Near‐Earth dipolarization: Evidence for a non‐MHD process

Abstract

We have investigated a near‐Earth dipolarization event in the midnight sector using simultaneous observations of Polar and Geotail. We have found evidence for near‐Earth dipolarization to be a non‐MHD process: dipolarization occurring without significant plasma flow or with tailward flow and during dawnward electric field different from that inferred based on the frozen‐in condition. These observations are inconsistent with the idea that dipolarization is an MHD process of magnetic flux pileup from braking of sunward plasma flow. Possible variances of the flow braking scenario are considered but none is satisfactory in accounting for the observed features. On the other hand, these findings are quite consistent with the expectations from the current disruption scenario.