Faculty Profiles - OTSUKA, Yuichi

Papers - OTSUKA, Yuichi

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Medium-scale traveling ionospheric disturbances detected with dense and wide TEC maps over North America Reviewed Open Access

T. Tsugawa, Y. Otsuka, A. J. Coster, A. Saito

GEOPHYSICAL RESEARCH LETTERS Vol. 34 ( 22 ) 2007.11

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

Nighttime and daytime medium- scale traveling ionospheric disturbances ( MSTIDs) are detected with dense and wide detrended total electron content ( TEC) maps over North America using multiple GPS receiver networks. The TEC maps cover a wide region of 60-130 degrees W and 24 - 54 degrees N (30-65 degrees N in geomagnetic latitude), and have a spatial resolution of 1.05 degrees x1.05 degrees in latitude and longitude (0.15 degrees x0.15 degrees with 7x7 pixel smoothing) and a temporal resolution of 30 seconds. The TEC maps reveal, for the first time, that the nighttime MSTIDs propagate southwestward with 200 - 500 km wavelengths over North America and have wavefronts longer than similar to 2,000 km. We also observe that daytime MSTIDs with 300 - 1,000 km wavelengths propagate southeastward until mid-afternoon and southwestward in the late afternoon. In the mid-to-late afternoon, these MSTIDs propagating in the different directions are superimposed. The TEC maps can be a new powerful tool to investigate the MSTIDs.

DOI： 10.1029/2007GL031663

Summer-winter hemispheric asymmetry of the sudden increase in ionospheric total electron content and of the O/N2 ratio: Solar activity dependence Reviewed Open Access

T. Tsugawa, S. R. Zhang, A. J. Coster, Y. Otsuka, J. Sato, A. Saito, Y. Zhang, L. J. Paxton

Journal of Geophysical Research: Space Physics Vol. 112 ( A8 ) 2007.8

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

The solar activity dependence of the summer-winter hemispheric asymmetry (SWHA) of the sudden increase in total electron content (SITEC) due to solar flares and of the O/N2 ratio is statistically analyzed using global GPS-total electron content data and TIMED Global Ultraviolet Imager column O/N2 ratio data. We focus on observations with nonnegligible residuals of the solar zenith angle (SZA) dependency of SITEC. We examined 109 SITEC events associated with flares larger than M5 X-ray class flare from 2000 through 2006 and compared the residual SITEC (δ) to the O/N2 ratio. We observed that the latitude gradient of δ has not only an annual variation but also a year-to-year variation that is similar to those of the O/N2 ratio. The SWHA magnitude (defined as the annual maximum of latitude gradient) of both δ and O/N2 decreases as the solar activity declines toward its minimum. The correlation coefficient between the annual SWHA magnitudes of δ and those of O/N2 is 0.92, indicating strongly that the SWHA of O/N2 is responsible for that of SITEC in both the annual and year-to-year variations. The X-ray classes of the solar flares have no clear correlation with the solar activity, F10.7 index. We observe that the SWHA magnitude of δ does not depend on the magnitude of solar flare but rather on the background solar activity through the SWHA magnitude of the O/N2. Copyright 2007 by the American Geophysical Union.

DOI： 10.1029/2007JA012415

Low-latitude total electron content enhancement at low geomagnetic activity observed over Japan Reviewed Open Access

Ivan Kutiev, Yuichi Otsuka, Akinori Saito, Takuya Tsugawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 112 ( A7 ) 2007.7

