Extracting physics from substorm dynamics observed by IMAGE/HENA

P. C:son Brandt, D. G. Mitchell, S. Ohtani, E. C. Roelof, R. Demajistre


HENA has observed ENA emissions from the ring current (L<6) and out to 
L=13 for periods of of southward IMF when the geomagnetic field is 
stretched. During dipolarization the ENA flux from the L<8 region 
decreases rapidly. In this presentation we discuss what physical
mechanism may lie behind the observed dynamics in the light of in-situ
observations. It is believed that the flux decrease during
dipolarization is caused by the dipolarization induced E-field, which
brings ions to lower L-shells faster than they can be replinished by
the overall convection, creating a void of particles in the plasma
sheet. This scenario implies that the acceleration region must be
localized to, from L=8 out to, at least L=13. We also discuss what can
be said about transport during the dipolarization. Is it Earthward?
Can we see where the dipolarization starts? We show a few examples of
weak and still uncertain signatures of both Earthward and anti
earthward propagation. We present different scenarios of how these
signatures can be explained.

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To be presented at the 2002 Spring A.G.U. Meeting, 
Washington, D.C., U.S.A., 28-31 May 2002