Paper#: 75
Poster #: 24
Session Name: Poster Session 2
Room: East Ballroom
Day: Thursday
Time: 10:10 a.m.-Noon
Abstract Title: Magnetotail Deflation Associated with Substorm Expansions: A New Substorm Mode
PresentSurname: Nakai, H.
All Authors: Hitoshi Nakai, Y. Kamide, M. Brittnacher
Abstract : Utilizing UVI data from the Polar spacecraft and plasma/field data from the Geotail spacecraft, three clear-cut substorms are studied. A precursory auroral breakup occurred 30 - 40 min before a major expansion, developing to a full-scale auroral bulge. The bulge moved eastward with a decrease in the auroral intensity. The magnetic field fluctuated violently, by sustaining a rather constant total pressure at Geotail, even though the ionospheric foot point of the spacecraft was located within the bulge. No high-speed plasma flow was observed at Geotail in the plasma sheet. A major auroral breakup occurred in the premidnight sector near the western edge of the existing auroral bulge. The active auroral region expanded poleward and eastward. A magnetotail deflation, i.e., a drastic decrease in the total pressure accompanied by the dipolarization and the plasma-sheet expansion, occurred at Geotail when the active aurora reached the ionospheric foot point of the spacecraft. On the basis of the present observations, we suggest that the source region of the substorm wedge current at the precursory breakup was caused by distant-tail magnetic field reconnection, while the wedge current at the major breakup was originated from the mid-magnetotail neutral line. Instability at the duskside boundary between the sunward and tailward large-scale plasma flows is probably responsible for the onset of major breakups.