Distribution of alpha temperature anisotropy in the slow and fast solar wind: WIND observations and Vlasov theory

Abstract

Context. Alpha particles in the solar wind frequently show temperature anisotropies, which can be constrained by various kinetic instabilities. However, the detailed constraint mechanism remains unclear, especially in different types of solar wind. Aims. Using data obtained by the Wind spacecraft between 2005 and 2015, we investigate the distribution of alpha temperature anisotropy and the correlation with background plasma parameters in the slow and fast solar wind. Methods. We present a comparison between observations of the alpha temperature anisotropy by the Wind spacecraft and theoretical results obtained with the linear Vlasov theory. Results. The results show that the distribution of the alpha temperature anisotropy depends not only on the parallel temperature ratio of alpha to proton Tα∥/Tp∥ and the proton temperature anisotropy Tp⊥/Tp∥, but also on the drift velocity of alpha to proton vα/vA and the solar wind velocity vsw. In particular, there are clearly different constraining mechanisms on the distribution of alpha temperature anisotropy in the slow and fast solar wind. Conclusions. The alpha temperature anisotropy instability together with different types of free energy can be effective for regulating the alpha temperature anisotropy in the slow and fast solar wind.