Ground‐based optical observations from Svalbard of 0900–1100 MLT auroral structures and dynamics are presented. Strong and stable red line emissions with weak, transient auroral activity were initially observed. The IMP 8 spacecraft, located outside the dawnside bow shock, detected large changes in interplanetary magnetic field (IMF) direction including a sharp transition from negative to positive IMF Bz and a subsequent decrease in the solar wind density. A poleward expansion of the auroral display was seen in response to these variations, with a several minutes time delay, consistent with the location of IMP 8 relative to the magnetopause. The initial ionospheric response to these changes involved an intense brightening and northwestward drifts of discrete auroral forms. These effects may be connected with the decrease of external pressure and associated magnetopause perturbations. During the next 2–3 hours, quasi‐periodic transient, discrete auroral forms were observed to expand in the antisunward direction. The morphology of the auroral forms detected in the prolonged period of northward IMF Bz are compared with in situ measurements taken during a DMSP pass to the west of Svalbard and discussed in terms of several possible generation mechanisms. These include variations in external pressure, magnetic merging poleward of the cusp, and dynamo processes in the plasma mantle and/or low‐latitude boundary layer, powered by intruding magnetosheath plasma elements. The optical measurements appear most consistent with expected effects of a magnetosheath plasma penetration of the plasma mantle.
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Jacobsen et al. (1995) studied this question.