Categorization of the Time Sequence of Events Leading to Substorm Onset Based on THEMIS All-Sky Imager Observations
The sequence of events leading to substorm auroral onset has been a long-standing issue in substorm research. Based on statistical studies using THEMIS all-sky imager data, we have recently reported evidence that most substorm onset events are preceded by a pre-onset auroral form which is a distinct north-south arc originating from an poleward boundary intensification (PBI) and reaches the auroral onset region just before onset. This onset sequence was found to be a repetitive process; it is detected in 84% of 249 events between November 2007 and April 2008. A high occurrence of PBIs (84%) emphasizes an abrupt flux transport across the open-closed field line as initiation of the onset sequence. Here we present a variation of the onset sequence we have previously reported and two less frequently observed types of onset time sequence: poleward boundary contact and Harang aurora deformation. While poleward boundary contact events also start with PBIs, the auroral oval width becomes much narrower (∼2° MLAT) prior to onset, indicating that the plasma sheet is thin and the nightside magnetic separatrix is located closer to the near-Earth onset region. Harang auroral deformation events are not associated with an observed PBI, but the equatorward portion of a pre-existing Harang aurora bends equatorward, which indicates a rapid convection change leading to onset. All of those three categories of events suggest that new plasma intrusion toward onset location changes the pressure profile in the near-Earth region and leads to onset instability.
KeywordsPlasma Sheet Magnetic Local Time Auroral Oval Substorm Onset Onset Location
This work was supported by National Science Foundation grants ATM-0646233 and ATM-0639312, NASA grant NNX07AF66, NASA contract NAS5-02099, and JSPS Research Fellowships for Young Scientists. Deployment of the THEMIS ASIs was partly supported by CSA contract 9F007-046101. Alaska magnetometer data were obtained from Geophysical Institute of University of Alaska, Fairbanks. NOAA satellite data were provided through the NOAA’s National Geophysical Data Center.
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