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

[1] Numerous total electron content (TEC) values derived from GPS signals are averaged within 1.5 degrees x 1.5 degrees cells in a 1-hour time frame, and the relative deviations of these average values from corresponding monthly medians are used to produce latitude-time plots over Japan. The paper analyzes the appearance and development of enhancements of TEC of equatorial origin (ETEs), occurring outside initial and main phases of geomagnetic storms. ETE structures appear mainly as single-crest structures in the evening hours local time, with TEC peak around 1900 LT. TEC usually decreases with latitude, and the structures disappear below 40 degrees N. In some cases the TEC peak is found above the plot boundary of 24 degrees N, as depletions toward the equator are also observed. The observed enhanced structures are linked to the well-known evening prereversal enhancement of ion drift in the equatorial F region. Double-crest ETEs are also observed, with the second peak occurring in early morning hours. Most of the ETE events appear in periods of low geomagnetic activity, 1-3 days after the main phase of the storms. In some cases the time of rising of ETE structures coincides with the increase of interplanetary electric field (IEF), a fact showing the importance of directly penetrating electric field in formation of ETEs. Often, ETEs appear repeatedly in 2 or 3 consecutive days. It is supposed that planetary atmospheric waves are responsible for this phenomenon. Most of the observed features of ETEs can be explained by the published results of simulations of the coupled thermosphere-ionosphere-plasmasphere (CTIP) model and the thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). It is suggested that ETE structures are produced mainly by a disturbance winds dynamo electric field, built up after the main phase of the storms. Some ETE events, appearing at the end of a prolonged period of low geomagnetic activity, can be linked to directly penetrating IEF in equatorial ionosphere. The intensity of these electric fields is much smaller than is needed to explain the observations.

DOI： 10.1029/2007JA012385

Simultaneous observations of nighttime medium-scale traveling ionospheric disturbances and E region field-aligned irregularities at midlatitude Reviewed Open Access

Y. Otsuka, F. Onoma, K. Shiokawa, T. Ogawa, M. Yamamoto, S. Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 112 ( A6 ) 2007.6

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

[1] We report on simultaneous observations of very high frequency ( VHF) radar backscatter from field-aligned irregularities (FAI) in the E region and medium-scale traveling ionospheric disturbances (MSTID) in 630-nm airglow images. On the night of 6 August 2002, the powerful 46.5-MHz middle and upper atmosphere ( MU) radar with five beam directions at Shigaraki, Japan, observed typical quasiperiodic (QP) echoes from FAI in the E region. During this QP echo event, an all-sky charge-coupled device imager at the MU radar site detected MSTID in 630-nm airglow images. The MSTID also propagated southwestward at 80 m/s with fluctuation amplitudes of 50% from the background intensity and a wavelength of 380 km. Period of the MSTID was almost same as that of the Doppler velocity in the QP echoes. The airglow enhancement ( depletion) caused by the MSTID coincided with southeastward ( northwestward) velocity in the QP echo. These results indicate that the electric fields associated with the F region MSTID could be closely coupled to those associated with QP echoes in the E region.

DOI： 10.1029/2005JA011548

Ground observation and AMIE-TIEGCM modeling of a storm-time traveling ionospheric disturbance Reviewed Open Access

Shiokawa, K, Lu, G, Otsuka, Y, Ogawa, T, Yamamoto, M, Nishitani, N, Sato, N

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 112 ( A5 ) 2007.5

Simultaneous ground and satellite observations of an isolated proton arc at subauroral latitudes Reviewed Open Access

K. Sakaguchi, K. Shiokawa, A. Ieda, Y. Miyoshi, Y. Otsuka, T. Ogawa, M. Connors, E. F. Donovan, F. J. Rich

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 112 ( A4 ) 2007.4

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

[1] We observed an isolated proton arc at the Athabasca station (MLAT: 62 degrees N) in Canada on 5 September 2005, using a ground-based all-sky imager at wavelengths of 557.7 nm, 630.0 nm, and 486.1 nm (H beta). This arc is similar to the detached proton arc recently observed by the IMAGE satellite [Immel et al., 2002]. The arc appeared at 0500-0640 UT (2100-2240 MLT), coincident with strong Pc 1 geomagnetic pulsations in the frequency range of the electromagnetic ion cyclotron (EMIC) wave. The isolated arc did not change its structure and intensity during the late growth and expansive phases of a small substorm that occurred at 0550 UT. From particle data obtained by the NOAA 17 satellite, we found that the isolated arc was associated with the localized enhancement of ion precipitation fluxes at an energy range of 30-80 keV at L similar to 4. Trapped ion flux enhancements ( ring current ions) were also observed at two latitudinally separated regions. The localized ion precipitation was located at the outer boundary of the inner ring current ions. The DMSP F13 satellite observed signatures of an ionospheric plasma trough near the conjugate point of the arc in the Southern Hemisphere. The trough is considered to be connected to the plasmapause. These results indicate that the source region of the isolated arc was located near the plasmapause and in the ring current. We conclude that the observed isolated proton arc at subauroral latitudes was caused by the EMIC waves, which were generated near the plasmapause and resonantly scattered the ring current protons into the loss cone.

