Faculty Profiles - OTSUKA, Yuichi

Papers - OTSUKA, Yuichi

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Seasonal and Local Time Variations of E-Region Field-Aligned Irregularities Observed with 30.8-MHz Radar at Kototabang, Indonesia Reviewed

Otsuka, Y.

International Journal of Geophysics Vol. 2012 ( 695793 ) 2012.3

Polarization of Pc1/EMIC waves and related proton auroras observed at subauroral latitudes Reviewed Open Access

R. Nomura, K. Shiokawa, K. Sakaguchi, Y. Otsuka, M. Connors

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 117 2012.2

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

We have investigated the polarization of Pc1 geomagnetic pulsations and related proton auroras at subauroral latitudes, using an induction magnetometer and an all-sky camera at Athabasca, Canada (54.7 degrees N, 246.7 degrees E, magnetic latitude (mlat) 61.7 degrees N). Isolated proton auroras often appear in association with Pc1 pulsations, because of proton scattering by electromagnetic ion cyclotron (EMIC) waves in the magnetosphere. We used the proton aurora as a proxy for the location and size of the Pc1 ionospheric source. For 27 Pc1 events with simultaneously observed proton auroras over 4 years from September 7, 2005 to September 6, 2009, we calculated the distances between the Pc1 ionospheric source and the observation site, normalized by the scale size of the source. We tried three different definitions of the scale size of Pc1 ionospheric source to calculate the normalized distances. We found that the rotation angle theta between the Pc1 polarization ellipse and the direction to the proton aurora changes from 90 degrees to 0 degrees as the normalized distance increases. For the definition of the scale sizes that gives most clear theta-transition from 90 degrees to 0 degrees, the transition occurs at the normalized distance similar to 2.0-4.0, while it was similar to 1.0-2.0 by the model calculation of Fujita and Tamao (1988). The averaged major axes tend to point toward the proton aurora at larger distances. The difference of the transition location may imply that the Pc1 ionospheric sources are larger than the isolated proton auroras, or that the inhomogenuity of the ionospheric conductivity by the proton precipitation affects the transition distances.

DOI： 10.1029/2011JA017241

Web of Science

Ionospheric and thermospheric storms at equatorial latitudes observed by CHAMP, ROCSAT, and DMSP Reviewed Open Access

N. Balan, J. Y. Liu, Y. Otsuka, S. Tulasi Ram, H. Luehr

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 117 2012.1

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

Analysis of the dayside electron density (Ne) and neutral mass density (N) at 400 km height measured by CHAMP during 12 intense geomagnetic storms in 2000-2004, and ion densities at 600 km and 840 km heights measured by ROCSAT and DMSP during a few of the intense storms, reveal some new aspects. Thermospheric storms (change of N) reach the equator within 1.5 to 3 hours from the main phase (MP) onset of intense storms having short and steady MPs. The responses of the equatorial ionosphere (at CHAMP) to both MPs and RPs (recovery phases) of the storms are generally opposite to those at higher latitudes. In addition to the known opposite responses during MPs, the analysis reveals that positive ionospheric storms develop at equatorial latitudes (within about +/- 15 degrees magnetic latitudes) during daytime RPs, while conventional negative storms occur at higher latitudes. Ionospheric storms also extend to the topside ionosphere beyond 850 km height and are generally positive (at DMSP), especially during MPs. The positive storms around the equatorial ionospheric peak during RPs are interpreted in terms of the potential sources such as (1) zero or westward electric fields due to disturbance dynamo and/or prompt penetration, (2) plasma convergence due to the mechanical effects of storm-time equatorward neutral winds and waves, (3) increase of atomic oxygen density and decrease of molecular nitrogen density due to the downwelling effect of the winds, and (4) photoionization. The positive storms in the topside ionosphere during MPs involve the rapid upward drift of plasma due to eastward PPEFs, reduction in the downward diffusion of plasma along the field lines, and plasma convergence due to equatorward winds and waves.

