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

Abstract

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, κ e , of the electron distribution function, but that the maximum growth rate (γ m ) 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.,γ m /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 T e,⊥ > T e, , (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.

Destabilizing effects of the suprathermal populations in the solar wind

“…Precisely, in comparison with bi-Maxwellians, in biKappa plasmas with the same temperatures and the same temperature anisotropy, the resulting instabilities are not enhanced. Contrary to expectations, the instability growth rates are inhibited by the increase of suprathermal populations, i.e., decreasing kappa index (Xue et al 1996;Lazar & Poedts 2009;Mace & Sydora 2010;Lazar et al 2011Lazar et al , 2013Lazar 2012). An enhancing effect of the growth rates may also be apparent, but only for low values of the temperature anisotropy close to the marginal condition of stability where the growth rates Im(ω) ≡ γ → 0 (Summers & Thorne 1992;Xue et al 1993;Mace 1998).…”

Cyclotron Electromagnetic Instabilities in a Laboratory Dipole Magnetospheric Plasma with bi-Kappa Distributions

“…(18) and (19) are related to E to derive the dielectric tensor. We use the Fourier expansions of the 2D perturbed electric field and current density components over λ varying along the magnetic field line:…”

“…Refs. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] and references therein. However, the models of plasmas guided by straight magnetic field lines are not conform with LDM configurations in LDX and RT-1, which are axisymmetric and two-dimensional (2D).…”

“…In this paper we derive for the first time the dielectric characteristics and dispersion relations for the field-aligned electromagnetic waves in a LDM plasma with anisotropic bi-Kappa distributed particles [15,18,19]. These waves have been characterized in Ref.…”

“…The generalized Kappa power-laws are more appropriate to reproduce the velocity distributions with enhanced high-energy tails, which are frequently reported by the in-situ measurements in space plasmas, i.e., the solar wind and planetary magnetospheres [25,26]. On the other hand, cyclotron instabilities have been extensively investigated, but only in bi-Kappa plasmas embedded in a uniform magnetic field [15][16][17][18][19][20][21][22].…”