HE-2+ HEATING AT A QUASI-PARALLEL SHOCK

We present the first observations of solar wind He-2+ heating downstream from the Earth's quasi-parallel shock. These observations show that in conjunction with protons, two different regions are observed. In regions where the proton distribution is cooler, more dense, and similar to that observed downstream from quasi-perpendicular shocks, the He-2+ distributions is shell-like, also similar to that observed downstream from quasi-perpendicular shocks. In regions where the proton distribution is hotter, less dense, and Maxwellianlike, the He-2+ distribution is also Maxwellianlike without evidence for a shell. These observations support the interpretation that the nearly isotropic proton and He-2+ distributions are produced through the strong interaction of a very dense specularly reflected proton beam with the incident solar wind, while the cooler proton distributions and shell-like He-2+ distributions are produced in a manner similar to that at the quasi-perpendicular bow shock.