Papers - OTSUKA, Yuichi

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

　More details

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

Web of Science

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

　More details

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

Open Access

Web of Science

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

　More details

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

Open Access

Web of Science

Summer-winter hemispheric asymmetry of sudden increase in ionospheric total electron content induced by solar flares: A role of O/N₂ 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

　More details

Language：English  

DOI： 10.1029/2006JA011951

Web of Science

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

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

　More details

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

Open Access

Web of Science

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

　More details

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

Open Access

Web of Science

Scopus

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

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

GEOPHYSICAL RESEARCH LETTERS   Vol. 33 ( 15 )   2006.8

　More details

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

Open Access

Web of Science

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

　More details

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

Web of Science

Equatorial ionospheric scintillations and zonal irregularity drifts observed with closely-spaced GPS receivers in Indonesia Reviewed Open Access

Yuichi Otsuka, Kazuo Shiokawa, Tadahiko Ogawa

JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN   Vol. 84A   page： 343 - 351   2006.7

　More details

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

To investigate drift velocities of a few hundred meter-scale irregularities associated with equatorial plasma bubbles, we used three single-frequency GPS receivers at the Equatorial Atmosphere Radar (EAR) site at Kototabang, Indonesia (0.20 degrees S, 100.32 degrees E; geomagnetic latitude 10.6 degrees S), since January 2003. The GPS receivers sampled GPS signal intensity at a rate of 20 Hz. Distances between the receivers were 116, 127, and 152 m. An analysis of scintillation index (S-4) in two years (2003-2004) revealed that the scintillations often occurred between 2000-0100 LT at equinoxes and that their occurrence rate was higher during March-April than during September-October. Drift velocities of irregularities were measured using cross-correlation analysis with the time series of the GPS signal intensity obtained from the three receivers. From a statistical analysis of the drift velocities, the eastward component of drift velocity just after sunset is found to be greater during March-April than during September-October. Based on these results, for the first time, we suggest that the east-west component of plasma drift velocity (or vertical electric field) may be related to the evolution of plasma irregularities causing scintillations throughout the mechanism causing the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocity could be attributed to the equinoctial asymmetry of neutral winds in the thermosphere.

DOI： 10.2151/jmsj.84A.343

Web of Science

Ionospheric disturbances over Indonesia and their possible association with atmospheric gravity waves from the troposphere Reviewed Open Access

Tadahiko Ogawa, Yuichi Otsuka, Kazuo Shiokawa, Akinori Saito, Michi Nishioka

JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN   Vol. 84A   page： 327 - 342   2006.7

　More details

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

As part of the CPEA (Coupling Processes in the Equatorial Atmosphere) project, we have been conducting ground-based optical and radio observations of the ionosphere and thermosphere at Kototabang in Indonesia, Japan, and Australia. First, this paper gives a brief overview of some results, paying special attention to 100-1000 km scale plasma disturbances in the F region ionosphere, i.e., medium-scale traveling ionospheric disturbances (MSTIDs) and plasma bubbles. MSTIDs over the equator are observed within and in the south of the F region equatorial anomaly crest and have, on the average, a phase velocity of 300 m s(-1) toward the south, a period of 40 min, and a wavelength of 700 km. Plasma bubbles move to the east at similar to 100 m s(-1), have a scale of about 100 km with spacings of 200-250 km, and are embedded within plasma structures with a scale of about 1000 km. Interestingly, giant plasma bubbles can be simultaneously detected at lower midlatitudes in southern and northern hemispheres that are connected by the geomagnetic field line, and are very identical in appearance in the both hemispheres. Then, we present newly-obtained characteristics of equatorial ionospheric scintillations of 1.6-GHz GPS radio waves associated with plasma bubbles. Continuous scintillation observations for two years at Kototabang indicate that the scintillations appear predominantly from sunset to midnight in equinoctial months. Such a seasonal variation is also recognized from a statistical study of bubble occurrences over the Philippines, Singapore, and Indonesia. To investigate possible dynamical coupling between the ionosphere/thermosphere and troposphere over the equator, we compare the scintillation (bubble) activity and Earth's brightness temperature variation over the Indian Ocean measured by meteorological satellites. The results indicate that there can exist meaningful correlations between the scintillation occurrence and tropospheric disturbance at 80 degrees-95 degrees E longitudes, i.e. 5 degrees-20 degrees west of Kototabang. Possible processes to seed plasma bubbles are discussed.

