Auroral event sequence at the dayside polar cap boundary: Signature of time-varying solar wind-magnetosphere-ionosphere coupling

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A specific category of auroral event sequence taking place at the dayside polar cap boundary during southward-directed interplanetary magnetic field (IMF) is exemplified by three cases of combined ground and satellite observations. The detailed temporal and spatial evolution of these optical events were monitored by all-sky TV imagery mapped onto a geographic coordinate system. Profiles along the satellite track of particle precipitation and ionospheric convection in the vicinity of the optical events were important for the determination of the solar wind - magnetosphere coupling mode involved. The observed north-westward event motion in the prenoon sector is typical for periods of positive IMF B Y. Auroral events associated with negative IMF B Y were moving northeastward in the midday and early post-noon sector. The optical events mostly appear as a brightening of the 630.0 nm aurora near the poleward boundary of the persistent cleft/LLBL precipitation at the boundary of the field-of-view of the TV camera. A continuous motion into the polar cap (mantle precipitation) was observed before the optical emission faded out after 10–15 minutes, typically ∼500 km poleward of the continuous cleft arc. The auroral forms typically cover ∼500 km in the east - west direction and ∼200 km in the north - south direction. A glow of 630.0 nm emission spanning ∼500 km in latitudinal extent was observed during the decay phase of the Jan. 12, 1991 case presented here. The optical, particle and ion drift observations in combination with the available IMF and solar wind plasma data indicate that the events represent dynamical structures of IMF B Y-related Dungey cell convection over the polar magnetosphere, initiated by pulses of enhanced merging rate (B n ≠ 0) at the dayside magnetopause.