Effects of Thermal Fluctuations on Temperature Anisotropy Instabilities in the Solar Wind

The proton temperature anisotropy in the solar wind, when plotted as a two-dimensional histogram in (beta(parallel to), T-perpendicular to/T-parallel to) phase space, shows that the bulk of the data are broadly distributed in quasi-isotropic state, while the fringe of data distribution is apparently regulated by various marginal stability conditions. The physical origin of magnetic field fluctuations in the stable regime is an open question. Among plausible mechanisms is the spontaneous thermal emission process. Discrete particle effects are central to the theory of spontaneous emission and reabsorption of electromagnetic fluctuations in plasmas. The present paper formulates the quasi-linear theory of temperature anisotropy-driven instabilities in which discrete particle effects are taken into account. The hybrid simulation is also carried out with varying number of superparticles per cell in order to unveil the underlying relationship between the theoretical and simulated discrete particle effects. The present paper demonstrates that the inherent numerical noise in the particle simulation is not always undesirable but may actually be exploited in order to control the degree of particle discreteness. Such a finding may have a significant implication for simulating the solar wind plasmas.