DOI： 10.2151/jmsj.84A.327

Web of Science

Quasiperiodic southward moving waves in 630-nm airglow images in the equatorial thermosphere Reviewed Open Access

K. Shiokawa, Y. Otsuka, T. Ogawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 111 ( A6 )   2006.6

　More details

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

We report quasiperiodic southward moving waves, which are commonly observed in the OI 630-nm airglow images (emission altitudes of 200-300 km) near the equator, in 2-year airglow observations at Kototabang, Indonesia (0.2 degrees S, 100.3 degrees E, geomagnetic latitude of -10.4 degrees). The waves have predominantly east-west phase fronts and repeatedly propagate southward with a velocity of 310 +/- 110 m/s and a period of 40 +/- 15 min. They are frequently observed in May-July with an occurrence rate of 53% and are also observed in other seasons with occurrences of similar to 20%. The waves are observed in and to the south (geomagnetically poleward) of the equatorial ionospheric anomaly, which is identified as an airglow enhancement region moving gradually to lower geomagnetic latitudes at the premidnight local times. We suggest that gravity waves in the lower thermosphere below similar to 300 km are a plausible cause of the observed quasiperiodic waves in the airglow images.

DOI： 10.1029/2005JA011406

Open Access

Web of Science

Characteristics and implications of Doppler spectra of E region quasi-periodic echoes observed by the multibeam middle and upper atmosphere radar Reviewed Open Access

Ogawa, T, Otsuka, Y, Yamamoto, M

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 111 ( A5 )   2006.5

　More details

Language：Japanese  

DOI： 10.1029/2005JA011335

Web of Science

Climatological study of GPS total electron content variations caused by medium-scale traveling ionospheric disturbances Reviewed Open Access

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

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 111 ( A4 )   2006.4

　More details

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

Using global positioning system (GPS) data taken from the International GNSS Service (IGS), we investigated total electron content (TEC) perturbations associated with medium-scale traveling ionospheric disturbances (MSTIDs). We analyzed TEC data taken from four or five GPS receivers in each of six regions ( Japan, Europe, eastern United States, western United States, Australia, and South America) in 3 years ( 1998, 2000, and 2001). To derive perturbation components of TEC (I'), we subtracted the 1-hour running average form the time sequence of TEC for each satellite-receiver pair. Standard deviation of I' within 1 hour, delta I, was calculated every hour, and MSTIDs activity were defined as delta I/(I) over bar, where (I) over bar is the 1-hour average of absolute vertical TEC. We found that MSTIDs activity during daytime is different from that during nighttime with respect to seasonal, solar activity, longitudinal, and latitudinal dependences. Daytime MSTIDs activity are high in winter in all six regions. On the other hand, seasonal variation of nighttime MSTIDs activity is coupled with its longitudinal variation. In the Japanese and Australian longitudinal sector, nighttime MSTIDs are most active near the June solstice, whereas it is most active near the December solstice in the European longitudinal sector. Nighttime MSTIDs activity at the Japanese and Australian longitudinal sector shows negative correlation with solar activity, whereas solar activity dependence is not seen in daytime MSTIDs activity. These results suggest that mechanisms causing MSTIDs could be different between daytime and nighttime.

DOI： 10.1029/2005JA011418

Open Access

Web of Science

Geomagnetic conjugate observations of large-scale traveling ionospheric disturbances using GPS networks in Japan and Australia Reviewed Open Access

