On the origins of energetic ions in the Earth's dayside magnetosheath

Abstract

Energetic ion events in the Earth's dayside subsolar magnetosheath (0900 ‐ 1300 Local Time) are surveyed using data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) Hot Plasma Composition Experiment. Ion species carrying the signature of their origin (O+ and energetic He2+) are used to distinguish between magnetospheric and solar wind origins for the energetic ion events. The results of this survey indicate that the majority of energetic (10–17 keV/e) H+ and He2+ ions observed in the dayside magnetosheath are accelerated from the solar wind population. The energetic He2+ to H+ density ratio in the magnetosheath is consistent with that predicted from first‐order Fermi acceleration of solar wind ions in the turbulent regions upstream and downstream from the Earth's quasi‐parallel bow shock. Although the majority of the energetic ions appear to be of solar wind origin, magnetospheric O+ is also occasionally present in the magnetosheath. The simultaneous occurrence of both energetic He2+ and magnetospheric O+ indicates that, on occasion, both Fermi acceleration of solar wind ions and leakage of magnetospheric ions occurs in the dayside magnetosheath.