Faculty Profiles - SHINBORI Atsuki

Observations of very-low-energy (< 10 eV) ion outflows dominated by O+ ions in the region of enhanced electron density in the polar cap magnetosphere during geomagnetic storms Reviewed Open Access

N. Kitamura, Y. Nishimura, T. Ono, Y. Ebihara, N. Terada, A. Shinbori, A. Kumamoto, T. Abe, M. Yamada, S. Watanabe, A. Matsuoka, A. W. Yau

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 115 ( 11 ) 2010.11

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

Velocity distributions of upflowing ions in the polar ionosphere are crucial to understand their destinations. Natural plasma wave observations by the plasma wave and sounder experiments and thermal ion observations by the suprathermal ion mass spectrometer onboard the Akebono satellite at similar to 9000 km altitude in the polar magnetosphere during the geomagnetic storms showed that ions in the region of enhanced electron density in the polar cap were dominated by very-low-energy O+ ions (similar to 85%) with upward velocities of 4-10 km s(-1), corresponding to streaming energies of 1.3-8.4 eV. The fluxes of very-low-energy upflowing O+ ions exceeded 1 x 10(9) cm(-2) s(-1) (mapped to 1000 km altitude) across wide regions. These signatures are consistent with high-density plasma supplied by the cleft ion fountain mechanism. Trajectory calculations of O+ ions based on the Akebono observations as the initial condition showed the transport paths and accelerations of the O+ ions and indicated that the velocities of the very-low-energy upflowing O+ ions through the dayside polar cap are enough to reach the magnetosphere under strong convection. The calculations suggest the importance of the very-low-energy upflowing O+ ions with large fluxes in the total O+ ion supply toward the magnetosphere, especially the near-Earth tail region and inner magnetosphere. The initially very-low-energy O+ ions can contribute significantly to the ring current formation during geomagnetic storms since some of the O+ ions were transported into the ring current region with typical energies of ring current ions (several tens of keV) in the trajectory calculations.

DOI： 10.1029/2010JA015601

Anomalous occurrence features of the preliminary impulse of geomagnetic sudden commencement in the South Atlantic Anomaly region Reviewed Open Access

Atsuki Shinbori, Yukitoshi Nishimura, Yuji Tsuji, Takashi Kikuchi, Tohru Araki, Akihiro Ikeda, Teiji Uozumi, Roland E. S. Otadoy, Hisashi Utada, Jose Ishitsuka, Nalin Baual Trivedi, Severino L. G. Dutra, Nelson Jorge Schuch, Shinichi Watari, Tsutomu Nagatsuma, Kiyohumi Yumoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 115 ( 8 ) 2010.8

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

Occurrence features of the preliminary impulse (PI) of geomagnetic sudden commencement (SC) both in the Pacific Ocean and South Atlantic Anomaly (SAA) regions were investigated using the long-term magnetic field data obtained from the Circum-pan Pacific Magnetometer Network (CPMN) and NICT Space Weather Monitoring (NSWM) magnetometer networks. The low-latitude preliminary reverse impulse (PRI) at Okinawa (OKI: dip latitude = 37.97 degrees) in the Pacific Ocean region appeared in all the magnetic local time (MLT) sectors with the peak occurrence rate of 40% around noon. On the other hand, the PRI occurrence rate at Santa Maria (SMA: dip latitude = -34.35 degrees) near the center of the SAA region showed a significant enhancement in the daytime sector (0800-1600 h, MLT) with the peak value of 80%, which resembles the occurrence feature of the equatorial PRI. Moreover, the PRI amplitude around noon at SMA was about 3.0 times larger than that at OKI. From the calculation of the ionospheric conductivity derived from the IRI-2007 and NRLMSISE-00 models, it is shown that the height-integrated conductivity was more enhanced in the SAA region (SMA), where the ambient magnetic field intensity is weak, compared with that in the Pacific Ocean region (OKI). Therefore, the anomalous increase of the PRI occurrence and amplitude is caused by the significant enhancement of the ionospheric conductivity due to the weakness of the ambient magnetic field intensity in the SAA region.

