THEMIS observations of electron acceleration associated with the evolution of substorm dipolarization in the near-Earth tail

摘要

We present the evolution of dipolarizations in the near-Earth tail during a substorm on 15 March 2009, based on the two-point measurements in the nightside plasma sheet at X ～ -8.0 R_E. The earthward-moving dipolarization, the magnetic flux pileup, and the tailward-moving dipolarization were observed. For the 30-200 keV electrons, betatron acceleration was the dominant process, which was caused by the much larger gradient of the magnetic field there during the earthward-moving dipolarization or by a local compression of the magnetic field during the magnetic flux pileup and tailward-moving dipolarization. These near-perpendicular distributions for the 30-200 keV electrons are interpreted as produced by a two-step acceleration: Electrons were first accelerated in the dipolarization fronts in the midtail or the near-Earth tail and then were further accelerated near the tail current disruption region. For the more than 200 keV electrons, Fermi acceleration was the dominant process, which was caused by the shrinking length of magnetic field line during the tailward-moving dipolarization. The source region of the more than 200 keV electrons may be near the tail current disruption region, but these electrons were accelerated locally. These field-aligned electrons can precipitate into the ionosphere and form the discrete auroral arcs. Two parallel arcs were clearly observed around the substorm onset: one propagated equatorward, another propagated poleward. We suggest that the earthward-moving dipolarization, the magnetic flux pileup, and the tailward-moving dipolarization near the tail current disruption region can well explain the auroral evolution around the substorm onset.