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九州大学大学院理学研究院紀要

M E M O I R S
OF THEFACULTYOFSCIENCESKYUSHU UNIVERSITY

Series D Earth and Planetary Sciences
VOLUME XXXⅣ
No. 1

FUKUOKA, JAPANNovember, 2016

Contents

Hideaki Kawano, Akira Sessai Yukimatu, Yoshimasa Tanaka, Satoko Saita,
Nozomu Nishitani, and Tomoaki Hori:
SC-Triggered 1.6mHz Waves Including an Interval with Latitude-Dependent Phase Shift,
Observed by the SuperDARN Hokkaido East Radar in Mid Latitudes:
Possible Global Magnetospheric Cavity-Mode Waves andTheir Field-Line Resonance with Poloidal Alfven-Mode Waves …………… 1

Editorial Board
Tasuku Akagi, Chief Editor
Jun-ichiro Ishibashi

All communications relating to this Memoirs should be
addressed to the Dean of the Faculty of Sciences, Kyushu University
Motooka, Fukuoka 819-0395, JAPANMem. Fac. Sci., Kyushu Univ., Ser. D, Earth ＆ Planet. Sci., Vol. XXXⅣ, No. 1,
pp. 1-15, November 00, 2016

SC-Triggered 1.6mHz Waves Including an Interval with Latitude-Dependent Phase Shift,
Observed by the SuperDARN Hokkaido East Radar in Mid Latitudes: Possible Global
Magnetospheric Cavity-Mode Waves and Their Field-Line Resonance with Poloidal AlfvenMode Waves
Hideaki Kawano*, Akira Sessai Yukimatu**, Yoshimasa Tanaka**, Satoko Saita***,Nozomu Nishitani****, and Tomoaki Hori****,*****

Abstract
In this paper we present a new kind of wave event found in the ground/sea backscattered data of theSuperDARN Hokkaido East radar, located in the mid-latitude area at the AACGM latitude (longitude) of
36.5° (214.7°). The wave event started at the time of an SC at ~09:50 UT on Aug 19, 2009, and continued
for about an hour until ~10:50 UT: This relatively short-living wave event, probably triggered by the SC,
could be classified as a type different from the typical type seen in the ground/sea backscattered data of
SuperDARN radars, i.e., waves which start without a clearly identifiable trigger and continue for a longtime (4~5 hours) (e.g., Ponomarenko et al. 2003). Comparisons with ground magnetometer data suggest thatthe major part of this event consisted of global magnetospheric cavity-mode waves whose typical frequency
is in the order of 1mHz (e.g., Kivelson and Southwood 1985, 1986). In addition, in the interval of
10:26~10:42UT which was embedded in the above-stated one-hour interval, we have found the amplitude
and phase pattern typically observed when the field-line resonance (FLR) takes place; this pattern is found
for the first time in the mid-latitude ground/sea backscattered data, and suggests excitation of poloidal
Alfven-mode field-line eigenoscillations during this interval.
Keywords: S uperDARN Hokkaido East radar, mid-latitude observation, SC-triggered waves, global
magnetospheric cavity-mode waves, field-line resonance (FLR)

1. Introduction
SuperDARN stands for Super Dual Auroral Radar Network (Greenwald et al. 1995). It is a global network of
HF radars, and at present there exist more than thirty SuperDARN radars covering considerable portions of highand mid latitude ionosphere in the northern and southern hemispheres. SuperDARN radars emit HF (8-20MHz)
waves and receive echoes backscattered from either the ionosphere or the ground/sea. For the latter case, it is themost frequent that the emitted HF waves are reflected at the ionosphere, reaches the ground/sea, are backscattered
there, and reaches back to the radar site along the same ray path (this is called a single-hop case; there could be

Manuscript received on 5 September 2016; accepted on 3 October 2016*
Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, 744 Motooka, Nishiku, Fukuoka 819-0395, Japan, and International Center for Space Weather Science and Education, Kyushu
University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan**
National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan, and Departmentof Polar Science, School of Multidisciplinary Sciences, SOKENDAI (The Graduate University for Advanced
Studies), 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan*** Department of Electronics and Control Engineering, Kitakyushu National College of Technology, 5-20-1 Shii,
Kokuraminamiku, Kitakyushu, Fukuoka, 802-0985, Japan**** Institute for Space-Earth Environmental Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601,
Japan***** Present address: Department of Earth and Planetary Science, Graduate School of Science, The University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan2