DOI： 10.1029/2009JA015035

Temporal variations and spatial extent of the electron density enhancements in the polar magnetosphere during geomagnetic storms Reviewed

Anomalous Enhancement of Occurrence of the Preliminary Impulse of Geomagnetic Sudden Commencement (SC) at Low Latitude in the South Atlantic Anomaly (SAA) region Reviewed

Shinbori A, Y. Nishimura, Y. Tsuji, T. Kikuchi, T. Araki, A. Ikeda, T. Uozumi, R. Otadoy, H. Utada, J. Ishitsuka, N. Trivedi, S. Dutra, N. Schuch, S. Watari, T. Nagatsuma, K. Yumoto

Journal of Geophysical Research: Space Physics, Volume 115, Issue A8, CiteID: A08309, doi: 10.1029/2009JA015035 Vol. 115 ( A8 ) 2010.8

Kitamura, N, Y. Nishimura, T. Ono, A. Kumamoto, A. Shinbori, M. Iizima, A. Matsuoka, M. R. Hairston

J. Geophys. Res. Vol. 115 ( A7 ) 2010.7

Penetration of magnetospheric electric fields to the equator and their effects on the low-latitude ionosphere during intense geomagnetic storms Reviewed

B. Veenadhari, S. Alex, T. Kikuchi, A. Shinbori, Rajesh Singh, E. Chandrasekhar

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 115 ( A3 ) 2010.3

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

The penetration of magnetospheric electric fields to the magnetic equator has been investigated for two intense magnetic storms that occurred on 31 March 2001 and 6 November 2001. The digital ground magnetic data from equatorial station Tirunelveli (TIR, 0.17 degrees S geomagnetic latitude (GML)) and low-latitude station Alibag (ABG, 10.17 degrees N GML) have been used to identify the storm time electrojet index, EEJ(Dis), which is the difference of the magnetic field variations between TIR and ABG after removing the quiet day variations. The appearance of enhanced DP 2 currents and counterelectrojets (CEJ) during the main and recovery phases of the magnetic storms is possibly due to prompt penetration of electric fields from the high latitudes. These signatures can be interpreted as a clear indicator of the eastward and westward electric fields at the equator. The observed results suggest that the magnitude of the equatorial ionospheric currents driven by the penetrating electric fields is very sensitive to ionospheric conductivity (which depends on local time). Moreover, the intensity of the DP 2 currents started decreasing during the end of the main phase of the storm despite the large negative southward IMF Bz, indicating the dominance of a well-developed shielding electric field for 1 h. As an effect of penetrating electric fields at the equator, the equatorial ionization anomaly is enhanced during the main phase (because of strong eastward electric field) and is inhibited or reduced due to the strong CEJ (because of westward electric field) during the recovery phase.

DOI： 10.1029/2009JA014562

Development metadata database for upper atmosphere

Koyama, Y, Y.-M. Tanaka, T. Kouno, H. Hayashi, T. Hori, M. Kagitani, D. Yoshida, S. UeNo, S. Abe, A. Shinbori, N. Kaneda

Proceedings of CAWSES-II Kickoff Symposium in Japan page： 135-136 2010

Scientific perspectives powered by IUGONET metadata database and integrated analysis tool

Hori, T, Y. Koyama, Y.-M. Tanaka, M. Kagitani, H. Hayashi, A. Shinbori, S. UeNo, D. Yoshida, S. Abe, T. Kouno, N. Kaneda, H. Tadokoro, IUGONET project members

Proceedings of CAWSES-II Kickoff Symposium in Japan page： 71-74 2010

Response of convection electric fields in the magnetosphere to IMF orientation change Reviewed Open Access

