Faculty Profiles - OTSUKA, Yuichi

A statistical study of the response of the dayside equatorial F-2 layer to the main phase of intense geomagnetic storms as an indicator of penetration electric field Reviewed Open Access

N. Balan, M. Yamamoto, V. Sreeja, I. S. Batista, K. J. W. Lynn, M. A. Abdu, S. Ravindran, T. Kikuchi, Y. Otsuka, K. Shokawa, S. Alex

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 116 2011.3

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

The response of the dayside equatorial F-2 layer to the main phases of the 22 intense geomagnetic storms (Dst < -150 nT) in 1998-2008 is investigated using the digital ionosonde data from the equatorial stations in Brazilian, Indian, and Australian longitudes together with equatorial electrojet strength and IMF B-z; the storms include 15 superstorms (Dst < -200 nT). The observations show that there is a period during all MPs when the F-2 layer peak rises (and falls) rapidly with large peak electron density (Nmax) reduction, the rise velocity strongly correlates with the intensity (Dst) of the storms, and the duration of the Nmax reduction corresponds to that of strong eastward electrojet when IMF Bz remains highly negative. The observations indicate the occurrence of strong eastward prompt penetration electric fields (PPEF) during the rapid F-2 layer response. The PPEF drives the F-2 layer peak rapidly upward, which reduces Nmax due to vertical expansion and diffusion. The results therefore suggest that the rapid F-2 layer response (rapid rise (and fall) of peak height (hmax) with large Nmax reduction) observed by ionosondes can be used to detect the occurrence of the daytime eastward PPEF during intense geomagnetic storms irrespective of season and level of solar activity. The data also show two rare events of strong daytime westward electric fields due to disturbance dynamo and/or prompt penetration. The results are important when radars are not available to monitor the occurrence of the PPEF.

DOI： 10.1029/2010JA016001

First satellite-imaging observation of medium-scale traveling ionospheric disturbances by FORMOSAT-2/ISUAL Reviewed

Toru Adachi, Yuichi Otsuka, Masashi Yamaoka, Mamoru Yamamoto, Kazuo Shiokawa, Alfred B. Chen, Rue-Ron Hsu

GEOPHYSICAL RESEARCH LETTERS Vol. 38 2011.2

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

On the night of May 16, 2007, a satellite limb imager of FORMOSAT-2/ISUAL observed wave-like structures of the 630-nm airglow simultaneously with an all-sky imager deployed at Darwin in Australia. The height of the airglow layer was estimated as 220 km, and the structures were aligned in the northeast-southwest orientation with a wavelength of similar to 300 km and propagated toward the northwest with a phase velocity of similar to 100 m s(-1), showing typical characteristics of the nighttime medium-scale traveling ionospheric disturbances (MSTIDs). We conclude that ISUAL for the first time succeeded in observing the airglow layer altitude and airglow structures modulated by MSTID from space. Such a satellite limb airglow imaging could be a new tool to characterize ionospheric irregularities on a global level. Citation: Adachi, T., Y. Otsuka, M. Yamaoka, M. Yamamoto, K. Shiokawa, A. B. Chen, and R.-R. Hsu (2011), First satellite-imaging observation of medium-scale traveling ionospheric disturbances by FORMOSAT-2/ISUAL, Geophys. Res. Lett., 38, L04101, doi:10.1029/2010GL046268.

DOI： 10.1029/2010GL046268

Vertical connection from the tropospheric activities to the ionospheric longitudinal structure simulated by a new Earth's whole atmosphere-ionosphere coupled model Reviewed

H. Jin, Y. Miyoshi, H. Fujiwara, H. Shinagawa, K. Terada, N. Terada, M. Ishii, Y. Otsuka, A. Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 116 2011.1

