THEMIS observations of substorms on 26 February 2008 initiated by magnetotail reconnection

摘要

On 26 February 2008, the THEMIS satellites observed two substorms that occurred atabout 0405 and 0455 UT. Angelopoulos et al. (2008) made a comprehensive study of thesecond event. In this paper we display detailed features of the two substorms withemphasis on the first. In both substorms, a distinct auroral intensification occurred duringthe earliest stage of onset, about 1 to 2 min after midtail reconnection began. This initialintensification was weak and localized and thus had the signatures of a pseudobreakup.In both substorms, a second, major intensification occurred next in the substorm onsetsequence, followed by rapid and extensive poleward expansion. This secondintensification had the features of the major expansion onset and was nearly coincidentwith observations of earthward flows and magnetic dipolarization in the near-Earth tail.During the growth phase of the two substorms, open magnetic flux accumulated inthe polar cap; in the expansion/recovery phase the polar cap open flux was quicklyreduced. These observations are in agreement with the assertion that tail reconnectioninitiates the initial pseudobreakup and the ensuing major expansion and releases andtransports energy to eventually cause near-Earth dipolarization and the expansion phaseonset of these two substorms.