
concludes with a general discussion of the outcomes, thescientific challenges for the future, new satellite missions,and SCOSTEP's new VarSITI program. More results
related to TG4 can be found in the four special issues
listed in Table 2 and in a special issue of Earth, Planets
and Space dedicated to results presented at the International
CAWSES-II Symposium (2014) held at Nagoya University,
Japan, 18 to 22 November 2013.

Review
Project 1: How do atmospheric waves connect
tropospheric weather with ionosphere/thermospherevariability?

The impact of atmospheric waves on ionospheric structure
and variability has been realized for quite some time. Hines(1960) in his pioneering work was the first to propose GW
as the cause of ‘irregular motions’ in the thermosphere andionosphere. Since then, numerous studies have revealed
that GWs are present at heights up to the upper thermosphere and connected them to medium-scale travelingionospheric disturbances (MSTIDs), ionospheric irregularities, and plasma instabilities. See for example the more recent review by Fritts and Lund (2011) and references
therein. Before CAWSES-II, it was already known that
GW from convection and jet streams in the lower atmosphere propagate into the mesosphere, dissipate their energy near the mesopause region, and/or penetrate into thethermosphere. However, despite some speculation about
the initiation of various plasma instabilities by GW, the relation between GW and MSTID, day-to-day variability andzonal separation of plasma bubbles, and the scale size andpropagation of sporadic-E patches were not understood.
The examination of the relationship between these phenomena and an improved understanding of the importanceof GW in ionosphere/thermosphere dynamics were objectives for CAWSES-II.
The idea that global winds in the ionosphere may be asource of disturbance electric fields and currents goes

Table 1 Basic characteristics of atmospheric waves
Gravity (buoyancy) waves

Atmospheric tides

Planetary waves

Local; 10 to 1,000 s of kilometers

Global; 10,000 s of kilometers

Global; 10,000 s of kilometers

Temporal scale

Minutes to hours

1 day or harmonics

Days to weeks

Excitation

Mechanical distortions, latent heating

Solar heating, latent heating

Mechanical distortions, baroclinic instabilities

Spatial scale

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

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Table 2 Special issues with significant TG4 contributions
Title

JournalYear

TG4 relation

Coupling between the lower and upper atmosphere

JGR Space Physics

2010

General topic

Coupling between the Earth's atmosphere and its
plasma environment

Space Science Review, Vol. 168,
Issue 1-4, 20122012

ISSI workshop, see TG4 newsletter vol. 3

Recent progress in the vertical coupling in theatmosphere-ionosphere system

J. Atmos. Sol. Terr. Phys., Vol. 90-91,
pages 1-222, December 2012

2012

4th IAGA/ICMA/CAWSES-II TG4 workshop
on vertical coupling

Recent advances in equatorial, low-, and mid-latitude
aeronomy

J. Atmos. Sol. Terr. Phys., Vol. 103,
pages 1-194, October 20132013

ISEA-13 conference

back at least to the dynamo theory by Stewart (1882);
the connection to Sun-synchronous (migrating) atmospheric tides forced by solar radiation absorption was discussed by Fejer (1964). As tidal theory and observational
diagnostics progressed, it was realized that non Sunsynchronous (nonmigrating) tides forced by deep tropicalconvection are equally important for explaining longitudinal and local time variations in bulk neutral and plasma
properties of the ionosphere/thermosphere system. Satellite diagnostics (Forbes et al. 2006; Oberheide et al. 2006b;
Sagawa et al. 2005; Immel et al. 2006) and models (Hagan
and Forbes 2002; Hagan et al. 2007) resulted in a basic
quantitative knowledge of tidal forcing, propagation, andmorphology in the mesosphere and lower thermosphere,
the ionosphere, and a basic qualitative knowledge about
the coupling into the F-region through E-region dynamo
modulation. It should be noted that important contributions came from CAWSES-I activities, for example, fromthe CAWSES tidal campaigns (Ward et al. 2010) that effectively resolved the long-standing issue between groundbased radar and satellite optical measurements of winds.
See also Kishore Kumar et al. (2013) for a recent comparison between radar, satellite, and model results obtained
during the first CAWSES tidal campaign in 2005. Major
challenges for CAWSES-II included the elucidation of

tidal structures in the altitude range between 110 (upper
altitude observed by TIMED) and 400 km (in situ tidal
diagnostics from CHAMP) where suitable satellite observations are lacking, temporal variations of the tides
on timescales ranging from days to solar cycle, a better
separation of E-region dynamo modulation vs tidal
coupling at F-region heights, an assessment of the tidal
impacts on the energy balance and composition of thethermosphere, and wave-wave and wave-mean flow interactions. Similar questions are applied to PW as well,with special interest in their role in connecting polar
stratospheric warmings with F-region low latitude plasma
density variability (Goncharenko and Zhang 2008), as further detailed in project 3.
Gravity waves

Gravity waves generated by convective systems, hurricanes, and surface perturbations such as earthquakes
and tsunamis may seed plasma bubbles in the ionospherethrough Rayleigh-Taylor instability. Figure 1 shows GW in
airglow images at 630 nm (approximately 250 km) causedby the tsunami from the catastrophic Tohoku earthquakeon 11 March 2011 when it passed the Hawaiian Islands.
See Makela et al. (2011) for details and also for a movie
that includes the ionospheric response in total electron

Figure 1 GW in airglow images at 630 nm caused by the tsunami from the Tohoku earthquake. Example of 630.0 nm images processedusing length-8 FIR filters with passbands of (left) 0.3 to 1.7 mHz, (middle) 0.3 to 1.0 mHz to highlight the 26.2-min period waves, and (right) 1.0 to
1.7 mHz to highlight the 14.2-min period waves. The red line in each image indicates the tsunami location at the time of the image. The green
line in the left panel indicates a path perpendicular to the airglow wave fronts. Adapted with permission from Makela et al. (2011). See also themovie in the electronic supplement to Makela et al. (2011).

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

content (TEC). The ionospheric response over Japan(Figure 2) for the same event shows very pronounced
concentric waves in TEC that propagate in the radial
direction with a velocity between 138 and 3,457 m/swith an ‘ionospheric epicenter’ about 170 km southeast
from the earthquake epicenter (Tsugawa et al. 2011).
The initial signatures in Figure 2 with a propagation
velocity of 3,457 m/s are most likely caused by a Rayleighwave (a type of surface wave during earthquakes), followedby acoustic waves triggered by the Rayleigh wave. Finally,
medium-scale concentric waves with a propagation velocity of 138 to 423 m/s were detected 300 km awayfrom the ionospheric epicenter (Figure 2g,h). They are
the result of GW forced by the surface displacement(Matsumura et al. 2011).
Nishioka et al. (2013) discussed similar concentric waves
in the ionospheric TEC variation over the North-American
continent as an indicator of thermospheric gravity waves
generated by the 2013 Moore EF5 tornado. Fukushima
et al. (2012) showed correlation of MSTIDs observed by a
630 nm airglow imager over Indonesia with tropospheric
convective activity but with the average horizontal wavelength of the MSTIDs increasing with decreasing solar activity. These findings suggest that the observed MSTIDs in
the equator are caused by secondary gravity waves in thethermosphere, possibly generated by tropospheric convective activity. Smith et al. (2013) reported on thermosphericsecondary GWs generated from mountain wave breaking

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in the upper mesosphere using a simultaneous observationof 630 nm and 557.7 nm airglow images over New Zealand.
Progress in gravity wave theory (e.g., Vadas and Liu
2009; Vadas and Crowley 2010; Yiğit et al. 2012; Gavrilov
and Kshetvetskii 2014) now provides a much clearer picture of how gravity waves from the lower atmosphere canreach thermospheric altitudes, involving a multitude ofpossible pathways including direct propagation, reflectionin the upper atmosphere, and the generation of secondary
waves through upper mesospheric and thermosphericbody forces (Figure 3). As a result, substantial neutral
horizontal wind and plasma density variations from asingle convective plume are predicted to occur as far
away as adjacent continents, pointing to local wave
sources having a global impact: a significant challenge
for the interpretation of observations. An example ofthe ionospheric response to convective activity overBrazil on 1 October 2005 is shown in Figure 4 whichis adapted from Vadas and Liu (2013). Thermosphericbody forces resulting from convective plumes are computed with the Vadas model. Their incorporation into the
Thermosphere-Ionosphere-Mesosphere-Electrodynamics
General Circulation Model (TIME-GCM) results in largescale traveling ionospheric disturbances. Of note is thatthe hmF2′ perturbations are generally anticorrelated withthe foF2′ and TEC′ perturbations. This indicates thatfield-aligned transport instead of chemistry (i.e., [e ] +[O+] recombination) may be the dominant coupling

Figure 2 Time series (a)-(h) of detrended TEC during the Tohoku Earthquake on March 11, 2011. The star and cross marks represent theepicenter and the ionospheric epicenter, respectively. Gray circles are about the ionospheric epicenter. Adapted with permission from Tsugawa
et al. (2011).

