Probing the Evolution of Solar Wind Temperature Anisotropies in the Inner Heliosphere

Abstract Using solar wind measurements from Solar Orbiter, we investigated the distribution and evolution of the temperature anisotropy of alpha-particles (the ratio of perpendicular to parallel temperatures) in the inner heliosphere, between 0.29 and approximately 1 au. We also compared these observations with the corresponding proton temperature anisotropy to better understand the evolution of ion thermal properties in the expanding solar wind. We found that the slow wind (≤400 km s −1 ) exhibits higher alpha-particle anisotropy compared to the fast wind (≥600 km s −1 ), where the anisotropy values are mostly below one. The alpha-particle anisotropy increases as solar wind speed decreases, in contrast to protons, where anisotropy increases with solar wind speed. Additionally, we find that, regardless of the distance from the Sun, alpha-particle anisotropy is greater than proton anisotropy when the normalized differential speed between alpha-particles and protons (Δ V / V A ) is less than 0.3. However, when Δ V / V A exceeds 0.4, the trend reverses, and proton anisotropy becomes larger. The radial evolution also shows distinct differences between fast and slow wind. In the fast wind, proton anisotropy decreases with distance from 0.29 to 1 au, while alpha-particle anisotropy shows no clear radial trend and remains nearly constant. In the slow wind, both proton and alpha anisotropies decrease with distance from the Sun. These results reveal distinct behaviors of alpha-particles and protons in the expanding solar wind, highlighting the dependence of ion heating on solar wind speed and differential flow.