ON THE ORIGINS OF ENERGETIC IONS IN THE EARTHS DAYSIDE MAGNETOSHEATH

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.