DOI： 10.1029/2006JA012135

An intense gravity wave near the mesopause region observed by a Fabry-Perot interferometer and an airglow imager Reviewed Open Access

K. Shiokawa, S. Suzuki, Y. Otsuka, T. Ogawa, T. Nakamura, T. Horinouchi

JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Vol. 112 ( D7 ) 2007.4

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

[1] We report an intense gravity wave event observed at Shigaraki, Japan, at 1500 - 2000 UT (0000 - 0500 LT) on 22 December 2001 using a Fabry-Perot interferometer, which measures the Doppler shift of nocturnal airglow emission at a wavelength of 557.7 nm (emission altitude: 90 - 100 km). The wave had a period of 1.5 hours and a peak-to-peak amplitude of 68 m/s in the horizontal wind velocity. The hodograph of the east-west and north-south wind oscillations showed a polarization direction of the wave of ENE-WSW. An all-sky airglow imager at Shigaraki also recorded a similar southwestward moving wave with a period of similar to 1.5 hours and a velocity of 146 m/s in the 557.7-nm airglow images. The horizontal wavelength was estimated to be 790 km from the images. These facts indicate that the wave was generated in the northeast of Japan, where a well-developed low-pressure cell (968 hPa) and a distortion of the jet stream were observed in the troposphere. However, the large horizontal wavelength, the fast phase velocity, and the possible source location apart from the exit of the jet streak are different from those predicted from previous numerical simulations.

DOI： 10.1029/2006JD007385

Medium-scale traveling ionospheric disturbances observed by GPS receiver network in Japan: a short review Reviewed

Takuya Tsugawa, Nobuki Kotake, Yuichi Otsuka, Akinori Saito

GPS SOLUTIONS Vol. 11 ( 2 ) page： 139 - 144 2007.3

Language：English Publisher：SPRINGER HEIDELBERG

Medium-scale traveling ionospheric disturbances (MSTID) are wave-like perturbations of the ionospheric plasma with wavelengths of several hundred kilometres and velocities of several hundred metres per second. MSTID is one of the most common ionospheric phenomena that generally induce the perturbations of ionospheric total electron content (TEC) by similar to 10(16) electron/m(2), which corresponds to -54 ns (16.2 cm) delay at GPS L1 signal. In the past decade, several new characteristics on MSTIDs have been revealed by the TEC observations using the dense GPS receiver network in Japan. In this paper, we provide a short review of these new observations and summarize the morphological characteristics of MSTIDs in Japan.

DOI： 10.1007/s10291-006-0045-5

A concentric gravity wave structure in the mesospheric airglow images Reviewed

S. Suzuki, K. Shiokawa, Y. Otsuka, T. Ogawa, K. Nakamura, T. Nakamura

JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Vol. 112 ( D2 ) 2007.1

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

A concentric gravity wave pattern was observed by an all-sky airglow imager located at Shigaraki, Japan (34.9 degrees N, 136.1 degrees E) on 3 October 2002. The wave pattern was identified over 2.5 hours (1330-1600 UT or 2230-0100 LT) both in the OI (557.7 nm) and OH (720-910 nm) band images. It had a wavelength of about 35 km and propagated concentrically northeastward with a horizontal phase velocity of about 80 m/s. The wave had a short period of about 7 min and a momentum flux of 2.2 m(2)/s(2). We identified the source of the waves as a cumulonimbus in the satellite cloud images and in radar precipitation images near the center of the concentric pattern. We conclude that this rare wave pattern was caused by a weak wind system that extended from the stratosphere to the mesopause region in an autumn transition.

