Whistler-cyclotron spontaneous fluctuations. A proxy to identify thermal and non-thermal electrons?

Observed electron velocity distributions in the space plasmas exhibit a variety of non-thermal features which deviate from thermal equilibrium, in the form of temperature anisotropies, suprathermal tails, and field aligned beams. The state close to thermal equilibrium and its departure from it provides a source for spontaneous emissions of electromagnetic fluctuations. For example, the whistler cyclotron waves at electron scales. Here we present a comparative analysis of these fluctuations based upon anisotropic plasma modeled with thermal and non-thermal particle distributions. The analysis presented here considers the second-order theory of fluctuations and the dispersion relation of weakly transverse fluctuations, with wave vectors parallel to the uniform background magnetic field, in a finite temperature isotropic magnetized electron-proton plasma modeled with bi-Maxwellian and kappa-like distributions. Dispersion analysis and stability thresholds are derived for these non-thermal distributions and compared with similar results obtained from PIC simulations using plasma and field parameters relevant to space nearly collisionless environments. Our results indicate that there is a strong dependence between the shape of the velocity distribution function and the spontaneous magnetic fluctuations wave spectrum. This feature may be used proxy to identify the nature of electron populations in space plasmas when high resolution particle instruments are not available.