Abstract
Abstract. We present a statistical study of Polar electric field observations using auroral oval passes over Scandinavia above the acceleration region. We are especially interested in seeing whether we can find large perpendicular electric fields 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 field data to infer the existence of a field-aligned current (FAC) and conjugate ground-based magnetometers (the IMAGE magnetometer network) to check whether the event is substorm-related or not. We find several events with a FAC but only weak perpendicular electric fields at Polar. In those rare cases where the Polar electric field 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 fields. 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 field on the southern hemisphere, where Polar flies 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 fields at high altitudes we suggest an O-shaped potential model instead of the U-shaped one.Key words. Ionosphere (particle acceleration) · Magnetospheric physics (auroral phenomena; magnetosphere · ionosphere interactions)
Highlights
Due to low-orbiting satellite observations (McIlwain, 1960; Shelley et al, 1976), there is compelling evidence of the existence of upward directed parallel electric ®elds above auroral arcs, which was theoretically postulated a long time ago (Alfve n, 1958)
Thebottom'' of the U corresponds to the region of the parallel electric ®eld, the auroral acceleration region
If the ®eld lines are equipotentials above the acceleration region, the upward extensions of theU'' shape should be seen as perpendicular electric ®elds above the acceleration region, presumably extending all the way to the equatorial plane of the magnetosphere and closing only on the opposite hemisphere
Summary
Due to low-orbiting satellite observations (McIlwain, 1960; Shelley et al, 1976), there is compelling evidence of the existence of upward directed parallel electric ®elds above auroral arcs, which was theoretically postulated a long time ago (Alfve n, 1958). If the ®eld lines are equipotentials above the acceleration region, the upward extensions of the`U'' shape (which we will call uprightanks in this paper) should be seen as perpendicular electric ®elds above the acceleration region, presumably extending all the way to the equatorial plane of the magnetosphere and closing only on the opposite hemisphere. In the ISEE-1 case, the orbit was such that the passes were nearly tangential to the auroral oval, making estimation of potential drops associated with inverted-Vs dicult. In this paper we study whether the uprightanks can be seen in Polar satellite electric ®eld measurements at 25000±35000 km altitude, i.e. whether perpendicular electric ®elds of sucient magnitude are observed when Polar passes over the northern auroral oval. For comparison we con®rm that we ®nd the usual signatures of inverted-V regions in southern hemisphere low altitude passes
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