DOI： 10.1029/2005JD006558

Simultaneous observations of nighttime medium-scale traveling ionospheric disturbances and E-region field-aligned irregularities at midlatitude Reviewed

Otsuka, Y., F. Onoma, K. Shiokawa, T. Ogawa, M. Yamamoto, and S. Fukao

J. Geophys. Res page： doi:10.1029/2005JA011548 2007

Statistical study of medium-scale traveling ionospheric disturbances observed with the GPS networks in Southern California Reviewed Open Access

Nobuki Kotake, Yuichi Otsuka, Tadahiko Ogawa, Takuya Tsugawa, Akinori Saito

EARTH PLANETS AND SPACE Vol. 59 ( 2 ) page： 95 - 102 2007

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

Using global positioning system (GPS) data taken from 350 dual-frequency GPS receivers in Southern California in 2002, we investigated two-dimensional maps of total electron content (TEC) perturbations with a time resolution of 30 s and a spatial resolution of 0.15 degrees x0.15 degrees in longitude and latitude to reveal statistical characteristics of medium-scale traveling ionospheric disturbances (MSTIDs). We found that MSTIDs can be categorized into three types. One type is daytime MSTIDs, which frequently occur in winter and equinoxes. Since most of the daytime MSTIDs propagated southeastward, we speculate that the daytime MSTIDs could be caused by atmospheric gravity waves in the thermosphere. A second type is nighttime MSTIDs, which frequently occur in summer. Nighttime MSTIDs propagate southwestward. This propagation direction is consistent with the idea that polarization electric fields could play an important role in generating nighttime MSTIDs. The third is dusk MSTIDs, which frequently occur in summer and propagate northwestward. Dusk MSTIDs could be caused by gravity waves originating from the sunset terminator because they have wavefronts almost parallel to the sunset terminator.

DOI： 10.1186/BF03352681

The ionospheric response in the Brazilian sector during the super geomagnetic storm on 20 November 2003 Reviewed Open Access

F. Becker-Guedes, Y. Sahai, P. R. Fagundes, E. S. Espinoza, V. G. Pillat, W. L. C. Lima, Su. Basu, Sa. Basu, Y. Otsuka, K. Shiokawa, E. M. MacKenzie, X. Pi, J. A. Bittencourt

ANNALES GEOPHYSICAE Vol. 25 ( 4 ) page： 863 - 873 2007

Language：English Publishing type：Research paper (scientific journal) Publisher：COPERNICUS GESELLSCHAFT MBH

A very intense geomagnetic storm (superstorm) began with storm sudden commencement (SSC) at 08:03 UT on 20 November 2003, as a result of the coronal mass ejection (CME) by sunspot 484 hurled into space on 18 November 2003. The geomagnetic storm attained vertical bar D-st vertical bar(max)=472 nT at 20:00 UT (20 November). In this paper we present the simultaneous ionospheric sounding observations, using the Canadian Advanced Digital Ionosondes (CADIs), carried out from Palmas (PAL; 10.2 degrees S, 48.2 degrees W; dip latitude 5.5 degrees S; a near equatorial station) and Sao Jose dos Campos (SJC; 23.2 degrees S. 45.9 degrees W; dip latitude 17.6 degrees S; station located under the crest of equatorial ionospheric anomaly), Brazil. In addition, total electron content (TEC) measurements from several GPS receiving stations in the Brazilian sector during this storm are presented. The simultaneous ionospheric sounding observations carried out at SJC and PAL, and TEC observations on 3 consecutive days viz., 19 November (quiet), 20 November (disturbed) and 21 November (recovery phase) are presented. Salient features from the ionospheric observations in the Brazilian sector during the superstorm are discussed. The difference in the observed ionospheric response at the two stations (PAL and SJC) is considerable. This is not surprising given that PAL is close to the magnetic equator and SJC is near the crest of the equatorial ionospheric anomaly (EIA). It should be pointed out that soon after the SSC (about 4 h later), the F-region critical frequency (foF2), the F-region peak height (hpF2), and variations of virtual heights at different frequencies (iso-frequency plots) all show wavelike oscillations of the F-region during daytime at both the ionospheric sounding stations. Unusual rapid uplifting of F-region at PAL was observed during both the main and recovery phases of the storm.