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

This paper introduces a new Earth's atmosphere-ionosphere coupled model that treats seamlessly the neutral atmospheric region from the troposphere to the thermosphere as well as the thermosphere-ionosphere interaction including the electrodynamics self-consistently. The model is especially useful for the study of vertical connection between the meteorological phenomena and the upper atmospheric behaviors. As an initial simulation using the coupled model, we have carried out a 30 day consecutive run in September. The result reveals that the longitudinal structure of the F-region ionosphere varies on a day-to-day basis in a highly complex way and that a four-peak structure of the daytime equatorial ionization anomaly (EIA) similar to the recent observations appears as an averaged feature. The simulation reproduces and thus confirms the vertical coupling processes proposed so far with respect to the formation of the averaged EIA longitudinal structure; the excitation of solar nonmigrating tides in the troposphere, their propagation through the middle atmosphere, and the modulation of ionospheric dynamo, which in turn affects EIA generation. The simulation result indicates that not only the ionospheric averaged longitudinal structure but also the day-to-day variation can be modulated significantly by the lower atmospheric effect.

DOI： 10.1029/2010JA015925

Statistical Study of Medium-Scale Traveling Ionospheric Disturbances Observed with a GPS Receiver Network in Japan Reviewed

Otsuka, Y, Kotake, N, Shiokawa, K, Ogawa, T, Tsugawa, T, Saito, A

AERONOMY OF THE EARTH'S ATMOSPHERE AND IONOSPHERE Vol. 2 ( 3 ) page： 291 - 299 2011

A statistical study of the response of the dayside equatorial F2 layer to the main phase of intense geomagnetic storms as an indicator of penetration electric field Reviewed Open Access

N. Balan, M. Yamamoto, V. Sreeja, I. S. Batista, K. J.W. Lynn, M. A. Abdu, S. Ravindran, T. Kikuchi, Y. Otsuka, K. Shokawa, S. Alex

Journal of Geophysical Research: Space Physics Vol. 116 ( 3 ) 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：Blackwell Publishing Ltd

The response of the dayside equatorial F2 layer to the main phases of the 22 intense geomagnetic storms (Dst &lt
-150 nT) in 1998-2008 is investigated using the digital ionosonde data from the equatorial stations in Brazilian, Indian, and Australian longitudes together with equatorial electrojet strength and IMF Bz
the storms include 15 superstorms (Dst &lt
-200 nT). The observations show that there is a period during all MPs when the F2 layer peak rises (and falls) rapidly with large peak electron density (Nmax) reduction, the rise velocity strongly correlates with the intensity (Dst) of the storms, and the duration of the Nmax reduction corresponds to that of strong eastward electrojet when IMF Bz remains highly negative. The observations indicate the occurrence of strong eastward prompt penetration electric fields (PPEF) during the rapid F2 layer response. The PPEF drives the F2 layer peak rapidly upward, which reduces Nmax due to vertical expansion and diffusion. The results therefore suggest that the rapid F2 layer response (rapid rise (and fall) of peak height (hmax) with large Nmax reduction) observed by ionosondes can be used to detect the occurrence of the daytime eastward PPEF during intense geomagnetic storms irrespective of season and level of solar activity. The data also show two rare events of strong daytime westward electric fields due to disturbance dynamo and/or prompt penetration. The results are important when radars are not available to monitor the occurrence of the PPEF. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA016001

Acoustic resonance and plasma depletion detected by GPS total electron content observation after the 2011 off the Pacific coast of Tohoku Earthquake Reviewed Open Access

A. Saito, T. Tsugawa, Y. Otsuka, M. Nishioka, T. Iyemori, M. Matsumura, S. Saito, C. H. Chen, Y. Goi, N. Choosakul

EARTH PLANETS AND SPACE Vol. 63 ( 7 ) page： 863 - 867 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：TERRA SCIENTIFIC PUBL CO

Two-dimensional structures of the ionospheric variations generated by the acoustic resonance between the ground surface and the lower thermosphere was observed for the first time near the epicenter after the M 9.0 Tohoku earthquake on March 11, 2011. A short-period oscillation of total electron content was observed by a GPS receiver array after the earthquake for four hours in the vicinity of the epicenter. It was centered in the east of the epicenter where the tsunami was estimated to commence. The frequency of the dominant mode of the oscillation was 4.5 mHz, 222 seconds of period, while there were minor oscillations whose frequency were 3.7 mHz and 5.3 mHz. These periods are consistent with the periods of the acoustic resonance between the ground surface and the lower thermosphere, predicted by a numerical model. The amplitude of the TEC oscillation showed a gradual change of the amplitude. The two-dimensional distributions of TEC variations generated by this resonance had wave frontal structures that extended from northwest to southeast. The resonant oscillation of the TEC was accompanied by a depletion of TEC whose duration was about 60 minutes. The area of this depletion also centered on the epicenter.