Hideaki Kawano, Akira Sessai Yukimatu, Yoshimasa Tanaka,
Satoko Saita, Nozomu Nishitani, and Tomoaki Horicases with more reflections (for example, the HF waves that have reached the ground/sea are reflected there andreaches the ionosphere again), but such cases are less frequent than single-hop cases). The SuperDARN Hokkaido
East radar is among those that were built first at mid-latitudes (Nishitani et al. 2011). Its field of view is shown at,e.g., http://cicr.isee.nagoya-u.ac.jp/hokkaido/site1/intro_e.html .
From the ionosphere-backscattered signals, one can obtain the Doppler velocity of the ionospheric plasma
flow along the line of sight (LOS) of a radar beam. On the other hand from the ground/sea-backscattered signals,
one can obtain the velocity of the vertical motion of the ionosphere, because it is the only variable that can change
the ray path length of the single-hop signal: The ground/sea does not move much (compared to the ionosphere),
and the horizontal velocity of the ionospheric plasma does not change the height of the ionosphere, thus does notchange the ray path. The vertical velocity of the ionosphere is in general significantly smaller than the horizontal
velocity, and one can judge the observed backscatter is a ground/sea backscatter if its Doppler velocity is less than
~50 m/s and its spectral width is less than ~50m/s (See Ribeiro et al. (2011) for more details).
Ponomarenko et al. (2003) showed that the ground/sea backscatters, observed by the SuperDARN TIGER
radar located at ~ −55° AAGCM latitude in Tasmania, included continuous (lasting 4-5 hours a day on average)
periodic oscillations, which they attributed to ULF waves. Among thus found events, Ponomarenko et al. (2005)
showed a five-minute interval (embedded in the above-stated 4-5 hour interval) during which the latitudedependence of the power and the phase of the wave was consistent with those predicted from the field-line
resonance (FLR) theory (Tamao 1965, Southwood 1974, Chen and Hasegawa 1974) which is summarized below.
The resonance latitude of the FLR was estimated to be ~ −65° (i.e., L~6.0) in AACGM.
The FLR is the mechanism via which the fast-mode waves in the magnetosphere resonate with, and thus
excite, eigenoscillations of the magnetospheric magnetic field lines. In other words, since the field-line
eigenoscillations are maintained by the Alfven mode, the FLR is the mechanism in which the fast-mode waves areconverted to the Alfven-mode waves, which can take place in an inhomogeneous plasma.
The FLR-generated Alfven-mode wave has characteristic latitude (on the ground) dependence of theamplitude and phase, as follows: For a fixed frequency, the amplitude shows a single peak at the latitude (“FLR
latitude” below) where the (fixed) frequency is equal to the eigenfrequency of the field line there (theeigenfrequency is a function of latitude). On the other hand, the phase shows a monotonic change across the FLR
latitude. (e.g., Southwood 1974).
The above-stated excitation of the field-line eigenoscillations via the FLR have been identified in groundmagnetometer data (e.g., Samson et al. 1971, Kawano et al. 2002), satellite magnetic and electric fields data (e.g.,Singer and Kivelson 1979, Liu et al. 2009), ground-based radar data (e.g., Fenrich et al. 1995, Ponomarenko et al.
2005, Nedie et al. 2012, Liu et al. 2013), and combination of these data (e.g., Rae et al. 2005).
The energy source of the FLR is, as stated above, fast-mode waves. As such fast-mode waves, azimuthallypropagating waves such as the Kelvin-Helmholtz waves are often considered (e.g., Southwood 1974, Chen and
Hasegawa 1974; Fig. 7 of Glassmeier et al. (1999) is illustrative). On the other hand, cavity-mode waves, which
are fast-mode radial eigenoscillations of a magnetospheric cavity, have also been considered as the energy sourceof the FLR (e.g., Fenrich et al. 1995). In the cavity mode, the plasma in the entire cavity radially oscillates withthe same frequency. As the boundaries of the cavity region, the magnetopause (e.g., Kivelson and Southwood
1985, 1986), the plasmapause (e.g., Allan et al. 1986, Takahashi et al. 2010, and references therein), and theionosphere have been suggested. When the outer (inner) boundary is the plasmapause (ionosphere), such a cavity
is usually called the “plasmaspheric cavity” and it oscillates at frequencies in the order of 10 mHz (e.g., Takahashi
et al. 2010, Chi et al. 2013); when the outer (inner) boundary is the magnetopause (ionosphere), such a cavity isoften called the “global magnetospheric cavity” and it oscillates in the order of 1 mHz (e.g., Takahashi et al. 2010
and references therein).
In this paper we present a wave event identified in the ground/sea backscattered data obtained by theSuperDARN Hokkaido East radar, located in the mid-latitude area; its AACGM latitude (longitude) is 36.5°(214.7°). Our event, which took place at ~45° AACGM latitude (i.e., at L~2), had the following features, as
shown in Section 3.
(C1) It did not have a long-lasting (4-5 hours) nature as reported by Ponomarenko et al. (2003, 2005), but
continued for a relatively short time (about one hour), starting at the time of an SC on the ground.
(C2) The frequency of the observed wave was ~1 mHz, much smaller than the typically-observed frequency(in the order of 10 mHz) at L~2.
(C3) The one-hour event interval included a 16-minute interval which showed the amplitude and phase pattern
the same as that of FLR-generated Alfven-mode eigenoscillations.
Items (C1) and (C3) are new findings in the mid-latitude area, and reports on item (C2) are limited inliterature. The details will be presented below.

SC-Triggered 1.6mHz Waves Including an Interval with Latitude-Dependent Phase Shift, Observed by theSuperDARN Hokkaido East Radar in Mid Latitudes: Possible Global Magnetospheric Cavity-Mode Waves
and Their Field-Line Resonance with Poloidal Alfven-Mode Waves

3

2. DataIn this paper we study the above-stated wave event observed by the SuperDARN Hokkaido East radar. We
will first make a visual inspection of the SuperDARN data of this event, including comparisons withsimultaneous solar wind data and geomagnetic data. We will then perform spectral analyses of the SuperDARN
data. Finally we will discuss and summarize the obtained results.
The event of this paper took place on Aug. 19, 2009; on this day, the SuperDARN Hokkaido East radar was
run in a mode in which the cadence of each beam observation was 60 seconds and the field of view (FOV) of
each beam was divided into 220 range gates (RGs) in the radial direction with the range resolution of 15 km andwith the first RG having the distance of 135 km from the radar (for the case of ionospheric echoes). Figure 1(b)shows VLOS (the velocity along the LOS, defined to be positive when the beam’s reflector is approaching theradar site) along Beam 05, which had an angle of about 17° from the magnetic meridian at RG 91. (Fig. 1(a) and(c), showing Beam 00 and 10, will be discussed in Section 3.) In Fig. 1(b), the above-stated possible FLR took
place at 10:26~10:42UT near this RG 91; more details will be described and discussed in Section 3. Almost all
the data shown in Fig. 1 had VLOS’s smaller than 50m/s in magnitude and are classified as ground/sea
backscattered signals. The white parts in this figure corresponds to RGs which received no backscattered signal.
As the simultaneously observed solar wind data, we use 64-sec resolution ACE data, downloaded from
CDAWeb (http://cdaweb.gsfc.nasa.gov/istp_public/ ). Details of the ACE data are discussed in Section 3.
We also use the data of the geomagnetic indices AE and SYM-H provided by the World Data Center for
Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/ ). The AE index is that for substorm activities, and theSYM-H index can be regarded as a one minute-resolution version of Dst, which is an hourly index, widely used
to define the magnitudes of magnetic storms. Some more detail on SYM-H/Dst is that it is the ground equatorial
magnetic field deviation from its quiet-time value, and the deviation is caused by an increased ring current duringstorms. More details on AE and SYM-H/Dst are presented at the above website.
We also use ground magnetometer data from SuperMAG and Kakioka Magnetic Observatory of the Japan
Meteorological Agency; details will be presented below. We will compare them with the SuperDARN data.

