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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 29, issue 11 | Copyright
Ann. Geophys., 29, 2045-2059, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

Regular paper 17 Nov 2011

Regular paper | 17 Nov 2011

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

J. R. Kan1,2, H. Li1, C. Wang1, H. U. Frey3, M. V. Kubyshkina4, A. Runov5, C. J. Xiao6, L. H. Lyu7, and W. Sun2 J. R. Kan et al.
  • 1National Space Science Center, CAS, Beijing, China
  • 2Geophysical Institute, University of Alaska Fairbanks, AK, USA
  • 3Space Sciences Laboratory, UC Berkeley, CA, USA
  • 4St. Petersburg State University, St. Petersburg, Russia
  • 5Institute of Geophysics and Planetary Physics, UCLA, CA, USA
  • 6School of Physics, Peking University, Beijing, China
  • 7Institute of Space Science, National Central University, Chung-Li, Taiwan

Abstract. 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 ± 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 ∼20 min. The Alfvén 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 Alfvén wavefront is anti-parallel to the cross-tail current. Dipolarization onset occurs when the Alfvén 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.

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