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

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Figure 3 Sketch of possible pathways of a GW. Sketch of possible pathways of a GW from a single convective plume including direct
propagation (solid line), creation of body forces (grey ellipses), and subsequent generation of secondary GW with possible reflection (dotted anddashed-dotted lines). Adapted with permission from Vadas and Crowley (2010).

mechanism into the plasma. Observing such signatures still poses a significant challenge. Corresponding
temperature perturbations in the thermosphere (notshown) will be observed from geostationary orbit in
the near future by the Global-scale Observations of
the Limb and Disk (GOLD) instrument (a UV imager
to be launched in 2017; Eastes et al. 2008). For theionospheric signatures, much progress can be expected from Swarm, an ESA constellation mission(Haagmans et al. 2013) that was launched in 2013. The
role of GW for ionospheric instability seeding was thefocus of TG4 project 2 and is further discussed in thecorresponding section later in this paper.
Tides

The components of the tidal spectrum that follow theapparent westward propagation of the Sun relative tothe Earth's surface are called migrating tides, and thenon-Sun-synchronous components are called nonmigrating tides. Migrating tides are predominantly forced bysolar radiation absorption in tropospheric water vaporand stratospheric ozone. Nonmigrating tides are thought
to have two major sources: non-linear wave-wave interaction processes in the strato-/mesosphere (Hagan andRoble 2001) and latent heating due to large-scale deep
convection in the tropical troposphere (Hagan and Forbes
2002, 2003). Deep convection largely depends on land-sea
differences and sea-surface temperatures. Variations in the
periodic absorption of solar radiation at the surface thustransform to a longitudinal structure in raindrop formation (heat release) at roughly the same local time of the

day that acts as an efficient forcing mechanism for a number of nonmigrating tides including the diurnal eastward
propagating tide of zonal wavenumber 3 (DE3). Note thatthe DE3 appears as a zonal wavenumber 4 (wave-4) longitudinal structure when observed at constant local solartime, e.g., by precessing satellites in a low Earth orbit. Thisis simply a result of its eastward propagation and frequency.
Figure 5 summarizes the pre-CAWSES-II knowledgeof the cause-and-effect chain between latent heat forcing,tidal propagation, and coupling into the ionosphere, basedon various satellite observations. The most striking pattern is a 4-peaked ‘wave-4’ longitudinal modulation that isapparent in deep convective cloud occurrence, E-region
zonal winds, thermospheric constituents, F-region electron and ion density, and upper thermosphere neutral
mass density. Nonmigrating tidal winds in the low andmiddle latitude E-region move the partially ionized plasma
through Earth's magnetic field while the electrons with
their high gyro frequency/collision frequency ratio remain
fixed to the magnetic field lines. An electromotive
force is thus created with ensuing electric currents andpolarization electric fields. The E-region dynamo polarization
fields are further transmitted along magnetic field lines into
the overlying F-region where they drive vertical (approximately 20 m/s) and horizontal (approximately 100 m/s)
plasma drifts, which influence many important ionosphericprocesses. For example, vertical ExB drifts drive the plasma
fountain, which results in dense bands of plasma centerednear ±15 to 20 magnetic latitude. This equatorial ionization
anomaly can be seen in the top center panel of Figure 5 as
the two bluish bands north and south of the equator. The

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Figure 4 TIME-GCM response to convective activity over Brazil. (a) F2 plasma frequency perturbation (foF2′ in megahertz), (b) TEC perturbation(in TECU), (c) hmF2 perturbation (in kilometer), and (d) hmF2 altitude (in kilometer). These quantities are shown every 2 h from 22:30 to 28:30 UT, aslabeled. 1 TECU = 1016 electrons m−2. The contour lines are separated by 0.1 MHz, 0.2 TECU, 5 km, and 20 km in panels a to d, respectively. Note thatpanels a to c show the perturbed minus the unperturbed TIME-GCM solutions, while panel d shows only the perturbed TIME-GCM solution. Adaptedwith permission from Vadas and Liu (2013).

occurrence of the two ionization crests is predominantly the
result of migrating solar tidal winds in the E-region, while
the apparent longitudinal modulation in these bands is theresult of nonmigrating tides excited by latent heat release in
the tropical troposphere. For example, electron density varies
by a factor of 2 to 3 as a function of longitude along thecrests of the equatorial ionization anomaly at ±15 magnetic
latitude (top left panel in Figure 5) and reflects the largescale convective activity in the low-latitude troposphere. Thissurprising discovery that global conditions in the ionosphereand thermosphere are linked strongly to the terrestrial weather and climate below was initiated by IMAGE satelliteOberheide et al. Progress in Earth and Planetary Science (2015) 2:2

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Figure 5 Sketch of meteorological impacts on geospace due to non-Sun-synchronous (nonmigrating) tides. Sketch of meteorologicalimpacts on geospace due to non-Sun-synchronous (nonmigrating) tides as observed by different satellites in various neutral and plasma parameters.
The COSMIC figure is adapted with permission from Lin et al. (2007), the IMAGE figure is adapted with permission from Immel et al. (2006), the TIMED
figure uses data from Oberheide et al. (2011a,b), and the SNOE figure is adapted with permission from Oberheide and Forbes (2008). Deep convective
cloud data are from the ISCCP climatology. CHAMP neutral density data are provided by Dr X. Zhang and Dr. S. L. Bruinsma.

observations of the F-region plasma density (Sagawa
et al. 2005; Immel et al. 2006).
This general picture has been confirmed during the
CAWSES-II period by coupled ionosphere/thermospheremodels (e.g., Jin et al. 2011; Maute et al. 2012) and studies
using satellites, e.g., in temperature (Forbes et al. 2009),
infrared cooling (Oberheide et al. 2013), and plasma density (Chang et al. 2013). However, it has now been realizedthat alternative effects, in addition to the E-region dynamo modulation, may contribute to the coupling between the tides and the ionospheric plasma: for
instance neutral density variations, changes in thermospheric atomic oxygen to nitrogen ratio, and meridional
winds at F-region altitudes (Liu et al. 2009; England et al.
2010; Maute et al. 2012; see also the review article byEngland (2012)).
It should also be noted that tidal wind shears in the
E-region play a significant role in forming ionosphericintermediate layers, called sporadic E. See for example

the early work by Fujitaka and Tohmatsu (1973) andthe review by Haldoupis (2011). Much progress in thisfield, particularly in investigating the spatio-temporal
distribution of occurrence frequency, has been madeduring the CAWSES-II period. Important contributions
came from the concurrent analysis of COSMIC radio
occultation and TIMED tidal diagnostics. For example,
it has now been realized that, in addition to the diurnaland semidiurnal tides, the terdiurnal migrating tideplays an appreciable role in the sporadic E formation(Fytterer et al. 2014). There is also growing evidence
for an important role of nonmigrating tides on the
sporadic E formation but this topic needs to be studied
further in the future.
Some progress was also made in elucidating day-to-day
tidal variability. Forbes et al. (2011) combined CHAMP
and GRACE exosphere temperatures to perform daily
tidal fits for selected time periods and found mainly
solar-driven variability with a strong correlation with

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

the 27-day solar rotation period (Figure 6). Using combined WACCM-X/TIME-GCM simulations with constantsolar and geomagnetic conditions but realistic troposphericweather patterns, Liu et al. (2013a,b) showed day-to-day
tidal amplitude variability on the order of 50% for the
DW1, DE2, and DE3 tidal components in thermosphericwinds, respectively. This in turn produced ExB vertical drift
variability of the same magnitude and clearly indicates that
tropospheric weather variability imposes a significant
day-to-day variability on the ionosphere. At lower altitudes (mesosphere), Nguyen and Palo (2013) combinedsounding the atmosphere using broadband emission radiometry (SABER) and microwave limb sounder (MLS)
temperature data to derive day-to-day migrating diurnal
tide variability at low latitudes and found amplitudechanges of up to 15 K from one day to another. This isof the same order as variability deduced from ‘deconvolution’ approaches (Oberheide et al. 2002) using SABER alone(Figure 7). The causes for this variability still need to be

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understood. It is not yet clear what the relative roles of
tropospheric source variability and modulations imposedby wave-wave and wave-mean flow interaction processes
are in causing this variability. Addressing this challenge is
not only a matter of diagnosing short-term tidal variability
from the data but will also require a concentrated effort by
the modeling community. For example, a comparison of
the diurnal tide in models and ground-based observations
conducted as part of the 2005 equinox CAWSES tidal campaign (Ward et al. 2010) points to considerable differences
in model magnitudes and vertical wavelengths suggestingthat inconsistencies in model forcing, dissipation, and background winds exist (Chang et al. 2012).
On longer time-scales, it has become clear that changes
in global-scale weather patterns impose a considerable
variability in the thermosphere and ionosphere. A compelling example that has recently attracted attention is theeffect of the El Niño Southern Oscillation (ENSO). Pedatella
and Forbes (2009a) found a high correlation between the

Figure 6 Migrating diurnal tide exosphere temperature amplitudes and phases. Migrating diurnal tide exosphere temperature amplitudes(top) and phases (bottom) from daily CHAMP-GRACE fits (left) and NRLMSISE00 (right) for the period 3 November 2003 to 14 January 2004. The
solid line indicates the daily F10.7 solar flux, which ranges from 86 sfu (min) to 190 sfu (max). Adapted with permission from Forbes et al. (2011).

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

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Figure 7 Day-to-day DE3 tidal amplitude variability in 2008 at 100 km from SABER. Day-to-day DE3 tidal amplitude variability in 2008 at
100 km from SABER using a ‘deconvolution’ approach, plotted as a function of day of year and latitude. White color indicates data gaps.

