研究者詳細 - 西谷　望

論文 - 西谷　望

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Propagation of large amplitude ionospheric disturbances with velocity dispersion observed by the SuperDARN Hokkaido radar after the 2011 off the Pacific coast of Tohoku Earthquake 査読有り

Nishitani, N., T. Ogawa, Y. Otsuka, K. Hosokawa, and T. Hori

Earth Planets Space 63 巻頁： 891-896 2011年9月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

Ionospheric responses to the 2011 off the Pacific coast of Tohoku Earthquake are studied using the SuperDARN Hokkaido radar, which is located at (43.5^o N, 143.6^o E) and which monitors the ionosphere over a wide horizontal area. The radar observed an oscillation of the vertical motion of the ionosphere with a period of about 1 to 2 min. The disturbance propagated northward, away from the epicenter with the velocity of about 6.2, 4.5, 3.9 and 3.5 km/s. The latter three values are basically consistent with the propagation of the Earth's surface waves reported in several previous studies. The propagation velocities decreased with time, which has not been reported in previous studies for this propagation velocity range. The peak-to-peak amplitudes of Doppler velocities of ground/sea scatter echoes observed by the radar were up to 200 m/s, which is considerably larger than previously-reported values using HF Doppler measurements, although they are not extremely large for this historical earthquake (M = 9.0). This is the first time that ionospheric data have been obtained with high temporal (8 s) and spatial (22.5 km) resolutions following a giant earthquake, which enables us to discuss the detailed characteristics of the propagation of coseismic ionospheric disturbances.

DOI： doi:10.5047/eps.2011.07.003

Large-Scale traveling ionospheric disturbance observed by SuperDARN Hokkaido HF radar and GPS networks on 15 December 2006 査読有り

Hayashi, H., N. Nishitani, T. Ogawa, Y. Otsuka, T. Tsugawa, K. Hosokawa, and A. Saito

J. Geophys. Res. 115 巻 ( A06309 ) 頁： doi:10.1029/2009JA014297 2010年

*Medium-scale traveling ionospheric disturbancesobserved with the SuperDARN Hokkaido radar, all-sky imager and GPS network, and their relation to concurrent sporadic-E irregularities 査読有り

Ogawa, T., N. Nishitani, Y. Otsuka, K. Shiokawa, T. Tsugawa,and K. Hosokawa

J. Geophys. Res. 114 巻頁： A03316, doi:10.1029/2008JA013893 2009年

記述言語：英語掲載種別：研究論文（学術雑誌）

We present midlatitude medium-scale traveling ionospheric disturbances (MSTIDs)
observed with a Super Dual Auroral Radar Network (SuperDARN) HF radar at around
10 MHz in Hokkaido, Japan, in combination with a 630-nm all-sky imager and a GPS
network (GEONET) that provides total electron content (TEC) data. MSTIDs propagating
southward from high latitudes are detected at first with the HF radar and then with
the imager and GEONET. We analyze two MSTID events, one in winter (event 1) and
the other in summer (event 2), to find that MSTIDs appear simultaneously, at least, at
55–25N. It is shown that nighttime MSTIDs propagate toward the southwest over a
horizontal distance of about 4000 km, and daytime MSTIDs do so toward the southeast.
Daytime radar echoes are due to ground/sea surface (GS) scatter, while nighttime
echoes in event 1 return from 15-m-scale F region field-aligned irregularities (FAIs) and
those in event 2 are due to GS scatter. Doppler velocities of the nighttime F region
FAI echoes in event 1 are negative (motion away from the radar) within strong echo
regions and are positive (motion toward the radar) within weak echo regions. This fact
suggests that the strong (weak) echoes return from suppressed (enhanced) airglow/TEC
areas, in line with previous observations over central Japan. The nighttime MSTIDs in
events 1 and 2 are often accompanied by concurrent coherent echoes from FAIs in
sporadic E (Es) layers. The Es echo areas in event 2 rather coincide with suppressed
airglow/TEC areas in the F region that are connected with the echo areas along the
geomagnetic field, indicating the existence of E and F region coupling at night.