DOI： 10.1029/2011JA016903

Web of Science

A comparative study of equatorial daytime vertical e × B drift in the Indian and Indonesian sectors based on 150 km echoes Reviewed Open Access

A. K. Patra, P. Pavan Chaitanya, N. Mizutani, Y. Otsuka, T. Yokoyama, M. Yamamoto

Journal of Geophysical Research: Space Physics Vol. 117 ( 11 ) 2012

Language：English Publishing type：Research paper (scientific journal)

In this paper, we study for the first time the daytime vertical E × B drift velocities from Gadanki and Kototabang using the Doppler shifts of the 150-km echoes observed during 2008-2010, a period of low solar activity. Drift velocities are mostly positive and confined to 35 m s-1 at both the locations, except for Gadanki where on a few occasions negative drift velocities have been observed in the afternoon hours. Drift velocities generally show a decreasing trend with local time and the largest drift is generally observed in the forenoon hours consistent with extensively reported observations and models of E × B drift. Drift velocities from Gadanki and Kototabang compared exceeding well on some days and differed remarkably on many days despite the fact that they are longitudinally separated by only 20. The day-to-day variation in the drift velocity could be as high as 15 m s-1 at Gadanki and 7 m s-1 at Kototabang. Seasonal mean drifts over Gadanki are found to be generally larger than those of Kototabang. The observations have been compared in detail with those reported earlier based on ground-and satellite-based observations and also with the Scherliess-Fejer model. The observed differences in the drifts at the two locations, including the downward drifts, have been discussed in the light of current understanding of the longitudinal variability of E × B drift.

DOI： 10.1029/2012JA018053

Scopus

Development of low-cost sky-scanning Fabry-Perot interferometers for airglow and auroral studies Reviewed Open Access

K. Shiokawa, Y. Otsuka, S. Oyama, S. Nozawa, M. Satoh, Y. Katoh, Y. Hamaguchi, Y. Yamamoto, J. Meriwether

EARTH PLANETS AND SPACE Vol. 64 ( 11 ) page： 1033 - 1046 2012

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

We have developed new Fabry-Perot interferometers (FPIs) that are designed to measure thermospheric winds and temperatures as well as mesospheric winds through the airglow/aurora emissions at wavelengths of 630.0 nm and 557.7 nm, respectively. One FPI (FP01), possessing a large aperture etalon (diameter: 116 mm), was installed at the EISCAT Tromso site in 2009. The other FPIs, using 70-mm diameter etalons, were installed in Thailand, Indonesia, and Australia in 2010-2011 (FP02-FP04) by the Solar-Terrestrial Environment Laboratory, and in Peru (Nazca and Jicamarca) and Alaska (Poker Flat) by Clemson University. The FPIs with 70-mm etalons are low-cost compact instruments, suitable for multipoint network observations. All of these FPIs use low-noise cooled-CCD detectors with 1024 x 1024 pixels combined with a 4-stage thermoelectric cooling system that can cool the CCD temperature down to -80 degrees C. The large incident angle (maximum: 1.3 degrees-1.4 degrees) to the etalon achieved by the use of multiple orders increases the throughput of the FPIs. The airglow and aurora observations at Tromso by FP01 show wind velocities with typical random errors ranging from 2 to 13 m s(-1) and from 4 to 27 m s(-1) for mesosphere (557.7 nm) and thermosphere (630.0 nm) measurements, respectively. The 630.0-nm airglow observations at Shigaraki, Japan, by FP02-FP04 and by the American FPI instruments give thermospheric wind velocities with typical random errors that vary from 2 m s(-1) to more than 50 m s(-1) depending on airglow intensity.

