Parallel electric fields in the upward current region of the aurora: Indirect and direct observations

In this article we present electric field, magnetic field, and charged particle observations from the upward current region of the aurora focusing on the structure of electric fields at the boundary between the auroral cavity and the ionosphere. Over 100 high-resolution measurements of the auroral cavity that were taken by the Fast Auroral Snapshot (FAST) satellite are included in this study. The observations support earlier models of the auroral zone that held that quasi-static parallel electric fields are the primary acceleration mechanism. In addition to the statistical study, several examples of direct observations of the parallel electric fields at the low-altitude boundary of the auroral cavity are put forth. These observations suggest that the parallel electric fields at the boundary between the auroral cavity and the ionosphere are self-consistently supported as oblique double layers.