Authors

L. R. Lyons1,I. Voronkov 2, E. Donovan3,E. Zesta 1

1  Department of Atmospheric Sciences
University of California, Los Angeles
Los Angeles, CA 90095-1565

2  Department of Physics
University of Alberta
Edmonton, Alberta T6G 2J1

3  Physics and Astronomy Department
University of Calgary
Calgary, Alberta T2N 1N4

J. Geophys. Res., 107(A11), 1390, doi:10.1029/2002JA009317, 2002

Abstract

High-resolution CANOPUS meridian-scanning photometer and all-sky imager observations of pre-onset and expansion-phase auroral arcs are analyzed for expansion-phase onsets that evolve into full substorms and into pseudobreakups. One or more arcs are seen across the sky throughout the growth phase prior to onset. We find that auroral break-up at onset does not generally occur along one of these arcs, but instead often occurs along a thin break-up arc that forms equatorward of all growth phase arcs a few minutes prior to onset. The intensity of this breakup arc increases monotonically for the few minutes prior to the time normally identified as substorm onset and then typically increases dramatically. These results imply that the processes responsible for auroral breakup initiate a few minutes prior to the time normally identified as substorm expansion-phase onset. We also find that arcs poleward of the arc that breaks up appear to be unaffected by substorm onset until expansion-phase auroral activity moves poleward to the location of such arcs. Arcs poleward of the poleward-most extent of pseudobreakup auroral activity show no apparent effects of a pseudobreakup. This suggests that expansion phase does not significantly affect the portion of the plasma sheet lying anti-sunward of the field lines having expansion-phase auroral activity until expansion phase activity moves poleward to the field lines of those regions. For pseudobreakups, the observations imply that expansion phase activity never significantly affects plasma sheet regions anti-sunward of the poleward-most extent of field lines having auroral activity. Since the onset of full substorms and of pseudo-breakups appear to be by the same process, our observations imply that the process that initiates the onset of substorms does not require the occurrence of plasma sheet changes, significant enough to affect magnetosphere-ionosphere electrodynamics, along field lines that cross the equator tailward of the substorm onset region.

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