Joint observations of a traveling ionospheric disturbance with the Paratunka OMTI camera and the Hokkaido HF radar 査読有り

Koustov, A.V., N. Nishitani, K. Shiokawa, S. Suzuki, and B.M. Shevtsov

Ann. Geophys. 27 巻頁： 2399-2406 2009年

Subauroral polarization streams: observations with the Hokkaido and King SalmonSuperDARN radars and modeling 査読有り

Koustov, A.V., N. Nishitani, Y. Ebihara, T. Kikuchi, M.R. Hairston, and D. Andre

Ann. Geophys. 26 巻頁： 3317-3327 2008年

*Dynamical property of storm-time subauroral rapid flows as amanifestation of complex structures of the plasma pressure in the inner magnetosphere 査読有り

Ebihara, Y., N. Nishitani, T. Kikuchi, T. Ogawa, K. Hosokawa, M. -C. Fok and M. F. Thomsen

J. Geophys. Res. 114 巻頁： A01306, doi:10.1029/2008JA013614 2008年

記述言語：英語掲載種別：研究論文（学術雑誌）

During the intense magnetic storm of 15 December 2006, the midlatitude Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar observed a dynamical character of rapid, westward flows at 50–56 magnetic latitude. The simulation that couples the inner magnetosphere and the subauroral ionosphere was performed using a realistic boundary condition of the hot ion distribution determined from four Los Alamos National Laboratory satellites at 6.6 R E . The following results are obtained using the simulation: (1) In general, morphology of the azimuthal component of the simulated ionospheric plasma flow is consistent with that known as the subauroral polarization stream (SAPS), (2) an increase in the hot ion density in the plasma sheet results in the temporal reduction and subsequent intensification of the rapid flow at certain subauroral latitudes with a delay of ∼40 min, and (3) influence of the plasma sheet temperature on the rapid flow is not evident. The simulated line-of-sight velocity is compared with that obtained by the SuperDARN Hokkaido radar. Agreement between them is found in terms of the temporal and spatial variations of the rapid flows as well as the flow velocity. It is suggested that the dynamical character of the subauroral plasma flow is a direct manifestation of the plasma pressure distribution in the inner magnetosphere (the ring current) especially during the magnetic storm.

Two-dimensional observations of overshielding during a magnetic storm by the SuperDARN Hokkaido radar 査読有り

Ebihara, Y., N. Nishitani, T. Kikuchi, T. Ogawa, K. Hosokawa, and M. -C. Fok

Journal of Geophysical Research 113 巻 ( A01213 ) 頁： doi:10.1029/2007JA012641 2008年

記述言語：英語掲載種別：研究論文（学術雑誌）

Two-dimensional observations of ionospheric plasma flows possibly caused by overshielding are reported for the first time. The observations were made by the mid-latitude SuperDARN Hokkaido radar in Japan during a major magnetic storm on December 15, 2006. The magnetosphere was exposed continuously to a southward interplanetary magnetic field (IMF) for several hours during the main phase of the storm. Immediately following the subsequent northward turning of the IMF, an anti-sunward plasma flow was observed for about 14 min in the pre-dusk sector at magnetic latitudes of 50-60°, reaching a maximum line-of-sight speed of 70-80 m/s. These features are consistent with a simulation of coupling between the ring current and the ionosphere associated with an overshielding condition. Within 1 h of the first observation, a similar anti-sunward flow was observed during a period of southward-oriented IMF. However, the simulation cannot account for the anti-sunward flow in this case. It is suggested that the shielding/overshielding condition is not simply caused by the northward turning of IMF. This second overshielding-like condition is attributable to a sudden contraction of the polar cap associated with the substorm, or to a sudden strengthening of the inertial current converted from the abrupt injection of magnetospheric ions. However, none of both fully accounts for the observations.

