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 substorm duringsouthward IMF based on the THEMIS observations of two events on 1 March2008. The first event (E-1) was classified as a pseudo-breakup: brighteningof the onset arc preceded the first dipolarization onset by ∼71 ± 3 s,but the breakup arcs faded within ∼5 min without substantialpoleward expansion and the dipolarization stopped and reversed to thinning.The second event (E-2) was identified as a substorm: brightening of thesecond onset arc preceded the second dipolarization onset by ∼80 ± 3 s, leading to a full-scale expanding auroral bulge during the substormexpansion phase for ∼20 min. The Alfvén travel time from theionosphere to the dipolarization onset region is estimated at ∼69.3 sin E-1; at ∼80.3 s in E-2, which matched well with the observedtime delay of the dipolarization onset after the brightening of the onsetarc, respectively in E-1 and E-2. Brightening of the onset arc precedes thedipolarization onset suggest that the onset arc brightening is caused by theintense upward field-aligned currents originating from the divergence of theCowling electrojet in the ionosphere. The Cowling electrojet current loop(CECL) is formed to close the field-aligned currents at all times. Theclosure current in the Alfvén wavefront is anti-parallel to thecross-tail current. Dipolarization onset occurs when the Alfvénwavefront incident on the near-Earth plasma sheet to disrupt the cross-tailcurrent in the dipolarization region. Slow MHD waves dominate the disruptionof the cross-tail current in the dipolarization region.