Specularly reflected He²⁺ at high Mach number quasi‐parallel shocks

Upstream from the Earth's quasi‐parallel bow shock, the Lockheed Plasma Composition Experiment on ISEE 1 often observes two types of suprathermal He 2+ distributions. Always present to some degree is an energetic (several keV/ e to 17.4 keV/ e , the maximum energy of the detector) “diffuse” He 2+ distribution. Sometimes, apparently when the Alfven Mach number, M A , is high enough and the spacecraft is near the shock (within a few minutes of a crossing), a second type of suprathermal He 2+ distribution is also observed. This nongyrotropic, gyrating He 2+ distribution has velocity components parallel and perpendicular to the magnetic field that are consistent with near‐specular reflection of a portion of the incident solar wind He 2+ distribution off the shock. Specularly reflected and diffuse proton distributions are associated with these gyrating He 2+ distributions. The presence of these gyrating He 2+ distributions suggests that specular reflection is controlled primarily by magnetic forces in high Mach number quasi‐parallel shocks and that these distributions may be a seed population for more energetic diffuse He 2+ distributions.