CUSP CLEFT AURORAL FORMS AND ACTIVITIES IN RELATION TO IONOSPHERIC CONVECTION - RESPONSES TO SPECIFIC CHANGES IN SOLAR-WIND AND INTERPLANETARY MAGNETIC-FIELD CONDITIONS

This work is intended to be a first step toward a categorization of da yside auroral responses to various solar wind and interplanetary magne tic held (IMF) conditions and the corresponding states of plasma conve ction in the dayside magnetosphere. In this paper we relate cusp latit ude auroral emissions observed during a 4-hour interval on December 17 , 1992, to solar wind data supplemented by ground magnetograms. We foc us on different types of variability associated with an interplanetary shock and with an IMF directional discontinuity. We further investiga te the quasi-steady conditions related to periods of low activity vers us periods df high activity, as well as periods of IMF B-Y-related zon al convection in the cusp region versus periods of IMF B-Z-related con vection in the north-south direction. To these various conditions the dayside aurora responds differently. These observations are discussed in relation to entry of magnetosheath plasma into the magnetosphere as sociated with different modes-of solar wind-magnetosphere coupling. Tw o main categories of persistent auroral forms in the cusp/cleft region are observed, a latitudinally wide (greater than or equal to 200 km) zone of weak 630.0-nm emission with no sharp boundaries extending to m agnetic latitudes of similar to 78 degrees-79 degrees during quiet int ervals and another latitudinally narrow (similar to 100 km) zone of st rong red line emission, located equatorward of 75 degrees MLAT during disturbed periods. These forms occur in association with Sunward and a ntisunward convection in the dayside polar cap, respectively. It is su ggested that these two signatures of electron precipitation in the noo n/near-noon sector reflect plasma entry by lobe reconnection and recon nection at low (subcusp) latitudes, respectively. The observed sudden transition from the former to the latter auroral condition may thus re flect a change of reconnection site. The associated brightening of the cusp/cleft aurora then corresponds to the larger efficiency of magnet osheath plasma entry at low magnetopause latitudes when the subcusp re connection process is switched on, compared to the more limited plasma entry occurring at high latitude during lobe reconnection. The most i ntense and latitudinally narrow auroral forms at 630.0 nn are observed during intervals of strong IMF B-Y-related zonal convection and a str ong DPY mode of ground magnetic deflection. Information on the ionosph eric convection pattern in the vicinity of the magnetic field separatr ix is obtained for the different IMF orientations by combining the aur oral and magnetic observations in the Arctic sectors of Greenland and Svalbard.