DOI： 10.5047/eps.2012.05.004

Open Access

Web of Science

Early observation and future plan of the Ionosphere, Mesosphere, upper Atmosphere, and Plasmasphere mapping observation mission

SAITO Akinori, YAMAZAKI Atsushi, SAKANOI Takeshi, YOSHIKAWA Ichiro, ABE Takumi, OTSUKA Yuichi, TAGUCHI Makoto, SUZUKI Makoto, KIKUCHI Masayuki, NAKAMURA Takuji, YAMAMOTO Mamoru, KAWANO Hideaki, LIU Huixin, ISHII Mamoru, TSUGAWA Takuya, SAKANOI Kazuyo, FUJIWARA Hitoshi, KUBOTA Minoru, EJIRI Mitsumu K, SAITO Akinori

地球電磁気・地球惑星圏学会総会及び講演会予稿集(CD-ROM) Vol. 132nd page： ROMBUNNO.B005-18 2012

GPS total electron content variations associated with poleward moving Sun-aligned arcs Reviewed Open Access

P. T. Jayachandran, K. Hosokawa, K. Shiokawa, Y. Otsuka, C. Watson, S. C. Mushini, J. W. MacDougall, P. Prikryl, R. Chadwick, T. D. Kelly

Journal of Geophysical Research: Space Physics Vol. 117 ( 5 ) 2012

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

GPS total electron content (TEC) has shown quasiperiodic oscillations of varying amplitude associated with poleward moving Sun-aligned arcs. The amplitude of TEC variations showed a maximum of ∼3 TECU and seemed to decrease as the arcs moved poleward from the source/generation region. Simultaneous DMSP data showed that fluctuations in TEC and optical intensification were caused by precipitation of high-energy (&gt
500 eV) particles. Concurrent ionosonde observations also exhibited quasiperiodic variations (within limit of the resolution of the data) in peak ionospheric electron density of the ionosphere. Bottom height of the ionospheric layers produced by precipitating particles varied between 130km (upper E region) and 300km (F region), indicating variable particle precipitation energy. Frequency analysis of high-resolution TEC data showed a broad range of discrete frequency components from 1.60 mHz to 22.80 mHz present in the TEC oscillations, which may provide insight into the energization/modulation of precipitating particles by these oscillations. A broad distribution of equivalent vertical thickness of arcs was calculated using GPS TEC and ionosonde measurements of peak electron density. This distribution showed a minimum thickness of 21km, a maximum of 84km, and an average of 49km. The equivalent vertical thickness also showed a linear relationship with bottomside height of the ionospheric layer (auroral arc). The relationship showed an increase in the vertical thickness with an increase in bottomside height of the layer. This relationship is a consequence of variations in the energy of the precipitating particles causing different ionospheric production profiles. Copyright 2012 by the American Geophysical Union.

DOI： 10.1029/2011JA017423

Scopus

Low-latitude mesosphere, thermosphere, and ionosphere Reviewed

Y. Sahai, R. S. Dabas, Y. Otsuka, M. Klimenko

International Journal of Geophysics Vol. 2012 2012

Ionospheric and thermospheric storms at equatorial latitudes observed by CHAMP, ROCSAT, and DMSP Reviewed Open Access

N. Balan, J. Y. Liu, Y. Otsuka, S. Tulasi Ram, H. Lühr

Journal of Geophysical Research: Space Physics Vol. 117 ( 1 ) 2012

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

Analysis of the dayside electron density (Ne) and neutral mass density (N) at 400km height measured by CHAMP during 12 intense geomagnetic storms in 2000-2004, and ion densities at 600km and 840km heights measured by ROCSAT and DMSP during a few of the intense storms, reveal some new aspects. Thermospheric storms (change of N) reach the equator within 1.5 to 3 hours from the main phase (MP) onset of intense storms having short and steady MPs. The responses of the equatorial ionosphere (at CHAMP) to both MPs and RPs (recovery phases) of the storms are generally opposite to those at higher latitudes. In addition to the known opposite responses during MPs, the analysis reveals that positive ionospheric storms develop at equatorial latitudes (within about ±15° magnetic latitudes) during daytime RPs, while conventional negative storms occur at higher latitudes. Ionospheric storms also extend to the topside ionosphere beyond 850km height and are generally positive (at DMSP), especially during MPs. The positive storms around the equatorial ionospheric peak during RPs are interpreted in terms of the potential sources such as (1) zero or westward electric fields due to disturbance dynamo and/or prompt penetration, (2) plasma convergence due to the mechanical effects of storm-time equatorward neutral winds and waves, (3) increase of atomic oxygen density and decrease of molecular nitrogen density due to the downwelling effect of the winds, and (4) photoionization. The positive storms in the topside ionosphere during MPs involve the rapid upward drift of plasma due to eastward PPEFs, reduction in the downward diffusion of plasma along the field lines, and plasma convergence due to equatorward winds and waves.

