How does the U-shaped potential close above the acceleration region? A study using Polar data

We present a statistical study of Polar electric ®eld observations using auroral oval passes over Scandinavia above the acceleration region. We are especially interested in seeing whether we can ®nd large perpendicular electric ®elds associated with an upward extended classical U-shaped potential drop for these passes, during which Polar is in the northern hemisphere usually at about 4 RE altitude. We also use Polar magnetic ®eld data to infer the existence of a ®eld-aligned current (FAC) and conjugate ground-based magnetometers (the IMAGE magnetometer network) to check whether the event is substorm-related or not. We ®nd several events with a FAC but only weak perpendicular electric ®elds at Polar. In those rare cases where the Polar electric ®eld was large, its direction was mostly found to be incompatible with the U-shaped potential model, or it was associated with disturbed conditions (substorms), where one cannot easily distinguish between inductive and static perpendicular electric ®elds. We found only two cases which are compatible with the upward extended U-shaped potential picture, and even in those cases the potential value is quite small (1±2 kV). To check the validity of the analysis method we also estimate the perpendicular electric ®eld on the southern hemisphere, where Polar ̄ies within or below the acceleration region, and we found a large number of inverted-V-type signatures as expected from previous studies. To explain the lack of perpendicular electric ®elds at high altitudes we suggest an O-shaped potential model instead of the U-shaped one.