DOI： 10.5194/angeo-25-863-2007

Development of airglow temperature photometers with cooled-CCD detectors Reviewed Open Access

K. Shiokawa, Y. Otsuka, S. Suzuki, T. Katoh, Y. Katohl, M. Satoh, T. Ogawa, H. Takahashi, D. Gobbli, T. Nakamura, B. P. Williams, C.-Y. She, M. Taguchi, T. Shimomai

EARTH PLANETS AND SPACE Vol. 59 ( 6 ) page： 585 - 599 2007

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

We have developed three airglow temperature photometers with cooled-CCD detectors. The photometers measure rotational temperatures using the airglow emissions of OH and 02 near the mesopause region (altitude: 80-100 km). The photometers also measure six other airglow and auroral lines at wavelengths of 55.7, 630.0, 777.4, 589.3, 427.8, and 486.1 nm. The CCD detectors are used to distinguish the emission lines in these airglow bands, similarly to those used by the Spectral Airglow Temperature Imagers (SATI). In this paper, we describe the configuration of the photometers, their calibration, the data processing to extract rotational temperatures and emission intensities from the measured airglow spectra, as well as the initial deployment at Platteville, Colorado (40.2 degrees N, 255 degrees E), when their observations were compared with the concurrent and nearly collocated observations by a sodium lidar. We obtain a good correlation and some systematic difference of temperatures from the photometers and the lidar, and discuss possible causes of the temperature difference.

DOI： 10.1186/BF03352721

Response of nighttime equatorial and low latitude F-region to the geomagnetic storm of August 18, 2003, in the Brazilian sector Reviewed

Y. Sahai, F. Becker-Guedes, P. R. Fagundes, W. L. C. Lima, Y. Otsuka, C. -S. Huang, E. S. Espinoza, X. Pi, A. J. de Abreu, M. J. A. Bolzan, V. G. Pillat, J. R. Abalde, A. A. Pimenta, J. A. Bittencourt

ADVANCES IN SPACE RESEARCH Vol. 39 ( 8 ) page： 1325 - 1334 2007

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

This paper presents an investigation of geomagnetic storm effects in the equatorial and low latitude F-region in the Brazilian sector during the intense geomagnetic storm on 18 August, 2003 (SSC 14:21 UT on 17/08; Sigma Kp = 52+; Ap = 108; vertical bar Dst vertical bar(max) = 168 at 1600 UT on 18/08). Simultaneous ionospheric sounding measurements from two stations, viz., Palmas (10.2 degrees S, 48.2 degrees W; dip latitude 5.7 degrees S) and Sao Jose dos Campos (23.2 degrees S, 45.9 degrees W; dip latitude 17.6 degrees S), Brazil, are presented for the nights of 16-17, 17-18 and 18-19 August, 2003 (quiet, disturbed and recovery phases). Both stations are equipped with the Canadian Advanced Digital Ionosonde (CADI). Quiet and disturbed conditions of the F-region ionosphere are compared using data collected from the two stations. The relationship between magnetospheric disturbance and low-latitude ionospheric dynamics, and generation of ionospheric irregularities are discussed. On the disturbed nights (17-18 and 18-19 August), the low latitude station S. J. Campos showed strong enhancements in the F-region critical frequency (foF2), whereas the near equatorial station Palmas showed strong uplifting of the F-layer about 1 h earlier. Normally during the June solstice months (May-August) in the Brazilian sector, large-scale ionospheric irregularities in form of plasma bubbles are rarely observed. On the night of 17-18 August, ionsospheric sounding observations at Palmas showed the presence of bottomside spread-F, whereas on the night of 18-19 August, the observations at Palmas and S. J. Campos showed the presence of plasma bubbles when the storm recovery phase had just started. The complementary GPS data available from several stations in the "Rede Brasileira de MonitoramentoContinuo de GPS (Brazilian Network for Continuous GPSMonitoring)" are used to obtain the vertical total electron content (VTEC) and the rate of change of TEC per minute on UT days 18 and 19 August, 2003 and presented. Also, several global ionospheric TEC maps from the worldwide network of GPS receivers are presented, showing widespread latitudinal and longitudinal TEC changes during the different phases of the storm. All the observations (local ionospheric sounding and GPS network measurements, and global GPS measurements) presented in this investigation related to the geomagnetic storm on 18 August indicate that the equatorial and low latitude region in the Brazilian sector had much stronger effect during the recovery phase compared with the main phase. A comparison of the observed disturbance drifts with the Fejer-Scherliess storm-time model drifts indicate that the modeled drifts are not consistent with the present observations. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.asr.2007.02.064

Gravity wave momentum flux in the upper mesosphere derived from OH airglow imaging measurements Reviewed Open Access

Shin Suzuki, Kazuo Shiokawa, Yuichi Otsuka, Tadahiko Ogawa, Minoru Kubota, Masaki Tsutsumi, Takuji Nakamura, David C. Fritts

