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.

Solar Wind Helium Abundance as a Function of Speed and Heliographic Latitude: Variation through a Solar Cycle

“…Helium is generally observed to have a larger velocity then than hydrogen, especially in the fast solar wind (e.g., McKenzie et al 1978;Marsch et al 1982aMarsch et al , 1982bvon Steiger et al 1995). This phenomenon is probably an artifact of waveresonant heating and acceleration in the corona (e.g., Dusenbery & Hollweg 1981;Tu et al 2003).…”

Frontier Geoplasma Research

“…For example, the preferential heating of the helium ions over the protons due to wave-particle interactions translates into a higher 14 velocity downstream in the solar wind. That provides an explanation for the helium to be the faster major ion species in the solar wind, as observed, for example, by the Helios, WIND, and Ulysses spacecraft [Marsch et a/., 1982a;Steinberg et a/., 1996;Feldman et a/., 1996]. Moreover, the results of TC99 featured double peaks in the proton velocity distributions, which were occasionally observed by the Helios spacecraft [Marsch et a/., 1982b].…”

Nonclassical Transport and Particle-Field Coupling: from Laboratory Plasmas to the Solar Wind

“…From this figure, it is evident that, while the proton velocity distribution remains quite close to the Maxwellian spherical shape, the alpha-particles velocity distribution is evidently shaped by kinetic effects, displaying a certain elongation with the formation of a bubble structure in the direction perpendicular to the local magnetic field. These typical non-Maxwellian velocity distributions are common features of solarwind plasmas (Marsch et al 1982a(Marsch et al , 1982bBourouaine et al 2010Bourouaine et al , 2011aBourouaine et al , 2011b). …”