COUPLING OF IMF B-Y VARIATIONS INTO THE POLAR IONOSPHERES THROUGH INTERPLANETARY FIELD-ALIGNED CURRENTS

Studies of the correlation between the geomagnetic fields recorded at high-latitude observatories and the interplanetary magnetic field (IMF ) observed from the IMP 8 satellite in the solar wind have indicated t hat variations in the IMF B-Y component may generate perturbations of similar appearance in the geomagnetic north (H) components observed in the noon sector of the dayside polar cap. During southward interplane tary field conditions (IMF B-Z<0) these perturbations are observed to progress poleward from the cusp region across a large fraction of the dayside polar cap, changing only slightly in shapes and amplitudes. Th ese progressing geomagnetic variations are interpreted as the footprin ts of the variable IMF B-Y component present in the solar wind volume which is magnetically connected to the polar cap ionosphere. It is sug gested in this work that both the IMF B-Y variations and the correspon ding variations in the H(ground) components are related to interplanet ary field-aligned currents. The ''open'' magnetospheric topology, resu lting from the merging of the geomagnetic field and the interplanetary field during southward interplanetary field conditions, allows these interplanetary currents to flow along the merged field lines to the no rthern and southern polar cap ionospheres. Here they generate the conv ection-related Hall currents responsible for the observed geomagnetic perturbations. The closure from one polar cap to the other can be part ly in the form of ionospheric Pedersen currents and partly in the form of field-aligned currents along closed field lines in the inner magne tosphere.