Oceanic Niño Index (ONI) and longitudinal foF2 variability
from low latitude ionosondes and associated this with
ENSO-related changes in DE3 convective tidal heating.
In a follow-up modeling study using WACCM, Pedatella
and Liu (2012) showed that ENSO imposes a tidal
temperature variability on the order of 10% to 30%
during northern hemisphere winter. Interestingly, theyfound an enhanced DW1 and SW4 during the El Niño
phase but a reduced DE2 and DE3. The latter components
are enhanced during the La Niña phase. This is consistent
with recent TIMED Doppler interferometer (TIDI)
diagnostics focused on ENSO variability (Warner andOberheide 2014). The ENSO-imposed variability is generally smaller than the short-term tidal variability but similar
to variability due to the quasi-biannual oscillation (QBO)
determined by Oberheide et al. (2009). The ionospheric response to ENSO still needs to be studied in detail. A preliminary analysis (Figure 8) based on the COSMIC-based
TEC tides from Chang et al. (2013) and the TIDI-, TropicalRainfall Measuring Mission (TRMM)-, and Modern-Era

Retrospective Analysis for Research and Applications(MERRA)-based tidal wind and heating diagnostics from
Warner and Oberheide (2014) indicates a consistent 50%
enhancement in DE3 during the La Niña phase of thestrong 2009 to 2011 ENSO and no response during the El
Niño phase.
Tropospheric tidal forcing does not respond appreciatively to the solar cycle, and the E-region dynamo tidal
winds remain more or less unaffected (Oberheide et al.
2009). In the thermosphere above 120 km, however, increasing background temperatures during higher solar
activity cause more tidal dissipation because of thetemperature dependence of thermal conductivity. For
example, DE3 amplitudes during solar minimum aremuch larger than during solar maximum: a factor of 3
in the zonal wind, 60% in temperature and a factor of 5
in density (Oberheide et al. 2009; Häusler et al. 2013).
On the other hand, relative TEC tidal amplitudes fromCOSMIC (Chang et al. 2013) do not show any solar
cycle dependence. This can be understood as the result

Figure 8 DE3 amplitude anomaly in TEC, E-region zonal wind, and tidal heating. DE3 amplitude anomaly (deviation with respect to the2007 to 2011 climatological monthly means) in TEC (>200 km) from COSMIC at 15°N magnetic latitude (solid line), TIDI zonal winds at 100 km(averaged between 0°N to 20°N, dotted line), and TRMM- and MERRA-based convective and radiative DE3 heating, averaged from 3 to 16 km(plus symbols). Note the 50% enhancement in winter 2010/11 (La Niña phase) and the lack of any response in winter 2009/10 (El Niño phase).

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

of the absence of any solar cycle dependence in thezonal wind tides in the dynamo region which are themain driver of ionospheric tidal variability.
Connecting tides observed in the mesosphere lower
thermosphere (MLT) region and those in the upper
thermosphere and thus making the full connection to
tropospheric weather from a purely observational point isstill hampered by the lack of global wind or temperatureobservations in the ‘thermospheric gap’ between TIMED
observations (reliable below 110 km) and in situ observations from satellites like CHAMP, GRACE, or the new
Swarm mission at altitudes generally above 200 km. For
example, much of the tidal characteristics in this heightregion rely on fits to TIMED observations such as the Climatological Tidal Model of the Thermosphere (CTMT,
Oberheide et al. 2011a) that uses the observed tides in the
MLT region as a constraint for a physics-based empiricalmodel or general circulation models like WACCM-X (Liuet al. 2010a). The only dataset to date that turned out tobe suitable for thermospheric tidal diagnostics came fromHRDI and WINDII on UARS observations with tidal‘deconvolution’ applied (Lieberman et al. 2013a). Figure 9
exemplifies this for the DE3 using HRDI/WINDII diagnostics from 60 to 250 km compared to CTMT. While
CTMT does a reasonable prediction in this example, othercomponents are not well reproduced, particularly DW2Page 10 of 31

and D0 that have additional in situ thermospheric sources,
as shown by Jones et al. (2013) using the TIME-GCM.
The existence of in situ thermospheric tidal sources
for DW2 and D0 was confirmed by Forbes et al. (2014)
who used the CTMT and combined CHAMP/GRACE
tidal diagnostics to study tidal penetration to the upper
thermosphere.
In continuation of the CAWSES-I tidal campaigns (Wardet al. 2010), TG4 continued the data analysis (e.g., Changet al. 2012; Kishore Kumar et al. 2013) and conducted an
additional campaign in August to October 2011. These
campaign data still need to be analyzed and results will be
published elsewhere. Although requiring a significant international effort, observational campaigns such as these forvarious tropospheric and stratospheric conditions (ENSO,
QBO) are important for developing and confirming our understanding of tidal variability. See for example the recent
modeling results by Gan et al. (2014) that highlight interannual variability in satellite-borne tidal diagnostics and in a
general circulation model nudged to observed meteorological reanalysis data. It should also be noted that tides
provide the link between the ‘weather’ of the polar stratosphere and ionospheric variability close to the geomagnetic
equator, e.g., during sudden stratospheric warmings (SSW)
as a result of planetary wave - tidal interaction. This is further elaborated on in project 3.

Figure 9 DE3 amplitudes from CTMT and WINDII. (Left) DE3 amplitudes (m/s) computed from CTMT averaged over September to November
and mapped to 17:00 to 07:00 local time difference at 22.5°E. (Right) Same as left but from HRDI and WINDII analysis. White areas are data gaps.
Adapted with permission from Lieberman et al. (2013).

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Project 2: What is the relation between atmosphericwaves and ionospheric instabilities?

Equatorial plasma instabilities, commonly referred to as
equatorial spread-F, plasma bubbles, or depletions cancause radio signals propagating through the disturbed
region to scintillate resulting in a distortion or loss of
signal. First observed by Booker and Wells (1938), they
have been extensively studied due to the increasing importance of satellite-based communications and positioning. Much has been learned over the decades about
the growth mechanism and occurrence variability on aseasonal timescale. Dungey (1956) suggested the RayleighTaylor instability as the generating process during postsunset at the magnetic equator. At this time and location,
since the bottom side of the F layer has recombined while
the entire F layer itself has been raised by the pre-reversal
enhancement, a very sharp vertical density gradient is created that is unstable to vertical perturbations in the ionosphere. Identifying the contribution of atmospheric waves
as a perturbation source and studying the resulting instabilities was the leading motivation for two observational
campaigns carried out as part of TG4 activities.
The SpreadFEx-2 campaign

Based on the success of the first SpreadFEx campaign
held in northeastern Brazil (e.g., Fritts et al. 2008), which
focused on studying the interaction between the neutral
atmosphere and ionosphere, especially during periods ofionospheric irregularities, a second set of campaigns, called
SpreadFEx-2, was carried out in 2009 and 2010. A totalof eight institutions participated in these campaigns,
deploying instruments to the sites detailed in Table 3.
The addition of the coherent electromagnetic radio tomography (CERTO) beacon receivers and deployment ofmultiple wide-angle imaging systems added the possibilityof performing tomographic inversions of the ionosphere
during periods of ionospheric irregularities while the deployment of Fabry-Pérot interferometers (FPIs) added thePage 11 of 31

capability to directly measure neutral winds and temperatures and study the relationship between the neutral andplasma state.
The campaigns were operated from early September
until the end of November in both 2009 and 2010. Thesemonths correspond to the beginning of the ‘spread-F’
season, so called since this is when the occurrence ofionospheric irregularities (known as spread-F or equatorial plasma bubbles) is more common. Combining datafrom the imaging systems and FPIs, the coupling of theneutrals and plasma during periods of equatorial plasma
bubbles (EPBs) was studied. Chapagain et al. (2012)
found that the zonal neutral winds and EPB zonal drift
velocity, which is assumed to be indicative of the background plasma drift velocity, were tightly correlated, especially beginning several hours after sunset. The zonal
velocities showed similar patterns of both nightly andnight-to-night variability, indicating that the F-region dynamo was fully developed. Earlier in the evening, however, the EPB zonal speed was found to be slower thanthe background neutral winds, indicating that during theperiod of bubble development, the F-region dynamo
might not be fully activated. Examples of the comparisons made during the SpreadFEx-2 2009 campaign are
shown in Figure 10. These examples show the overallsimilarity between the winds and EPB drift velocities
as well as the variability in both of these parameters on
a night-to-night basis. Histograms showing the difference between the measured winds and estimated EPB
drift velocities are shown in Figure 11 and show thegood agreement between the two zonal speeds seen
later in the evening (after 23 LT).
The data collection that begun under the SpreadFEx-2
campaigns has been continued through 2013, allowing
for the study of the low latitude thermosphere throughthe transition from the deep solar minimum toward thesolar maximum of solar cycle 24. A summary of thezonal and meridional thermospheric neutral winds as

Table 3 Spread FEx2 observation sites(Latitude
longitude)

IonosondeSão Luís(2.6°S, 44.2°W)

DPS4Fortaleza(3.9°S, 38.4°W)

DPS4Cariri(7.4°S, 36.5°W)

CADI

ImagerFPI

Photometer

Meteor VHF CERTO
GPS
radar radar receiver receiverOK

All sky 6300, OH

6300

Cajazeiras

6300

6300

Petrolina

All sky 6300, OH

OH temperature

3 point
sounding

OK

OK

Transmission
receiver
OK

OK

OK

Caico

Transmission

Campina
GrandeOK

C. Paulista (22.7°S, 45.0°W)

DPS4

All sky 6300, OH

Institution

INPE

INPE, USU, UI

Multichannel
UI and Clemson

UFCG

Transmission.
OK

INPE

INPE

NRL

UI, INPE

UWO, INPE

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Page 12 of 31

Figure 10 Comparisons of thermospheric neutral winds and equatorial plasma bubble drift velocities. Comparisons of thermospheric
neutral winds (red) and equatorial plasma bubble drift velocities (blue) collected during the SpreadFEx-2 campaign in 2009. Adapted withpermission from Chapagain et al. (2012).

well as thermospheric neutral temperatures approximately 250 km through the end of 2013 are presented in
Figure 12. Analysis of the data collected through the endof July 2012 by Makela et al. (2013) showed the expected
strong dependence of the neutral temperature on thesolar flux, with the average temperature increasing byseveral hundred Kelvin over the study period. However,a solar flux dependence was not seen in the neutral winds,
at least over the range of solar fluxes observed during theperiod. Strong seasonal and day-to-day variability, however,is seen in the data indicating the influence of tidal and,possibly, gravity waves in the thermospheric neutral flows.
The collection of this type of multi-year dataset will provide important validation to models of the thermosphere

Figure 11 Histogram of differences between the thermospheric neutral wind and equatorial plasma bubble drift velocities. Binned by
local time. Total number of coincident observations in each bin is indicated by the blue line, referenced to the right hand axis.

