Particle-in-cell Simulations of Firehose Instability Driven by Bi-Kappa Electrons

We report the first results from particle-in-cell simulations of the fast-growing aperiodic electron firehose instability driven by the anisotropic bi-Kappa distributed electrons. Such electrons characterize space plasmas, e.g., solar wind and planetary magnetospheres. Predictions made by the linear theory for full wave-frequency and wave-vector spectra of instabilities are confirmed by the simulations showing that only the aperiodic branch develops at oblique angles with respect to the magnetic field direction. Angles corresponding to the peak magnetic field fluctuating power spectrum increase with the increase in the anisotropy and with the decrease in the inverse power-law index κ. The instability saturation and later nonlinear evolutions are also dominated by the oblique fluctuations, which are enhanced by the suprathermals and trigger a faster relaxation of the anisotropic electrons. Diffusion in velocity space is stimulated by the growing fluctuations, which scatter the electrons, starting with the more energetic suprathermal populations, as appears already before the saturation. After saturation the fluctuating magnetic field power shows decay patterns in the wave-vector space and a shift toward lower angles of propagation.