Faculty Profiles - OTSUKA, Yuichi

Papers - OTSUKA, Yuichi

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Statistical study of relationship between medium-scale traveling ionospheric disturbance and sporadic E layer activities in summer night over Japan Reviewed

Y. Otsuka, T. Tani, T. Tsugawa, T. Ogawa, A. Saito

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS Vol. 70 ( 17 ) page： 2196 - 2202 2008.12

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We investigate the relationship between medium-scale traveling ionospheric disturbance (MSTID) and sporadic E (E-s) layer activities in summer nights by analyzing total electron content (TEC) data obtained from a global positioning system (GPS) network in Japan and ionosonde data obtained at Kokubunji, Japan during May-August in 2001-2005. MSTID activity is defined as delta I/($) over bar, where delta I is standard deviation of the TEC perturbations over Kokubunji within 1 h, and 7 is the background TEC. By analyzing nighttime-averaged (19-02 LT) values of MSTID activity and E, layer parameters, we find that the MSTID activity is closely correlated with f(0)E(s) and f(0)E(s) -f(b)E(s). This result suggests that MSTID and the spatial structures of E, layer could be generated by an electro-dynamical coupling process between the Es layer and F region through polarization electric fields. Furthermore, we suggest that the appearance of the E, layer in the summer hemisphere could play an important role in generating MSTIDs in both hemispheres. (C) 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2008.07.008

Decay of 3-m-scale ionospheric irregularities associated with a plasma bubble observed with the Equatorial Atmosphere Radar Reviewed Open Access

Susumu Saito, Shoichiro Fukao, Mamoru Yamamoto, Yuichi Otsuka, Takashi Maruyama

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 113 ( A11 ) 2008.11

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

We investigated the decay processes of 3-m-scale ionospheric irregularities associated with plasma bubbles by multi-instrument observations. The observations were made using the Equatorial Atmosphere Radar (EAR), a 630-nm airglow imager, and ionosondes. The EAR's electronic beam steering capability allowed us to see the temporal evolution of 3-m-scale irregularities associated with plasma bubbles in two-dimensional views. Around midnight on 28 and 31 March 2006, the EAR detected plasma bubbles that drifted into its field of view. When their eastward drift motion slowed and they finally stopped moving eastward, the 3-m-scale irregularities decayed. These results suggest that instability driven by the eastward neutral wind kept the 3-m-scale irregularities in matured plasma bubbles, which are in the well-developed stage, active for several hours after sunset. The stagnated plasma bubbles drifted downward with the ambient westward electric field. This drift resulted in the backscatter echo patterns seen in the range-time-intensity (RTI) plots which look like those associated with mid-latitude plumes.

DOI： 10.1029/2008JA013118

F-3 layer during penetration electric field Reviewed Open Access

Balan, N, Thampi, SV, Lynn, K, Otsuka, Y, Alleyne, H, Watanabe, S, Abdu, MA, Fejer, BG

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 113 2008.9

Simultaneous appearance of isolated auroral arcs and Pc 1 geomagnetic pulsations at subauroral latitudes Reviewed Open Access

K. Sakaguchi, K. Shiokawa, Y. Miyoshi, Y. Otsuka, T. Ogawa, K. Asamura, M. Connors

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 113 ( A5 ) 2008.5

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

We have been conducting observations of aurora and geomagnetic pulsations at Athabasca, Canada, located at a subauroral latitude ( magnetic latitude: 62 degrees, L similar to 4.6), using an all-sky imager and an induction magnetometer. Isolated auroral arcs at wavelengths of 557.7 nm, 630.0 nm, and 486.1 nm ( H-beta) were often observed at latitudes separated equatorward from the main auroral oval. From a 1-year observation ( 4 September 2005 to 3 September 2006), we found 13 isolated arc events. All these isolated arcs occurred coincidentally with Pc 1 geomagnetic pulsations, although there were nine other Pc 1 events without isolated arcs in the field of view of the imager. The arcs were observed in both pre- and post-midnight sectors and tended to appear during the late recovery phase of geomagnetic storms. The isolated arcs had limited latitudinal and longitudinal widths of less than 230 km and 250 - 800 km, respectively. We found that as isolated arcs moved equatorward ( poleward), the frequencies of the simultaneous Pc 1 pulsations increased ( decreased). Using the Tsyganenko-02 magnetic field model, the observed Pc 1 frequencies were almost the same as the frequencies of He+ electromagnetic ion cyclotron ( EMIC) waves at the equatorial plane connected to observed isolated arcs. These results indicate that interactions of spatially localized EMIC waves with ring current ions cause high-energy ion precipitation and associated isolated auroras at subauroral latitudes. These results also imply that the dynamics and instabilities in the inner magnetosphere can be monitored as low-latitude auroral emissions away from the ordinary auroral oval.

