Our analysis of the EISCAT radar data disclosed that S_P div E and E · grad S_P produced FACs with the same direction inside a stable broad arc around 05:00 MLT, when the EISCAT radar presumably crossed the boundary between the large-scale upward and downward current regions. In the most successfully observed case, in which the conductances and the electric field were spatially varying with little temporal variations, the contribution of S_P div E was nearly twice as large as that of E · grad S_P . On the other hand, the contribution of (b × E) · grad S_H was small and not effective in closing FACs. The present EISCAT radar mode along with auroral images also enables us to focus on the temporal or spatial variation of high electric fields associated with auroral arcs. In the present experiment, the electric field associated with a stable arc was confined in a spatially restricted region, within ~ 100 km from the arc, with no distinct depletion of electron density. We also detected a region of the high arc-associated electric field, accompanied by the depletion of electron density above 110 km. Using auroral images, this region was identified as a dark spot with a spatial scale of over 150 × 150 km. The dark spot and the electron depletion were likely in existence for a limited time of a few minutes.

Key words. Ionosphere (auroral ionosphere; electric fields and currents; particle precipitation)

• Annales Geophysicae ; 20, no. 9