Acceleration signatures in the dayside boundary layer and the cusp

Abstract

Freja data show various electron acceleration signatures in the cusp and the dayside boundary layer: (1) time dispersive super-Alfvénic electrons followed by strong wave activity which accompanies transient downward super-thermal electron burst in both the boundary layer and the cusp; (2) quasi-stationary bidirectional electron burst coinciding with localized intense field-aligned current in the boundary layer; (3) downgoing electron burst without visible time dispersion in the cusp; and (4) thermal electrons accelerated by electrostatic potential in both the boundary layer and the cusp. The first and last signatures are different between two regions for typical energies and fluxes, and these differences probably reflect the different auroral emission in the cusp proper (red) and the boundary layer (green). Contributions of these electrons to the large-scale field-aligned currents are also different between two regions. The dispersed electron burst is probably accelerated within 1 Re above the ionosphere. From this result we believe that the cusp red aurora is caused mainly by accelerated electrons, but not by the smoothly entering magnetosheath electrons without acceleration. This also requires revisions of flux transfer event models for the structured cusp red aurora.