Substorm Triggering by New Plasma Intrusion 1: THEMIS All-Sky Imager Observations

Details Substorm Triggering by New Plasma Intrusion 1: THEMIS All-Sky Imager Observations Substorm Triggering by New Plasma Intrusion 1: THEMIS All-Sky Imager Observations

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm41b1708n&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SM41B-1708

Physics

[2407] Ionosphere / Auroral Ionosphere, [2790] Magnetospheric Physics / Substorms

Scientific paper

A critical and long-standing problem for physical understanding of substorms is determination of the sequence of events that leads to onset. We have identified a remarkably repeatable sequence of events in observations from the multi-station array of THEMIS all-sky imagers (ASI). First, a poleward boundary intensification (PBI) forms or intensifies near the polar cap boundary of the auroral oval. Next an approximately north-south (NS) oriented arc extends equatorward from the PBI and extends equatorward toward the equatorward portion of the auroral oval. In most cases, the aurora merges with a preexisting east-west growth phase arc equatorward of the Harang flow reversal, which is inferred from clockwise auroral motions. The motion of the precursor aurora then turns westward leading to breakup when the westward motion reaches the breakup region, which is generally located in the premidnight sector. In some cases, the equatorward moving NS arc contacts the breakup region from the west, leading to breakup closer to or dawnward of midnight. We have identified arcs moving toward the breakup region in 81% of 260 events having good ASI coverage of the breakup region, the initiating PBI and ensuing NS arc often also being seen when imaging from an appropriate more poleward location is available. No precursor auroral activity is detected for only 6% of events. The above sequence of auroral motions demonstrates a clear association between poleward auroral activity and auroral breakup. We suggest that this temporal sequence results from new plasma entering the plasma sheet with lower global entropy than that of the pre-existing plasma sheet plasma. This new lower entropy plasma then moves equatorward (earthward in the equatorial plane) adjacent to and on the eastward side of the observed PBI and NS arc with enhanced flows, bringing the new plasma toward the onset region.

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