Authors

L. R. Lyons
Department of Atmospheric Sciences
University of California, Los Angeles
Los Angeles, CA 90095-1565

R. L McPherron
Institute of Geophysics and Planetary Physics
University of California, Los Angeles
Los Angeles, CA 90095-1567

E. Zesta
Department of Atmospheric Sciences
University of California, Los Angeles
Los Angeles, CA 90095-1565

G. D. Reeves
NIS-2 Mail Stop D436
Los Alamos National Laboratories
Los Alamos, NM 87545

J. Sigwarth and L. A. Frank
Department of Physics and Astronomy
The University of Iowa
Iowa City, Iowa 52242

Journal of Geophysical Research, 106, 349, 2001

Abstract

An excellent data set has been gathered for the November 24, 1996 GEM substorm interval that included an ~95 min interval of strongly southward IMF. There were two expansion phase onsets and a pseudo breakup during this period. For both onsets, the classical signatures of onset in the auroral ionosphere (ground magnetometer signatures of electrojet formation, ground observations of Pi 2 pulsations, and auroral brightening) all occurred within less than a minute of each other, indicating consistency between these onset indicators and giving reliable identification of onset times. On the other hand, low-latitude positive bay observations that were most likely made within the longitude range of substorm onset did not provide accurate onset identifications or accurate onset timing. Low-latitude Pi 2šs identified both onsets, though one was delayed by 1 min from the auroral zone onset. Our most important and unexpected result is that current wedge formation, as indicated by magnetic field dipolarization, was observed at synchronous orbit ~2.5 min prior to both expansion onsets in the auroral ionosphere. We suggest that the early detection of dipolarization for these onsets was at least in part due to the unusually low latitude (63° magnetic) of the auroral-zone onsets, which maps closer to synchronous orbit that usual. Onsets more often occur at somewhat higher latitudes, which map to a few RE beyond synchronous orbit. As a result, dipolarizations at synchronous orbit are generally delayed with respect to the time of current wedge initiation We do not currently know whether or not current wedge initiation generally occurs ~2.5 min prior to expansion onset; however we have no reason to believe that the timing that we have observed is unique.

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Paper (pdf format)
Figures: 1-4, 5-9(PDF format)