Towards a general quasi-linear approach for the instabilities of bi-Kappa plasmas. Whistler instability

TL;DR

This paper develops a quasi-linear model to study the evolution of bi-Kappa electron distributions in space plasmas, revealing how wave-particle interactions influence the plasma's temperature anisotropy and the Kappa parameter over time.

Contribution

It introduces a novel quasi-linear approach that accounts for time-dependent changes in the Kappa parameter during plasma relaxation, advancing understanding of wave-particle dynamics in space plasmas.

Findings

Kappa parameter can initially increase then decrease during relaxation.

Electromagnetic turbulence influences plasma suprathermalization.

Kappa variation depends on initial electron anisotropy conditions.

Abstract

Kappa distributions are ubiquitous in space and astrophysical poorly collisional plasmas, such as the solar wind, suggesting that microscopic and macroscopic properties of these non-equilibrium plasmas are highly conditioned by the wave-particle interactions. The present work addresses the evolution of anisotropic bi-Kappa (or bi-$\kappa$-)distributions of electrons triggering instabilities of electromagnetic electron-cyclotron (or whistler) modes. The new quasi-linear approach proposed here includes time variations of the $\kappa$ parameter during the relaxation of temperature anisotropy. Numerical results show that $\kappa$ may increase for short interval of times at the beginning, but then decreases towards a value lower than the initial one, while the plasma beta and temperature anisotropy indicate a systematic relaxation. Our results suggest that the electromagnetic turbulence plays an important role on the suprathermalization of the plasma, ultimately lowering the parameter $\kappa$. Even though the variation of $\kappa$ is in general negative ($\Delta \kappa <0$) this variation seems to depend of the initial conditions of anisotropic electrons, which can vary very much in the inner heliosphere.