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Figure 12 Climatologies of thermospheric neutral winds andneutral temperatures. Climatologies of thermospheric neutral winds(zonal - top; meridional - middle) and neutral temperatures (bottom)
collected by the RENOIR FPIs through the end of 2013.

and ionosphere being developed. Such a validation
study for the WAM using the dataset that begun under
the CAWSES SpreadFEx-2 campaign is presented in
Meriwether et al. (2013).
The LONET campaign

The LONgitudinal NETwork (LONET) campaign was
carried out as a TG4 activity during the September toNovember period in 2010 and 2011 with the goal of
creating a robust, global, multi-instrument database to
study the longitudinal variability of planetary-scale
waves. Table 3 lists the observation sites and instrumentation (13 ionosondes, 1 meteor radar, 2 MF radars, and1 optical Fabry-Pérot interferometer) participating in
LONET. TEC data obtained from the COSMIC satellite were also included. As can be seen from Table 4,
the data collected in 2010 were more complete than
2011 in terms of the longitudinal coverage. Figure 13
shows the observation sites distributed along the equator marked on a map. The rectangular boxes depict
COSMIC TEC data sampling along the magnetic equator. The ground-based data collection was limited inSouth America, Asia, and Oceania. No data were available from the African continent.
As the first step in understanding the longitudinalvariability of waves, the LONET team studied whether
any global-scale periodic oscillations were present in theobserved parameters and how the amplitude of thosePage 13 of 31

oscillations varied with longitude. From the temporalvariations of the ionospheric parameters foF2, h′F, TEC,
and mesospheric winds, the 2- to 16-day oscillations
were studied using wavelet analysis. In Figure 14, thewavelet power spectra for the three parameters, h′F at
Fortaleza (FZA, 321.6°E), TEC at around 345°E, and MF
radar zonal wind component at Pameungpeuk (107.7°E)
are presented for the period consisting of day of year
(DOY) 240 to 330. The h′F and TEC are approximately
in the same longitudinal zone. Among the common oscillations seen in these parameters is a distinct 16-day
period oscillation during DOY 260 to 300.
In order to further investigate the 16-day oscillation,
the amplitude and phase of the oscillation observed in
the h′F parameter observed by the network of ionosondes participating in the LONET campaign are plottedin Figure 15 for Fortaleza (FZA, top) to Darwin (DWN,
second from the bottom). The mesospheric wind components (zonal winds at 89 km altitude), obtained from
the MF radar at Pameungpeuk (PAM), are also shown(bottom) for reference. From the PAM wind oscillationsin Figures 14 and 15, clear evidence of a 16-day oscillation during the period DOY 260 to 320 centered ataround DOY 290 is found. The h′F at Fortaleza shows
clear 16-day oscillations during the same period with anamplitude that reaches approximately 4 km. However, theamplitudes of oscillation in the Indian and Asian sectors
are somewhat lower compared to the American sites.
In Figure 16, we plot the results of a 16-day harmonic
analysis of the COSMIC TEC data for different longitudinal zones. Each line corresponds to a given longitudinal zone, starting at 195°E (top) to 345°E (middle) to165°E (bottom). During DOY 270 to 300, the 16-day oscillation feature can clearly be seen. The phase of the
maximum is slightly shifted from DOY 290 at 15°E to
DOY 300 at 195°E, indicating that the phase is propagating
westward. These observations suggest that the 16-day oscillations might be generated by the Rossby mode 16-day
planetary wave (Forbes 1995). Furthermore, there appear
to be low amplitude regions (195 to 255°E) and high amplitude regions (315 to 105°E) in the global distribution of theoscillations. The Rossby 16-day waves normally appear in
the troposphere to mesosphere mainly through the windfield and propagate into the lower thermosphere, modulating the ionosphere (Lastovicka 2006). The present results
provide further understanding regarding the latitudinaland longitudinal variability of the 16-day waves.
In addition to the ionospheric data presented above,
data on geomagnetic field variations were also collectedduring the campaign period. The results of comparison
with the ionospheric parameters will be presented elsewhere. Longitudinal variability of the spread F activity was
also investigated using ground-based TEC observation
over South America. Large day-to-day variability of spreadOberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Page 14 of 31

Table 4 LONET campaign observation sites
Item

Site

SymbolLatitudeLongitude

2010

2011

Contact

1

Jicamarca

JCM

12.0°S

76.9°W

Partial

YES

Karim Kuyeng/Chau2

Fortaleza

FZA

3.9°S

38.4°W

YES

YES

Inez Batista

3

C. Paulista

CPT

22.7°S

45.0°W

YES

YES

Inez Batista

4

São Luís

SLS

2.5°S

44.3°W

YES

YES

Inez Batista

5

Tirunelveli

TIR

8.7°N

77.8°E

YES

NO

Diwarker/Gurubaran

6

Gadanki

GAD

13.5°N

79.2°E

YES

NO

A.K. Patra

7

Cocos Islands

COC

12.2°S

96.8°EYES

YESPhil Wilkinson

8

Chumphon

CPN

10.7°N

99.4°E

YES

NO

Nagatsuma

9

Bac Lieu

BCL

9.3°N

105.7°E

YES

NO

Nagatsuma

10

Sanya

SYA

18.3°N109.6°E

YES

YES

Guozhu Li

11

Wuhan

WHN

30.5°N

114.3°E

YES

YES

Guozhu Li

12

Cebu

CEB

10.3°N

123.9°E

YES

NO

Nagatsuma

13

DarwinDWN

12.5°S

130.9°E

YES

YES

Phil Wilkinson

Jicamarca

JCM

12.0°S

76.9°W

PartialYES

Luiz/Chau

Ionosonde/foF2, h′F

Meteor radar/wind

MF radar/wind
1

Tirunelveli

TIR

8.7°N

77.8°E

PartialYES

Narayanan/Gurubaran

2

Pameungpeuk

PAM

7.6°S

107.7°E

YES

YES

T. Tsuda/IUGONET

CAR

7.5°S

36.4°W

YES

YES

Makela/Meriwether

YES

YES

COSMICFPI (thermospheric winds)
Cairi
COSMIC satellite data (TEC)
Equatorial

Geomag. ±20

Figure 13 Ground-based observation sites and COSMIC TEC data sampling areas along the geomagnetic equator for LONET campaign.

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Page 15 of 31

Figure 14 Wavelet spectral density for 2- to 32-day period oscillations during the LONET campaign in 2010. (Top) h′F at Fortaleza,(middle) COSMIC TEC at 345°E, and (bottom) mesospheric zonal wind (89 km height) at Pameungpeuk.

F activity was observed, and the results were presented in
the CAWSES TG4 News letter (vol. 7, page 5, 2012).
Progress of understanding for plasma bubble seeding by
GWs

During the CAWSES-II interval, and in addition to theabovementioned campaigns, several additional interesting results were obtained relevant to the relation between atmospheric waves and ionospheric instabilities.
Takahashi et al. (2009) showed a positive correlation between plasma bubble spacing and the spatial scale ofmesospheric GW, which were observed simultaneously
using airglow imagers in 630 nm and OH airglow emissions. Makela et al. (2010) found that the distribution of
periodic spacing of equatorial plasma bubble compares
favorably to the spectrum of GW-induced traveling ionospheric disturbances (TIDs) measured by Vadas and Crowley(2010) from a similar geographic latitude in the northernhemisphere. These results suggest that the periodic spacingof plasma bubbles are determined by GWs in the lower
thermosphere. Several other studies also suggest seeding of plasma bubbles by GWs in the lower thermosphere(e.g., Taori et al. 2010, 2011; Paulino et al. 2011).
Distinct from these small-scale (approximately 100 km)
GWs, large-scale wave structures (LSWSs, approximately
1,000 km) have been considered as another controlling
factor of the plasma bubble/equatorial spread F (ESF) generation (e.g., Tsunoda 2005). Thampi et al. (2009) and Tsunoda et al. (2010) demonstrated that a close relationshipexists between LSWS and the generation of ESF when the
post-sunset rise (PSSR) of the F layer was absent. Tsunoda
et al. (2011) showed that the amplification of LSWS in thelate afternoon mostly occurs during the post-sunset rise of
the equatorial F layer. Narayanan et al. (2014) showed thatthe occurrences of ionogram satellite traces, which areused as a proxy of LSWS, were followed by ESF in about
71% of the cases, supporting the view that LSWS appears
to be an important parameter in the formation of ESF.

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Page 16 of 31

Figure 15 16-day period oscillations of h′F and mesosphericwinds. 16-day period oscillations of h′F and mesospheric winds(bottom) during the LONET campaign period in 2010.

It is also interesting to note that Otsuka et al. (2012)
and Shiokawa et al. (2014) reported observations of thedissipation of plasma bubbles in the field-of-view of 630
nm airglow images due to collision of the bubbles with a
mid-latitude MSTID and a large-scale TAD, respectively.
In both cases, the bubbles seem to be dissipated bypolarization electric field associated with these waves
in the ionosphere and thermosphere. This process suggests an additional role of GWs in the ionosphere,
namely the suppression of ionospheric instabilities.
Project 3: How do the different types of waves interact asthey propagate through the stratosphere to theionosphere?

