Dayside Auroral Particle Acceleration Mechanisms Derived from Dynamics Explorer Data

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Data from Dynamics Explorer hot-plasma and magnetic-field instruments have been used to investigate the characteristics of dayside electron acceleration regions, or inverted-V's, and the nature of the field-aligned currents that flow in their vicinity. The data set examined in this study was obtained in the pre-noon sector, where the region-1 field-aligned currents are typically flowing downward. Although not permanent features of the dayside auroral region, inverted-V events are commonly observed to occur just equatorward of the cusp or cleft in a region which contains trapped electrons and which is believed to map to the boundary layer. The downward region-1 currents are observed either just poleward of the trapping boundary in the equatorward part of the cleft or, at times, within the same flux tubes as the inverted-V events. In either location, the region-1 currents are carried by accelerated ionospheric electrons flowing out of the ionosphere. Therefore, when the region-1 currents flow through the inverted-V events, both upward electron beams and upward ion beams are observed together. Although the inferred potential drops associated with the dayside inverted-V events are about a factor of ten smaller than those observed on the night side, a downward field-aligned current is not consistent with common assumptions regarding the generation of auroral acceleration regions. Moreover, the existence of upward-accelerated ionospheric electrons within the inverted-V events is inconsistent with their acceleration by parallel potential drops.