Wave generation and heat flux suppression in astrophysical plasma systems

TL;DR

This study uses particle-in-cell simulations to investigate heat flux suppression mechanisms in collisionless astrophysical plasmas across different plasma beta values, revealing a transition from whistler to double-layer dominance.

Contribution

It identifies a transition between whistler-dominated and double-layer-dominated heat flux suppression in collisionless plasmas, highlighting the role of ion-electron coupling and scale-dependent physics.

Findings

Whistler waves saturate at small amplitudes in low-beta plasmas.

Double layers suppress heat flux to a constant fraction of free streaming.

Ion heating scales with plasma beta and associated instabilities.

Abstract

Heat flux suppression in collisionless plasmas for a large range of plasma $\beta$ is explored using two-dimensional particle-in-cell simulations with a strong, sustained thermal gradient. We find that a transition takes place between whistler-dominated (high-$\beta$) and double-layer-dominated (low-$\beta$) heat flux suppression. Whistlers saturate at small amplitude in the low beta limit and are unable to effectively suppress the heat flux. Electrostatic double layers suppress the heat flux to a mostly constant factor of the free streaming value once this transition happens. The double layer physics is an example of ion-electron coupling and occurs on a scale of roughly the electron Debye length. The scaling of ion heating associated with the various heat flux driven instabilities is explored over the full range of $\beta$ explored. The range of plasma-$\beta$s studied in this work makes it relevant to the dynamics of a large variety of astrophysical plasmas, including the intracluster medium of galaxy clusters, hot accretion flows, stellar and accretion disk coronae, and the solar wind.