Reimei Satellite Observations of Alfvénic Interaction Modulating Inverted-V Electrons and Filamentary Auroral Forms at the Poleward Edge of a Discrete Arc

We present an event based on Reimei satellite observations in the low-altitude midnight auroral region, showing that intense and clear energy-dispersed electron precipitations, repetitively generated by field-aligned accelerations due to dispersive Alfvén waves, were modulating inverted-V electrons. These Alfvénic electrons had peak energies equal to or slightly larger than those of the inverted-Vs and were associated with the filamentary auroral forms rapidly streaming at the poleward edge of a broad discrete arc. This arc was caused by the inverted-V accompanied by ion depletions produced by quasi-electrostatic parallel potential drop. Assuming instantaneous electron accelerations over a wide energy range in a single location and a simple time-of-flight effect for the energy-time dispersions, the Alfvénic source distances were estimated 1,500 ± 500 km above the satellite altitude of ∼676 km, a lower bound since the interaction locations are realistically distributed in altitudinally extended regions. The electron characteristics in detailed energy-pitch angle distributions obtained at high time resolution can be categorized into: (a) original inverted-V fluxes energized by quasi-electrostatic upward electric field, (b) accelerated and decelerated/reduced inverted-V fluxes, (c) field-aligned energy-dispersed precipitations accelerated by dispersive Alfvén waves, and (d) upwelling secondary components effectively produced by the field-aligned precipitations particularly at energies of a few tens of eV. This event is useful to reveal the interactions between the inverted-V and Alfvénic electrons and their related ionospheric effects in the magnetosphere-ionosphere coupling processes. The detailed energy-pitch angle distributions presented here provide constraints for models of these interactions and processes.