Determination of substorm onset timing and location using the THEMIS ground based observatories

Abstract

The NASA THEMIS mission is studying substorms by timing the substorm signatures at multiple satellite locations in the magnetosphere and in the aurora using 20 ground based observatories (GBO‐s). The time resolution requirement is ∼10 sec. The GBO‐s provide a near contiguous array over North America. Each contains an all sky imager (3 s exposure cadence) and a magnetometer (with 2 Hz sampling rate). In one example substorm, the onset brightening of the equatorward arc was a gradual process (>27 seconds) with minimal morphology changes until the arc break up. The break up was timed to the nearest frame (<3 sec) and occurred at 58°N latitude and 256 ± 3°E longitude geographic (67°N magnetic latitude 22.1 hours MLT). The brightening of the arc was accompanied by a slow increase of the westward electrojet but this was too gradual for accurate timing of the event. High pass filtered magnetic data showed some wave activity but with significant delay (∼40 sec) after the arc break up. Similar break up occurred in Alaska ∼10 minutes later highlighting the need for an array to distinguish prime onset.