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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, A12312, doi:10.1029/2008JA013417, 2008
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Northeastward motion of nighttime medium-scale
traveling ionospheric disturbances at middle latitudes
observed by an airglow imager
K. Shiokawa,1 Y. Otsuka,1 N. Nishitani,1,2 T. Ogawa,1,2 T. Tsugawa,2 T. Maruyama,2
S. E. Smirnov,3 V. V. Bychkov,3 and B. M. Shevtsov3
Received 22 May 2008; revised 3 September 2008; accepted 15 October 2008; published 30 December 2008.
[1] Nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed
in 630-nm airglow images at middle latitudes are known to have a predominantly
northwest-southeast phase surface and to move southwestward in the Northern
Hemisphere of Earth. However, the mechanisms of MSTID generation and their
systematic southwestward motion have not been clarified. In this paper, we report the
‘‘northeastward’’ motion of the MSTIDs observed at Paratunka, Far East Russia (52.97°N,
158.25°E), using an all-sky 630-nm airglow imager at 2000–2300 LT on 19 August 2007.
The MSTIDs moved first southwestward but then back northeastward in the northern
part of the images. The northeastward motion of the MSTIDs took place coincident with a
F layer height decrease observed by an ionosonde at Paratunka. The F layer height
decrease was also confirmed by an enhancement of the 630-nm airglow intensity, which
seemed to propagate from northeast to southwest. This fact suggests that the F layer
height decrease was caused by poleward wind enhancement rather than westward electric
field. These observations imply that the F layer height decrease or the poleward
thermospheric wind has some role in the northeastward turning of the MSTID propagation
direction.
Citation: Shiokawa, K., Y. Otsuka, N. Nishitani, T. Ogawa, T. Tsugawa, T. Maruyama, S. E. Smirnov, V. V. Bychkov, and B. M.
Shevtsov (2008), Northeastward motion of nighttime medium-scale traveling ionospheric disturbances at middle latitudes observed by
an airglow imager, J. Geophys. Res., 113, A12312, doi:10.1029/2008JA013417.
1. Introduction
[2] Nighttime medium-scale traveling ionospheric disturbances (MSTIDs) are frequently observed at middle latitudes. Recent statistical studies of two-dimensional MSTID
images over Japanese and American sectors using 630-nm
airglow imagers and total electron content (TEC) maps
obtained by Global Positioning System (GPS) receivers
indicate that (1) the occurrence of MSTIDs is very high in
the summer in Japan (>50% of the total observation hours),
(2) they have horizontal-scale sizes of 50– 500 km and
phase speeds of 50– 170 m/s, and (3) they have a predominantly northwest-southeast phase surface and propagate
southwestward [Garcia et al., 2000; Shiokawa et al.,
2003a; Kotake et al., 2006, 2007]. The ionospheric Perkins
instability is a likely mechanism in generating these
MSTIDs, since it can explain the northwest-southeast phase
surface [Perkins, 1973]. However, the growth rate of the
1
Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya,
Japan.
2
National Institute of Information and Communications Technology,
Koganei, Japan.
3
Institute of Cosmophysical Research and Radio Wave Propagation, Far
Eastern Branch of the Russian Academy of Sciences, Paratunka, Russia.
Copyright 2008 by the American Geophysical Union.
0148-0227/08/2008JA013417$09.00
instability is too small to develop in the ordinary nighttime
ionosphere at middle latitudes [e.g., Kelley and Fukao,
1991; Garcia et al., 2000; Shiokawa et al., 2003b]. Moreover, the instability cannot explain the observed systematic
southwestward motion of MSTIDs. Kelley and Makela
[2001] introduced a northwest-directed polarization electric
field parallel to the MSTID phase surface to explain the
southwestward motion.
[3 ] In this paper, we report an event of nighttime
MSTIDs, which moved first southwestward and then
northeastward, using a 630-nm airglow imager. This turning
of MSTID direction provides a good opportunity for
investigating the mechanism that causes the systematic
southwestward motion of MSTIDs. We conclude that the
thermospheric neutral wind or associated F layer height
variation has some role in determining the direction of
MSTID motion.
