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

¹ Institute of Space Physics, Luoyang Normal University, 471934 Luoyang, PR China
e-mail: fenghq9921@163.com
² Henan Key Laboratory of Electromagnetic Transformation and Detection, Luoyang Normal University, 471934 Luoyang, PR China
³ School of Physics and Electronics, Hunan Normal University, 410081 Changsha, PR China
⁴ Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, 210034 Nanjing, PR China

Received: 4 May 2023
Accepted: 7 July 2023

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 T_α∥/T_p∥ and the proton temperature anisotropy T_p⊥/T_p∥, but also on the drift velocity of alpha to proton v_α/v_A and the solar wind velocity v_sw. 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.