A variety of satellite- and ground-based data sets along withmodel simulations have been used during the CAWSES-II
time frame to delineate how large-scale waves propagate
through the middle atmosphere to the ionosphere. Several
dedicated campaigns, e.g., coordinated ISR observations

Figure 16 16-day period oscillations of COSMIC TEC during the
LONET campaign period in 2010.

during SSWs, and workshops (Table 2) involved the TG4community and were supported by TG4.
PW signatures in the ionosphere

For example, the global distribution and the climatological features of the 5- to 6-day planetary waves derived from SABER/TIMED temperature measurements
over the height range 20 to 120 km for a full 6-year
period were delineated by Pancheva et al. (2010). This
study determined that the 5-day wave, the gravest symmetric wave number 1 Rossby wave, has a vertically
propagating phase structure with a mean vertical wavelength of approximately 50 to 60 km, whereas the 6-day
wave is an equatorially trapped wave number 1 eastward
propagating wave with a mean vertical wavelength of approximately 25 km. Liu et al. (2010a,b) attributed temporal hmF2 ‘wave-4’ variations diagnosed from COSMIC

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

data to the interaction of the DE3 with the 5- and 2-day
planetary waves. In another study, Mukhtarov et al.
(2010) reported on the climatology of stationary planetary waves with zonal wave numbers 1 and 2 observed in
SABER/TIMED temperatures for the same period. Further,using COSMIC data on ionospheric parameters, foF2 andhmF2 and electron density at fixed altitudes and the
SABER temperature data, Pancheva and Mukhtarov (2012)
diagnosed the global-scale spatial and temporal variabilityof the approximately 23-day zonally symmetric planetary
wave during the Northern winter of 2008/2009. The ionospheric oscillations were shown to be forced by planetary
waves propagating from below. Pedatella and Forbes(2009b) demonstrated the presence of 16-day waves thatwere simultaneously observed in the CHAMP electron
densities at 350 km, total electron content from global
positioning system (GPS) observations, and in SABER
temperature data at altitudes of 110 km, indicating thatvertically propagating planetary waves induce significant variability in the low latitude F-region ionosphereand that these effects are seen at all longitudes. Recently, Chang et al. (2013) analyzed COSMIC TEC
data and found strong stationary planetary wavenumber 4 (SPW4) signals (Figure 17), most likely caused bya strong SPW4 in neutral zonal wind in the E-region(Oberheide et al. 2011b) that in turn is the result of amodulation of the migrating diurnal tide DW1 by theDE3 nonmigrating tide (Hagan et al. 2009).
While the presence of PW signatures in the ionospheric plasma is without dispute and the same neutral/
plasma coupling processes as for the tides (predominantly
E-region dynamo modulation, see project 1) are thought
to apply to PWs as well, the mechanism through which
they enter the E-region is still under debate. For example,

Page 17 of 31

planetary waves do not penetrate much above 100 km, butinstead are thought to impose their periodicities on theionosphere and thermosphere by modulating the tides
and gravity waves that do penetrate to higher altitudes.
Other mechanisms include secondary PW forcing. For instance, Lieberman et al. (2013b) using MLS, SABER, and
TIDI data, along with Navy Operational Global Atmospheric Prediction System - Advanced Level Physics-High
Altitude (NOGAPS-ALPHA) model simulations foundobservational evidence for wintertime SPWs in the lower
thermosphere forced in part by drag imparted by gravity
waves that have been modulated by underlying stratospheric SPWs. Their results supported earlier model andcase studies by Liu and Roble (2002), Smith (2003), andOberheide et al. (2006a) that already suggested theplausibility of this mechanism.
SSW impact on the ionosphere and thermosphereA prime example for tidal modulation by PWs occurs
during sudden stratospheric warmings. An SSW is anextreme atmospheric disturbance event that results in a
warmer polar stratosphere and reversed zonal mean flow at
mid-latitudes and provides the link between the ‘weather’of the polar stratosphere and ionospheric variability close
to the geomagnetic equator. As a consequence of publication of the observational evidence for the ionospheric signatures associated with a SSW event by Goncharenko andZhang (2008), several studies in the recent past have addressed the effect of SSW on the ionosphere due to planetary wave propagation from below. It is now understoodthat the planetary waves modulate the propagation characteristics of predominantly the semidiurnal migrating tide,enhancing its amplitude, and thereby modulating theE-region ionospheric dynamo, vertical plasma drifts,

Figure 17 Relative SPW4 TEC amplitude in percent from COSMIC as a function of latitude and year. TEC was computed from theobserved electron densities above 200 km. Adapted with permission from Chang et al. (2013).

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

and plasma density in the F-region, in a manner similar to the effects of nonmigrating tides (Liu et al.
2010b; Fuller-Rowell et al. 2011; Chau et al. 2012; Jinet al. 2012). This is exemplified in Figure 18 in which
GAIA model simulations are compared with COSMIC
and SABER satellite observations during the January
2009 SSW.
A recent ground-based study by Laskar et al. (2014)
conducted in the Indian sector suggests that the 16-day
modulation of the semidiurnal tide is particularly strong
during strong SSW events and as such exerts a significant impact on ionospheric variability even during highsolar activity. Although SSWs only happen at most a few
times per year, they nevertheless provide an excellent
opportunity to test current theories of neutral-plasma
coupling on a global scale and for testing the predictive
capabilities of space weather models, as SSWs can beforecasted on a 1-week timescale (Fuller Rowell et al.
2011; Wang et al. 2014).
Pancheva and Mukhtarov (2011) presented for the firsttime the global spatial (latitude and altitude) structure of
the mean ionospheric response to SSW events during
winters of 2007/2008 and 2008/2009 using COSMIC
foF2 and hmF2 and electron density data at fixed altitudes.
Several studies were carried out to understand the processes underlining the SSW control of the electrodynamics

Page 18 of 31

at low latitudes. Chau et al. (2009) provided strong evidence
indicating that the distinctive behavior of the vertical ExB
drifts over the magnetic equator during daytime was associated with a minor SSW. Following this work, Chau et al.
(2010) identified SSW signatures in several ionospheric parameters using the incoherent scatter radar (ISR) electron density and temperature measurements from theArecibo Observatory, as well as relative TEC variations
derived from a dual-frequency GPS receiver. To examine
the response of the mid-latitude ionosphere to SSW,
Goncharenko et al. (2013) undertook a case study ofthe day-to-day variability in the ion temperature at altitudes between 200 and 400 km and detected disturbances
at tidal periods as well as at non-tidal and multi-day periods. As planetary waves are not expected to reach middle and upper thermospheric altitudes, as noted in theprevious subsection, these results have once again raisedquestions about the underlying mechanisms coupling thelower and upper atmosphere.
A TIME-GCM simulation by Liu and Roble (2002) revealed that the resonant SPW amplification prior to thepeak warming causes a deceleration of the mean wind in
the high latitude winter stratopause and mesosphere andreverses to westward with a critical layer near the zero
wind line. This changes the filtering of gravity waves by
allowing more eastward GW to propagate into the MLT

Figure 18 Comparison of the amplitude of the SW2 semidiurnal migrating tidal component. Comparison of the amplitude of the SW2
semidiurnal migrating tidal component at different altitudes extracted from GAIA model simulations (left) and from COSMIC and SABER satellite
observations (right). Adapted from the Jin et al. TG4 Newsletter 13 (2013) article using results from Jin et al. (2012).

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

region and a resulting change of the meridional circulation in the upper mesosphere from poleward/downward
to equatorward/upward with corresponding stratosphericwarming, mesospheric cooling, and thermospheric warming patterns in the mean temperatures at high latitudes
during SSWs. At low latitudes, this temperature patternreverses and produces a thermospheric cooling and consequently a thermospheric density decrease at a fixed altitude by reducing the scale height.
SSW and lunar tides

Recent ground-based and satellite magnetometer observations in conjunction with other ionospheric measurements
have provided insights of how lunar tidal modulation during SSW events can drive the spatio-temporal variabilitiesof the equatorial electrojet and the electrodynamics (Fejer
et al. 2010, Park et al. 2012, Yamazaki et al. 2012). However, Stening (2011) argued against a strong lunar tidal
contribution to the observed variability during SSWs sincestrong lunar tidal signals exist during non-SSW periods sothat these correlations might be coincidental. Fuller-Rowell
et al. (2011) indeed found SSW-induced phase shifts in thesemidiurnal solar tide consistent with an apparent lunar
tidal signal using WAM simulations, but it should be notedthat the model did not include the semidiurnal lunar tide
as an input. Wang et al. (2014) in a coupled WAM/ionosphere model simulation also found a phase shift of theSW2 solar component during SSWs caused by SSWinduced variability in its main source, stratospheric ozone,
and/or additional middle atmosphere circulation changes.
On the other hand, Pedatella et al. (2014) reported notableenhancements of the semidiurnal lunar tide during SSW
events in WACCM-driven TIME-GCM simulations and abetter agreement with COSMIC electron density observations when the lunar tide was included in the model. It
must thus be concluded that the impact of lunar tides on
ionospheric variability during SSWs is not yet finally resolved and requires further studies by the aeronomy community over the coming years. It should also be noted that
there is increasing evidence that the ionospheric plasma
variability during SSWs is not solely caused by E-region
electric field variability but also by variability in F-region
meridional neutral winds and thermospheric compositionchanges (e.g., Pedatella et al. 2014). This is consistent withthe earlier modeling work by England et al. (2010) and
Maute et al. (2012) that was focused on nonmigratingtides.
SSW and GW

Small-scale variability of the upper atmosphere driven
by major atmospheric disturbances from below like SSW
has also gathered attention recently. Adopting a firstprinciple nonlinear hydrostatic GCM extending fromthe lower atmosphere to the thermosphere, Yiğit et al.

