On the field‐aligned electric field in the polar cap

Abstract

AbstractThe Johns Hopkins University Applied Physics Laboratory open‐field line particle precipitation model predicts downward field‐aligned electric field to maintain charge quasi‐neutrality. Previous studies confirmed the existence of such electric fields. However, the present study shows that upward field‐aligned electric field can be found within upward field‐aligned current (FAC) region. In the upward FAC region, upward electric field that accelerates electron downward is seen with the occurrence rates of 82%–96%. In contrast, the occurrence rates in the downward FAC regions are 3%–11%. Polar rain electrons located in the upward FAC region adjacent to closed field lines often show a ramping up of energy with increasing latitude before reaching a plateau. This plateau may be attributed to the magnetosheath electrons that progressively have higher antisunward velocity and lower density with increasing distance from the subsolar point before they asymptotically reach the solar wind values.