4

Hideaki Kawano, Akira Sessai Yukimatu, Yoshimasa Tanaka,
Satoko Saita, Nozomu Nishitani, and Tomoaki Hori

Fig. 1. Shows the line-of-sight Doppler velocity (VLOS) observed by Beam 00 (Panel a), 05 (Panel b), and 10 (Panel c) of theSuperDARN Hokkaido East radar at 09:00~12:00UT on Aug. 19, 2009. The time resolution is one minute. The vertical axis shows
all the range gates (RGs). Note that the negative (positive) VLOS is painted in reddish (bluish) color. See text for more details.

SC-Triggered 1.6mHz Waves Including an Interval with Latitude-Dependent Phase Shift, Observed by theSuperDARN Hokkaido East Radar in Mid Latitudes: Possible Global Magnetospheric Cavity-Mode Waves
and Their Field-Line Resonance with Poloidal Alfven-Mode Waves

5

3. Data Analysis and Results
In Fig. 1(b) we can see a wide vertical red band centered around 09:58UT. Before this red band, there is anarrower green vertical band starting at around 09:50UT. After these bands, we can visually identify at least six
bands with similar horizontal width (i.e., time length) before 10:50UT. Since these bands alternately change theircolors between reddish and bluish (negative (positive) VLOS is painted in reddish (bluish) color), it can be said
that they have the nature of a wave, although their periodicity is rather irregular. Before 09:50UT and after
10:50UT, a similar band structure is not recognizable.
We also notice in Fig. 1(b) that the bands show a clear bending feature during the 10:26~10:42UT interval;
this corresponds to the possible FLR feature we mentioned above. Further details will be described below.
Figure 2 shows the 64-sec solar wind data from the ACE spacecraft, and 1-min geomagnetic indices: From
top, IMF Bz, the solar wind dynamic pressure (Pdyn), AE, and SYM-H are shown. The ACE data is shifted intime by 44 minutes, and as a result, Pdyn starts to make a clear and significant increase at about the same time as
the SC identified in SYM-H (i.e., 09:50UT). To support this number 44, we have estimated the propagation lag
for all the ACE data in the 44 minutes preceding 09:50UT (in Fig. 2) by using the following equation: (X∙ByY∙Bx)/(-Vx∙By+Vy∙Bx), where all the quantities (X, Y: position, B: magnetic field, V: plasma velocity, all inGSE) are the data at ACE. This equation is a simplified version of that used by Lockwood et al. (1989), andshows the time lag for a straight IMF field line to convect from the ACE position to the position of the Earthcenter (in case there is no magnetospheric obstacles). At 09:06UT X was 244 Re and Y was 33 Re, thus theseparation of ACE from the Sun-Earth line was not ignorable. As a result of the above calculation, we haveobtained the median value of 120 min, with its 50±34.13 percentile range (which corresponds to the standard
deviation range for normal distributions) of 40~342 min. That is, this range includes the above 44 min.
Furthermore, during this 40~342 min interval, there was only one step-like increase in Pdyn from a quiet level(which is the one shown in Fig. 2); Thus, from the cause-and-effect point of view, it is the most reasonable toassociate it with the SC starting at 09:50UT (identified in the SYM H data; see Fig. 2). It is also likely that the
step-like increase in Pdyn also excited the waves in VLOS starting ~09:50UT (Fig. 1(b)); further support of thisassociation with SC is presented just below.

Fig. 2. Shows, from top, the IMF Bz [nT], the solar wind dynamic pressure [nPa], the AE index [nT], and the SYM H index [nT] forthe same time interval as Fig. 1. The solar wind propagation lag is corrected. See text for more details.

6

Hideaki Kawano, Akira Sessai Yukimatu, Yoshimasa Tanaka,
Satoko Saita, Nozomu Nishitani, and Tomoaki Hori

Fig. 3. Shows, from top, SuperDARN VLOS of RG106 (running through the upper part of the bending bands around 10:30UT in
Fig. 1(b)), VLOS of RG84 (running through the lower part of the same bands), and mid- to low-latitude ground magnetometerdata located at a wide range of longitudes (The local times shown in the legend are those at 10:00UT). See text for more details.

Figure 3 shows, from top, VLOS of RG106 (running through the upper part of the bending bands around10:30UT in Fig. 1(b)), VLOS of RG84 (running through the lower part of the same bands), and mid- to lowlatitude ground magnetometer data located at a wide range of longitudes (the local times shown in the legend arethose at 10:00UT). These ground magnetometer data come from SuperMAG, except for the MMB data from the
Kakioka Magnetic Observatory of the Japan Meteorological Agency. The time resolution of the SuperMAG(MMB) data shown in Fig. 3 is one minute (one second). We selected mid- to low-latitude stations to better
identify compressional components. Shown in the figure is the northward component (“N component” below) of
the magnetic field vector. Magnetic longitudes and latitudes [deg] of the stations in Fig. 3 are as follows: TUC:(－43.96, 39.32), MBO: (58.44, 1.2), AAE: (111.51, －0.06), PHU: (178.77, 14.11), MMB: (－143.33, 37.08), and
HON: (－89.1, 20.88).
The VLOS data in this figure are bandpass-filtered (BPF’ed) with the period range of 320 seconds to six
hours: The 320 seconds corresponds to the central frequency of the fourth-left column of Fig. 5, so that thefrequency range of the BPF includes the frequencies of the leftmost three columns for which the wave power was
relatively large (details of Fig. 5 will be explained below), and the six hours is twice the time length of Fig. 3. No
bandpass filtering has been applied to the ground magnetometer data.
Except near the local midnight, the perturbations in the ground magnetometer data are commonly andsimultaneously observed, and had the frequency of ~1mHz, suggesting that these are global magnetosphericcavity-mode waves (e.g., Kivelson and Southwood 1985, 1986). We also note that the initial biggest perturbations
starting around 09:50UT are likely to correspond to the SC.

SC-Triggered 1.6mHz Waves Including an Interval with Latitude-Dependent Phase Shift, Observed by theSuperDARN Hokkaido East Radar in Mid Latitudes: Possible Global Magnetospheric Cavity-Mode Waves
and Their Field-Line Resonance with Poloidal Alfven-Mode Waves

7

Fig. 4. Shows a closeup of Fig. 1(b), and the superposed black rectangle (corresponding to 10:26~10:42UT and RG=77~109)
indicates the wave bands that are bending. For each RG, all the 16 timeseries data in the rectangle (corresponding to a row in therectangle) are Fast Fourier Transformed (FFT’ed). See text for more details.

