Suprathermal He²⁺ in the Earth's foreshock region

ISEE 1 and 2 H + and He 2+ observations upstream from the Earth's bow shock are used to investigate the origin of energetic (or diffuse) ion distributions. Diffuse ion distributions have energies from a few keV/ e to >100 keV/ e and have near solar wind concentrations (i.e., an average of about 4% He 2+ ). These distributions may evolve from suprathermal ion distributions that have energies between 1 and a few keV/e. Upstream intervals were selected from the ISEE data to determine which suprathermal distributions have He 2+ concentrations similar to those of diffuse ion distributions. The type of distribution and the location in the foreshock were similar in all events studied. Two intervals that represent the results from this study are discussed in detail. The results suggest that diffuse ion distributions evolve from suprathermal distributions in the region upstream from the quasi‐parallel bow shock. For He 2+ , the suprathermal distribution is a nongyrotropic partial ring beam and has characteristics consistent with specular reflection off the quasi‐parallel bow shock. The suprathermal proton distributions associated with these He 2+ distributions are nongyrotropic partial ring beams or nearly gyrotropic ring beams also approximately consistent with specular reflection. The location in the quasi‐parallel foreshock and the similarity of the suprathermal He 2+ and H + distributions suggest that these are the seed population for diffuse distributions in the foreshock region.