Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan

In the framework of IACG Campaign #2, we study a case in which the GEOTAIL spacecraft was close to the mid-tail magnetopause when three substorms took place. GEOTAIL was mainly located within the dawnside plasmasheet, and its average speed normal to the nominal magnetopause was very small (~0.2 R_E/hr) because it was near its apogee. It exited to the LLBL ~ magnetosheath during the expansion to recovery phases of each substorm, stayed there for a while, and then returned to the plasmasheet near the end of the recovery phase. This pattern suggests a shrinkage of the mid-tail magnetosphere after the expansion onset so that the tail radius becomes smaller than that in the quiet time. We interpret it as a result of strong tailward flows from the near-Earth neutral line which transports plasma in the mid-tail region to the distant tail and causes a rarefaction in the mid-tail region.

Adv. Space Sci., Vol. 25, No. 7/8, 1689-1696, 2000