Solar wind helium ions: observations of the Helios solar probes between 0.3 and 1 AU

A survey of solar wind helium ion velocity distributions and derived parameters as measured by the Helios solar probes between 0.3 and 1 AU is presented. Nonthermal features like heat flux or He2+ double streams and temperature anisotropies have been frequently observed. Fairly isotropic distributions have only been measured close to sector boundaries of the interplanetary magnetic field. The role of Coulomb collisions in limiting Δv/sub alphap/ and the ion temperature ratio T/sub α//T/sub p/ is investigated. Collisions are shown to play a negligible role in fast solar wind, possibly a minor role in intermediate speed solar wind and a distinct role in low-speed wind in limiting the differential ion velocity and temperature