Abstract
Abstract This study investigates two substorm onset events with favorable constellations of spacecraft, TC-1 and Cluster, separated by several Earth radii. The substorms have been identified in both auroral regions. One is believed to be triggered by a northward turning of the interplanetary magnetic field (IMF), while the other occurs under generally southward IMF. For both events, Cluster was located tailward of TC-1, but observed the dipolarization at earlier time for one event while at later time for the other. The timing difference of dipolarization at different positions could be explained by the earthward or tailward propagation of the field disturbances in the radial direction. The earthward dipolarization front was found in one case to bounce back and forth at TC-1. The earthward propagation was accompanied by a fast earthward plasma flow for the 21 September 2005 event. The event analysis suggests that substorms can be quite different from case to case due to differences in the solar wind condition and magnetotail configuration.
Highlights
A magnetospheric substorm is a complicated phenomenon that is not fully understood
The cross-tail current instability (CCI) model regards the near Earth region (6–10RE) as the initial region (Lui, 1996). It postulates that plasma instabilities generate a cross-tail current disruption and a divergence into the ionosphere via field-aligned currents (FACs) to form the substorm current wedge (SCW)
For the 2004 substorm event, TC-1 observed the onset of the field dipolarization earlier than Cluster, but observed the general dipolarization feature later than Cluster
Summary
A magnetospheric substorm is a complicated phenomenon that is not fully understood. One of the most controversial topics concerns the exact physical conditions that initiate a substorm. The CCI and NENL models can be distinguished by comparing the time at which reconnection is observed in the tail, utilizing in situ satellite data, with the time when the substorm onset is detected at near-Earth. Wang et al (2006) reported a typical substorm onset event with a fortunate constellation of Cluster and Double Star near the current sheet, which could be one “textbook” example of a substorm due to its distinct character. Both ionosphere and magnetotail observations have shown a clear dawnward propagation of the substorm current wedge. These previous studies have exhibited a current wedge with a large radial extent, which may be another class of substorm that is different from the CCI model and
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