HR: 10:50h
AN: SM12A-03    [Abstracts]
TI: Equatorial measurement of SAID electric fields and relation with the plasmapause location
AU: * Nishimura, Y
EM: yukitoshi@stpp1.geophys.tohoku.ac.jp
AF: Department of Geophysics, Tohoku University, Japan., 6-3, Aramaki-Aza-Aoba, Sendai, MI 980-8578, Japan
AU: Wygant, J
EM: wygant@ham.space.umn.edu
AF: School of Physics and Astronomy, University of Minnesota, USA., Tate Lab, 116 Church Street, S.E., Minneapolis, MN 55455, United States
AU: Ono, T
EM: ono@stpp1.geophys.tohoku.ac.jp
AF: Department of Geophysics, Tohoku University, Japan., 6-3, Aramaki-Aza-Aoba, Sendai, MI 980-8578, Japan
AU: Iizima, M
EM: iizima@stpp1.geophys.tohoku.ac.jp
AF: Department of Geophysics, Tohoku University, Japan., 6-3, Aramaki-Aza-Aoba, Sendai, MI 980-8578, Japan
AU: Kumamoto, A
EM: kumamoto@stpp1.geophys.tohoku.ac.jp
AF: Department of Geophysics, Tohoku University, Japan., 6-3, Aramaki-Aza-Aoba, Sendai, MI 980-8578, Japan
AU: Brautigam, D
EM: Donald.Brautigam@hanscom.af.mil
AF: Air Force Research Laboratory, 29 Randolph Road, Hanscom AFB, MA 01731, United States
AU: Rich, F
EM: Frederick.Rich@hanscom.af.mil
AF: Air Force Research Laboratory, 29 Randolph Road, Hanscom AFB, MA 01731, United States
AB: In order to investigate the equatorial source of subauroral ion drifts (SAID) and its association with the plasmapause position, multi-spacecraft measurements of SAID are presented using the CRRES, Akebono, and DMSP. Direct measurement of the convection electric field and plasmapause density close to the equator is measured by the electric field instrument onboard the CRRES satellite, and the plasmasheet electrons and low energy part of the ring current ions are measured by the low energy plasma instrument. The CRRES satellite is on the dusk inner magnetosphere, and the DMSP-F8 and Akebono satellites are approximately on the same field line. Associated with a substorm onset at 16:40 UT on February 20, 1991, the DMSP-F8 satellite at 19 MLT measures SAID with a maximum westward velocity of 1,500 m/s. The CRRES satellite is on outbound in the inner magnetosphere at ~21 MLT and ~5 RE at the onset of the substorm. It measures increase of DC electric field with 0.4 mV/m in the plasmasphere just after the substorm onset. Thirty minutes later, injection of ring current ions are observed in the plasmasphere with Bz decrease. After the crossing of the plasmapause, the electric field increases to 0.8 mV/m. At the same time, the spacecraft enters the plasmasheet, and the DC electric field disappears. The same time sequence is also identified in other SAID events detected on the dusk inner magnetosphere. The above CRRES measurement indicates that DC electric field is intensified in a narrow region between the ring current and electron plasmasheet after the onset of the substorm. Although the E*B drift points sunward in this region, this region with enhanced electric field is filled with plasmaspheric plasma without abrupt density change. The position where the convection electric field is equal to the corotation electric field locates inside the plasmapause. The plasmapause coincides with inner edge of the plasmasheet. This association suggests that the plasmaspheric plasma is depleted by the plasmasheet electrons, possibly by the enhanced E*B drift earthward of the plasmasheet. During the SAID event on 16:40 UT on February 20, 1991, the Akebono satellite was approximately on the same field line of the CRRES satellite (21 MLT and 5 RE) 40 minutes later the substorm onset. It measures enhancement of electric field with 2 mV/m between L=5 and 6. The inner edge of the electric field corresponds to the inner edge of ring current ions, and the outer edge coincides with the plasmasheet electrons. This signature of the electric field intensification in the charge-separated region is in accordance with the CRRES measurement. This study has clarified that the equatorial source of SAID electric fields is charge separation of ring current ions and plasmasheet electrons by electric field associated with substorms. This is consistent with the theoretical study by Southwood and Wolf [1978] and low-altitude measurements by Anderson et al. [2001] by that the charge separation provides current and voltage sources and the electric field is increased by the low conductance of the subauroral ionosphere.
UR: http://stpp1.geophys.tohoku.ac.jp
DE: 2730 Magnetosphere: inner
DE: 2736 Magnetosphere/ionosphere interactions (2431)
DE: 2760 Plasma convection (2463)
DE: 2768 Plasmasphere
DE: 2788 Magnetic storms and substorms (7954)
SC: SPA-Magnetospheric Physics [SM]
MN: 2007 Fall Meeting