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

Abstract 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.