Ion Inverted-V Precipitation Adjoining Auroral Arc: Low-Altitude Observations With High Time/Space Resolution by the Reimei Microsatellite

Previous studies have verified that upward geomagnetic field-aligned electric fields (FAEFs) produce inverted-V signatures of precipitating electrons that generate discrete auroras in upward field-aligned currents (FACs). However, clear ion inverted-V signatures have rarely been reported in the polar magnetosphere. Plasma measurements with high time resolution at altitude of 630-670 km in the nightside auroral oval often detected inverted-V signatures of precipitating ions adjoining active electron inverted-Vs, suggesting downward ion acceleration by downward FAEFs associated with intense downward FAC. The energy-pitch angle distributions indicate that the altitudes of these FAEF accelerations producing ion inverted-V precipitation are estimated to be a few thousand kilometers, lower than those of electron inverted-Vs. The energies, fluxes, and latitudinal extents of ion inverted-Vs tend to be smaller than those of frequently observed electron inverted-Vs. However, their downward FAC densities are similar to or often larger than those of the upward FACs carried by inverted-V electrons.