2. Observations
[4] The airglow imager (imager number 10) used in the
present study is part of the Optical Mesosphere Thermosphere Imagers (OMTIs) [Shiokawa et al., 1999, 2008].
The automatic airglow imaging observations at Paratunka,
Kamchatka, Russia (52.97°N, 158.25°E, dipole magnetic
latitude: 45.8°N) have been conducted since 17 August
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Figure 1. Three representative 630-nm airglow images obtained at Paratunka on 19 August 2007. The
top shows deviation images in % from 1-h running averages. The bottom shows images in absolute
intensity. The all-sky images are converted to geographical images by assuming an airglow altitude of
250 km. The dashed line in the images at 1100 UT indicates the baseline to take the cross section of the
images in Figure 3.
2007. The event reported in this paper was observed on
19 August 2007, during a test operation of the imager. The
imager has a 180° field of view fisheye lens, several band
pass filters, and a thermoelectrically cooled CCD with
512 512 pixels. We used the CCD with 2 2 binning
(used as 256 256 pixels) to increase the output counts.
The 630-nm airglow images were taken every 1.5 min with
an exposure time of 40 s, using a band pass filter with a
band width of 1.58 nm. The background continuum intensity of the sky was monitored at 572.5 nm every 10 min, to
obtain the absolute intensity of the 630-nm line emission.
[5] Animation 1 shows a movie of sequential 630-nm
airglow images obtained at 1010 –1640 UT (2040– 0310 LT)
on 19 August 2007, at Paratunka.1 The MSTIDs appear
from the northeast (top left) from the beginning of the
observation and move southwestward (toward bottom
right). At later time intervals, they move back northeastward, particularly in the northeastern part of the images.
Figures 1 and 3 indicate these motions more quantitatively.
[6] Figure 1 shows three representative 630-nm airglow
images at 1100, 1200, and 1300 UT on 19 August 2007, at
Paratunka. The all-sky coordinates were converted into
geographical coordinates with 20° 20° in latitude and
longitude, by assuming a 630-nm airglow altitude of 250 km.
The bottom shows images in raw intensities, and the top
shows deviations obtained by subtracting 1-h running averages to show the wave structures more clearly. The deviations are calculated as (I(t) Ia(t))/Ia(t), where I(t) and Ia(t)
are the airglow intensity at time t and the average intensity
over t ± 30 min, respectively, for each pixel of images.
1
Animations are available in the HTML.
[7] The MSTIDs, which have a phase surface from
northwest to southeast, are clearly seen in the top; they
can also be recognized in the bottom. From these images, we
estimate the wavelength of the MSTIDs to be 100 – 400 km.
It is noteworthy that the northwestern and southeastern
edges along the phase surface of the MSTIDs are often
seen in the field of view of the images. The scale sizes along
the phase surface are 500– 1500 km. The amplitude of the
MSTIDs decreases with time from 1100 to 1300 UT. The
airglow intensity itself also decreases in time.
[8] To demonstrate the spatial extent of the MSTIDs at
lower latitudes, we show a two-dimensional map of the
TEC variations obtained by GPS receivers over Japan at
1300 UT on 19 August 2007 (Figure 2). A running average
of TEC over 1 h was subtracted from the raw TEC values to
show the TEC variations clearly. In the top right of this
map, the airglow image at Paratunka at 1300 UT is also
shown in the same scale size. The locations of the stations
used in this paper are also indicated. The MSTIDs are
recognized as waves with northwest-southeast phase surfaces
over Japan. The horizontal wavelengths are 100– 400 km,
similar to those observed at Paratunka. We should note that
the MSTIDs were most distinct before 1300 UT in the
airglow images at Paratunka, while they became distinct
after 1300 UT over Japan in this TEC map.