DOI： 10.5047/eps.2011.06.034

Decay of polar cap patch Reviewed

K. Hosokawa, J. I. Moen, K. Shiokawa, Y. Otsuka

Journal of Geophysical Research: Space Physics Vol. 116 ( 5 ) 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：Blackwell Publishing Ltd

We report an event in which a polar cap patch was detected with an all-sky imager (ASI) at Resolute Bay, Canada (74.73° N, 265.07° E
AACGM latitude 82.9°), on the nightside. The patch stopped its antisunward motion associated with a northward turning of interplanetary magnetic field and stayed within the field of view of the ASI for more than 1 h. When the patch stagnated, its luminosity decreased gradually, which allows us to investigate how the patch plasma decayed in a quantitative manner. The decay of the patch can be quantitatively explained by the loss through recombinations of O+ with ambient N2 and O2 molecules, if we assume the altitude of the optical patch to be around 295 km. The derived altitude of the patch around 295 km is much higher than the nominal value at 235 km obtained from the MSIS-E90 and IRI-2007 models, indicating that climatological models such as IRI are not suitable for describing the actual density profile of patches. This is probably because the loss process was much faster in the lower-altitude part of the patch
thus, the peak altitude of the patch increased as it traveled across the polar cap because of rapid recombination at the bottomside of the F region. This suggests that we should employ higher emission altitude when we investigate optical patches transported deep into the nightside polar cap. Such information is important when we compare the optical data with other instruments such as coherent radars and GPS scintillation measurements by mapping the all-sky image on the geographic coordinate system with an assumption of the patch emission altitude. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA016297

Equatorial electrodynamics and neutral background in the Asian sector during the 2009 stratospheric sudden warming Reviewed Open Access

Huixin Liu, Mamoru Yamamoto, S. Tulasi Ram, Takuya Tsugawa, Yuichi Otsuka, Claudia Stolle, Eelco Doornbos, Kiyohumi Yumoto, Tsutomu Nagatsuma

Journal of Geophysical Research: Space Physics Vol. 116 ( 8 ) 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：Blackwell Publishing Ltd

Using ground observations of total electron content (TEC) and equatorial electrojet (EEJ) in the Asian sector, along with plasma and neutral densities obtained from the CHAMP satellite, we investigate the ionospheric electrodynamics and neutral background in this longitude sector during the major stratospheric sudden warming (SSW) in January 2009. Our analysis reveals the following prominent features. First, the TEC response in tropical regions is strongly latitude dependent, with monotonic depletion at the dip equator but a semidiurnal perturbation at low latitudes. Second, the TEC semidiurnal perturbation possesses a significant hemispheric asymmetry in terms of onset date and magnitude. It starts on the same day as the SSW peak in the Northern Hemisphere but 2 days later in the Southern Hemisphere. Its magnitude is twice as strong in the north than in the south. Third, strong counter electrojet occurs in the afternoon, following the strengthening of the eastward EEJ in the morning. Fourth, semidiurnal perturbation in both TEC and EEJ possesses a phase shift, at a rate of about 0.7 h/day. Comparisons with results reported in the Peruvian sector reveal clear longitude dependence in the amplitude and hemispheric asymmetry of the semidiurnal perturbation. Finally, thermospheric density undergoes ∼25% decrease at low latitudes in the afternoon local time sector during the SSW, indicating significant cooling effects in the tropical upper thermosphere. Copyright © 2011 by the American Geophysical Union.

