Timing and localization of near-Earth tail and ionospheric signaturesduring a substorm onset

摘要

[1] On 16 February 2008, the THEMIS spacecraft (probes) bracketed the near-Earthsignatures of a substorm onset as identified in the THEMIS ground-based observatoriesmeasuring an AETH index up to 180 nT. The main onset was associated with the formationand tailward release of a plasmoid (a proto-plasmoid) at X_(GSM) =—18.3R_Eand adipolarization in the inner part of the plasma sheet at X_(GSM) = —11.0R_E.The time historyand geometry of the event in the tail are consistent with magnetic reconnection as thecause of the substorm expansion onset process. Two activations of the plasma sheet,evidenced by tailward streaming of energetic ions and southward or bipolar signatures ofthe magnetic field, preceded the main substorm. The first activation was associatedwith an intensification of a high-latitude arc, while the second was associated with theonset of ULF pulsations at midlatitude and low-latitude stations. We conclude thatnear-Earth plasma sheet activity that may also be due to reconnection and may be relatedto nonsubstorm arc intensifications can precede substorm onset by several minutes.In particular, high-latitude arcs do not appear to result in substorm initiation eventhough they may occur in close temporal and spatial proximity to the substorm arc.