T Tsugawa, K Shiokawa, Y Otsuka, T Ogawa, A Saito, M Nishioka

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 111 ( A2 )   2006.2

　More details

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

The geomagnetic conjugacy of large-scale traveling ionospheric disturbances (LSTIDs) was studied using total electron content (TEC) data derived from GPS networks in Japan and Australia. The number of simultaneous occurrences of LSTIDs within +/- 1 hour in both hemispheres was 5 out of 20 (21) events in Japan (Australia) when Kp >= 5- and 0 out of 15 (10) when Kp <= 4+. As for the LSTIDs observed simultaneously in both hemispheres, the propagation velocities of equatorward LSTID were comparable between the two hemispheres, with differences of 10-40%. The crossing times at 30 degrees geomagnetic latitude of the simultaneous LSTIDs over Japan and Australia were also different by several tens of minutes for all five events. These observational results indicate that the LSTIDs observed almost simultaneously in both hemispheres are not connected electromagnetically through the geomagnetic field but are generated by atmospheric gravity waves propagating to the equator independently in the two hemispheres.

DOI： 10.1029/2005JA011300

Open Access

Web of Science

GPS detection of total electron content variations over Indonesia and Thailand following the 26 December 2004 earthquake Reviewed Open Access

Otsuka, Y., N. Kotake, T. Tsugawa, K. Shiokawa, T. Ogawa, Effendy, S. Saito, M. Kawamura, T. Maruyama, N. Hemmakorn, and T. Komolmis

Earth, Planets and Space   Vol. 58   page： 159-165   2006

　More details

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1186/BF03353373

Open Access

Equatorial ionospheric scintillations and zonal irregularity drifts observed with closely-spaced GPS receivers in Indonesia Reviewed Open Access

Otsuka, Y., K. Shiokawa, and T. Ogawa

J. Meteor. Soc. Japan   Vol. 84A   page： 343-351   2006

　More details

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.2151/jmsj.84A.343

A Statistical Study of Ionospheric Irregularities Observed with a GPS Network in Japan Open Access

Y. Otsuka, T. Aramaki, T. Ogawa, A. Saito

Recurrent Magnetic Storms: Corotating Solar Wind Streams   Vol. 167   page： 271 - +   2006

　More details

© 2006 by the American Geophysical Union. All rights reserved. The Geographical Survey Institute of Japan has installed a network of about 1000 dual-frequency GPS receivers in Japan with their mutual distance of about 25 km. Phases and pseudo ranges of dual-frequency GPS signals are recorded every 30 s. The dense distribution of the GPS receivers allows us to reveal twodimensional structures of the ionospheric plasma density irregularities with scale sizes of the order of several kilometers. We analyzed TEC data obtained from the GPS network of Japan in 2000. It was found that the irregularity characteristics over Japan depended on latitude. The results are as follows: (1) at the northern part, the irregularities appeared only during geomagnetic storms. (2) At the middle part, they had the most frequent occurrence in summer nighttime and were usually accompanied by Medium-Scale Traveling Ionospheric Disturbances (MSTID). (3) At the southern part, they were associated with the equatorial plasma bubbles and their occurrences were highest in the equinoctial nighttime.

DOI： 10.1029/167GM21

Open Access

Web of Science

Scopus

GPS observations of post-storm TEC enhancements at low latitudes Reviewed Open Access

Ivan Kutiev, Yuichi Otsuka, Akinori Saito, Shigeto Watanabe

EARTH PLANETS AND SPACE   Vol. 58 ( 11 ) page： 1479 - 1486   2006

　More details

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

In a previous work (J. Geophys. Res., 110(A01308), 1-11, 2005), the authors developed an original approach to the processing of total electron content (TEC) data obtained by GPS signals from the Japan receiver network. This approach includes removing the diurnal and seasonal variation carried by 27-day medians and the solar rotation periodicity. The relative deviations of TEC from the median-from all measured locations at a given hour-were then approximated by a regression line along the main prolongation of the Japan islands, between latitudes 24 degrees and 45 degrees N. The two variables of the regression line, the average value at the center and the slope were obtained as a time series, and their behavior during geomagnetic storms in the period 2000-2002 were analyzed. One interesting result was the observed enhancement of TEC at the end of the recovery phase of the storms. The slope variations clearly showed that this enhancement started from the south and was interpreted as a poleward expansion of equatorial crest. In the present paper we further analyze this post-storm phenomenon, adding foF2 data from Japanese Kokubunji and Okinawa ionosondes. We also show the latitude extension of the poleward expansion by using lat/UT contour plots. The results confirm that most of the post-storm TEC enhancements are part of the equatorial crest region which extends poleward during nighttime. In some cases, the enhanced TEC structures develop by separating from the crest region. Daytime TEC enhancements were also observed. Their structures are not confined to the equatorial crests region, but occupy the whole latitude range considered in this study. TEC post-storm enhancements were generally found to be in agreement with foF2 variations.

