Search for: All records

The frequency distribution of solar wind protons, measured in the vicinity of Earth’s orbit, is customarily plotted in (β_∥,T_⊥/T_∥) phase space. Here,T_⊥/T_∥is the ratio of perpendicular and parallel temperatures, andβ_∥= 8πnT_∥/B²is the ratio of parallel thermal energy to background magnetic field energy, the so-called “parallel beta,” with ⊥ and ∥ 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-β_∥side. The low-β_∥side remains largely unexplained despite some efforts. Work by Vafin et al. recently showed that certain contours of collisional relaxation frequency,ν_pp, when parameterized byT_⊥/T_∥andβ_∥, 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 (β_∥,T_⊥/T_∥) 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.

The expanding-box model of the solar wind has been adopted in the literature within the context of magnetohydrodynamics, hybrid, and full particle-in-cell simulations to investigate the dynamic evolution of the solar wind. The present paper extends such a method to the framework of self-consistent quasilinear kinetic theory. It is shown that the expanding-box quasilinear methodology is largely equivalent to the inhomogeneous steady-state quasilinear model discussed earlier in the literature, but a distinction regarding the description of wave dynamics between the two approaches is also found. The expanding-box quasilinear formalism is further extended to include the effects of a spiraling solar-wind magnetic field as well as collisional age effects. The present finding shows that the expanding-box quasilinear approach and the steady-state global-kinetic models may be employed interchangeably in order to address other more complex problems associated with the solar-wind dynamics.

This paper formulates a velocity moment-based quasi-linear theory that combines the impacts of weakly unstable proton–cyclotron- (or, equivalently, electromagnetic ion cyclotron) and proton-mirror instabilities on the solar wind plasma initially characterized by an excessive perpendicular proton temperature anisotropy. The present formalism is an alternative to the existing model in that the weakly unstable modes are characterized by analytical formalism that involves the assumption of weak growth rate and/or fluid-theoretical dispersion relation, in place of numerical root-finding method based on the transcendental plasma dispersion function. This results in an efficient numerical platform for analyzing the quasi-linear development of the said instabilities. Such a formalism may be useful in the larger context of global solar wind modelling effort where an efficient calculation of self-consistent wave–particle interaction process is called for. A direct comparison with spacecraft observations of solar wind proton data distribution shows that the present weak growth rate formalism of quasi-linear calculation produces results that are consistent with the observation.