[9] Figure 3 shows a comprehensive summary of the
MSTID event of 19 August 2007. Figure 3a shows echo
power obtained by the SuperDARN Hokkaido HF radar at
Rikubestu, Japan (43.5°N, 143.6°E, near the northern edge
of Japan) along beam 5, shown by the thin dashed-dotted
line in Figure 2; Figure 3b shows cross sections (keograms)
of 630-nm airglow images along the northeast to southwest
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Figure 2. A two-dimensional map of the total electron content (TEC) variations obtained by the GPS
receivers over Japan, and an airglow image (deviation from 1-h running average, same as the top right of
Figure 1) obtained at Paratunka at 1300 UT on 19 August 2007. A running average of TEC over 1 h was
subtracted from the raw TEC values to show the TEC variations clearly. The thick dashed line indicates
the baseline to take the cross section of the TEC variations in Figure 3. The thin dashed-dotted line
indicates the line of sight of beam 5 of the Hokkaido HF radar used in Figure 3.
baseline (dashed line) in Figure 1 (deviation from 1-h running
averages); Figure 3c shows absolute intensity in units of
Rayleigh; Figure 3d shows ionospheric height (h0F2); and
Figure 3e shows foF2, which corresponds to the F layer peak
electron density; the h0F2 and foF2 values were obtained by
ionosondes at Paratunka and Wakkanai (45.4°N, 141.7°E,
270 km northwest of Rikubetsu); Figure 3f shows foEs
and foEs-fbEs, obtained at Paratunka; Figure 3g shows
H-component geomagnetic field variations observed at
Moshiri, Kagoshima in Japan, and Kototabang in Indonesia,
and Figure 3h shows cross section (keogram) of GPS-TEC
variations along the northeast to southwest baseline (thick
dashed line in Figure 2).
[10] The airglow deviation keogram in Figure 3b clearly
shows the turning of the MSTID motion from southwestward at 1000– 1200 UT to northeastward afterward. It is
very interesting to note that the turning point moves from
the northeastern edge of the keogram at 1200 UT to lower
latitudes at 1300 UT at the zenith of Paratunka. At lower
latitudes below the zenith of Paratunka, the MSTIDs continue to propagate southwestward with a phase speed of
100 m/s. At further lower latitudes in Japan, Figure 3h
shows that the MSTIDs move southwestward continuously
over 1100– 1500 UT over the entire region of Japanese
latitudes with a phase speed of 70 m/s.
[11] The propagation of the turning point from the northeastern edge at 1200 UT to the zenith of Paratunka at 1300 UT
in Figure 3b coincides with the enhancement of the 630-nm
airglow intensity in Figure 3c, which also seems to propagate from the northeastern edge (Figure 3c, top) at 1200 UT
to the zenith at 1300 – 1400 UT. The airglow intensity
enhancement is not so clear around the zenith of Paratunka.
Another airglow-enhanced region appeared at 1230–1400 UT
in the southwest of Paratunka (Figure 3c, bottom). In
Figure 3d, the F layer virtual height (h0F2) at Paratunka
suddenly decreased from 268 km (1230 UT) to 238 km
(1245 UT), when the airglow enhancement reached near the
zenith of Paratunka. This feature is consistent with the idea
that the enhancement of the 630-nm airglow intensity was
caused by the F layer height decrease. Since the 630-nm
airglow is produced through the interaction between O2
molecules and O+ ions, which are the main ions in the
ionospheric F layer, the F layer height decrease gives more
interaction of O+ with the O2-rich atmosphere at lower
altitudes, and hence the 630-nm airglow intensity increases.
This relation can be confirmed after 1400 UT, when the F
layer height drastically increases with the decrease in the
630-nm airglow intensity over the whole sky of Paratunka.
[12] The Hokkaido HF radar echoes in Figure 3a also
confirm these F layer height variations in the north of Japan.
