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

Abstract

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 fluxes 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. At times in slow solar wind, persistent double‐humped helium ion distributions constituting a temperature anisotropy T∥α/T⊥α > 1 have been reliably identified. Distributions in high‐speed wind generally have small total anisotropies (T∥α/T⊥α ≳ 1) with a slight indication that in the core part the temperatures are larger parallel than perpendicular to the magnetic field, in contrast to simultaneous proton observations. The anisotropy tends to increase with increasing heliocentric radial distance. The average dependence of helium ion temperatures on radial distance from the sun is described by a power law ∼ R−β with 0.7 ≲ β ≲ 1.2 for T∥α and 0.87 ≲ β ≲ 1.4 for T⊥α. In fast solar wind the T⊥α profile is compatible with nearly adiabatic cooling. Pronounced differential ion speeds Δvαp have been observed with values of more than 150 km/s near perihelion (0.3 AU). In fast streams Δvαp tends to approach the local Alfvén velocity vA, whereas in slow plasma values around zero are obtained. Generally, the differential speed increases with increasing proton bulk speed and (with the exception of slow plasma) with increasing heliocentric radial distance. The role of Coulomb collisions in limiting Δvαp and the ion temperature ratio Tα/Tp 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.