Abstract

The frequency distribution of solar wind protons, measured in the vicinity of Earth's orbit, is customarily plotted in (beta parallel to, T perpendicular to/T parallel to) phase space. Here, T perpendicular to/T parallel to is the ratio of perpendicular and parallel temperatures, and beta parallel to = 8 pi nT parallel to/B 2 is the ratio of parallel thermal energy to background magnetic field energy, the so-called "parallel beta," with perpendicular to and parallel to denoting directions with respect to the ambient magnetic field. Such a frequency distribution, plotted as a two-dimensional histogram, forms a peculiar rhombic shape defined with an outer boundary in the said phase space. Past studies reveal that the threshold conditions for temperature anisotropy-driven plasma instability partially account for the boundary on the high-beta parallel to side. The low-beta parallel to side remains largely unexplained despite some efforts. Work by Vafin et al. recently showed that certain contours of collisional relaxation frequency, nu pp, when parameterized by T perpendicular to/T parallel to and beta parallel to, could match the overall shape of the left-hand boundary, thus suggesting that the collisional relaxation process might be closely related to the formation of the left-hand boundary. The present paper extends the analysis by Vafin et al. and carries out the dynamical computation of the collisional relaxation process for an ensemble of initial proton states with varying degrees of anisotropic temperatures. The final states of the relaxed protons are shown to closely match the observed boundary to the left of the (beta parallel to, T perpendicular to/T parallel to) phase space. When coupled with a similar set of calculations for the ensemble in the collective instability regime, it is found that the combined collisional/collective effects provide the baseline explanation for the observation.