Since most of the echoes are due to ground scatters, echoes
in the 3000-km range have raypaths whose ionospheric
reflection points are approximately above Paratunka. Some
structures with ranges that decrease with time were observed
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Figure 3. (a) Echo power obtained by the Hokkaido HF radar along beam 5 (along the thin dasheddotted line in Figure 2), cross sections (keograms) of 630-nm airglow images along the northeast to
southwest baseline (dashed line in Figure 1) for (b) deviation from 1-h running averages and (c) absolute
intensity in units of Rayleigh, (d) ionospheric height (h0F2), (e) foF2, which corresponds to the F layer
peak electron density, (f) foEs and foEs-fbEs, obtained by ionosondes at Paratunka and Wakkanai
(45.4°N, 141.7°E, northern edge of Japan), (g) H-component geomagnetic field variations observed at
Moshiri (44.4°N, 142.3°E), Kagoshima (31.5°N, 130.7°E), and Kototabang (0.2°S, 100.3°E), and (h) cross
section (keogram) of GPS-TEC variations along the northeast to southwest baseline (thick dashed line in
Figure 2).
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at 1130 – 1250 UT at 1500- to 2500-km ranges. These
structures were probably caused by complicated reflections
associated with the MSTIDs seen in the Paratunka images,
as shown by, e.g., Bristow et al. [1994]. The overall feature
of decreasing ranges with time in these structures may
indicate the F layer height decrease or the southwestward
motion of the MSTIDs. The increasing range with time
observed at 1350– 1410 UT in Figure 3a probably corresponds to the drastic F layer height increase observed in the
h0F2 data at Paratunka in Figure 3d.
3. Discussion
[13] There are two possible mechanisms that cause the
observed F layer height decrease at 1230 – 1245 UT at
Paratunka: poleward neutral wind and westward electric
field. The poleward neutral wind pushes the ionosphere
down along the geomagnetic field line which has a finite
angle to the horizontal plane at middle latitudes. The
westward electric field causes downward and equatorward
E B drift perpendicular to the geomagnetic field line. The
observed downward speed of the F layer height decrease
was 33.3 m/s (=15 km/15 min). Considering the inclination
of geomagnetic field at Paratunka (65.0°), this velocity
corresponds to a poleward neutral wind speed of 87 m/s
(=33.3 m/s/sin(65°)/cos(65°)) or to the westward electric
field of 3.4 mV/m (=33.3 m/s/cos(65°) 43000 nT). These
values are quite large as a short-term variation of the
thermosphere and are comparable to the amplitude of background tidal variations of thermospheric wind (100 m/s =
4.3 mV/m).
[14] As shown in Figure 3g, a geomagnetic field impulse
was observed at the three stations in Japan and Indonesia at
1230 – 1245 UT, coincident with the F layer height decrease.
This magnetic impulse was caused by a sudden enhancement of solar wind dynamic pressure which was observed
by the ACE satellite (not shown). The magnetic impulse
causes electric field in the polar and low-latitude ionosphere. However, the electric field propagate from the polar
ionosphere to lower latitudes instantaneously by the speed
of light as the TM0 mode waves in the Earth-ionosphere
waveguide [e.g., Kikuchi et al., 1978]. On the other hand,
the 630-nm airglow intensity enhancement, and hence the
F layer height decrease, seem to propagate with much slower
speed from northeast to southwest at 1200 –1300 UT in
Figure 3c, although it is difficult to identify the propagation
conclusively because the enhancement also occurs southwest of Paratunka. Assuming that the airglow enhancement
of the present event propagates from north to south, the
speed is estimated to be 200 m/s (10° in latitude from
1200 UT to 1330 UT). Thus, it is not likely that the electric
field associated with the observed magnetic impulse caused
the F layer height decrease.
[15] Because of the observed propagation of airglow
enhancement, the F layer height decrease is more likely to
be caused by the poleward neutral wind enhancement (or
more correctly, decrease in the prevailing southward wind
speed in the nighttime thermosphere at middle latitudes).
