AURORAL ELECTRODYNAMICS AT THE CUSP/CLEFT POLEWARD BOUNDARY DURING NORTHWARD INTERPLANETARY MAGNETIC-FIELD

Combined latitude profiles of optical aurora, magnetic disturbances, particle precipitation and field-aligned currents are used to investigate the electrodynamics of the cusp/cleft ionosphere during northward interplanetary magnetic field (Bz = 12 nT; BY = - 17 nT). An active zone of strong azimuthal plasma convection, manifested by the Svalgaard-Mansurov effect in ground magnetic deflections (due to Hall current between pairs of Birkeland currents) and auroral structure at the poleward boundary of the cusp/cleft is observed within approximately 1030-1230 MLT. Contrary to the typical cusp/cleft auroral activity during negative IMF Bz the discrete auroral forms in the present case move into the cusp/cleft arc, from the poleward edge. Both the optical observations and the inferred field-aligned current/plasma flow pattern indicate a generator mechanism poleward of the cusp/cleft (i.e. associated with the plasma mantle), giving rise to strong momentum transfer and discrete electron precipitation within a approximately 200 km wide latitudinal zone at the cusp/cleft poleward boundary, at the time of a poleward moving cusp/cleft arc. From this and other similar studies of the cusp/cleft ionosphere, we conclude that a qualitatively similar mode of solar wind-magnetosphere interaction is operating both during northward and southward IMF.