Page 19 of 31(2014) investigated the influence of small-scale gravity
waves originating in the lower atmosphere on the variability of the high latitude thermosphere during an SSW
event. The numerical experiments revealed that thegravity wave penetration into the thermosphere increased the momentum deposition rates above 150 km
in the high latitude northern hemisphere by up to afactor of 3 to 6 during the warming. This demonstrates
that gravity wave-induced variations during SSWs constitute a significant source of high latitude thermosphericvariability.
Project 4: How do thermospheric disturbances generated
by auroral processes interact with the neutral and ionizedatmosphere?
Upper thermosphere

From the CHAMP satellite observations, Lühr et al.
(2004) showed the existence of strong heating in the cusp
(and/or near the cusp) region. This heating causes upwelling of the air and enhancement of the neutral mass density in the altitude region of about 400 km. In addition, theCHAMP satellite observed some thermospheric disturbances during geomagnetically disturbed periods. For example, Ritter et al. (2010) identified substorm-related
thermospheric density and wind disturbances from theCHAMP observations. They reported mass density enhancements, a density bulge propagating as a traveling atmospheric disturbance (TAD), and wind variations duringa substorm event. Liu and Yamamoto (2011) reported
geomagnetic storm effects on the formation and characteristics of the mid-latitude summer nighttime anomaly(MSNA), which is a phenomenon during which the diurnal variation of the plasma density maximizes at night instead of day. They pointed out the role of the effective
neutral wind in the formation of MSNA. Some results obtained from the CHAMP observations were summarized
in the review paper presented by Lühr et al. (2012).
Vickers et al. (2013) developed a new technique to obtain thermospheric neutral density at approximately 350
km altitude from incoherent scatter radar data. In situ
comparisons with the CHAMP satellite show good agreement. Vickers et al. (2014) used this technique on the EISCAT Svalbard radar for the period 2000 to 2013 andshowed that F10.7 solar irradiance is a very good proxy forthermospheric density variations with only a small seasonal dependence. In addition, the long-term trend of declining thermospheric density was observed.
Co-rotating interaction region (CIR) and subsequent
high speed solar wind stream (HSS) is one of importantcauses of the thermospheric disturbances in association
with auroral phenomena during a solar minimum period(e.g., Thayer et al. 2008; Lei et al. 2011; Verbanac et al.
2011). Pedatella and Forbes (2011) described responsesof the ionosphere and thermosphere to high speed solar

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

wind streams from the DMSP F13 satellite observations
and the Thermosphere-Ionosphere-Electrodynamics General
Circulation Model (TIE-GCM) simulations. They concluded
that the disturbance dynamo would be an important mechanism for driving the electrodynamic response at dawn anddusk to recurrent geomagnetic activity driven by HSS.
Gardner et al. (2012) investigated the HSS heating effects
from numerical simulations and observations with the EISCAT Svalbard radar (ESR) and PFISR. They showed that the
HSS heating can cause temperature enhancement as high as
100 K at high latitudes, but the global increase in thermospheric temperature would be low.
Coronal mass ejections (CMEs) are also one of the important causes of the geomagnetic storms. The thermospheric density variations during CME- and CIR-induced
geomagnetic activities were compared by Chen et al.
(2012). The total changes in the thermospheric density observed during periods of CIR storms were greater thanthose of the CME storms because the CIR storms lasted
longer than CME storms, while the CME storms werestronger than the CIR storms on average.
For smaller scale waves, several airglow imagers investigated thermospheric and ionospheric waves identified
as MSTIDs near the auroral zone through the 630 nm
airglow emissions at altitudes of 200 to 300 km. Kubota
et al. (2011) reported characteristics of MSTIDs observed by an airglow imager at Alaska. They concluded
that these southwestward-moving MSTIDs are not caused
by ionospheric instabilities, as usually suggested at middle
latitudes, but are likely to be caused by auroral disturbances as TADs because the observed background thermospheric wind was poleward, stabilizing the ionosphericinstability. On the other hand, Shiokawa et al. (2013)
reported similar southwestward-moving MSTIDs over
Norway and northern Canada and suggested that they
are caused mainly by the Perkins and E-F coupling instabilities (Perkins 1973; Yokoyama et al. 2009) similar
to those at middle latitudes and that an additionalsource by atmospheric gravity waves from lower altitudes
also comes into play. Shiokawa et al. (2012, 2013) reported
sudden movements of these MSTIDs at subauroral latitudes concurrent with auroral activity, indicating instantaneous penetration of auroral electric fields to subauroral
latitudes.
MLT region

Dynamics and Energetics of the Lower Thermosphere in
Aurora (DELTA) and DELTA-2 campaigns were carriedout in December 2004 and January 2009, respectively.
Simultaneous observations with the EISCAT radar, FPI,
and sounding rockets were successfully made during thetwo campaigns. For example, Kurihara et al. (2009) identified temperature enhancement in the lower thermosphere in association with the auroral heating during thePage 20 of 31

DELTA campaign. Oyama et al. (2010) also reported lower
thermospheric wind fluctuations measured with an FPI
during pulsating aurora at Tromsø, Norway, in the periodof the DELTA-2 campaign (Figure 19). Some studies concerning the occurrence of atmospheric gravity waves in
the polar thermosphere in response to auroral activity
were summarized in the review paper of Oyama andWatkins (2012).
Following initial studies of thermospheric meso-scalevariability using tristatic Fabry-Pérot interferometer (FPI)
observations at EISCAT (Aruliah et al. 2004, 2005), theneutral wind variations in the vicinity of an auroral arc
were studied by some researchers. Oyama et al. (2009)
showed spatial evolution of frictional heating and increases in ion flow and temperature in the vicinity of anauroral arc from observations with the Sondrestrom
incoherent-scatter radar and the Reimei satellite. Theyclarified localized ionospheric structures associated withlocalized soft particle precipitation or F-region ionization,suggesting the presence of a narrow thermospheric wind
shear of about 10 km width. Kosch et al. (2010) reported
the first observational results of the E-region neutral windfields and their interaction with auroral arcs during geomagnetically quiet conditions at Mawson, Antarctica, witha scanning Doppler imager (SDI). The 144° field-of-view
Doppler images of the sky over Mawson are shown in
Figure 20. They found that E-region wind could rotate
90° within approximately 10 min when close to (withinapproximately 50 km) of an auroral arc and then recoverto the background flow when the arc disappeared. The
F-region wind remained unaffected. This was attributed to the electric field associated with auroral arcs causing localized strong ion drag. Anderson et al. (2011)
performed bistatic FPI observations of F-region thermospheric vertical winds in Antarctica. They foundstrong upward winds poleward of the auroras as wellas correlated vertical wind responses over horizontal
distances of approximately 150 to 480 km.
Kosch et al. (2011) made meso-scale observations of
the ionosphere and thermosphere with the EISCAT
Svalbard radar and SDI, partly in the vicinity of an auroral arc, within the polar cap at approximately 110 km
altitude. They found that Joule heating had a meso-scale(approximately 64 km) spatial variability of up to >100%
even during steady-state geomagnetic conditions. Figure 21
shows the spatial distribution of Joule heating in 22.5°
sectors about the EISCAT Svalbard radar projected to
110 km for 23:36 to 23:54 UT on 2 February 2010.
They estimated the E-region ion-neutral collision frequency at 112.8 (±0.8) km altitude by combining E- andF-region ion flow with E-region neutral flow data andfound good agreement with the MSIS model. Anderson
et al. (2013) also made observations of the ionosphere andthermosphere with PFISR and SDI. They found theOberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Page 21 of 31

Figure 19 Temporal variations in the vertical wind speed and the fringe peak count at the zenith. Top and middle panels show the
temporal variations in the vertical wind speed and the fringe peak count at the zenith, respectively, measured with the FPI (557.7 nm) from 00:30
to 02:00 UT on 26 January 2009. Vertical bars are the 2 sigma uncertainty (see in the text of Oyama et al. 2010). The bottom color panels show
the horizontal aurora images taken with the ASC at 557.7 nm from 01:15 to 01:39 UT during pulsating aurora (corresponding time interval ismarked by black dashed line in the top two panels). The color scale of optical intensity is shown at the right middle. These images are mappedin geographic coordinate assuming the peak emission height is 110 km. The five black dots and the four red crosses indicate the location of the
FPI and the EISCAT UHF radar observation positions, respectively. The black arrows correspond to the horizontal component of the FPI-derived
neutral wind velocity (the scale is presented at the right bottom corner of the first panel from the left). White dashed lines in the colored panels
are drawn to easily identify the darker area. Adapted with permission under Creative Commons Attribution 3.0 License from Oyama et al. (2010).

neutral wind dynamo contribution to Joule heating was
36% to 64% on average and was positive when the horizontal magnetic field perturbation was weak (|ΔH| <approximately 40 nT) and vice versa. They estimatedthe F-region ion - neutral collision frequency at 260 km
altitude using the ambipolar diffusion equation - andalso found good agreement with the MSIS model.
Recently, Xu et al. (2013) showed longitudinal variations of the thermospheric temperature from satellite
observations with SABER and Michelson Interferometer
for Passive Atmospheric Sounding (MIPAS) and simulations with TIE-GCM. Their simulations clarified thatauroral heating would cause the observed longitudinaltemperature variations. In addition, the satellite observations indicated that impacts of auroral heating on
the neutral atmosphere can penetrate down to about
105 km. The lidar observations are quite useful tostudy thermospheric disturbances in the MLT region.
For example, Chu et al. (2011) observed temperatureenhancements caused by Joule heating and fast gravity
waves propagating from the lower atmosphere in theneutral Fe layers (110 to 155 km) in Antarctica. Tsudaet al. (2013) showed decreases in sodium density in association with auroral particle precipitation from

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Figure 20 (See legend on next page.)

