Ionospheric conductivities, electric fields and currents associated with auroral substorms measured by the EISCAT radar

E-region electron density profiles with high resolution in time and altitude (5 s and 2 km, respectively) measured by the EISCAT incoherent scatter radar are used to examine the conductivity changes during substorm growth, onset and expansion phases for seven substorms occurring in the local evening sector. The measurements are related to electric fields and neutral winds measured by the radar, to ground magnetometer and riometer records, and to optical features, including the westward-travelling surge and auroral bulge. Auroral features are identified using all-sky camera photographs and images from the Viking satellite. Conductances and electric fields in the zone of diffuse aurora corresponding to the westward substorm electrojet are found to be consistent with existing models. Conductances in the discrete auroral arcs marking the expanding edge of the substorm are found to be much higher, and electric fields rather lower, than previously assumed. The magnetic signatures of the discrete arcs are found to be best explained by Hall and Pedersen currents driven by a southward neutral wind, as is observed by the radar. The highest conductances observed, with Hall and Pedersen conductances reaching 120 and 48 S, respectively, are found to be associated with arcs appearing at the southern edge of activity in the vicinity of a westward-travelling surge.