DOI： 10.1029/2011JA016903

Scopus

Motion of high-latitude nighttime medium-scale traveling ionospheric disturbances associated with auroral brightening Reviewed Open Access

K. Shiokawa, M. Mori, Y. Otsuka, S. Oyama, S. Nozawa

Journal of Geophysical Research: Space Physics Vol. 117 ( 10 ) 2012

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

We report for the first time the rapid oscillating motion of nighttime medium-scale traveling ionospheric disturbances (MSTIDs) based on airglow imaging observations at Troms (magnetic latitude: 67.1N), Norway on 8 December, 2009. The MSTIDs appeared in 630-nm airglow images at 1530 UT as wave-like structures south of Troms with a horizontal wavelength of ∼200km and a phase surface of north to south. They moved eastward with velocities of 30-60m/s. The velocity was faster in the poleward-side of the MSTIDs, forming a northeast-southwest phase surface at later times. This phase surface direction is opposite to that of midlatitude MSTIDs. The MSTIDs show sudden oscillations and phase jump in the east-west direction with a timescale of ∼10min at 1730 UT. The oscillations were associated with an auroral brightening observed at the poleward edge of the images and small magnetic field perturbations observed by ground magnetometers. The Doppler measurement of the 630-nm airglow by a Fabry-Perot interferometer at Troms showed a stable southeastward thermospheric wind with a velocity of ∼150m/s. These observations indicate that the MSTID oscillations were linked to auroral electric field in the ionosphere, implying that the observed MSTIDs are ionospheric plasma structures. We suggest that the observed MSTIDs were created by atmospheric gravity waves at the beginning, left as fossil plasma structures even after the gravity wave packet dissipated in the thermosphere, moved eastward according to the background electric field driven by the F-region dynamo, and oscillated associated with the auroral electric field. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017928

Scopus

Polarization of Pc1/EMIC waves and related proton auroras observed at subauroral latitudes Reviewed Open Access

R. Nomura, K. Shiokawa, K. Sakaguchi, Y. Otsuka, M. Connors

Journal of Geophysical Research: Space Physics Vol. 117 ( 2 ) 2012

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

We have investigated the polarization of Pc1 geomagnetic pulsations and related proton auroras at subauroral latitudes, using an induction magnetometer and an all-sky camera at Athabasca, Canada (54.7°N, 246.7°E, magnetic latitude (mlat) 61.7°N). Isolated proton auroras often appear in association with Pc1 pulsations, because of proton scattering by electromagnetic ion cyclotron (EMIC) waves in the magnetosphere. We used the proton aurora as a proxy for the location and size of the Pc1 ionospheric source. For 27 Pc1 events with simultaneously observed proton auroras over 4years from September 7, 2005 to September 6, 2009, we calculated the distances between the Pc1 ionospheric source and the observation site, normalized by the scale size of the source. We tried three different definitions of the scale size of Pc1 ionospheric source to calculate the normalized distances. We found that the rotation angle θ between the Pc1 polarization ellipse and the direction to the proton aurora changes from 90° to 0° as the normalized distance increases. For the definition of the scale sizes that gives most clear θ-transition from 90° to 0°, the transition occurs at the normalized distance ∼2.0-4.0, while it was ∼1.0-2.0 by the model calculation of Fujita and Tamao (1988). The averaged major axes tend to point toward the proton aurora at larger distances. The difference of the transition location may imply that the Pc1 ionospheric sources are larger than the isolated proton auroras, or that the inhomogenuity of the ionospheric conductivity by the proton precipitation affects the transition distances. Copyright 2012 by the American Geophysical Union.