Dynamic variations of a convection flow reversal in the subauroral post-midnight sector as seen by the SuperDARN Hokkaido HF radar 査読有り

Ryuho Kataoka, Nozomu Nishitani, Yusuke Ebihara, Keisuke Hosokawa, Tadahiko Ogawa, Takashi Kikuchi, and Yoshizumi Miyoshi

Geophysical Research Letters 32 巻頁： L21105, doi:10.1029/2007GL031552 2007年

記述言語：英語掲載種別：研究論文（学術雑誌）

SuperDARN Hokkaido HF radar, capable of measuring the subauroral ionospheric plasma convection especially during storms, has been in continuous operation since the beginning of December 2006. We report the first two-dimensional observation of a dynamic variation of convection flow reversal in subauroral postmidnight sector during the storm main phase on 29 January 2007. The flow reversal region is extended over 20° in longitude and 5° in latitude, lasting for about 10-15 min, and the maximum flow speed is about 0.5-1.0 km/s. The flow reversal structure is reasonably reproduced by the ring current simulation coupled with the ionosphere, suggesting that it is produced by the region 2 field-aligned current associated with the ring current enhancement during the storm main phase. The dynamic variation of the flow reversal structure is interpreted as a transient eastward extension of the elongated dusk convection cell to the postmidnight and equatorward of the dawn cell, associated with the variation of the ring current whose structure is controlled by the interplanetary magnetic field and solar wind dynamic pressure. It is suggested that the ring current variation is highly coupled with the interplanetary parameters and is much more complicated than ever thought.

Model calculations of possible ionospheric backscatter echo area for a mid-latitude HF radar 査読有り

Nishitani, N., and T. Ogawa

Adv. Polar Upper Atmos. Res. 19 巻頁： 55-62 2005年8月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

HF ray path calculation is performed in order to identify possible
ionospheric backscatter echo area for an HF radar at mid-latitude. The calculation is
made on the basis of the R.M. Jones and J. J. Stephenson (U.S. Dept. of Commerce,
OT Rep. 75-76, 1975) HF ray path tracing algorithm plus the IRI-2001 ionosphere
model. It is shown that depending on the local time and geomagnetic activity, the
possible ionospheric backscatter regions have different distributions. In any case the
backscatter region is large enough, indicating the capability of a planned HF radar in
Hokkaido (43.5 N, 143.6 E), Japan.

*Unusual ionospheric echoes with high velocity and very low spectral width observed by the SuperDARN radars in the polar cap during high geomagnetic activity 査読有り

Nishitani, N., M. Lester, S. E. Milan, T. Ogawa, N. Sato, H. Yamagishi, A. S. Yukimatu, and F. J. Rich

Journal of Geophysical Research 109 巻 ( A2 ) 頁： A02311, doi:10.1029/2003JA010048 2004年2月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

The spectral width of the ionospheric backscatter echoes obtained with the SuperDARN radars has been regarded as a useful tool for locating specific ionospheric regions such as the cusp. In this paper we report the presence of ionospheric echoes with high (> 450 m/s) Doppler velocity and very low (< 60 m/s) spectral width, observed by the CUTLASS and Syowa East and South SuperDARN radars. These echoes have the following characteristics. (1) They have a close correlation with geomagnetic activity such that as the Dst index decreases, the radars tend to observe ionospheric echoes with high Doppler velocity and very low spectral width more frequently. (2) Their existence does not depend on magnetic local time. (3) They are located preferably in the polar cap region, where anti-sunward convection prevails. (4) They sometimes exist over a wide range, so they are more likely to be F-region rather than E-region echoes. The occurrence of these echoes during active periods is associated with the suppression of the electric field turbulence. The present result appears consistent with the previous paper by Golovchanskaya et al. [2002], who showed the negative correlation between the electric field turbulence level and geomagnetic activity.

*Interhemispheric asymmetry of the high latitude ionospheric convection on May 11-12, 1999 査読有り

Nishitani, N., V. Papitashvili, T. Ogawa, N. Sato, H. Yamagishi, A.S. Yukimatu, and F.J. Rich

Journal of Geophysical Research 108 巻 ( A5 ) 頁： 1184, doi:10.1029/2002JA009680 2003年5月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

Ionospheric convection over the southern polar cap on May 11-12, 1999 has been studied by using the Syowa East and South HF radar data and the DMSP ion driftmeter data, when the solar wind density was very low and geomagnetic activity was low. The overall convection pattern is consistent with the previous results by Ohtani et al. [2000]. However, the Syowa radars and the DMSP satellites observed very high (> 1500 m/s) westward plasma flows at dusk directed from the nightside toward the dayside only in the southern (dark) hemisphere. The high-speed flow was observed continuously across the fields of view of both radars from 1530 UT on May 11 to 0200 UT on May 12, when the solar wind density was close to minimum. Comparison with the DMSP particle and auroral image data shows that the westward flow regions were located in the middle of the auroral precipitation area. The strong asymmetry of the convection between the two hemispheres indicates the importance of the presence (absence) of solar illumination for the absence (presence) of the strong and localized ionospheric flows.