Vertical lines in the figure run through maxima and minima in the SuperDARN VLOS at RG 84. In general,
the VLOS was at maximum (minimum) when the ground N component was increasing (decreasing). This feature
can be explained as follows: For ground/sea echoes, an increase (decrease) in VLOS actually reflects the vertical
downward (upward) motion of the ionosphere where the echoes are reflected, because it decreases (increases) the
length of the ray path between the radar site and the ground/sea backscattering point. The downward (upward)
motion of the ionosphere is caused by the compression (rarefaction) of the magnetosphere via the fast mode
waves: Differently put, this compression (rarefaction) corresponds to the westward (eastward) wave electric field
which causes, with the tilted field line topology, the downward (upward) motion of the ionosphere.
At ground high latitudes, magnetic pulsations starting just after SCs, having frequencies similar to the abovementioned waves of this paper, have been reported (e.g., Araki et al. 1997) and have sometimes been referred toas Psc. However, at mid-latitudes, we do not know a previous paper which reported SC-triggered waves having
frequencies similar to the waves of this paper.
We have noticed in Fig. 1(b) that the bands around 10:30UT bend toward the top right. Figure 4 is a closeup
of Fig. 1(b), and the bending bands are surrounded by a black rectangle. Although the time resolution is rather
coarse with this scaling, the bending feature is apparent in the middle blue band surrounded by the rectangle, clear
enough for the leftmost red band, and barely identifiable for the rightmost red band. It is to be noted that thecavity mode cannot generate this bending feature by itself.
The right three vertical lines in Fig. 3 are drawn in the time range of the rectangle in Fig. 4. The red verticalarrows in the top panel of Fig. 3 show that the peaks at RG 106 were delayed from those at RG 84, (naturally)
consistent with Fig. 4.
To further analyze this bending feature, we have performed the spectral analysis, as follows. Each RG yields
timeseries data of VLOS, and for each of the RGs surrounded by the black rectangle in Fig. 4, that is, RG =77~109, we have applied FFT to all of its timeseries data surrounded by the rectangle, that is, all of its VLOS datafrom 10:26UT to 10:42UT; this means applying FFT to 16 timeseries datapoints, because the data time resolution
is one minute. We note we have applied no bandpass filter before applying the FFT. We also note we have applied
the Hanning window before applying the FFT.
After applying the above-stated analysis procedure, we obtain, for each RG, the real and imaginary parts of
the FFT results as functions of frequency. The frequency resolution is 1/16/60[s] = 1.042[mHz]. From theobtained real and imaginary parts, we can calculate the power and phase. Figure 5 shows thus obtained power as afunction of frequency (horizontal axis) and RG (vertical axis). From this figure it is clear that the wave power isthe largest at 1.042 mHz, or if we take into account the frequency resolution, that the wave power is the largest in
the 0.521-1.563mHz range (corresponding to the second-left column of Fig. 5). Thus, in the following analyses tostudy the latitude dependence of the wave power and phase, we will use only the FFT results at 1.042 mHz.

8

Hideaki Kawano, Akira Sessai Yukimatu, Yoshimasa Tanaka,
Satoko Saita, Nozomu Nishitani, and Tomoaki Hori

Fig. 5. Shows, by grey colors, the wave power obtained by applying the FFT to the data in the rectangle of Fig. 4. The horizontal
axis shows the frequency, which has the 1.042mHz resolution. The vertical axis shows the RG numbers. See text for more details.

By fixing the frequency to 1.042 mHz, we obtain one value of the wave power (phase) for one given RG.
Since the latitude increases with increasing RG number, we can study the latitude dependence of the wave power
and phase by plotting them as a function of the RG number. The result of doing so is shown in Figure 6: The top
panel shows the wave power, and the bottom panel shows the wave phase, as functions of the RG number(horizontal axis). Since the original FFT results are rather scattered as a function of the RG number (which ismainly because the original number of timeseries data are small (16)), we have smoothed the latitude profiles bymoving-averaging the data from adjacent nine RG’s, and the result is shown in Fig. 6.

Fig. 6. From the FFT results, (top) the power and (bottom) the phase of the data at 1.042mHz (corresponding to the second-leftcolumn in Fig. 5) are shown as a function of the RG number. They are smoothed in the direction of the horizontal axis by movingaveraging nine adjacent data. In the bottom panel, a constant offset is subtracted from all the data so that the phase at RG 91(center of the event) is zero. The combination of the two panels’ patterns is consistent with FLR-generated waves. See text for
more details.

SC-Triggered 1.6mHz Waves Including an Interval with Latitude-Dependent Phase Shift, Observed by theSuperDARN Hokkaido East Radar in Mid Latitudes: Possible Global Magnetospheric Cavity-Mode Waves
and Their Field-Line Resonance with Poloidal Alfven-Mode Waves

