Analysis of substorm expansion and surge development

We have used simultaneous magnetogram, radar, and Viking imager data from evening local time to study several substorm expansion phase onsets and an expansion phase auroral surge that formed near the radar field of view. At the onsets, auroral brightenings developed within the region of the electric field gradient of the Harang discontinuity, and if not already present, a westward electrojet began poleward of the eastward electrojet. Immediately following onsets, the poleward boundary of the westward electrojet moved poleward along with the poleward boundary of the aurora; however, no consistent motion of the Harang discontinuity was found in the ionosphere. The auroral surge formed along the poleward boundary of the westward electrojet (and of the aurora). We argue that the distortion of the poleward boundary of the aurora associated with the surge was a manifestation of a distortion of the separatrix between open and closed magnetic field lines, the distortion being caused by the field-aligned currents within the head of the surge. We suggest that rapid surge development may result from a tapping of the westward electrojet current at the surge head. We also suggest that the separatrix distortion could lead to the escape of trapped plasma sheet particles along open field lines in the tail.

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