This wind change might propagate equatorward as a largerscale wave to the zenith of Paratunka, causing the propagation of the observed airglow enhancement in Figure 3c.
The F layer peak electron density (foF2) in Figure 3e at
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Paratunka slightly increases just after the F layer height
decrease at 1245 – 1315 UT. This feature can be explained
by the accumulation of F layer electrons due to the vertical
wind shear associated with the poleward wind enhancement. The F layer height decrease at 1230 – 1300 UT at
Wakkanai, Japan, in Figure 3d is more gentle and slow. This
may indicate that the equatorward moving large-scale wave
did not fully reach Wakkanai.
[16] All these features (630-nm airglow enhancement,
F layer height decrease, foF2 increase, and associated
decrease in southward neutral wind speed) were observed
in Japan during the large-scale traveling ionospheric disturbances (LSTIDs) reported by Shiokawa et al. [2002,
2003c, 2007]. The propagation speed of airglow enhancement (200 m/s) estimated for the present event is less than
half of the typical equatorward speed of LSTIDs (500 m/s).
We should also note that any geomagnetic storm/substorm
activities were not occurring in this time interval (Kp < 1 at
0000– 1200 UT and provisional AE index < 100 nT at
0000– 1240 UT). Tsugawa et al. [2004] showed from a
statistical study of LSTIDs using GPS-TEC maps over
Japan that 28% of LSTIDs were observed during geomagnetically quiet intervals with Kp 3. The present event
might correspond to such an LSTID during geomagnetically
quiet interval.
[17] Now we discuss how the turning of the MSTID
direction occurred in association with the poleward thermospheric wind and the F layer height decrease. So far, Kelley
and Makela [2001] seem to propose the only model to
explain the systematic southwestward motion of nighttime
MSTIDs at middle latitudes. They consider a northwestward
polarization electric field parallel to the MSTID phase
surface due to the prevailing northeastward Pedersen current
and a finite-sized low-conductivity region along the MSTID
phase surface [Kelley and Makela, 2001, Figure 2]. The
finite size (500– 1500 km) along the phase surface was seen
in the MSTID structures in the images at Paratunka in
Figure 1. The northwestward polarization electric field can
cause southwestward MSTID motion through the E B
drift. The northeastward Pedersen current is basically
caused by the background southeastward neutral wind in
the nighttime thermosphere at middle latitudes. If the wind
direction turns to the opposite direction (northwestward),
the directions of the Pedersen current, polarization electric
field, and thus the MSTID propagation direction would turn
to the opposite direction.
[18] The above discussion is a possible explanation of the
present observation, which is hence consistent with the idea
of Kelley and Makela [2001]. However, since the present
event was observed during a geomagnetically quiet interval,
it may be difficult to expect the LSTID-associated poleward
wind to have enough amplitude to cancel the prevailing
southeastward tidal wind (100 m/s) in the nighttime
thermosphere. The LSTID-like equatorward moving structure in the 630-nm airglow enhancement seems to cease
around the latitude of Paratunka. This fact also suggests that
the amplitude of the LSTID-associated disturbance is small.
[19] It should also be noted that Kelley and Makela
[2001] considered a region of low conductivity, which
corresponds to the airglow depletion region. In a highconductivity structure, the directions of a polarization electric field and the E B MSTID motion become opposite.
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Thus, the model by Kelley and Makela [2001] does not
explain the southwestward motion of wave-like features that
have both an enhanced and a depleted airglow region. Even
if the observed MSTID structure consists of only the
airglow depletion regions, the procedures to subtract 1-h running averages from the 630-nm airglow images and GPS
TEC maps would cause pairs of enhanced and depleted
regions of airglow and TEC in Figure 1 (top) and Figure 2.
It is difficult to figure out from the raw airglow images in
Animation 1 and Figure 1 (bottom) whether the observed
MSTIDs are only the airglow-depleted regions or pairs of
depleted and enhanced regions.
