Statistical characteristics of plasma flows associated with magnetic dipolarizations in the near-tail region of r < 12 RE

Abstract

The magnetic dipolarization in the tail close to Earth is often associated with plasma flows of some magnitude, and the flow direction can be critically related to the substorm triggering issue. In this work we identified 167 events of dipolarizations in the near-Earth tail (r ≈ 7-12 RE) using Time History of Events and Macroscale Interactions during Substorms (THEMIS) tail observations and examined plasma flow patterns associated with the dipolarizations. We first find that ∼50% of the dipolarizations in this near-tail region initiate with plasma flows of a significant magnitude (>100 km/s). Out of such fast flow events, ∼78% are earthward flows and ∼22% are tailward flows, which we interpret as rebound of the earthward flows. The overall occurrence rate of the fast flows (including both earthward and tailward flows) noticeably decreases earthward: It drops from ∼47% at r ≈ 8-9 RE to ∼27% at r ≈ 7-8 RE. This implies that the earthward flows can seldom penetrate inside of 7-8 RE, consistent with previous reports. Incidentally, we find that the flow directions often change during the course of the entire dipolarization process whether dipolarization begins with an earthward or tailward flow. Nevertheless, the net magnetic transport is earthward during the main duration of most of the dipolarizations, even if they initiate with dominantly tailward flows at the onset. Copyright 2012 by the American Geophysical Union.