Timing and localization of near-Earth tail and ionospheric signatures during a substorm onset.

On 16 February 2008, the THEMIS spacecraft bracketed the near-Earth signatures of a substorm onset as identified in the THEMIS ground-based observatories measuring an AETH index up to 180 nT. The main onset was associated with the formation and tailward release of a plasmoid (a proto-plasmoid) at XGSM = - 18.3 RE, and a dipolarization in the inner part of the plasma sheet at XGSM = -11.0 RE. Using timing analysis between the spacecraft we approximate where and when the main onset occurs. The time history and geometry of the event in the tail is consistent with magnetic reconnection as the cause of the substorm expansion onset process. Two activations of the plasma sheet, evidenced by tailward streaming of energetic ions and southward or bipolar signatures of the magnetic field preceded the main substorm. The first activation was associated with an intensification of an arc observed separately from the substorm arc, while the second with the onset of ULF pulsations at mid- and low-latitude stations. We conclude that near-Earth plasma sheet activity that may also be due to reconnection and may be related to non-substorm arc intensifications can precede substorm onset by several minutes.