DOI： 10.1029/2011JA016607

Imaging observation of the Earth's mesosphere, thermosphere and ionosphere by VISI of ISS-IMAP on the International Space Station Reviewed

Sakanoi, T, Akiya, Y, Yamazaki, A, Otsuka, Y, Saito, A, Yoshikawa, I

IEEJ Transactions on Fundamentals and Materials Vol. 131 ( 12 ) page： 983 - 988 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan

We present the scientific targets, instrumental design, principle of measurement, and development of the Visible and near-Infrared Spectral Imager (VISI) which will be launched onto International Space Station (ISS) in January 2012. VISI is equipped with fast optics including a grism, and a high-sensitivity CCD to obtain airglow emissions at wavelengths of O (630 nm), OH Meinel band (650 nm) and O<sub>2</sub> (0-0) atmospheric band (762 nm). VISI has two field-of-views (FOVs), 45 degrees forward and 45 degrees backward to nadir, to subtract background contaminations. The data acquisition of VISI will be continuously performed with short exposure cycle less than several seconds when ISS is orbiting in the night side of the earth, and then provide the seamless line scanning images of the airglows with a spatial resolution better than 50 km. After manufacturing VISI, we checked its performance though optical and electric function tests as well as environmental tests that are necessary to be launched.

DOI： 10.1541/ieejfms.131.983

CiNii Research

Other Link： https://jlc.jst.go.jp/DN/JALC/00383579674?from=CiNii

Long-distance propagation of ionospheric disturbance generated by the 2011 off the Pacific coast of Tohoku Earthquake Reviewed Open Access

C. H. Chen, A. Saito, C. H. Lin, J. Y. Liu, H. F. Tsai, T. Tsugawa, Y. Otsuka, M. Nishioka, M. Matsumura

EARTH PLANETS AND SPACE Vol. 63 ( 7 ) page： 881 - 884 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER HEIDELBERG

Propagation of the initial ionospheric total electron content (TEC) disturbances generated by the 2011 off the Pacific coast of Tohoku Earthquake at 05: 46: 23 UT on March 11, 2011, was investigated with ground-based Global Positioning System (GPS) receivers in the east-Asian region. It was found that the initial ionospheric disturbance formed a zonal wave front after the earthquake occurrence. Four zonal wave fronts of this initial ionospheric disturbance were observed to travel southward from Japan to Taiwan with a velocity of about 1,0001,700 m/s. This study further found that the direction of the wave vector rotated from the south-southwest to the south-southeast as it traveled from Japan to Taiwan. The meridional propagation of the coseismic ionospheric disturbances is consistent with those observed after previous intense earthquakes. The temporal evolutions of initial ionospheric disturbances, after the earthquake, near the epicenter was observed in two-dimensions. The directivity of the disturbances was caused by a geomagnetic field effect.

DOI： 10.5047/eps.2011.06.026

ISS-IMAPミッションによって明らかになる超高層大気の姿

斎藤昭則, 山崎敦, 阿部琢美, 鈴木睦, 坂野井健, 藤原均, 吉川一朗, 大塚雄一, 田口真, 山本衛, 中村卓司, 江尻省, 菊池雅行, 河野英昭, 石井守, 久保田実, 星野尾一明, 坂野井和代

第25回大気圏シンポジウム 2011

Ionospheric multiple stratifications and irregularities induced by the 2011 off the Pacific coast of Tohoku Earthquake Reviewed Open Access

Takashi Maruyama, Takuya Tsugawa, Hisao Kato, Akinori Saito, Yuichi Otsuka, Michi Nishioka

EARTH PLANETS AND SPACE Vol. 63 ( 7 ) page： 869 - 873 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：TERRA SCIENTIFIC PUBL CO

A strong earthquake with a magnitude of 9.0 occurred at 1446: 23 JST on March 11, 2011, in Japan. Ionospheric disturbances were detected at 1500 JST at four ionosonde stations. An irregular distortion of echo trace was observed at Kokubunji, which is the nearest station to the epicenter and is 440 km from it. Multiple-cusp-type trace indicating extra stratification was observed at Wakkanai and Yamagawa, which are 870 and 1410 km away from the epicenter. A small wavy fluctuation was observed at Okinawa 1910 km away from the epicenter. The real height analysis of the ionograms showed a vertical structure with a scale size of 20 similar to 30 km.