DOI： 10.1029/2011JA017241

Scopus

On post-midnight field-aligned irregularities observed with a 30.8-MHz radar at a low latitude: Comparison with F-layer altitude near the geomagnetic equator Reviewed

M. Nishioka, Y. Otsuka, K. Shiokawa, T. Tsugawa, Null Effendy, P. Supnithi, T. Nagatsuma, K. T. Murata

Journal of Geophysical Research: Space Physics Vol. 117 ( 8 ) 2012

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

We investigated the relationship between post-midnight F-region field aligned irregularities (FAIs) and F-layer altitude by analyzing data of a 30.8-MHz radar installed 5at Kototabang, Indonesia (0.2S, 100.3E
geomagnetic latitude 10.4S) and an ionosonde installed at Chumphon, Thailand (10.7N, 99.4E
geomagnetic latitude 3.3N). Chumphon is located near the geomagnetic equator on approximately the same meridian as Kototabang. Case studies show that the altitude of the F-layer rose at Chumphon a half hour before the post-midnight FAIs appeared at Kototabang. The Doppler velocity of the E-region FAIs observed simultaneously by the 30.8-MHz radar was downward, indicating that the F-layer uplift was not caused by the electric field. We also investigated seasonal variations of the post-midnight FAI occurrence and the F-layer altitude. Both the post-midnight FAIs and the uplift of the F-layer were frequently seen around midnight between May and August. The seasonal variation of the midnight F-layer uplift around the geomagnetic equator coincided with that of the post-midnight FAI occurrence at Kototabang. These results suggest that the uplift of the F-layer would play an important role in the generation of post-midnight FAIs. We evaluated the linear growth rate of the Rayleigh-Taylor instability based on the altitude of the F-layer observed at Chumphon. The result shows that the uplift of the F-layer can enhance the growth rate because gravity-driven eastward electric current increases. Therefore, we interpret that the observed FAIs were accompanied by plasma bubble, the growth rate of which was reinforced by the uplifted F-layer. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017692

Scopus

Observation of equatorial nighttime medium-scale traveling ionospheric disturbances in 630-nm airglow images over 7 years Reviewed

D. Fukushima, K. Shiokawa, Y. Otsuka, T. Ogawa

Journal of Geophysical Research: Space Physics Vol. 117 ( 10 ) 2012

Language：English Publishing type：Research paper (scientific journal)

[1] We report on nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed at Kototabang, Indonesia (geographic longitude: 100.3°E
geographic latitude: 0.2°S
and geomagnetic latitude: 10.6°S) during a 7-year period from October 2002 to October 2009. MSTIDs were observed in 630-nm nighttime airglow images by using a highly sensitive all-sky airglow imager at Kototabang. The averages and standard deviations of horizontal phase velocity, period, and horizontal wavelength of MSTIDs observed during the 7 years were 320 ±170 m/s, 42 ±11 min, and 790 ± 440 km, respectively. The occurrence rate of the observed MSTIDs decreased with decreasing solar activity. The average horizontal wavelength of MSTIDs increased with decreasing solar activity. Southward MSTIDs were dominant throughout the 7 years of observations. These facts are consistent with the hypothesis that the observed MSTIDs are caused by gravity waves in the thermosphere. Moreover, we compared the propagation directions of the observed MSTIDs with the locations of tropospheric convection activity for the events where gravity waves producing the observed MSTIDs could have existed in the lower atmosphere. Strong tropospheric convection was found within ±30 degrees from the source directions of MSTIDs in 81% of the MSTID events. In such events, gravity waves were possibly generated from deep convection in the troposphere and directly propagated into the thermosphere. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017758

Scopus

Response of low-latitude ionosphere to medium-term changes of solar and geomagnetic activity Reviewed

