%0 Journal Article
%T Brightening of onset arc precedes the dipolarization onset: THEMIS observations of two events on 1 March 2008
%A J. R. Kan
%A H. Li
%A C. Wang
%A H. U. Frey
%A M. V. Kubyshkina
%A A. Runov
%A C. J. Xiao
%A L. H. Lyu
%A W. Sun
%J Annales Geophysicae (ANGEO)
%D 2011
%I Copernicus Publications
%X We present a new M-I coupling model of substorm during southward IMF based on the THEMIS observations of two events on 1 March 2008. The first event (E-1) was classified as a pseudo-breakup: brightening of the onset arc preceded the first dipolarization onset by ¡«71 ¡À 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 ¡À 3 s, leading to a full-scale expanding auroral bulge during the substorm expansion phase for ¡«20 min. The Alfv¨¦n travel time from the ionosphere to the dipolarization onset region is estimated at ¡«69.3 s in E-1; at ¡«80.3 s 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-1 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 Alfv¨¦n wavefront is anti-parallel to the cross-tail current. Dipolarization onset occurs when the Alfv¨¦n 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.
%U http://www.ann-geophys.net/29/2045/2011/angeo-29-2045-2011.html