DOI： 10.5047/eps.2011.06.008

Ionospheric disturbances detected by GPS total electron content observation after the 2011 off the Pacific coast of Tohoku Earthquake Reviewed Open Access

T. Tsugawa, A. Saito, Y. Otsuka, M. Nishioka, T. Maruyama, H. Kato, T. Nagatsuma, K. T. Murata

EARTH PLANETS AND SPACE Vol. 63 ( 7 ) page： 875 - 879 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：TERRA SCIENTIFIC PUBL CO

All the details of ionospheric disturbances following the 2011 Tohoku Earthquake were first revealed by the high-resolution GPS total electron content observation in Japan. The initial ionospheric disturbance appeared as sudden depletions following small impulsive TEC enhancements similar to 7 minutes after the earthquake onset, near the epicenter. Then, concentric waves appeared to propagate in the radial direction with a velocity of 138-3,457 m/s. Zonally-extended enhancements of the TEC also appeared in the west of Japan. In the vicinity of the epicenter, short-period oscillations with a period of similar to 4 minutes were observed. This paper focuses on the concentric waves. The concentric pattern indicates that they had a point source. The center of these structures, termed the "ionospheric epicenter", was located about 170 km from the epicenter in the southeast direction. According to the propagation characteristics, these concentric waves could be caused by atmospheric waves classified into three types: acoustic waves generated from a propagating Rayleigh wave, acoustic waves from the ionospheric epicenter, and atmospheric gravity waves from the ionospheric epicenter. The amplitude of the concentric waves was not uniform and was dependent on the azimuth of their propagation direction, which could not be explained by previously-proposed theory.

DOI： 10.5047/eps.2011.06.035

Motion of polar cap arcs Reviewed

Hosokawa, K, J. I. Moen, K. Shiokawa, Y. Otsuka

Journal of Geophysical Research Vol. 116 page： doi:10.1029/2010JA015906 2011

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 Reviewed Open Access

Nozomu Nishitani, Tadahiko Ogawa, Yuichi Otsuka, Keisuke Hosokawa, Tomoaki Hori

EARTH PLANETS AND SPACE Vol. 63 ( 7 ) page： 891 - 896 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：TERRA SCIENTIFIC PUBL CO

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 degrees N, 143.6 degrees 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： 10.5047/eps.2011.07.003

On postmidnight low-latitude ionospheric irregularities during solar minimum: 2. C/NOFS observations and comparisons with the Equatorial Atmosphere Radar Reviewed Open Access

T. Yokoyama, R. F. Pfaff, P. A. Roddy, M. Yamamoto, Y. Otsuka

Journal of Geophysical Research: Space Physics Vol. 116 ( 11 ) 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：Blackwell Publishing Ltd

A detailed comparison between the observations of the Communication/ Navigation Outage Forecasting System (C/NOFS) satellite and the 47 MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (10.36S dip latitude) on the postmidnight irregularities is presented. The zonal and meridional E × B drift velocities measured by the vector electric field instrument on the C/NOFS are consistent with the westward propagation of backscatter echoes and the line-of-sight Doppler velocities observed with the EAR, respectively. The plasma density depletions are observed in the postmidnight sector for several consecutive orbits, which suggests the depletions grow slowly during the premidnight period and reach the spacecraft altitude around local midnight. The convergence of the equatorward wind which could be responsible for the midnight temperature maximum may produce a preferable condition for the growth of the Rayleigh-Taylor instability around midnight. Electric field fluctuations of medium-scale traveling ionospheric disturbances may play an important role in seeding the instability. Both equatorial and midlatitude-type plasma instabilities could be operational at the EAR latitude sector, which together would foster a high occurrence of postmidnight irregularities during solar minimum. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2011JA016798

On postmidnight low-latitude ionospheric irregularities during solar minimum: 1. Equatorial Atmosphere Radar and GPS-TEC observations in Indonesia Reviewed

T. Yokoyama, M. Yamamoto, Y. Otsuka, M. Nishioka, T. Tsugawa, S. Watanabe, R. F. Pfaff

Journal of Geophysical Research: Space Physics Vol. 116 ( 11 ) 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：Blackwell Publishing Ltd

