FINE-STRUCTURE OF FIELD-ALIGNED CURRENT SHEETS DEDUCED FROM SPACECRAFT AND GROUND-BASED OBSERVATIONS - INITIAL FREJA RESULTS

Due to its special orbit-over large segments tangential to the auroral oval and at altitudes around 1600 km-FREJA provides measurements from a sparsely sampled region. Here we will introduce initial results fro m the magnetic field experiment. For one event occurring about an hour after midnight we investigate the fine structure of the field-aligned currents associated with a westward electrojet. From simultaneous gro und-based observations we deduce the orientation and the spatial and t emporal variations of the electrojet. Space-borne magnetic field measu rements provide estimates of the field-aligned current density. We ded uce a lower limit of 1.75 km for the thickness of field-aligned curren t filaments. The comparison between electric and magnetic field record ings on FREJA suggests a poor correlation of these quantities for this event.