9

4. Discussion
The essential features of the wave event of this paper observed at L~2 for 09:50~10:50UT are summarized asfollows (they are more detailed version of what we wrote at the end of Section 1):(C1) It was probably triggered by a step-like increase in Pdyn which reached the ionosphere around 09:50UT,
and continued for a relatively short time (about one hour; see Figs. 1(b) and 2). These features are incontrast to the waves reported by Ponomarenko et al. (2003, 2005), which continued for 4-5 hours
without clearly identifiable triggers.
(C2) This wave was observed by the SuperDARN Hokkaido East radar and by the mid- to low-latitude groundmagnetometers in a synchronized manner (Fig. 3). The frequency of the observed wave was ~1mHz (Fig.
3), much smaller than the typically-observed frequency (in the order of 10 mHz) at L~2 (e.g., Takahashi
et al. 2010, Chi et al. 2013).
(C3) The one-hour event interval included a 16-minute interval (10:26~10:42 UT; see Fig. 4) which showed theamplitude and phase pattern the same as that of FLR-generated Alfven-mode eigenoscillations (Fig. 6).
The SuperDARN Hokkaido East radar observed Items (C1) and (C3) for the first time as the mid latitude
phenomena, and reports on item (C2) are quite limited in literature.
Item (C1) is self-explanatory and we do not have much to add at this point; we will refer to (C1) in theSummary section. We then proceed to discussing Item (C2). Fig. 1 shows that, except for the 16-min interval of
10:26~10:42UT (Item C3), the bands were vertical, meaning that the phase was the same at all the latitudes. Thisfeature suggests that these waves (except for the waves in the above-mentioned 16-min interval) correspond to thecavity mode. However, as stated above, the typical frequency of the plasmaspheric cavity mode is in the order of
10 mHz (e.g., Takahashi et al. 2010, Chi et al. 2013). Thus, we interpret the frequency of our event (~1 mHz) at
L~2 in terms of the global magnetospheric cavity-mode waves (Kivelson and Southwood, 1985, 1986) which has
the same frequency in both the trough region and the plasmasphere. It is only such a global cavity mode that canhave the frequency of ~1 mHz in the plasmasphere. On the other hand, observations of such a global mode arequite limited: Kokubun et al. (2013) is the only paper we are aware of. Thus, this paper constitutes a usefuladdition to the publication of such phenomena.
We next discuss Item (C3). Fig. 6 shows the results of the spectral analysis of the above-stated 16 min intervalof 10:26~10:42UT, for RG = 77~109. It shows that the wave power has a maximum near RG 91, and that thephase continuously decreases with increasing RG (latitude) across RG 91. This combination of the power profileand the phase profile (as a function of latitude) is typically observed at the FLR (e.g., Pilipenko and Fedorov
1994, Fenrich et al. 1995, Kawano et al. 2002).
Here we note that, due to the resonance width (explained in the above references), the frequency dependenceof the power and the phase becomes gradual, not showing a step-like change across the resonance point. In
addition, the power of the FLR-generated wave is smaller at lower latitude (e.g., Chi et al. 2013). Thus, at midlatitudes, it is often difficult to observe a full 180deg phase shift, because at distances from the FLR resonancepoint, the FLR signal becomes smaller than the background signal level; this signal-noise ratio effect is notincluded in the standard FLR theory.
If the amplitude-phase pattern observed at 10:26~10:42UT (Figs. 4-6) is caused by the FLR which couples theabove-stated global magnetospheric cavity-mode waves with the Alfven-mode eigenoscillations, another expectedfeature is that the FLR frequency increases with decreasing latitude, because the field line length becomes shorterand enables quicker eigenoscillations of the field line (although the cavity-mode frequency is essentially the same
inside the cavity, it has a finite frequency range (half width) like any natural phenomena, and the FLR-generated
Alfvenic eigenfrequency changes within that finite frequency range). Fig. 5 appears to support this; that is, at
small RG numbers, the wave power is large not only at 1.042 mHz but also at 2.083mHz (see the RG numbers
less than ~90 in the third-left column).
If we regard this pattern during the interval 10:26~10:42 UT as a result of the FLR, what is not typical about
this interval is that the FLR generated poloidal-mode Alfven waves in this interval while the most of thepreviously identified FLR events were associated with toroidal-mode Alfven waves (e.g., Fenrich et al. 1995,
Sakaguchi et al. 2012). In the following we will call the former case “poloidal FLR” and the latter case “toroidal
FLR” for simplicity, and describe more details of them, starting with the “toroidal FLR” and then the “poloidal
FLR.”
The toroidal FLR couples magnetopause surface waves (such as the Kelvin-Helmholtz waves), which

10

Hideaki Kawano, Akira Sessai Yukimatu, Yoshimasa Tanaka,

SatokoSaita,
Nozomu(i.e.,
Nishitani,
On the other hand for smaller
beam
numbers
Beamand
00Tomoaki
and 01, Hori
having the angles from themagnetic meridian of 1° and 4°), the three bands at 10:26~10:42UT (corresponding to the central

propagate tailward and radially inward via the fast mode, to toroidal-mode Alfven waves (Fig. 7 of Glassmeier etal. (1999) is in
illustrative).
toroidal-mode
waves
move
(i.e.,
cause inthetheE×Bdrift of) therectangularFig. 4) didThenotgenerated
show a clear
bending Alfven
shape butwerefairlystraight
verticalionospheric plasma in the east-west horizontal direction (not in the vertical direction), meaning that the ray path

length between
radar andthe shownbackscattering
the ground/sea
not ischangeand thus
the VLOS
direction(data theof Beam00 arein Fig. region1(a)). Aonpossible
reason does
for this
as follows:
Asthatstatedis unchanged, regardless of the beam direction. In more detail, since SuperDARN estimates the location of areflectorweof thinka radarthat
beam
an HFverticalwave beam)
theinrayFig.
path1(b)lengthbeamofspeed,
no change inabove,
the(i.e.,straight
parts byof dividing
the bands
are bythetheresult
the global
the ray path length means no motion of the reflector and thus no change in the VLOS. Thus, ground/sea

backscattered signal
shows no wavy
perturbation
whenandtoroidal-mode
Alfventakeplace.
magnetosphericcavity-mode
oscillations(Kivelson
Southwood,
1985,waves
1986),
that
the FLR in theOn the other hand, the poloidal FLR couples fast-mode waves (including the cavity modes mentioned above)

to poloidal-mode interval
Alfven waves,
andAlfven waves verticallymove andthe ionosphere,
10:26~10:42UT
coupledthethus-generated
cavity-mode poloidal-mode
wave and the poloidal-mode
Alfven wave,
thatbecause the magnetic field line (which oscillates radially in the magnetosphere) is tilted in the mid- to low-

latitudeionosphere.
Therefore,
the poloidal-mode
Alfvenwaves
can change
the beam’s
ray inpath
thegenerated
Alfvenwave yielded
the bending
featureclearly
seen aroundRG 91
thelength,
centraland thus

can change the VLOS. We note Ponomarenko et al. (2005) also interpreted that the FLR-generated Alfvenic

pulsations ofwhich
observedin the ground/sea
signalsforwere
in thepoloidalmode.
As statedrectangle
Fig. they
4. Then,the no-bending
feature backscatteredof the same bands
Beam
00 and01 maymean

