Emergence of the active magnetotail plasma sheet boundary from transient, localized ion acceleration

摘要

Observations of the Earth's magnetotail plasma sheet boundary layer (PSBL) have been typically accompanied by field-aligned crescent-shaped ion beams, thought to emanate at distant or mid-tail semi-permanent or impulsive acceleration sites. Typically such observations, and the theoretical and modeling efforts to explain them, have been disjoint from the adjacent plasma sheet properties near the equatorial projection of the observation. Thus the plasma sheet boundary layer has been thought of as a harbinger of remote, rather than local plasma sheet activity, exception of plasma sheet expansions during the recovery phase of substorms. Using case and statistical studies from THEMIS, obtained simultaneously at the near-Earth PSBL and at its adjacent central plasma sheet (CPS), we study the transient and impulsive nature of PSBL beams and their inherent connection with CPS bursty bulk flows and associated dipolarization fronts. We show that PSBL beams typically commence a few minutes before CPS flow bursts, which in turn are seen tens of seconds ahead of the arrival of dipolarization fronts. These timing correlations, the crescent shapes of PSBL ion beams, the CPS ion flux enhancements in the earthward and dawnward directions, and other particle distribution characteristics can all be well reproduced by a simple model of ion reflection and acceleration at earthward-propagating dipolarization fronts associated with CPS flow bursts. The emerging paradigm, therefore, unifies impulsive transport phenomena across latitudes in the near-Earth magnetotail plasma sheet.