H⁺ and He⁺ ⁺ in the dawnside magnetosheath

Mass and energy analyses of energetic ions in the dawnside magnetosheath have been made by the energetic ion composition experiment on the ISEE‐1 spacecraft over the energy range 10 eV/e ⩽ E/Q ⩽ 17 keV/e. The significant observations are: The bulk flows of the H + and He + + components determined from first moments of the distributions averaged over several minutes are equal within the uncertainties of the measurement. The H + and He + + distributions are similar in that they both show temperature anisotropies with T ⊥ ⩾ T ∥ ; however, the distributions are not well characterized by temperatures derived from second moments. In at least one case high energy tails of both distributions can be characterized by an additional higher temperature component. The major difference between the H + and He + + distributions is the depletion of the low velocity (in the center of mass frame) part of the He + + distributions, relative to the H + distributions.