Survey and analysis of auroral arcs in the dusk and midnight sector

Abstract

Ultraviolet auroral imagery obtained by the Freja satellite between October 26 , 1992 and December 20, 1992 is calibrated and surveyed in search of auroral arcs. Of the 512 orbits surveyed, 80 provide imagery of auroral arcs, 50 of which are suitable for a comparison of imagery and Frej a in-situ measurements. The auroral arcs identified in the imagery consist of both undistorted and distorted arcs (ripples , pre-spirals and spirals). Characteristics of the processes involved in the formation of these arcs are suggested by temporal and spatial characteristics of the arcs as extracted from the auroral imagery and near simultaneous electric field measurements. The continuity of current within the region of the arc's current sheet, is found to be satisfied by both polarization electric fields and fields associated with enhanced field-aligned current. Characteristics of the distorted arcs suggest that shear-driven current-sheet instabilities are involved in their formation.