Page 22 of 31

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Page 23 of 31(See figure on previous page.)
Figure 20 The 144° field-of-view Doppler images of the sky over Mawson. The right column shows F layer 630 nm Doppler images, the left
column shows E layer 557.7 nm Doppler images on 10 June 2008. Each plot is labeled with UT. The optical intensity is not calibrated. The optical
data are plotted in geographic coordinates with horizontal wind vectors overlaid for each zone (yellow). The wind vectors are centered on theirmeasurement points. The vector for the zenith sector shows the average wind field, not the vertical wind. The top row (18:16 to 18:27 UT) shows
before the arc appeared, the middle row (19:36 to 19:40 UT) shows when the arc appeared, and the bottom row (20:33-20:40 UT) shows after thearc disappeared. The blue box highlights the region of interest.

simultaneous and common volume observations by
the EISCAT VHF radar and a sodium lidar at Tromsø,
Norway.
Model simulations in association with auroral/highlatitude phenomenaSome general circulation models (GCMs) have been developed to understand the energetics, dynamics, and composition variations of the upper atmosphere. These simulations
clarified disturbances in the polar ionosphere and thermosphere in association with auroral activity and/or high latitude energy inputs. For example, Qian et al. (2010)
simulated thermospheric response to recurrent geomagnetic forcing with the NCAR TIE-GCM. Neutral
density and nitric oxide (NO) cooling rates were simulated for the declining phase of solar cycle 23. The
simulated results were compared to neutral density derived from satellite drag and to NO cooling measured
by TIMED/SABER. Gardner and Schunk (2011) performed numerical simulations to investigate characteristics of the large-scale gravity waves with a high-resolution
global thermosphere-ionosphere model which can represent
global distributions of mass density, temperature, and all
three components of the neutral wind at altitudes from 90
to 500 km without the assumption of hydrostatic equilibrium. They noted that a secondary wave was generated

which propagated around the entire globe, while the original
gravity wave was localized. Deng et al. (2011) also investigated nonhydrostatic effects in the thermosphere from
Global Ionosphere Thermosphere Model (GITM). Theirsimulations demonstrated that most of the nonhydrostatic
effects at high altitudes (300 km) arise from sources below
150 km and propagated vertically through the acoustic
wave. Basic structures of the polar ionosphere andthermosphere during solar minimum and geomagnetically quiet periods were investigated by Fujiwara et al.
(2012) from the EISCAT Svalbard radar observations(March 2007 to February 2008) and simulations with awhole atmosphere GCM. Their results indicated thatboth the ions and neutrals would show larger variations than those described by the empirical models,suggesting significant heat sources in the polar cap region even under solar minimum and geomagneticallyquiet conditions. In addition to the global model simulations, de Larquier et al. (2010) performed one- and twodimensional numerical simulations with a finite-differencetime-domain (FDTD) model to provide quantitative interpretation of the recently reported infrasound signaturesfrom pulsating aurora. They discussed pressure perturbations caused by particle heating due to pulsating aurora(heat source is located between 90 and 110 km on average). The simulation results were roughly an order of

Figure 21 The spatial distribution of Joule heating in 22.5° sectors. The spatial distribution of Joule heating in 22.5° sectors about the
EISCAT Svalbard radar projected to 110 km 23:36 to 23:54 UT on 2 February 2010. The aurora (areas in green color) is overlaid for 110 km altitude.

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

magnitude smaller than those observed, suggesting theneed for an additional source, e.g., Joule heating.

Conclusions
Scientific outcome and challenges

Overall, TG4 brought together the historically separate
neutral atmosphere and plasma communities in a waythat allowed for much progress in understanding how
neutral variability originating in the lower atmosphere
impacts and interacts with Earth's ionosphere, from
low to polar latitudes and from the troposphere to theF-region. For example, the role of waves due to convection, polar stratosphere dynamics, or auroral processes
in causing substantial ionospheric variability through dynamo processes and Rayleigh-Taylor instabilities is now
much better understood than before. This was achieved
through a combination of dedicated campaign activities
and workshops resulting in four special issues in the peerreviewed literature (Table 2). The aeronomy community iswell on its way to separating ionospheric variability introduced by driving from below and from the magnetosphereand Sun above, an essential task toward achieving predictability of space weather. In this respect, it is important
to note that the solar activity at the beginning of the
CAWSES-II period was extremely quiet, allowing thepure effects from the lower atmosphere to be observed
in isolation without disturbances from above.
There are a number of critically important open questions
toward the goal of achieving space weather predictability.
Significant amounts of variability remain unexplained and/or are very difficult to detect and interpret, for example, theimprints of convectively forced gravity waves in the thermosphere. Short-term tidal variability in the neutrals andplasma is largely unknown because of the lack of suitablesatellite data since local time coverage by a single satellite is limited. It is not clear what is more important:wave propagation into the thermosphere and then into
to the ionosphere; or electrodynamic coupling between
the E- and F-region and then into the thermosphere.
The dynamics community still knows little about interactions between the various types of waves (tides, PWs,
and GWs) and the mean flow. A major observational andtechnological challenge is the lack of global wind observations, day and night, throughout the 120 to 400 km region.
As overviewed below, new space-borne and groundbased assets, along with progress in geospace modeling,
will help to close this knowledge gap that is also targetedthrough elements of SCOSTEP's new VarSITI program.
Program outreachAs a dedicated effort of the international scientific community, and in accordance with SCOSTEP's mission not
only to run scientific programs but also to promotesolar-terrestrial physics and dissemination of the derived

Page 24 of 31

knowledge for the benefit of society, Task Group 4 published a quarterly newsletter. Each newsletter was about
eight pages long with updates on recent campaign activities, short news, and a portrait of a young scientist. All articles were written to be understandable for non-specialist
and published on the task group wiki (www.cawses.org/
wiki/index.php/Task_4) and through an extensive mailinglist. A total of 13 issues have been published with 64 articles from authors from 20 different countries, including
10 articles featuring young scientists. TG4 also organized
and/or supported 12 dedicated workshops and special sessions and held annual business meetings during major
conferences.
New and future satellite missions

Space agencies and national science foundations in the U.S.,
Europe, and Asia by now recognize the importance of lower
atmosphere driving of the ionosphere-thermosphere system
not only as a domain of compelling scientific inquiry but also
as highly relevant for understanding and predicting space
weather; a task highly relevant for technological societies.
In the United States, the Decadal Survey on Solar andSpace Physics 2013 to 2022 conducted by the National
Academy of Sciences (National Research Council 2013)
recognized the realization that weather systems in the
troposphere impact space weather through tides andgravity waves as one of the significant discoveries in thepast decade, making ‘the comprehensive understandingof the variability in space weather driven by loweratmosphere weather on Earth’ its second highest priority
and recommended a dedicated satellite constellation(DYNAMIC) to be launched around the year 2020.
While NASA budget constraints may delay the implementation of DYNAMIC; two new satellite missions
dedicated to ionosphere-thermosphere physics, ICON
and GOLD, will be launched in 2017. Both ICON andGOLD can be seen as pathfinder missions for DYNAMIC. ICON will collect data from a low Earth orbit
to compare the impacts of direct solar driving andlower atmosphere driving on variability in the ionosphere/
thermosphere system, and GOLD will image the thermosphere from a geostationary orbit, allowing for an unprecedented global view of short-term variability.
A very exciting new mission, launched on 22 November 2013, is the European Space Agency (ESA) Swarm
mission. Swarm is a unique three satellite constellation
with two satellites orbiting at 480 km and one at 510 km
to measure Earth's magnetic field, the electric field in
the ionosphere, and thermospheric density and winds.
See the special issue in Earth, Planets and Space on
Swarm Science Data Processing and Products - theSwarm Satellite Constellation Application and Research
Facility (2013) for details. Each of the identical Swarmsatellites has a vector and scalar magnetometer, aOberheide et al. Progress in Earth and Planetary Science (2015) 2:2

Langmuir probe, an accelerometer, and GPS instrumentation on board, similar to the tremendously successful
CHAMP satellite. During its anticipated 10-year mission,
the Swarm data will allow the ionosphere/thermospherecommunity to obtain an unprecedented view of smallscale structures in the thermosphere and the ionosphericplasma.
Due to the success of COSMIC (a particularly important
dataset for the ionosphere/thermosphere community because of its global electron density profiles from radio occultation), the United States and Taiwan will launch COSMIC-2
between 2015 and 2018. COSMIC-2 consists of six satellites
in a low-inclination orbit (launch in 2015) and another six
satellites in high inclination orbits (launch in 2018).
Along with emerging cubesat and nanosat capabilities,
these new space-borne assets will form the backbone ofstudying the signatures and impact of lower atmospherevariability versus magnetosphere/solar-driven variability
in the next decade. Their synergistic use will most likely
allow the community to make significant progress toward understanding geospace as a system and also toward predictability of space weather, a significant needfor a technological society. A particular challenge, however, will remain: global thermospheric winds throughout the whole thermosphere during day- and nighttime;
dataset airglow interferometers cannot provide. The implementation of new emerging technologies, such as the
Doppler wind and temperature sounder (DWTS; Gordley
and Marshall 2011; Lieberman et al. 2012), will be needed.

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New ground-based assets

Ground-based facilities are powerful tools to measure vertical coupling from the atmosphere to geospace. They arebasically radio techniques (various types of radars, ionosondes, satellite beacon receivers including Global Navigation Satellite System (GNSS) receivers, natural/artificial
wave receivers, and magnetometers) and optical techniques (imagers and interferometers for airglow and aurora; and lidars). Figure 22 summarizes the altitude ranges
and measurement parameters of various ground-based
techniques. Because a single instrument covers only acertain range of altitudes and can measure a limited
set of physical parameters, it is essential to combine
several types of instruments at the same place to understand the connection from the troposphere to geospace.
Multi-point networks of these various instruments are
becoming available, providing global information ofthese parameters, e.g., Yumoto (2011) - MAGDAS andYizengaw and Moldwin (2009) - AMBER for magnetometers, Makela et al. (2009) - RENOIR, Shiokawa
et al. (2009) - OMTIs, Makela et al. (2012) - NATION,
and Meriwether et al. (2014) - Peruvian FPIs for optical instruments, and Cohen et al. (2010) - AWESOME
for LF/VLF receivers. It is also important to combine theseground-based observations with satellite measurements
to separate temporal and spatial variations.
One of the great progress of ground-based observation
during CAWSES-II was that the multi-point GNSS receiver network is becoming a powerful tool to provide

Figure 22 Various ground-based instruments to observe the upper atmosphere, their altitude ranges, and parameters of measurements.

