AURORAL ACTIVITY ASSOCIATED WITH UNSTEADY MAGNETOSPHERIC EROSION - OBSERVATIONS ON DECEMBER 18, 1990

We report auroral observations made on December 18, 1990, when interpl anetary conditions should lead to large-scale erosion of the dayside m agnetosphere during a substorm growth phase. A long interval of strong ly northward pointing interplanetary magnetic field (IMF) was succeede d by several hours of strongly southward pointing IMF. The interval of southward pointing IMF was punctuated by a number of IMF directional discontinuities during which the IMF north-south component, Bz changed polarity abruptly. The auroral responses, monitored at Ny Alesund (75 degrees magnetic latitude) by meridian scanning photometers and all-s ky cameras, were as follows: The interval of negative IMF Bz was chara cterized by a net equatorward migration of the equatorward boundary of the dayside cusp/cleft aurora, as expected from previous studies. On this occasion, however, we find that the latitudinal shift occurred in steps which consisted of an initial brightening of individual auroral events at similar to 0.5 degrees MLAT equatorward of the preexisting luminosity, followed by a steady poleward retreat lasting typically 4- 5 min. The net effect over the first hour of IMF B-Z<0 conditions was to move the equatorvard boundary toward the geomagnetic equator by sim ilar to 2.7 degrees MLAT. The auroral data suggest that in this instan ce dayside magnetosphere erosion took place intermittently bursts of r econnection (initial brightenings) are followed by a switch-off of the reconnection electric field (subsequent poleward retreat). The bursts of reconnection may be identified with flux transfer events or, equiv alently, flux erosion events.