[20] We should also note that strong sporadic-E (Es)
activities were observed simultaneously during the present
event, as shown in the values of foEs and foEs-fbEs in
Figure 3f. The foEs, which represents the peak electron
density of the Es layer, is very high at 6 MHz throughout
the night. The foEs-fbEs, which represents the amplitude of
density inhomogeneity in the Es layer [Ogawa et al., 2002],
is also high (3 – 4 MHz). The Es layer can cancel the
polarization electric field in the F layer through the fieldaligned current circuit. Thus, the above discussion of the
polarization electric field should be modified by considering
the three-dimensional current circuit between the F and Es
layers. Tsunoda and Cosgrove [2001], Cosgrove et al.
[2004], and Yokoyama et al. [2008] investigated these E-F
coupling processes using theoretical modeling and threedimensional simulation. Cosgrove et al. [2004] discussed
that a westward (southward) electric field leads to propagation of the instability structure to the southwest (northeast)
through the E B drift for the northwest-to-southeast phase
front structures in the Northern Hemisphere. For the present
event, however, the F layer height decrease, which may be
related to the westward electric field, was observed with the
northeastward turning of the MSTIDs.
4. Conclusion
[21] Using a 630-nm airglow imager at Paratunka, we
reported an event of middle-latitude nighttime MSTIDs
(wavelength is 100 – 400 km, phase speed is 70– 100 m/s)
that shows the turning of their propagation direction from
typical southwestward to northeastward. The turning coincided with the F layer height decrease, simultaneously
observed by the ionosonde at Paratunka, and with associated
airglow intensity enhancement. The turning point propagated southwestward to the zenith of Paratunka. The
airglow enhancement seems to propagate southwestward
simultaneously. A poleward neutral wind enhancement (or
decrease of tidal equatorward wind at nighttime thermosphere), propagating equatorward as a large-scale wave,
might cause this F layer height decrease along the geomagnetic field line and the 630-nm airglow enhancement. This
implies that the poleward neutral wind enhancement possibly caused the observed turning of the MSTID propagation
direction from southwestward to northeastward. On the
other hand, if the observed F layer height decrease was
caused by a westward electric field, the MSTIDs should
move southwestward according to the E B drift, opposite
the observation. Strong Es activities were observed simultaneously during the present event, indicating that electro-
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dynamical coupling between the ionospheric E and F layers
should be taken into account.
[ 22 ] Acknowledgments. We thank H. Hamaguchi, Y. Katoh,
M. Satoh, T. Katoh, and R. Nomura of the STEL, Nagoya University,
and S. Gubanov and I. Babakhanov of the IKIR, Far Eastern Branch of the
Russian Academy of Sciences, for their helpful support in the installation
and operation of the all-sky airglow imager at Paratunka. The multipoint
GPS data were supplied by the Geographical Survey Institute, Japan. This
work was carried out under the agreement between IKIR and STEL on the
international project ‘‘Ground and Satellite Measurements of Geospace
Environment in the Far Eastern Russia and Japan.’’ This work was
supported by a Grant in Aid for Scientific Research (18403011 and
20244080) and Dynamics of the Sun-Earth-Life Interactive System (G-4,
the 21st Century COE Program) of the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.
[23] Zuyin Pu thanks the reviewers for their assistance in evaluating
this paper.
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V. V. Bychkov, B. M. Shevtsov, and S. E. Smirnov, Institute of
Cosmophysical Research and Radio Wave Propagation, Far Eastern Branch
of the Russian Academy of Sciences, 7 Mirnaya Street, Elizovo District,
Kamchatka Region, Paratunka, 684034 Russia. ([email protected])
T. Maruyama and T. Tsugawa, National Institute of Information and
Communications Technology, 4-2-1 Nukui-Kita, Koganei, Tokyo 1848795, Japan. ([email protected])
N. Nishitani, T. Ogawa, T. Otsuka, and K. Shiokawa, Solar-Terrestrial
Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan.
([email protected])
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