Energy Dissipation and Entropy in Collisionless Plasma

TL;DR

This paper explores how collisionless plasmas, despite lacking particle collisions, still undergo energy dissipation and entropy increase through processes like magnetic reconnection and turbulence, using theoretical analysis and simulations.

Contribution

It derives evolution equations for fluid entropy in collisionless plasma and highlights the role of heat flux in dissipation, verified by particle-in-cell simulations.

Findings

Entropy increases during magnetic reconnection.

Heat flux significantly contributes to dissipation.

Simulation confirms theoretical entropy evolution equations.

Abstract

It is well known that collisionless systems are dissipation free from the perspective of particle collision and thus conserve entropy. On the other hand, processes such as magnetic reconnection and turbulence appear to convert large-scale magnetic energy into heat. In this paper, we investigate the energization and heating of collisionless plasma. The dissipation process is discussed in terms of fluid entropy in both isotropic and gyrotropic forms. Evolution equations for the entropy are derived and they reveal mechanisms that lead to changes in fluid entropy. These equations are verified by a collisionless particle-in-cell simulation of multiple reconnecting current sheets. In addition to previous findings regarding the pressure tensor, we emphasize the role of heat flux in the dissipation process.