above, the vertical bands in Fig. 1(b) are likely to have been caused by the cavity mode; thus, their bending (Fig.
4) suggest
the not
coupling
the cavity-mode
theshape
poloidal-mode
waves,
sincecavitythat
FLR wasactiveofthere.
(The changewaves
in theandband
for Beam Alfven
01 through
Beam
05the
was
not modecannot bend the bands by itself.
In thiswe havegradual
analyzed(notBeam
05 only,
butgenerally
Beams 02thought
throughthat
06 theshowedabruptbutpapermore-or-lessshown).)It is
FLRmore-or-lessstructure hassimilara largefeatures(not shown). These beams had the angles from the magnetic meridian from 7° (Beam 02) to 21° (Beam 06). For
larger beam
numbers
to morebeamsetinal.thefield-of-viewof FLR-driven
the radar, looking
at lowereast-west
size,
but we(referringnote that Fig.
2(d)eastsideof Sakaguchi(2012)
shows that
Pc5’s atlatitudes), the three bands marked in Fig. 4 became less clear (data of Beam 08 is shown in Fig. 1(c)). A possible
reasonlatitude
for thiswere
is as observed
follows: The
ground/sea
tendedto be understanding
observed at similar
RGlongituderanges for manyhigh
at 20MLT
but backscatters
not at 19MLT.
Furtherof thebeams (not shown), which means that the eastside beams (having larger angles to the magnetic meridian)observed ground/sea
at lower
latitudes
the westside
beams;
then,events,
if the FLR
effectdependent
feature (ofbackscatters
our mid-latitude
event)
needsthanstatisticalanalysesof morewhich
is a(taking
topic placeat similar latitudes) enhances the wave bands in some manner, the eastside beams were reflected at latitudes lower
thanthe latitudesof the enhanced wave bands, thus missed the bands.
offutureresearch.
On the other hand for smaller beam numbers (i.e., Beam 00 and 01, having the angles from the magnetic
meridianof 1° toandthe4°),
the threebandsofatthe
10:26~10:42UT
to thecentral pattern
rectangularFig. 4) did
ReturningbendingfeatureVLOS bands,(corresponding
if we regard theobserved
as a inresultnot show a clear bending shape but were fairly straight in the vertical direction (data of Beam 00 are shown in
Fig.the1(a)).
A possible
for thisis asfollows: As
above,
we think thethatequatorial
the straightplasma
verticalmass
parts of theofFLR,then thereason
observed
wave
frequency
canstated
be usedto estimate
bands in Fig. 1(b) are the result of the global magnetospheric cavity-mode oscillations (Kivelson and Southwood,
1985, 1986),
FLR
the 10:26~10:42UT
intervalcavity-mode
wave
andthetheevent
poloidal-mode
densityalongthatthe thefield
lineinwhichis under the FLR;
this coupledfield linetheran
through RG
91 forof
Alfven wave, and that the generated Alfven wave yielded the bending feature clearly seen around RG 91 in thecentralrectangle
Fig. 4.ofThen,theisno-bendingof the same
bands
for Beam
00 and 01
may meanthispaper.
The Lofvalue
RG 91
L = 1.992 feature(corresponding
to theAACGM
magnetic
latitudeof that
FLR was not active there. (The change in the band shape for Beam 01 through Beam 05 was not abrupt but moreor-less gradual (not shown).) It is generally thought that the FLR structure has a large east-west size, but we note
44.88°).
that Fig. 2(d) of Sakaguchi et al. (2012) shows that FLR-driven Pc5’s at high latitude were observed at 20MLT
but There
not at are
19MLT.
understandingof the longitude-dependent(ofmethod
our mid-latitude
manyFurther
methods
for the estimation,and in this paper wefeature
use theof Singer event)et al. needs
statistical analyses of more events, which is a topic of future research.
Returningof thatthe VLOS
bands, if wethe observed
pattern as a result
the FLR,(1981)
whichtoisthea bending
general feature
methodcan calculate
the regard
poloidal-mode
eigenfrequency
for ofany
then the observed wave frequency can be used to estimate the equatorial plasma mass density along the field line
which is under the
FLR;andthisany
fieldline ran throughRG 91Thefor method
the eventnumericallyof this paper.solves
The Lthevalueof RG 91 is L =magnetic-fieldmodel
plasma-density
model.
differential
1.992 (corresponding to the AACGM magnetic latitude of 44.88°).
There ofareAlfvenmany methods
the estimation,and in this(forpaper
we details,
use the method
Singeral. As(1981)
equationwaves tofor
obtainthe eigenfrequency
morerefer to ofSingeret etal.).
the whichis a general method that can calculate the poloidal-mode eigenfrequency for any magnetic-field model and any
plasma-density
model.input
The into
methoddifferential05equationof Alfven waves
to obtain themagnetic
field model
this numericallymethod, we solves
use thethe
Tsyganenko
model (Tsyganenko
and Sitnov,
eigenfrequency (for more details, refer to Singer et al.). As the magnetic field model input into this method, we
use theAs
Tsyganenko
modelmodel(Tsyganenko
andSitnov,2005).
As theplasma density
model input into this2005).
the plasma05
density
input into
this
method,we usethe following
model:
method, we use the following model:௅௔ ௠

is thethe LL value,
value, aa isis thethe Earth
Earthradius,
radius,rrisisthetheradialradialdistancedistanceofofany
anypoint
pointalong
alongthe fieldߩ ൌ ߩ଴ ቀ ቁ , where
where LL is௥

line in question, m is a parameter usually pre-defined, and ρ0 (the quantity to obtain, the only one unknown
parameter) is the equatorial mass density of the field line. This functional form is quite popularly used, and there13
are many studies on the proper value of m; in this paper we follow Goldstein et al. (2001) and set m to 1.17; theyobtained this number by using the plasma wave data observed by the Polar spacecraft in the L range from 2.5 to
6.3. This L range barely misses 1.992 for our event, but to our knowledge it is the closest. We note here that Chi etal. (2000) and Takasaki et al. (2006) used m = 4 for ground-based observations at L = 2 and 1.4, respectively. Chi

SC-Triggered 1.6mHz Waves Including an Interval with Latitude-Dependent Phase Shift, Observed by theSuperDARN Hokkaido East Radar in Mid Latitudes: Possible Global Magnetospheric Cavity-Mode Waves
and Their Field-Line Resonance with Poloidal Alfven-Mode Waves

