Brightening of onset arc precedes the dipolarization onset: THEMIS observations of two events on 1 March 2008

摘要

We present a new M-I coupling model of sub-storm during southward IMF based on the THEMIS observations of two events on 1 March 2008. The first event (E-l) was classified as a pseudo-breakup: brightening of the onset arc preceded the first dipolarization onset by ～71 ± 3 s, but the breakup arcs faded within ～5 min without substantial poleward expansion and the dipolarization stopped and reversed to thinning. The second event (E-2) was identified as a substorm: brightening of the second onset arc preceded the second dipolarization onset by ～80 ± 3 s, leading to a full-scale expanding auroral bulge during the substorm expansion phase for ～20min. The Alfven travel time from the ionosphere to the dipolarization onset region is estimated at ～69.3s in E-l; at ～80.3s in E-2, which matched well with the observed time delay of the dipolarization onset after the brightening of the onset arc, respectively in E-l and E-2. Brightening of the onset arc precedes the dipolarization onset suggest that the onset arc brightening is caused by the intense upward field-aligned currents originating from the divergence of the Cowling electrojet in the ionosphere. The Cowling electrojet current loop (CECL) is formed to close the field-aligned currents at all times. The closure current in the Alfven wavefront is anti-parallel to the cross-tail current. Dipolarization onset occurs when the Alfven wavefront incident on the near-Earth plasma sheet to disrupt the cross-tail current in the dipolarization region. Slow MHD waves dominate the disruption of the cross-tail current in the dipolarization region.