Observations of the phases of the substorm

Abstract

Following the database of large-scale vortices during pseudo-breakup and breakup registered by the Gillam All-Sky Imager, we selected one event (19 February 1996) for a detailed consideration. This event is a sequence of pseudo-breakup and local substorm, and breakup followed by the large substorm, which is isolated from the previous pseudobreakup by the second growth phase. Commencement of these elements of auroral activity was clearly seen above the Churchill line of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS; pseudo-breakup was completely covered by the field of view of the Gillam All-Sky Imager). Geotail was located at ∼19 RE in the equatorial plane of midnight sector, which, along with supporting observations from two geostationary satellites (GOES 8 and 9), allowed for a comparison of ground-based, geostationary orbit and midtail signatures. The pseudo-breakup consisted of two distinct stages: a near-exponential arc intensity growth and a poleward vortex expansion that started simultaneously with dipolarization in the inner magnetosphere. The latter corresponded to explosive onset of short-period (tens of millihertz) pulsations observed at geostationary orbit and on the ground in the vicinity of the arc. No significant disturbances poleward of the vortex were observed. Pseudo-breakup was followed by the second growth phase, which involved a significant thinning of the plasma sheet. Breakup was of a similar two-stage character as the pseudo-breakup. Full onset of the expansive phase that followed breakup was seen simultaneously by all instruments including Geotail, which detected strong perturbations in the midtail. The expansive phase onset launched the second postbreakup package of Pi2 pulsations that were of larger amplitude. Finally, during the substorm recovery phase, the poleward boundary intensifications (PBIs) were observed as long-period, on the order of 10 min, pulses of electron precipitation. PBI commencement coincided with bursty flows and pulses of plasma energization in the midtail. Observed features support recent ideas claiming that we are dealing with processes (breakup, full onset of the expansive phase, and PBIs) of a distinct physical nature that require different commencement thresholds, namely, the inner plasma sheet instability (pseudo-breakup and breakup), midtail reconnection (expansive phase onset), and further magnetotail dynamics during the recovery phase (PBIs). Copyright 2003 by the American Geophysical Union.