*A study of the dusk convection cell's response to an IMF southward turning 査読有り

Nishitani, N., T. Ogawa, N. Sato, H. Yamagishi, M. Pinnock, J.-P. Villain, G. Sofko, and O. Troshichev

Journal of Geophysical Research 107 巻 ( A3 ) 頁： 10.1029/2001JA900095 2002年3月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

One example of the response of ionospheric convection and the polar cap boundary to a sudden change in the IMF orientation has been studied when the IMF suddenly changed from northward (+6 nT) to strongly southward (-19 nT), at 1716 UT on September 5, 1995, The Bz component was fairly constant for about 2 hours before and about 25 min after the sudden IMF change. The convection flow changed almost simultaneously over a global extent. This initial change of the convection pattern can be characterized by a sudden formation of a large flow vortex in the afternoon sector. On the other hand, the response of the polar cap boundary (or its proxy) is more complicated. The radar and particle data indicate the immediate equatorward expansion of the precipitation regions in the noon and premidnight sectors. About 10 to 20 minutes after the initial change, there were changes observed in the dusk region, namely an equatorward expansion of the current reversal boundary in the dusk sector between 1740 and 1750 UT, and an equatorward expansion of the convection reversal boundary. The delayed responses were observed 18 to 8 minutes before a substorm onset was recorded at mid-latitude stations at 1756 UT. These observations indicate that there were two kinds of ionospheric responses to the southward turning of the IMF; the first response is the formation of the convection vortex and the equatorward shift of the polar cap boundary at noon and at 21 MLT, and the second response is the equatorward expansion of the convection reversal boundary in the dusk sector. We make the case that the first response is associated with the propagation of magnetosonic waves, and that the second response is consistent with the Cowley and Lockwood [1992] picture of the redistribution of the newly created open flux in the polar cap region.

A very large scale flow burst observed by the SuperDARN radars 査読有り

Nozomu Nishitani, Tadahiko Ogawa, Mike Pinnock, Mervyn Freeman, John Dudeney, Jean-Paul Villain, Kile B. Baker, Natsuo Sato, Hisao Yamagishi, and Haruhisa Matsumoto

Journal of Geophysical Research 104 巻 ( 10 ) 頁： 22469 1999年10月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

We examined the dynamics of the ionospheric plasma in the dayside sector by using the HF radar data at Iceland West and at Finland from 1100 to 1230 UT on September 5, 1995. During that period, the solar wind density was high and the IMF was strongly southward. The dayside magnetopause was highly compressed nearly to the geosynchronous orbit. The two radars simultaneously detected a poleward flow burst in the noon sector which, assuming uniformity of flow in the region of the data gap (1.5 h of MLT) between the two radars, showed a magnetic local time extent of 5 hours. This local time extent is 2 to 3 hours wider than previous results. The maximum poleward plasma velocity of the flow burst is about 750 m/s, and the latitudinal size of the flow burst region is about 100 to 200 km. This flow burst region initially expanded in longitude up to 5 hours, and then shifted poleward with a phase speed of 400 to 670 m/s. The flow burst has a duration of about 20 minutes. This large-scale poleward flow burst is likely to be due to large-scale reconnection occurring at the dayside magnetopause and subsequent convection as the magnetic field lines are transported across the polar cap.

Averaged Pattern of Ionospheric Echo Region and Convection : Initial Results From the Syowa Station HF radar

Proceedings of the NIPR Symposium on Upper Atmospheric Physics 10 巻頁： 42 1997年

Spatial and Temporal Characteristics of Giant Undulations

Geophysical Research Letters 21 巻 ( 24 ) 頁： 2673 1994年

Auroral activity and corresponding magnetic signatures at synchronous orbit

Journal of Geomagnetism and Geoelectricity 40 巻頁： 4 1988年

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