FAST/Polar conjunction study of field‐aligned auroral acceleration and corresponding magnetotail drivers

Abstract

The discrete aurora results when energized electrons bombard the Earth's atmosphere at high latitudes. This paper examines the physical processes that can cause field‐aligned acceleration of plasma particles in the auroral region. A data and theoretical study has been carried out to examine the acceleration mechanisms that operate in the auroral zone and to identify the magnetospheric drivers of these acceleration mechanisms. The observations used in the study were collected by the Fast Auroral Snapshot (FAST) and Polar satellites when the two satellites were in approximate magnetic conjunction in the auroral region. During these events FAST was in the middle of the auroral zone and Polar was above the auroral zone in the near‐Earth plasma sheet. Polar data were used to determine the conditions in the magnetotail at the time field‐aligned acceleration was measured by FAST in the auroral zone. For each of the magnetotail drivers identified in the data study, the physics of field‐aligned acceleration in the auroral region was examined using existing theoretical efforts and/or a long‐system particle in cell simulation to model the magnetically connected region between the two satellites. Results from the study indicate that there are three main drivers of auroral acceleration: (1) field‐aligned currents that lead to quasistatic parallel potential drops (parallel electric fields), (2) earthward flow of high‐energy plasma beams from the magnetotail into the auroral zone that lead to quasistatic parallel potential drops, and (3) large‐amplitude Alfvén waves that propagate into the auroral region from the magnetotail. The events examined thus far confirm the previously established invariant latitudinal dependence of the drivers and show a strong dependence on magnetic activity. Alfvén waves tend to occur primarily at the poleward edge of the auroral region during more magnetically active times and are correlated with intense electron precipitation. At lower latitudes away from the poleward edge of the auroral zone is the primary field‐aligned current region which results in the classical field‐aligned acceleration associated with the auroral zone (electrons earthward and ion beams tailward). During times of high magnetic activity, high‐energy ion beams originating from the magnetotail are observed within, and overlapping, the regions of primary and return field‐aligned current. Along the field lines where the high‐energy magnetotail ion beams are located, field‐aligned acceleration can occur in the auroral zone leading to precipitating electrons and upwelling ionospheric ion beams. Field‐aligned currents are present during both quiet and active times, while the Alfvén waves and magnetotail ion beams were observed only during more magnetically active events. During quiet times (no storm or substorm), the field‐aligned currents were relatively weak with the resulting electron precipitation in the primary current region correspondingly weak, and for the quiescent events examined here, upwelling ion beams were not observed. The results presented here support the overall contention that processes in the magnetotail lead to field‐aligned auroral acceleration and discrete aurora during active events, however, it is not a single process but several processes acting at the same time and at different locations in latitude.