Geotail spacecraft observations of plasma flow rotations at magnetic field dipolarization in near-tail during substorm expansion

摘要

The magnetic field dipolarization at near-tail is one of the key elements in the substorm phenomena, and it often accompanies the plasma flow in tail in some pattern. In this paper, we present 4 events of dipolarization observed from the Geotail satellite, in which the plasma flow rotates in X-Y plane. We found them out of 30 events, identified for 8 months period while Geotail was in the near-tail plasma sheet region defined by X_(GSE) > -15R_E and |Y_(GSE)| < 10R_E. In some cases of those 4 events we find flow rotations that begin nearly simultaneously with the dipolarization initiation and continues afterward for some time. In the other cases, they even precede the initiation of dipolarization by some minutes and continue throughout the maximum interval of the remaining dipolarization period. These flow rotations reveal no features of bursty bulk flows (BBF) and the speed of flow components is moderate, less than ～300 km/s. Also, flow rotations presented here are distinguished from the previously reported ones which appear only after, and thus presumably caused by, the passage of the earthward BBF (thus seen only well after the initiation of the dipolarization). Overall, we find difficulties and inconsistencies in relating the observed flow rotations at dipolarization to BBF as a possible cause. Based on intuitive ground, instead we suggest an alternative, though tentative, possibility that they may be a manifestation of some near-tail instability during the substorm expansion.