Proton temperature relaxation in the solar wind by combined collective and collisional processes

It is well established that various plasma instabilities regulate the proton temperature anisotropy in the solar wind. However, the question of why majority of data in (beta(parallel to), T-perpendicular to/T-parallel to) phase space are broadly distributed near isotropic conditions has not been successfully addressed. Among possible causes are local perpendicular heating, binary collisions, and geometrical effects. The present paper examines the influence of binary collisions, based upon recently formulated collisional transport theory for magnetized plasmas combined with macroscopic quasilinear theory for proton cyclotron and parallel firehose instabilities. The spatial development of perpendicular and parallel temperatures for protons as well as spatial distribution of excited wave magnetic field energy density are studied by making use of model solar wind. The present finding suggests that binary collisions may contribute toward the isotropization process but does not completely explain the isotropization.