EARTH PLANETS AND SPACE Vol. 59 ( 5 ) page： 421 - 428 2007

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

We report procedures to identify small-scale (20-100 km) atmospheric gravity waves from OH airglow images to estimate momentum fluxes carried by the waves. We also deduce contamination of background continuum emission in OH image, by comparing a simultaneous observation of OH lines measured by the Spectral Airglow Temperature Imager (SATI). We applied the procedures to a one-night dataset obtained at Shigaraki, Japan (34.9 degrees N, 136.1 degrees E) on November 19, 1999. The background wind, which is essential for deriving the intrinsic parameters of gravity waves, was measured by the Middle and Upper Atmosphere (MU) radar. Contamination of background continuum emission with the OH filter was deduced to be 30%. From these procedures, we found that the gravity waves identified in the OH images were mainly propagating southward or southeastward with horizontal wavelengths of 60-90 km and apparent phase speeds of 40-80 m/s. The estimated momentum fluxes on this night was 1-15 m(2) s(-2), with an average of 4.9 m(2) s(-2).

DOI： 10.1186/BF03352703

Summer-winter hemispheric asymmetry of sudden increase in ionospheric total electron content induced by solar flares: A role of O/N<sub>2</sub> ratio Reviewed Open Access

T. Tsugawa, T. Sadakane, J. Sato, Y. Otsuka, T. Ogawa, K. Shiokawa, A. Saito

Journal of Geophysical Research A: Space Physics Vol. 111 ( A11 ) 2006.11

A fast-propagating, large-scale atmospheric gravity wave observed in the WAVE2004 campaign Reviewed

Minoru Kubota, Seiji Kawamura, Makoto Abo, Yoshiko Koizumi, Yasuhiro Murayama, Miho Yamamori, Kazuo Shiokawa, Yuichi Otsuka, Michihiro Uchiumi, Kiyoshi Igarashi, Takumi Abe, Koh-Ichiro Oyama, Naomoto Iwagami

JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Vol. 111 ( D21 ) 2006.11

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

The Waves in Airglow Campaign in 2004 (WAVE2004), which aimed to elucidate the formation process of waves in airglow structures from both dynamical and chemical perspectives, was conducted using rocket-borne and ground-based instruments in Japan on 17 January 2004. In this experiment, we observed a large-scale atmospheric gravity wave (AGW), which appeared in both the vertical profiles of sodium density obtained by a Na lidar and the horizontal distributions of airglow emission obtained by an all-sky imager (ASI). Vertical propagation of the AGW accompanied by a shortening of its vertical wavelength was clearly visualized using the Na lidar data. The horizontal wavelength, horizontal phase velocity, period, and propagation direction of the AGW were estimated from the ASI data as 673-774 km, 107-122 m/s, similar to 1.75 hours, and north-northeastward, respectively. Using these parameters and the MF radar wind, vertical wavelengths of the wave were calculated from the dispersion relation of gravity waves. The calculated vertical wavelengths were comparable at altitudes of 85.5 km and 93.25 km to those estimated from the variation of the sodium density. Using a simple ray tracing technique, the AGW was traced back to the southern edge of the distorted jet stream near tropopause. This result strongly suggests that an unstable baroclinic wave associated with ageostrophic motions in the jet stream was the wave source of the large-scale AGW observed in the WAVE2004.

DOI： 10.1029/2005JD006788

Detection of ruptures of Andaman fault segments in the 2004 great Sumatra earthquake with coseismic ionospheric disturbances Reviewed Open Access

Kosuke Heki, Yuichi Otsuka, Nithiwatthn Choosakul, Narong Hemmakorn, Tharadol Komolmis, Takashi Maruyama