11

et al. (2000) did not mention what let them use this number. Takasaki et al. (2006) used m = 4 so that theiranalysis procedure would be consistent with Chi et al. (2000). Takahashi et al. (2004) made a detailed study on
the value of m by using spacecraft data, and they obtained m = 0.5 for 4 ≤ L < 6; this number is popularly used atpresent, but the spatial coverage of Takahashi et al. (2004) was L = 4 at minimum. We also note that Obana et al.
(2015) used m = 1 for their density estimation at L = 2.33. To our knowledge there is no paper which changed thevalue of m depending on the storm or SC activities. The actual procedure to obtain ρ0 is to search for ρ0 so that thecalculated eigenfrequency (by inputting the ρ0 to the Singer et al.’s method) equals to the actually observed
frequency (for more details, refer to Singer et al.).
As stated above, the observed wave frequency is in the range of 0.521-1.563 mHz, with the center being 1.042
mHz. In the following we use 1.563 mHz, because visual inspection of Fig. 4 suggests that there are 1.5 wave
periods in the rectangle having the time length of 16 minutes; the frequency calculated from these numbers equals
to 1000/(16×60/1.5) = 1.563 [mHz].
By inputting this 1.563 (mHz, as the poloidal eigenfrequency) and 1.992 (as the L value) into Singer et al.’s
method, we obtain 2.643×105 amu/cm3 as the equatorial density (‘amu’ refers to the atomic mass unit, using
which one H+ is counted as one, one He+ is counted as four, one O+ is counted as 16, etc.). Since the electron
number density at L = 1.992 is usually in the order of 104/cm3 (for example, in Fig. 3 of Gallagher et al. (2000)
which shows the direct density observations by spacecraft at Kp < 1.3, the maximum density at L~2.1 is ~1.3×104/
cm3), this number 2.643×105 appears unrealistic at first sight.
There are four factors which could explain the above-stated large density, as follows:(E1) In the 10:26~10:42UT interval, the global cavity-mode waves were coupled to quarter-wave-mode (notfundamental-mode) poloidal Alfven waves (eigenoscillations) via the FLR. With this, the density
estimated from the observed wave frequency (i.e., 1.563 mHz) has the same order as those in pastsatellite observations at L~2.
(E2) The compression of the plasmasphere by the sudden increase in Pdyn at the time of the SC directly
increased the plasma density in the plasmasphere.
(E3) The compression of the ionosphere by the increased Pdyn yielded, as a reaction, ionospheric heavy ion
outflow to the plasmasphere.
(E4) Ionospheric heavy ion outflow to the plasmasphere was enhanced as the effect of the magnetic storm
main phase.
(E2) is so straightforward that we do not have further discussion on it. The other three factors are described in
more detail just below. It is possible that all these four factors took effect during the event of this paper, and the
combination of these four effects have a good chance of explaining the observation. (In addition to these four
factors, we have considered the possible effect of the deformation of the magnetic field at L~2 due to theenhanced ring current during the storm. Takasaki et al. (2006) estimated how much field lines (at L~1.4) were
deformed during their storm event, having Dst = −300 [nT], by using the Tsyganenko 05 model. The estimatedchanges in the field-line length and the equatorial magnetic field strength were both less than 1%. From this,although our observation was made at L~2, i.e., larger than 1.4, we think the effect of the deformation of themagnetic field was small.)(E1) Quarter waves (Obana et al. 2008) take place near the dawn and the dusk: On the dayside, theionospheric conductance is large due to the sunlight, and the both ends of the field line are nodes because theionospheric currents associated with the wave tend to stop the motion of the field line there. On the other handnear the dawn and the dusk, it can happen that one end is on the dayside and the other end is on the nightside. The
ionospheric conductance is small on the nightside, and thus the nightside end is an antinode. This leads to thequarter wave.
The above means that, while the wavelength of the fundamental-mode Alfven wave (eigenoscillation) is twice
the field-line length, the wavelength of the quarter wave is four times the field-line length. Thus, the quarter-wave
frequency is about half of the fundamental-mode eigenfrequency; this is analogous to the fact that the second
harmonic-mode eigenfrequency is about twice the fundamental-mode eigenfrequency.
Our event took place around 10:30UT, which is 19:30LT, i.e., near the dusk. Thus, it is possible that theobserved waves were quarter waves; if we employ this possibility, the fundamental-mode eigenfrequency is about
3.1 mHz (twice the observed frequency 1.563 mHz), leading to the density of about 6.6×104 amu/cm3, i.e., one
fourth of the above-obtained 2.643×105 amu/cm3 (because the eigenfrequency is roughly proportional to theinverse of the Alfven-wave travel time along the field line, and because the Alfven speed is proportional to theinverse of the square root of the density). This 6.6×104 amu/cm3 is in the same order as the usual density range at
L = 1.992 near 104 amu/cm3; as we mentioned above, Fig. 3 of Gallagher et al. (2000) shows that, even during

12

Hideaki Kawano, Akira Sessai Yukimatu, Yoshimasa Tanaka,
Satoko Saita, Nozomu Nishitani, and Tomoaki Hori

Fig. 7. Shows the area where the bending of the band of VLOS (i.e., the possible signature of FLR) was observed. It was observedby Beam 02 through 06, and for RG 77 through 109, at 10:26~10:42UT. This figure shows this area near the central time of theevent duration.