DOI： 10.1029/2007JA012888

Daytime 150-km echoes observed with the Equatorial Atmosphere Radar in Indonesia: First results Reviewed Open Access

A. K. Patra, T. Yokoyama, Y. Otsuka, M. Yamamoto

GEOPHYSICAL RESEARCH LETTERS Vol. 35 ( 6 ) 2008.3

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

Results of the daytime 150-km echoes from 10.36 degrees S magnetic latitude observed using the Equatorial Atmosphere Radar (EAR), which are first of its kind from magnetic southern hemisphere, are presented. The echoing region of 145-165 km shows forenoon descent and afternoon ascent with signal intensity modulation with time resembling a necklace and the Doppler spectra are narrow, which are quite similar to those reported earlier from other locations. Westward and upward/southward irregularity drifts are found to be consistent with daytime background electric fields, providing a tool to investigate low latitude daytime electrodynamics. Detectability of these echoes suggests the absence of strong latitudinal dependence in contrast to the earlier belief, suggesting a possibility of their detection at mid-latitudes. Also the echoes show zonal anisotropy that is opposite to that observed over Pohnpei. The implications of these observations are discussed in the light of present understanding of 150-km echoing phenomenon.

DOI： 10.1029/2007GL033130

First three-dimensional simulation of the Perkins instability in the nighttime midlatitude ionosphere Reviewed Open Access

Tatsuhiro Yokoyama, Yuichi Otsuka, Tadahiko Ogawa, Mamoru Yamamoto, David L. Hysell

GEOPHYSICAL RESEARCH LETTERS Vol. 35 ( 3 ) 2008.2

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

A three-dimensional numerical simulation in the nighttime midlatitude ionosphere is developed for the first time and applied to the Perkins instability in the midlatitude F region. Growth of the Perkins instability is successfully reproduced under nighttime condition, and the numerical results basically agree with a linear theory and previous two-dimensional numerical studies. Northwest-southeast (NW-SE) alignment of density perturbations is generated from random seeding by applying a southeastward neutral wind. The perturbations are dominant at altitudes of 200 300 km where a steep density gradient exists, which is consistent with the altitude of 630-nm airglow emission that often shows NW-SE alignment. Further simulation in terms of the E-F coupling can be done in the near future.

DOI： 10.1029/2007GL032496

Optical and radio observations and AMIE/TIEGCM modeling of nighttime traveling ionospheric disturbances at midlatitudes during geomagnetic storms Open Access

K. Shiokawa, T. Tsugawa, T. Tsugawa, Y. Otsuka, T. Ogawa, G. Lu, A. Saito, M. Yamamoto

Geophysical Monograph Series Vol. 181 page： 271 - 281 2008

© 2008 by the American Geophysical Union. This paper summarizes our recent observations of storm-time traveling ionospheric disturbances (TIDs) over Japan. The storm-time TIDs, which are often referred as large-scale TIDs or traveling atmospheric disturbances, are generated at high-latitude auroral zone and propagate predominantly equatorward at midlatitudes as atmospheric waves in the thermosphere. The imaging measurements by airglow imagers and GPS networks give reliable wave parameters (wavelength, phase velocity, and amplitude) of TIDs. Comparison of imaging observations of TIDs between the northern and southern hemispheres revealed their nonconjugacy. Combinations of these imaging observations with vertical sounding measurements by ionosondes and radars provide a comprehensive description of TID structures at midlatitudes. Comparison between these observations and the assimilative mapping of the ionospheric electrodynamics technique with the thermosphere-ionosphere electrodynamics general circulation model results in several insights into the generation and propagation of TIDs at the high-latitude source region as well as limitations of the current global model.

DOI： 10.1029/181GM24

Discovery of anoumoulous stripes over the amazon by the PALSAR onboard ALOS satellite Reviewed

Masanobu Shimada, Yasushi Muraki, Yuichi Otsuka

International Geoscience and Remote Sensing Symposium (IGARSS) Vol. 2 ( 1 ) page： II387 - II390 2008

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

We discovered anomalous stripes over the large area of Amazon when the region was scanned by the radar (PALSAR) onboard ALOS satellite. The stripes appeared on the local midnight along the geomagnetic field line with a typical characteristic width of 600m. ©2008 IEEE.

DOI： 10.1109/IGARSS.2008.4779009

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

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