Ivan Kutiev, Yuichi Otsuka, Dora Pancheva, Rod Heelis

Journal of Geophysical Research: Space Physics Vol. 117 ( 8 ) 2012

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

The paper presents the medium-term quasi periodic (∼9-27 day) response of middle and low-latitude ionosphere to solar [F10.7) and geomagnetic (Kp-index) forcing. The ionospheric response is examined by wavelet analysis of the relative deviations of TEC over Japan for the period of time 2000-2008. It is found that the ∼27-day rTEC oscillations correlate well with the same oscillations of the solar index F10.7 particularly in the solar maximum and its early declining phase (2001-2005). During the declining phase of solar activity (for example, year of 2005) the Kp-index variability exhibits additionally strong oscillations with periods 13.5- and 9-days. Similar oscillations are found in rTEC as well but they do not follow the geomagnetic forcing as faithfully as those associated with F10.7. During solar minimum the quasi periodic rTEC variability is shaped mainly by the recurrent geomagnetic activity. An attempt is made to investigate the latitudinal dependence of the ∼9-27-day rTEC response over Japan as well as the phase relationship between the forcing and response. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017641

Scopus

Seasonal and local time variations of e -Region field-aligned irregularities observed with 30.8-mhz radar at kototabang, indonesia Reviewed

Y. Otsuka

International Journal of Geophysics Vol. 2012 2012

Language：English Publishing type：Research paper (scientific journal)

A VHF backscatter radar with operating frequency 30.8 MHz has been operated at Kototabang (0.20°S, 100.32°E
dip latitude 10.36°S), Indonesia, since February 2006. We analyzed E-region field-aligned irregularities (FAIs) observed by this radar through a year of 2007 and found that the E-region FAI observed at Kototabang can be classified into two groups. One is "descending FAI". Altitude of the FAI echo region descends with time from 102 km to 88 km altitude during 0700-1000 and 1900-0000 LT in June solstice season. The other is "low-altitude FAI", which is observed in an altitude range from 88 to 94 km mainly during nighttime. The observed Doppler velocity show distinct local time and altitude dependence. The seasonally averaged zonal velocity above (below) approximately 94 km altitude is westward (eastward) during daytime and eastward (westward) during nighttime. Meridional/vertical velocity perpendicular to the geomagnetic fields is upward during daytime and downward during nighttime. The direction of the FAI velocity above approximately 94 km altitude is consistent with that of the background E × B plasma drifts reported previously. © 2012 Y. Otsuka.

DOI： 10.1155/2012/695793

Scopus

Simultaneous observations at Darwin of equatorial bubbles by ionosonde-based range/time displays and airglow imaging Reviewed Open Access

K. J. W. Lynn, Y. Otsuka, K. Shiokawa

GEOPHYSICAL RESEARCH LETTERS Vol. 38 2011.12

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

Airglow observations of ionospheric electron density depletions made at Darwin, Australia have demonstrated that the tree-like structure of bubbles developed at the magnetic equator are mapped along magnetic field lines with considerable accuracy to the base of the ionosphere at higher latitudes. Ionosonde range-time displays made at Darwin and other equatorial sites in the Australian region show characteristic approaching and receding echoes which converge on a typical spread-F event. These off-angle echoes have often been referred to in the literature as satellite traces and associated with spread F with little recognition of their true significance. All four optical depletions previously reported in the literature as being seen at Darwin are found in this paper to be accompanied by such typical off-angle/spread F events. The zonal drift velocity of the moving reflectors can be measured from the speed at which such echoes approach and recede. Since digital ionosondes in equatorial sites have existed for many years, existing ionogram data, when suitably processed into range-time displays, may allow the occurrence of such events over several sunspot cycles to be found. A question remains as to whether all or only some of such equatorial range-time events correspond to electron density depletions. Citation: Lynn, K. J. W., Y. Otsuka, and K. Shiokawa (2011), Simultaneous observations at Darwin of equatorial bubbles by ionosonde-based range/time displays and airglow imaging, Geophys. Res. Lett., 38, L23101, doi:10.1029/2011GL049856.

DOI： 10.1029/2011GL049856

Web of Science

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

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

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 116 2011.11

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

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

DOI： 10.1029/2011JA016797

Web of Science

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

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

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 116 2011.11

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

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

DOI： 10.1029/2011JA016798

Web of Science

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

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

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 116 2011.8

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

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

DOI： 10.1029/2011JA016607

Web of Science

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

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

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 116 2011.7

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

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

DOI： 10.1029/2010JA016399

Web of Science

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Papers - OTSUKA, Yuichi