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 (beta parallel to, T perpendicular to/T parallel to) phase space show that their respective occurrence distributions are confined within an area enclosed by outer boundaries. Here, T perpendicular to/T parallel to is the ratio of perpendicular and parallel temperatures and beta parallel to is the ratio of parallel thermal energy to background magnetic field energy. While it is known that the boundary on the high-beta parallel to side is constrained by the temperature anisotropy-driven plasma instability threshold conditions, the low-beta parallel to boundary remains largely unexplained. The present paper provides a baseline explanation for the low-beta parallel to 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 (beta parallel to, T perpendicular to/T parallel to) phase space is adequately explained, both for the "core" and "halo" components.