geomagnetically quiet intervals, the maximum density directly observed by spacecraft at L~2.1 is ~1.3×104/cm3.
Here one may think 19:30LT is well in the nightside. So that the quarter waves take place along a field line,
the conductance difference between the two footpoints of the same field line is important, and the sunlightincreases the conductance. Obana et al. (2015) estimated the ratio of the interhemispheric Pedersen conductanceand the same-definition ratio of the Hall conductance for observed quarter waves, and suggested the threshold offive for the both quantities above which quarter waves could take place.
To address this issue, we have first identified the area where the bending of the band of VLOS (i.e., thepossible signature of FLR) was observed. As stated above, it was observed by Beam 02 through 06, and for RG
77 through 109, for 10:26 to 10:42UT. Figure 7 shows this area near the central time of the event duration.
Then, by using the IRI model (Bilitza et al. 2011) for the ionosphere and the NRLMSISE-00 model for theatmosphere (Piscone et al. 2002), we have calculated the height-integrated (80-500km) Pedersen and Hall
conductance values at the central point of this possible FLR area as shown in Fig. 7 at the event time, andobtained 0.47mho and 0.33mho, respectively. We have also calculated the same-definition conductance values atthe conjugate point, and obtained the Pedersen (Hall) conductance value of 0.13mho (0.13mho). Thus, theinterhemispheric Pedersen and Hall conductance ratios are 3.7 and 2.5. The former value is close to five. Thus we
think that the possibility of the quarter wave is not negligible for the event of this paper.
(E3) We note that Kale et al. (2009) reported an increase in the plasmaspheric density just after an SC; they
attributed it to the ionospheric O+ outflow to the plasmasphere, as a result of the ionospheric density enhancement
due to the ionospheric compression by the increased Pdyn. The O+ outflow increases the plasmaspheric ion amu
density much larger than the electron number density (if all the ions are H+, then the ion amu density is equal tothe electron number density). It is quite possible that the same effect took place for the event of this paper.
(E4) SYM-H increased at SC and then decreased (Fig. 2, bottom panel), indicating a magnetic storm (it was a

SC-Triggered 1.6mHz Waves Including an Interval with Latitude-Dependent Phase Shift, Observed by theSuperDARN Hokkaido East Radar in Mid Latitudes: Possible Global Magnetospheric Cavity-Mode Waves
and Their Field-Line Resonance with Poloidal Alfven-Mode Waves

13

very small storm, with SYM-H reaching its minimum value －26nT at 19:48~19:50UT on this day, but having thetime profile of SYM-H (Dst) typical to storms (not shown; readers are referred to the website of the WDC Kyoto
and/or OMNIweb for the plots)). In the possible FLR interval (10:26~10:42UT), SYM-H was on the decreasing
trend (Fig. 2. bottom panel, from ~10:00UT to the rightside edge of the figure), meaning the enhancement of thering current, a typical feature of the storm main phase; this is also supported by the simultaneously observed
continuously negative IMF Bz (Fig. 2, top panel), which in general leads to a ring current buildup. Then, we note
Takasaki et al. (2006) reported a case in which the mass density at L~1.4 was increased during a magnetic storm.
As its cause, they suggested an outflow of heavy ions (e.g., O+) from the ionosphere as a result of the enhanced
ionospheric activity due to precipitating particles and enhanced FACs.

5. Summary
Many waves have been observed in the ground/sea backscattered signals of mid-latitude SuperDARN radars,
but past reports mainly looked at waves lasting for a long time. In this paper we have reported for the first time in
the mid latitudes a relatively short-living wave after an SC and during the following magnetic storm. It may
constitute a distinct category with unique characteristics, such as having a small wave frequency and including
FLR interval(s) associated with an increased mass density. It is a topic of future research to conduct a statisticalanalysis of this type of events to clarify its nature, including what controls the occurrence of ~1 mHz waves andwhat controls the duration of the waves after SCs (about one hour for the event of this paper). It is also a topic offuture research to study data from ground magnetometers near the event location.

6. Acknowledgements
This research was supported by the National Institute of Polar Research through its General Collaboration
Projects no.24-11, 25-7 and 26-8. We would like to thank all the staff who contributed to the managements andoperations of the SuperDARN Hokkaido East radar. This research was also supported by Special Funds for
Education and Research (Energy Transport Processes in Geospace) and IUGONET project of the Ministry of
Education, Culture, Sports, Science and Technology of Japan. The computer software package UDAS, which later
became a part of SPEDAS, was made in the IUGONET project and is used in this work, and we appreciate it. The
fitacf CDF data and the data analysis software of the SuperDARN Hokkaido East radar were distributed by ERGScience Center (ERG-SC) operated by ISAS/JAXA and ISEE/Nagoya University, and the work of T. H. was done
at ERG-SC. We acknowledge use of NASA/GSFC's Space Physics Data Facility's CDAWWeb service, and the
ACE spacecraft data obtained from it. We also acknowledge use of the AE and Dst indices distributed by theWorld Data Center for Geomagnetism, Kyoto. For the MMB ground magnetometer data we gratefully
acknowledge the Kakioka Magnetic Observatory of the Japan Meteorological Agency. For the other groundmagnetometer data used in this paper we gratefully acknowledge: Intermagnet; USGS, Jeffrey J. Love;CARISMA, PI Ian Mann; CANMOS; The S-RAMP Database, PI K. Yumoto and Dr. K. Shiokawa; The SPIDR
database; AARI, PI Oleg Troshichev; The MACCS program, PI M. Engebretson, Geomagnetism Unit of theGeological Survey of Canada; GIMA; MEASURE, UCLA IGPP and Florida Institute of Technology; SAMBA, PI
Eftyhia Zesta; 210 Chain, PI K. Yumoto; SAMNET, PI Farideh Honary; The institutes who maintain the IMAGE
magnetometer array, PI Eija Tanskanen; PENGUIN; AUTUMN, PI Martin Conners; DTU Space, PI Dr. Jürgen
Matzka; South Pole and McMurdo Magnetometer, PI's Louis J. Lanzarotti and Alan T. Weatherwax; ICESTAR;
RAPIDMAG; PENGUIn; British Artarctic Survey; McMac, PI Dr. Peter Chi; BGS, PI Dr. Susan Macmillan;
Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN); GFZ, PI Dr.
Jürgen Matzka; MFGI, PI B. Heilig; IGFPAS, PI J. Reda; University of L’Aquila, PI M. Vellante; SuperMAG, PI
Jesper W. Gjerloev. We used a webpage at Kyoto University, Japan (http://wdc.kugi.kyoto-u.ac.jp/ionocond/
sigcal/index-j.html ), to calculate the ionospheric conductance, and are grateful for it. We also appreciate S. Abeand S. Imajo for providing computer programs used in this study. Thanks are due to Dr. Kentarou Kitamura at the
National Institute of Technology, Tokuyama College who reviewed this paper.

14

Hideaki Kawano, Akira Sessai Yukimatu, Yoshimasa Tanaka,
Satoko Saita, Nozomu Nishitani, and Tomoaki Hori

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and Their Field-Line Resonance with Poloidal Alfven-Mode Waves

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