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Stormtime substorm onsets: occurrence and flow channel triggering
Larry R Lyons1, Ying Zou2,3, Yukitoshi Nishimura4
11Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095-1565 USA.
Earth, Planets, and Space : EPS
|July 2, 2019
Summary
Auroral streamers, observed by THEMIS all-sky-imagers, frequently trigger geomagnetic substorm onsets. Substorm occurrences are more common during high-speed stream (HSS) storms than coronal mass ejection (CME) storms.
Area of Science:
- Space Physics
- Geomagnetism
- Plasma Physics
Background:
- Geomagnetic storms feature bright auroral emissions, offering opportunities to study substorm onset triggers.
- Previous proposals suggest plasma sheet flow bursts, appearing as ionospheric auroral streamers, frequently initiate substorm onsets.
Purpose of the Study:
- To test the hypothesis that substorm onsets are triggered by plasma sheet flow bursts manifesting as auroral streamers.
- To compare substorm occurrence rates between coronal mass ejection (CME) and high-speed stream (HSS) related geomagnetic storms.
Main Methods:
- Utilized high-resolution THEMIS all-sky-imagers (ASIs) for broad ionospheric mapping of the plasma sheet.
- Analyzed main phases of 9 CME-related and 9 HSS-related geomagnetic storms.
- Identified 60 substorm auroral onsets characterized by brightening followed by poleward expansion.
Main Results:
- A detectable streamer was observed heading towards the substorm onset location for all 60 identified onsets.
- Substorm onsets were significantly less frequent during CME storms compared to HSS storms.
- Results contradict previous findings of equal substorm occurrences between CME and HSS storms.
Conclusions:
- Substorm onsets are frequently triggered by plasma intrusions, indicated by auroral streamers, though direct plasma measurement is lacking.
- The observed lower substorm frequency during steady southward IMF in CME storms challenges existing models of substorm repetition.
- Higher substorm occurrence during HSS periods is likely due to fluctuating interplanetary magnetic field (IMF) conditions.
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