Particle-in-cell simulations of whistler waves excited by an electron κ distribution in space plasma

Satellite observations clearly reveal that superthermal electrons in space plasma generally possess a pronounced non-Maxwellian distribution that can be well modeled by a kappa distribution. In this paper, one-dimensional (1-D) particle-in-cell simulations are performed to investigate the evolution of whistler waves driven by superthermal electrons with a typical kappa distribution in the presence of a cold plasma population. The results obtained from the linear theory are first confirmed: with the increase of the spectral index kappa for the kappa distribution, the linear growth rate of the excited waves increases and instability threshold for the temperature anisotropy (A = T-perpendicular to/T-parallel to-1) decreases. Then we further find that with the increase of kappa, the fluctuating magnetic field energy density at the saturation stage also increases. Therefore, from both the linear growth rate and the fluctuating magnetic field energy density at the saturation stage, we can find that a bi-Maxwellian distribution (kappa -> infinity) overestimates the importance of whistler waves, since the observed value of kappa lies in the range 2 <= kappa <= 6. We also find that the kappa values of the electron distributions become smaller with the excitation of the whistler waves.