Energetic particle precipitation in sub-auroral polarization streams

Sub-auroral polarization streams (SAPS) are one of the most intense manifestations of magnetosphere-ionosphere coupling. Magnetospheric energy transport to the ionosphere within SAPS is associated with Poynting flux and the precipitation of thermal energy (0.03−30 keV) plasma sheet particles. However, much less is known about the precipitation of high-energy (≥ 50 keV) ions and electrons and their contribution to the low altitude SAPS physics. This study examines precipitation within one SAPS event using a combination of equatorial THEMIS and low-altitude DMSP and ELFIN observations, which, jointly, cover from a few eV up to a few MeV energy range. Observed SAPS are embedding the ion isotropy boundary, which includes strong 300−1000 keV ion precipitation. SAPS are associated with intense precipitation of relativistic electrons (≤ 3 MeV), well equatorward of the electron isotropy boundary. Such relativistic electron precipitation is likely due to electron scattering by electromagnetic ion cyclotron waves at the equator.