A comparison of energetic ions in the plasma depletion layer and the quasi‐parallel magnetosheath

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Energetic ion spectra measured by the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) downstream from the Earth's quasi‐parallel bow shock (in the quasi‐parallel magnetosheath) and in the plasma depletion layer are compared. In the latter region, energetic ions are from a single source, leakage of magnetospheric ions across the magnetopause and into the plasma depletion layer. In the former region, both the magnetospheric source and shock acceleration of the thermal solar wind population at the quasi‐parallel shock can contribute to the energetic ion spectra. The relative strengths of these two energetic ion sources are determined through the comparison of spectra from the two regions. It is found that magnetospheric leakage can provide an upper limit of 35% of the total energetic H+ population in the quasi‐parallel magnetosheath near the magnetopause in the energy range from ∼10 to ∼80 keV/e and substantially less than this limit for the energetic He2+ population. The rest of the energetic H+ population and nearly all of the energetic He2+ population are accelerated out of the thermal solar wind population through shock acceleration processes. By comparing the energetic and thermal He2+ and H+ populations in the quasi‐parallel magnetosheath, it is found that the quasi‐parallel bow shock is 2‐3 times more efficient at accelerating He2+ than H+. This result is consistent with previous estimates from shock acceleration theory and simulations.