Y. Nishimura, T. Kikuchi, J. Wygant, A. Shinbori, T. Ono, A. Matsuoka, T. Nagatsuma, D. Brautigam

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 114 ( 9 ) 2009.9

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

The transient response of convection electric fields in the inner magnetosphere to southward turning of the interplanetary magnetic field (IMF) is investigated using in-situ electric field observations by the CRRES and Akebono spacecraft. Electric fields earthward of the inner edge of the electron plasma sheet show quick responses simultaneously with change in ionospheric electric fields, which indicates the arrival of the first signal related to southward turning. A coordinated observation of the electric field by the CRRES and Akebono spacecraft separated by 5 RE reveals a simultaneous increase in the dawn-dusk electric field in a wide region of the inner magnetosphere. A quick response associated with the southward turning of the IMF is also identified in in-situ magnetic fields. It indicates that the southward turning of the IMF initiates simultaneous (less than 1 min) enhancements of ionospheric electric fields, convection electric fields in the inner magnetosphere, and the ring or tail current and region 2 FACs. In contrast, a quick response of convection electric fields is not identified in the electron plasma sheet. A statistical study using 161 events of IMF orientation change in 1991 confirms a prompt response within 5 min for 80% of events earthward of the electron plasma sheet, while a large time lag of more than 30 min is identified in electric fields in the electron plasma sheet. The remarkable difference in the response of electric fields indicates that electric fields in the electron plasma sheet are weakened by high conductance in the magnetically conjugated auroral ionosphere.

DOI： 10.1029/2009JA014277

Magnetic latitude and local time dependence of the amplitude of geomagnetic sudden commencements Reviewed

Magnetic latitude and local time dependence of the amplitude of geomagnetic sudden commencements Reviewed

Shinbori Atsuki, Tsuji Yuji, Kikuchi Takashi, Araki Tohru, Watari Shinichi

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 114 2009.4

Shinbori A., Tsuji Y., Kikuchi T., Araki T., Watari S.

Journal of Geophysical Research: Space Physics Vol. 114 ( 4 ) 2009.4

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Journal of Geophysical Research: Space Physics

Statistical analysis of the main impulse (MI) amplitude of geomagnetic sudden commencements (SCs) in a region from the middle latitudes to equator has been made using the long-term geomagnetic field data obtained from the Yap (geomagnetic latitude, θ = 0.38°), Guam (θ = 5.22°), Okinawa (θ = 16.54°), Kakioka (θ = 27.18°), Memanbetsu (θ = 35.16°), and St. Paratunka (θ = 45.58°) stations. The magnetic local time (MLT) dependence of SC amplitude in the middle latitudes showed magnetic field variations produced by two-cell ionospheric currents (DP 2-type currents) which are driven by the dawn-to-dusk electric field accompanying a pair of field-aligned currents (FACs). The effect of the DP 2-type currents at least expands to the low latitude (θ = 16.54°). In this region, the DL part of SC produced by the enhanced Chapman-Ferraro currents can be dominant, but the DP part of SC contaminated 7% of the DL one. On the other hand, at the daytime equator between 8:00 and 16:00 (MLT), the SC amplitude is considerably enhanced with its peak amplitude of 3.24 (normalized SYM-H value) around 11:00 (MLT) due to the Cowling effect. Another interesting point is that the SC amplitude in the nighttime sector was enhanced at all the stations again, and its peak value increases with increasing magnetic latitude. This result suggests that the effect of the FACs associated with the MI phase of SC expands to the equator. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2008JA013871

Seasonal variations of the electron density distribution in the polar region during geomagnetically quiet periods near solar maximum Reviewed Open Access

N. Kitamura, A. Shinbori, Y. Nishimura, T. Ono, M. Iizima, A. Kumamoto

Journal of Geophysical Research: Space Physics Vol. 114 ( A1 ) page： n/a - n/a 2009.1

Language：English Publishing type：Research paper (scientific journal) Publisher：American Geophysical Union (AGU)

Meridional electron density distributions above 45 degrees invariant latitude (ILAT) during geomagnetically quiet periods are statistically studied. Electron density data were obtained from plasma waves observed by the Akebono satellite from March 1989 to February 1991 (near solar maximum) in an altitude range of 274-10,500 km. Field-aligned electron density profiles were fitted by the sum of exponential and power law functions. The transition height, where the power law term equals the exponential term, is highest in the summer (at low solar zenith angle (SZA)) at similar to 4000 km and lowest in the winter (at high SZA) at similar to 1800 km in a region of ILAT >= 70 degrees; this is caused by the larger scale height in the summer (similar to 550 km) than that in the winter (similar to 250 km). The largest seasonal variation and SZA dependence of the electron density are found at an altitude of similar to 2000 km with a factor of similar to 50 (similar to 10(4) /cc in the summer, similar to 10(3) /cc in the winter) in the trough, auroral, and polar cap regions. The seasonal variation and SZA dependence are smaller, about a factor of 5-10, above similar to 5000 km. Day-night asymmetries in each season (within a factor of 5) are smaller than the seasonal variation. The scale height is larger in the dayside than in the nightside in each season. These results indicate that photoionization processes in the ionosphere strongly control electron density distributions up to at least similar to 5000 km in the trough, auroral, and polar cap regions.