Journal of Geophysical Research: Solid Earth Vol. 111 ( B9 ) 2006.9

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

We Near-field coseismic perturbations of ionospheric total electron content (TEC), caused by direct acoustic waves from focal regions, can be observed with Global Positioning System (GPS). They appear 10-15 min after the earthquake with typical periods of ∼4-5 min and propagate as fast as ∼1 km/s toward directions allowed by ambient geomagnetic fields. Ionospheric disturbance, associated with the 2004 December 26 great Sumatra-Andaman earthquake, was recorded with nine continuous GPS receiving stations in Indonesia and Thailand. Here we explore the possibility to constrain the rupture process of the earthquake with the observed ionospheric disturbances. We assumed linearly distributed point sources along the zone of coseismic uplift extending ∼1300 km from Sumatra to the Andaman Islands that excited acoustic waves sequentially as the rupture propagate northward by 2.5 km/s. TEC variations for several satellite-receiver pairs were synthesized by simulating the propagation of acoustic waves from the ground to the ionosphere and by integrating the TEC perturbations at intersections of line of sights and the ray paths. The TEC perturbations from individual point sources were combined using realistic ratios, and the total disturbances were compared with the observed signals. Prescribed ratios based on geodeticatly inferred coseismic uplifts reproduced the observed signals fairly well. Similar calculation using a rupture propagation speed of 1.7 km/s degraded the fit. Suppression of acoustic waves from the segments north of the Nicobar Islands also resulted in a poor fit, which suggests that ruptures in the northern half of the fault were slow enough to be overlooked in short-period seismograms but fast enough to excite atmospheric acoustic waves. Coseismic ionospheric disturbance could serve as a new indicator of faulting sensitive to ruptures with timescale up to 4-5 min. Copyright 2006 by the American Geophysical Union.

DOI： 10.1029/2005JB004202

Estimating drift velocity of polar cap patches with all-sky airglow imager at Resolute Bay, Canada Reviewed

K. Hosokawa, K. Shiokawa, Y. Otsuka, A. Nakajima, T. Ogawa, J. D. Kelly

GEOPHYSICAL RESEARCH LETTERS Vol. 33 ( 15 ) 2006.8

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

Highly sensitive all-sky airglow imager has been operative at Resolute Bay, Canada (74.73 degrees N, 265.07 degrees E; AACGM latitude 82.9 degrees) since January 2005. We present, as a first result from the imager, an event of polar cap patches drifting anti-sunward during the southward IMF conditions. Magnitude and direction of patch drift velocities are computed with a temporal resolution of 2 min by using the newly developed patch-tracking algorithm based on 2D cross correlation analysis. It is well visualized that the patches change their moving speed and direction drastically in a short time scale ( a few minutes). Speed of the patch is primally controlled by the IMF Bz. Dawn-dusk component of the patch drift velocities is well correlated with the IMF By in agreement with published By dependence of the nightside polar cap convection. However, response of the patch drift direction to the IMF By is found to be much slower ( approximate to 20 min) than that of the drift speed to the IMF Bz ( almost instantaneous).

DOI： 10.1029/2006GL026916

A multi-instrument measurement of a mesospheric front-like structure at the equator Reviewed Open Access

Kazuo Shiokawa, Shin Suzuki, Yuichi Otsuka, Tadahiko Ogawa, Takuji Nakamura, Martin G. Mlynczak, James M. Russell

JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN Vol. 84A page： 305 - 316 2006.7

Language：English Publishing type：Research paper (scientific journal) Publisher：METEOROLOGICAL SOC JAPAN

We have made a comprehensive measurement of a front-like structure in the mesosphere at the equator at Kototabang, Indonesia (0.2 degrees S, 100.3 degrees E), using an airglow imager, an airglow temperature photometer, a meteor radar, and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. The event was detected in airglow images of both OH-band (peak emission altitude: 87 km) and 557.7-nm (96-km) emissions, as an east-west front-like structure propagating northward with a velocity of 52-58 m/s. Wave trains with a horizontal wavelength of 30-70 km were observed after the passage of the front. The airglow intensity decreased for all the mesospheric emissions of OI (557.7 nm), OH-band, O-2-band (altitude: 94 km), and Na (589.3 nm) (90 km) after the front passage. The rotational temperatures of both the OH-band and the O-2-band also decreased similar to 10 K. An intense shear in northward wind velocity of 80 m/s/6 km was observed at altitudes of 84-90 km by the meteor radar. The kinetic temperature profile at altitudes of 20-120 km was observed near Kototabang by TIMED/SABER. The front-like structure and trailing waves were similar to those of the mesospheric tidal bore. However, we found that the ducting condition, which is necessary to form a bore, was not satisfied for the observed wave parameters. We speculate that the intense wind shear may play some role for the generation of the front-like structure.

DOI： 10.2151/jmsj.84A.305

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