The oblique firehose instability in a bi-kappa magnetized plasma

Abstract

In this work, we derive a dispersion equation that describes the excitation of the oblique (or Alfvén) firehose instability in a plasma that contains both electron and ion species modelled by bi-kappa velocity distribution functions. The equation is obtained with the assumptions of low-frequency waves and moderate to large values of the parallel (respective to the ambient magnetic field) plasma beta parameter, but it is valid for any direction of propagation and for any value of the particle gyroradius (or Larmor radius). Considering values for the physical parameters typical to those found in the solar wind, some solutions of the dispersion equation, corresponding to the unstable mode, are presented. In order to implement the dispersion solver, several new mathematical properties of the special functions occurring in a kappa plasma are derived and included. The results presented here suggest that the superthermal characteristic of the distribution functions leads to reductions to both the maximum growth rate of the instability and of the spectral range of its occurrence.

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

“…Numerical simulations may offer a complete and dynamical picture providing valuable insights from both the linear and nonlinear phases of the relaxation, to resolve the interplay of kinetic instabilities that may develop for the same plasma conditions and to decode the role played by different populations, in our case the suprathermal electrons. For bi-Kappa distributed plasmas, a theoretical characterization of the whole wave-vector spectrum of kinetic instabilities is complicated even in a linear limit (Summers & Thorne 1991;Summers et al 1994;Astfalk et al 2015;Kim et al 2017Kim et al , 2018Meneses et al 2018;, and a quasilinear approach is even less straightforward. Here we use particle-in-cell (PIC) simulations and seek to provide answers to these questions from linear and extended quasilinear and nonlinear evolution of the firehose instabilities driven by the bi-Kappa distributed electrons.…”

Linear dispersion theory of parallel electromagnetic modes for regularized Kappa-distributions

“…The regularized Kappa-distribution, therefore, can reproduce the results of a standard Kappa-distribution with the benefit of removing its limitations by being defined for all κ > 0, having no diverging velocity moments and no contribution to macroscopic quantities (e.g., pressure) by superluminal particles (provided that α is choosen properly). As a continuation of the wave dispersion properties of plasma systems described by the regularized Kappa-distribution, future studies must consider oblique modes, 47 e.g., the aperiodic mirror 48 and firehose 45,49,50 instabilities. However, along with these strengths, we have encountered a few issues which need only to be mentioned at this stage.…”