Using the 47 MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (10.36S dip latitude), it is shown that postmidnight irregularities during solar minimum are morphologically different from those detected during solar maximum and are quite similar to those observed with the middle and upper atmosphere (MU) radar in midlatitudes (29.3N dip latitude). Utilizing the rapid beam-steering capability of the EAR, the spatial structure of the postmidnight irregularities is clearly presented for the first time. It is found that they usually propagate westward and can be categorized into two types. One shows sharp upwelling plumes near local midnight, which should not be a mere passage of fossil plasma bubbles. The other has successive tilted structures which have the same orientation as medium-scale traveling ionospheric disturbances typically observed at midlatitudes. We suggest that the convergence of the equatorward thermospheric wind which is believed to be responsible for the midnight temperature maximum may be an important factor to produce a preferable condition for the upwelling plumes in the postmidnight sector. The displacement between geographic and magnetic equators may also be important for seasonal/longitudinal variation of the postmidnight irregularities. © 2011 by the American Geophysical Union.

DOI： 10.1029/2011JA016797

Numerical simulations of atmospheric waves excited by the 2011 off the Pacific coast of Tohoku Earthquake Reviewed Open Access

M. Matsumura, A. Saito, T. Iyemori, H. Shinagawa, T. Tsugawa, Y. Otsuka, M. Nishioka, C. H. Chen

EARTH PLANETS AND SPACE Vol. 63 ( 7 ) page： 885 - 889 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER HEIDELBERG

Numerical simulations are performed to simulate atmospheric perturbations observed at ionospheric heights just after the 2011 off the Pacific coast of Tohoku Earthquake. A time-dependent, two-dimensional, nonlinear, non-hydrostatic, compressible and neutral, numerical model is developed to reproduce the atmospheric perturbations. An impulsive upward surface motion is assumed as the source of the perturbations. Simulated atmospheric perturbations at 300-km altitude show remarkable agreement with oscillations observed in the ionospheric total electron content (TEC) when the source width is about 250 km. In the vicinity of the source, the acoustic resonance modes between the ground surface and the lower thermosphere are dominant. They have three dominant frequencies for the interval between 20 and 60 min after the impulsive input. The perturbation with the maximum amplitude has a frequency of 4.4 mHz. The other dominant modes have frequencies of 3.6 and 5.1 mHz. The beats between the dominant modes are also seen. In the distance, the gravity modes are dominant. The horizontal phase velocities are about 220 to 300 m/s, and the horizontal wavelengths are about 200 to 400 km. The good agreement between the simulation and the observations indicates that ionospheric oscillations generated by the earthquake are mainly due to the motion of the neutral atmosphere.

DOI： 10.5047/eps.2011.07.015

New aspects of thermospheric and ionospheric storms revealed by CHAMP Reviewed Open Access

N. Balan, M. Yamamoto, J. Y. Liu, Y. Otsuka, H. Liu, H. Lühr

Journal of Geophysical Research: Space Physics Vol. 116 ( 7 ) 2011

Language：English Publishing type：Research paper (scientific journal) Publisher：Blackwell Publishing Ltd

The neutral mass density N and electron density Ne at 400 km height measured by CHAMP during nine intense geomagnetic storms bring out some new aspects of the thermospheric and ionospheric storms. The thermospheric storms (increase of N) develop with the onset of the main phases (MP) of the geomagnetic storms and reach their peak phases before or by the end of the MPs. The ionospheric storms (change of Ne) in general undergo an initial negative phase (with the equatorial ionization anomaly (EIA) crests shifting poleward) before turning positive, and the positive storms reach their peak strengths (or phases) centered at 25-30 magnetic latitudes
in some (4) cases the positive storms develop without an initial negative phase and with the EIA crests shifting equatorward
in all cases the positive storms reach their peak phases before the end of the MPs and turn to conventional negative storms by the end of the MPs. The observations agree with the different aspects of a physical mechanism of the positive storms. The observations also reveal that the Halloween storms of 30 October 2003 with a short MP without fluctuations produced the strongest positive ionospheric storms through impulsive response, and there is strong equinoctial asymmetry in the ionosphere and thermosphere during geomagnetic storms. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA016399