Electromagnetic electron whistler-cyclotron instability in bi-Kappa distributed plasmas

Context. Recent studies of the electromagnetic electron whistler-cyclotron instability in anisotropic bi-Kappa distributed plasmas claim that the instability threshold conditions do not depend on the power index, κe, of the electron distribution function, but that the maximum growth rate (γm) strongly depends on this parameter. But these two statements contradict each other because the instability threshold conditions are derived with respect to the threshold levels of the maximum growth rates (e.g.,γm/Ω = 10-1,10-2, etc.). Aims. This paper proposes to clarify this inconsistency, refining the analysis of the electron-whistler cyclotron instability. In anisotropic plasmas far from Maxwellian equilibrium, this instability represents one of the most plausible constraints for the electron temperature anisotropy Te, ⊥ > Te, ∥, (where ∥ and ⊥ denote directions relative to the local stationary magnetic field). Methods. In the context of a suprathermal solar wind, where the electron populations are well fitted by the advanced Kappa distribution functions, these models are expected to provide a more realistic description for the critical stability conditions. The unstable solutions are derived exactly numerically, providing accurate physical correlations between the maximum growth rates and the threshold conditions. Results. Thresholds of the temperature anisotropy are derived for the full range of values of the plasma beta including both the solar wind and magnetospheric plasma conditions. The lowest thresholds, which are the most relevant for the marginal stability, are found to decrease with the increase in density of the suprathermal populations. This result is correlated with an opposite effect on the corresponding growth rates (at low anisotropies), because their maximum values are enhanced in the presence of suprathermal electrons. The new marginal thresholds calculated with a bi-Kappa model are expected to provide better predictions for the limits of the temperature anisotropy in the solar wind.