DOI： 10.1029/2008JA013288

Magnetic latitude and local time dependence of the amplitude of geomagnetic sudden commencements Reviewed

A. Shinbori, Y. Tsuji, T. Kikuchi, T. Araki, S. Watari

Journal of Geophysical Research A: Space Physics Vol. 114 ( 4 ) 2009

SC-associated Ionospheric Electric Fields at Low Latitude : FM-CW Radar Observation

Ikeda Akihiro, Yumoto Kiyofumi, Shinohara Manabu, Nozaki Kenro, Yoshikawa Akimasa, Shinbori Atsuki

Memoirs of the Faculty of Science, Kyushu University. Series D, Earth and planetary sciences Vol. 32 ( 1 ) page： 1 - 6 2008.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Faculty of Science, Kyushu University

We present the low-latitude ionospheric electric field during the main impulse (MI) phase of two geomagnetic sudden commencements (SCs), observed for the first time by an FM-CW radar. From our observation, the direction of the ionospheric electric field is eastward in the daytime and westward in the nighttime sectors. This difference indicates the penetration of the dawn-to-dusk polar electric field associated with the SCs into the low-latitude ionosphere.

DOI： 10.5109/11811

CiNii Research

SC-associated ionospheric electric fields at low latitude: FM-CW radar observation Reviewed

Ikeda, A, K. Yumoto, M. Shinohara, K. Nozaki, A. Yoshikawa, A. Shinbori

Mem. Fac. Sci., Kyushu Univ., Ser. D, Earth and Planet. Sci.,XXXII, 1-6 Vol. 32 ( 1 ) page： 1 - 6 2008.2

Generation mechanism of Z-mode waves in the equatorial plasmasphere Reviewed

Nishimura Y., Ono T., Iizima M., SHINBORI A., KUMAMOTO A.

Earth, planets and space Vol. 59 ( 9 ) page： 1027 - 1034 2007.9

Evolution of ring current and radiation belt particles under the influence of storm-time electric fields Reviewed Open Access

Y. Nishimura, A. Shinbori, T. Ono, M. Iizima, A. Kumamoto

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

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

Electric field and potential distributions in the inner magnetosphere during geomagnetic storms have been investigated using the Akebono/EFD data. Using this electric field, we study injection of ring current particles and acceleration of radiation belt electrons by single-particle calculations. During the main phase, the dawn-dusk electric field is intensified especially in a range of 2 < L < 5 with a maximum amplitude of 6 mV/m on the duskside, and a two-cell convection pattern with a potential difference of 180 kV is identified. The convection pattern on the equatorial plane is significantly distorted with a large potential drop of 70 kV on the dawn and dusk sectors, indicating an intrinsic source of large-scale electric field in the inner magnetosphere. The plasma sheet ions are gathered into the dusk to premidnight sector in the inner magnetosphere in the region of enhanced electric field due to the strong E x B drift. The ions are transported into around 4 R-E with an acceleration of more than 1 order of magnitude within 40 min, conserving the first adiabatic invariants. Relativistic electrons with initial energy of some hundreds of kiloelectron volts at 5 R-E are energized to more than 100 keV for 3 hours. The energy spectrum during the recovery phase of 9 October 1990 geomagnetic storm observed by the CRRES satellite is reproduced without the radial diffusion or nonadiabatic acceleration by plasma waves. It is possible that this acceleration process is the inhomogeneity of the large-scale electric field, which corresponds to the del x E term along orbits of electrons around the Earth.

DOI： 10.1029/2006JA012177