Auroral activity associated with unsteady magnetospheric erosion: Observations on December 18, 1990

Abstract

We report auroral observations made on December 18, 1990, when interplanetary conditions should lead to large‐scale erosion of the dayside magnetosphere during a substorm growth phase. A long interval of strongly northward pointing interplanetary magnetic field (IMF) was succeeded 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, B z changed polarity abruptly. The auroral responses, monitored at Ny Ålesund (75° magnetic latitude) by meridian scanning photometers and all‐sky cameras, were as follows: The interval of negative IMF B z was characterized 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 ∼0.5° 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 equatorward boundary toward the geomagnetic equator by ∼2.7° MLAT. The auroral data suggest that in this instance dayside magnetosphere erosion took place intermittently: bursts of reconnection (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, equivalently, flux erosion events.