DOI： 10.1186/BF03352647

Open Access

Web of Science

GPS detection of total electron content variations over Indonesia and Thailand following the 26 December 2004 earthquake Reviewed Open Access

Yuichi Otsuka, N. Kotake, Takuya Tsugawa, Kazuo Shiokawa, Tadahiko Ogawa, E. Effendy, Susumu Saito, Masabumi Kawamura, Takashi Maruyama, N. Hemmakorn, T. Komolmis

Earth, Planets and Space   Vol. 58 ( 2 ) page： 159 - 165   2006

　More details

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan , The Geodetic Society of Japan , The Japanese Society for Planetary Sciences  

We report the response of the ionosphere to the large earthquake that occurred in West Sumatra, Indonesia, at 0058 UT on December 26, 2004. We have analyzed Global Positioning System (GPS) data obtained at two sites in Sumatra and at three sites in Thailand to investigate total electron content (TEC) variations. Between 14 and 40 min after the earthquake, TEC enhancements of 1.6-6.9 TEC units (TECU) were observed at subionospheric points located 360-2000 km north of the epicenter. From the time delays of the observed TEC enhancements, we find that the TEC enhancements propagated northward from the epicenter. The time delays between the earthquake and rapid increases in TEC, which occurred near the epicenter, are consistent with the idea that acoustic waves generated by the earthquake propagated into the ionosphere at the speed of sound to cause the TEC variations. A small TEC enhancement of 0.6 TECU was observed south of the epicenter, while no TEC enhancements were seen east of the epicenter. From a model calculation, we find that this directivity of the TEC variations with respect to the azimuth from the epicenter could be caused partially by the directivity in the response of the electron density variation to the acoustic waves in the neutral atmosphere. © 2006, The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan, The Geodetic Society of Japan, The Japanese Society for Planetary Sciences. All rights reserved.

DOI： 10.1186/BF03353373

Open Access

Web of Science

Scopus

CiNii Research

Ionospheric variations at midlatitude detected by a dense GPS receiver array in Japan Reviewed

A. Saito, Y. Otsuka, T. Tsugawa, K. Matsunaga, K. Hoshinoo

Proceedings of the Annual Meeting - Institute of Navigation     page： 425 - 428   2005.12

　More details

Language：English   Publishing type：Research paper (international conference proceedings)  

Ionospheric variations over Japan were investigated with high spatial and temporal resolution data of GEONET, which consists of about 1,200 GPS receivers. Total Electron Content (TEC) data derived from GEONET clarified that various ionospheric phenomena cause the variation of the ionosphere in the region around 30 degrees of the geomagnetic latitude, and affect on the navigation system. The large day-to-day variation of TEC gradient over Japan was found to be attributed to the day-to-day variation of the Equatorial Ionization Anomaly (EIA) because Japan is located on the northern slope of EIA. The amplitude and location of EIA have large variability because they are controlled by a complex system among the neutral and the ionized atmospheres, lower and upper atmospheres, and low-latitude and high-latitude ionospheres. As a result, the variation of EIA is difficult to be predicted. Meso-scale ionospheric structures, such as medium-scale traveling ionospheric disturbances (MSTID) and plasma bubbles, drift in about 100m/s of velocity and cause variations of short temporal scale. They were often observed to be accompanied by smaller scale structures that generate phase and amplitude scintillations of the GPS radio waves. The activity of MSTID has maxima in summer and winter. During these maximum periods, they were detected by GEONET almost everyday. Plasma bubbles were detected in the lowest latitude area of Japan, and found to reach the central part of Japan in a few days in a year. Most of them were in the geomagnetically disturbed periods. The geomagnetic disturbances also generate large-scale traveling ionospheric disturbances and reduction of TEC.

Scopus