Regulation of Solar Wind Electron Temperature Anisotropy by Collisions and Instabilities

Typical solar wind electrons are modeled as being composed of a dense but less energetic thermal “core” population plus a tenuous but energetic “halo” population with varying degrees of temperature anisotropies for both species. In this paper, we seek a fundamental explanation of how these solar wind core and halo electron temperature anisotropies are regulated by combined effects of collisions and instability excitations. The observed solar wind core/halo electron data in (β _∥, T _⊥/T _∥) phase space show that their respective occurrence distributions are confined within an area enclosed by outer boundaries. Here, T _⊥/T _∥ is the ratio of perpendicular and parallel temperatures and β _∥ is the ratio of parallel thermal energy to background magnetic field energy. While it is known that the boundary on the high-β _∥ side is constrained by the temperature anisotropy-driven plasma instability threshold conditions, the low-β _∥ boundary remains largely unexplained. The present paper provides a baseline explanation for the low-β _∥ boundary based upon the collisional relaxation process. By combining the instability and collisional dynamics it is shown that the observed distribution of the solar wind electrons in the (β _∥, T _⊥/T _∥) phase space is adequately explained, both for the “core” and “halo” components.