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

The magnetic field dipolarization at near-tail is one of the key elements i n the substorm phenomena, and it often accompanies the plasma flow in tail in some pattern. In this paper, we present 4 events of dipolarization obser ved from the Geotail satellite, in which the plasma flow rotates in X-Y pla ne. We found them out of 30 events, identified for 8 months period while Ge otail was in the near-tail plasma sheet region defined by X-GSE > - 15R(E) and \ YGSE \ < 10R(E). In some cases of those 4 events we find flow rotatio ns that begin nearly simultaneously with the dipolarization initiation and continues afterward for some time. In the other cases, they even precede th e initiation of dipolarization by some minutes and continue throughout the maximum interval of the remaining dipolarization period. These flow rotatio ns reveal no features of bursty bulk flows (BBF) and the speed of flow comp onents is moderate, less than similar to 300 km/s. Also, flow rotations pre sented here are distinguished from the previously reported ones which appea r only after, and thus presumably caused by, the passage of the earthward B BF (thus seen only well after the initiation of the dipolarization). Overal l, we find difficulties and inconsistencies in relating the observed flow r otations at dipolarization to BBF as a possible cause. Based on intuitive g round, instead we suggest an alternative, though tentative, possibility tha t they may be a manifestation of some near-tail instability during the subs torm expansion. (C) 2001 Published by Elsevier Science Ltd.