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

[1] 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.