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

two-dimensional images of the ionospheric total electron
content in the spatial resolution of less than approximately
100 km, as shown in Figure 2 as a striking example oftsunami- and tornado-induced gravity wave penetration to
the ionosphere (Tsugawa et al. 2011; Nishioka et al. 2013).
Such high-resolution TEC maps become available not only
over Japan and US, but also in other places, e.g., over SouthAmerica (Takahashi et al. 2014) and Europe (Otsuka et al.
2013). The number of GNSS receivers will grow further
even after the CAWSES-II era and will become an important facility to monitor ionospheric disturbances.
The widest altitude coverage from the troposphere to
the ionosphere can be achieved by powerful facilities ofincoherent scatter (IS)/mesosphere-stratosphere-troposphere (MST) radars. Currently, major facilities of IS/
MST radars are available at Resolute Bay (RISR), Poker
Flat (PFISR), Sondrestrom, Milstone Hill, Arecibo, andJicamarca in the American longitudinal sector; Irkutsk,
Shigaraki (MU), Wuhan, Gadanki, and Kototabang (EAR) in
the Asian sector; and Svalbard (ESR), Scandinavia (EISCAT),
Andøya (MAARSY), Kharkov, and Syowa (PANSY) in theEuropean sector. The EISCAT radar will be renewed into
the EISCAT_3D radar (Wannberg et al. 2010) to obtain
three-dimensional information of plasma-neutral interactionin the auroral ionosphere/thermosphere. This renewal will
also stimulate measurements of tropospheric andstratospheric parameters using its powerful facilities.
The Equatorial Atmosphere Radar (EAR) is planned tobe renewed to the EMU (Equatorial Middle and Upper
atmosphere) radar with more than five times larger
transmission power to perform IS observations of theionosphere.
VarSITI - SCOSTEP's 2014 to 2018 program

The new 2014 to 2018 SCOSTEP program Variability of
the Sun and Its Terrestrial Impact (VarSITI) will focus
on the unusual low solar activity during solar cycle 24
and its consequences on Earth. One of the four VarSITI
projects is called Role of the Sun and the Middle Atmosphere/Thermosphere/Ionosphere in Climate (ROSMIC)
and is directed toward identifying the effects of external
forcing on and predicting the effects of internal changes
to the atmosphere/ionosphere. One goal of ROSMIC is
to understand the impact of the Sun on the terrestrial
middle atmosphere/lower thermosphere/ionosphere relative to anthropogenic forcing including coupling within
the atmosphere-ionosphere system. As such, ROSMIC
continues the science investigations related to understanding the geospace response to lower atmosphere variability
that were carried out during CAWSES-II, using similar
approaches such as observing campaigns, workshops, etc.
Abbreviations
AMBER: African Meridian B-Field Education and Research;
AWESOME: Atmospheric Weather Electromagnetic System for Observation

Page 26 of 31

Modeling and Education; CAWSES: Climate and Weather of the Sun-Earth
System; CERTO: coherent electromagnetic radio tomography;
CHAMP: CHAllenging Minisatellite Payload; CIR: co-rotating interaction
region; CME: coronal mass ejection; COSMIC: Constellation Observing System
for Meteorology; CTIPe: Coupled Thermosphere Ionosphere PlasmasphereElectrodynamics Model; CTMT: Climatological Tidal Model of the
Thermosphere; D0: diurnal stationary tide of zonal wavenumber 0;
DE2: diurnal eastward propagating tide of wavenumber 2; DE3: diurnal
eastward propagating tide of wavenumber 3; DELTA: Dynamics andEnergetics of the Lower Thermosphere in Aurora; DMSP: Defense
Meteorological Satellite Program; DOY: day of year; DW1: diurnal westward
propagating tide of wavenumber 1; DW2: diurnal westward propagating tideof wavenumber 2; DWN: Darwin; DWTS: Doppler wind and temperaturesounder; DYNAMIC: dynamical atmosphere ionosphere coupling;
EISCAT: European Incoherent Scatter Scientific Association; ENSO: El Niño
Southern Oscillation; EPB: equatorial plasma bubble; ESA: European Space
Agency; FPI: Fabry-Pérot interferometer; FDTD: finite-difference time-domain;
FZA: Fortaleza; GAIA: Ground-to-Topside Model of Atmosphere andIonosphere for Aeronomy; GCM: General Circulation Model; GITM: Global
Ionosphere Thermosphere Model; GOLD: Global-scale Observations of the
Limb and Disk; GNSS: Global Navigation Satellite System; GPS: global
positioning system; GRACE: Gravity Recovery and Climate Experiment;
GW: gravity wave; HRDI: high-resolution Doppler interferometer; HSS: highspeed solar wind stream; ICON: Ionospheric Connection Explorer;
IMAGE: Imager for Magnetopause-to-Aurora Global Exploration;
ISR: incoherent scatter radar; ISCCP: International Satellite Cloud Climatology;
LONET: LONgitudinal NETwork; LSWS: large-scale wave structure;
MAGDAS: MAGnetic Data Acquisition System; MERRA: Modern-Era
Retrospective Analysis for Research and Applications; MIPAS: Michelson
Interferometer for Passive Atmospheric Sounding; MLS: microwave limb
sounder; MLT: mesosphere lower thermosphere; MSNA: midlatitude summernighttime anomaly; MSTID: medium-scale traveling ionospheric disturbance;
NASA: National Aeronautics and Space Administration; NATION: North
American Thermosphere Ionosphere Observation Network; NCAR: National
Center for Atmospheric Research; NOGAPS-ALPHA: Navy Operational Global
Atmospheric Prediction System - Advanced Level Physics-High Altitude;
OMTIs: Optical Mesosphere Thermosphere Imagers; ONI: Oceanic Niño Index;
PAM: Pameungpeuk; PFISR: Poker Flat Advanced Modular Incoherent Scatter
Radar; PW: planetary wave; QBO: quasi-biannual oscillation; RENOIR: The
Remote Equatorial Nighttime Observatory of Ionospheric Regions;
ROSMIC: Role of the Sun and the Middle Atmosphere/Thermosphere/
Ionosphere in Climate; SABER: sounding the atmosphere using broadband
emission radiometry; SCOSTEP: Scientific Committee on Solar-Terrestrial
Physics; SDI: scanning Doppler imager; SPW: stationary planetary wave;
SSW: sudden stratospheric warming; SW4: semidiurnal westward
propagating tide of wavenumber 4; TAD: traveling atmosphericdisturbance; TEC: total electron content; TG4: Task Group 4; TIDI: TIMED
Doppler interferometer; TIE-GCM: Thermosphere-Ionosphere-Electrodynamics
General Circulation Model; TIME-GCM: Thermosphere-Ionosphere-MesosphereElectrodynamics General Circulation Model; TIMED: Thermosphere Ionosphere
Mesosphere Energetics and Dynamics; TRMM: Tropical Rainfall Measuring
Mission; VarSITI: Variability of the Sun and Its Terrestrial Impact; VHF: very highfrequency; WACCM-X: Whole Atmosphere Community Climate Model thermosphere extension; WAM: Whole Atmosphere Model; WINDII: wind
imaging interferometer.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
JO, KS, and SG have been co-leaders of Task Group 4 (TG4) ‘What is the geospace
response to variable inputs from the lower atmosphere?’ of SCOSTEP's CAWSES-II
program. WW is the leader of the ROSMIC sub-program of SCOSTEP's VarSITI
program and also led project 1. JM and HT organized the LONET campaign andled project 2. HF and MK led project 4. MK organized various EISCAT and
PFISR campaigns. All authors read and approved the final manuscript.
Acknowledgements
JO, KS, and SG are grateful for the enthusiasm and contributions of all 250
members of TG4, especially the project leaders W. Ward, M. A. Abdu,
J. Chau, J. Makela, H. Takahashi, D. Pancheva, M. Yamamoto, H. Fujiwara,

Oberheide et al. Progress in Earth and Planetary Science (2015) 2:2

and M. Kosch. Our special acknowledgement goes to Dr. Michi Nishioka
who edited the quarterly newsletter. JM and HT thank all of the LONET
and SpreadFEx-2 campaign participants. JM and HT acknowledge the
COSMIC data center team who provided the TEC data. The Pameungpeuk
MF radar data were provided through IUGONET under agreement of RISH,
Kyoto University. JO was supported by NSF award 1139048 and NASA
grants NNX11AJ13G and NNH12CF66C. KS is supported by the JSPS
Grants-in-Aid for Scientific Research (20244080, 23403009, and 25247080),
JSPS Core-to-Core Program, B. Asia-Africa Science Platforms, and the STEL
Cooperative and IUGONET Projects from MEXT, Japan.
Author details
1
Department of Physics and Astronomy, Clemson University, Clemson,
SC 29634-0978, USA. 2 Solar-Terrestrial Environment Laboratory, Nagoya
University, Nagoya 464-8601, Japan. 3Indian Institute of Geomagnetism,
Navi Mumbai 410 218, India. 4 Department of Physics, University of New
Brunswick, Fredericton, NB E3B 5A3, Canada. 5 Faculty of Science andTechnology, Seikei University, Tokyo, Japan. 6 South African National Space
Agency, Hermanus 7200, South Africa. 7Lancaster University, Lancaster LA1 4YB, UK.
8
Currently at National Institute of Polar Research, Tachikawa, Japan. 9University of
Illinois at Urbana-Champaign, Urbana, IL, USA. 10Instituto Nacional de Pesquisas
espaciais, São José dos Campos, SP, Brazil.
Received: 31 May 2014 Accepted: 7 December 2014

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