Direct Observation of Low-Energy Electron Precipitation in the Subauroral Region

This study used data collected by the Defense Meteorological Satellite Program (DMSP) and the Radiation Belt Storm Probes (RBSP) to identify a unique subauroral arc in the duskside, which is associated with precipitations of both ions and electrons. The auroral arc was found to be separated from the auroral oval, extending from 14 to 19 magnetic local time hours. Subauroral arcs are usually generated by energetic protons with energies above approximately 10 keV, while enhanced fluxes of low-energy (<200 eV) electrons were revealed to occur and last for a few hours in this arc region from in situ observations by the DMSP satellite. The observations from RBSP-B indicate that the low-energy electron fluxes, with pitch angles near the loss cone, were accompanied by intensified electromagnetic ion cyclotron (EMIC) waves. It is suggested that the enhanced fluxes of cold electrons were heated by the Landau damping of the observed H+ band EMIC waves, which was in the frequency range below the local H+ gyrofrequency and above the local He+ gyrofrequency. Additionally, both electron density and temperature in the ionosphere increased significantly. This study, using conjugate observations of magnetospheric and ionospheric spacecraft, providing evidence of precipitation of low-energy electrons within the subauroral arc region, thereby offering new insight into understanding the ionospheric effects of cold electron heating by EMIC waves.