Solar wind protons: Three‐dimensional velocity distributions and derived plasma parameters measured between 0.3 and 1 AU

Abstract

A survey of solar wind three‐dimensional proton velocity distributions as measured by the Helios solar probes between 0.3 and 1 AU is presented. A variety of nonthermal features like temperature anisotropies, heat fluxes, or proton double streams has been observed. The relative speed of the second proton component increases on the average with increasing wind speed and decreasing heliocentric radial distance and shows a correlation with the local Alfvén speed. A marked anisotropy in the core of proton distributions with a temperature larger perpendicular than parallel to the magnetic field (T∥c < T∥c) is a persistent feature of high speed streams and becomes most pronounced in the perihelion (≈0.3 AU). Fairly isotropic distributions have only been measured very close to and directly at magnetic sector boundaries. Low and intermediate speed distributions usually show a total temperature anisotropy T∥p/T⊥p > 1 frequently caused by ‘high‐energy shoulders’ or a resolved second proton component. No clear radial gradient of the temperature anisotropy could be established in these cases. The average dependence of the proton temperature on heliocentric radial distance is given by a power law R−α, where α ≈ 1 for T⊥p and 0.7 < α < 1 for T⊥p are compatible neither with isothermal nor adiabatic expansion. Flattest radial temperature profiles are obtained in high‐speed streams. These observations indicate that local heating or considerable proton heat conduction occurs in the solar wind. Some consequences of nonthermal features of proton distributions for plasma instabilities are discussed as well as kinetic processes that may shape the observed distributions.