Energy partition in collisionless counterstreaming plasmas

TL;DR

This study uses 3D kinetic simulations to explore how energy is distributed among particles in collisionless counterstreaming plasmas, revealing a two-stage magnetic amplification process and electron heating dependent on mass ratio.

Contribution

It provides new insights into energy partition mechanisms in collisionless plasmas, highlighting the roles of Weibel instability and filament kinking in magnetic field amplification.

Findings

Electrons are mainly heated during the late-stage magnetic dynamo process.

Final electron-to-ion temperature ratio depends on ion-to-electron mass ratio.

Electron thermal energy reaches only a few percent of initial ion kinetic energy in proton flows.

Abstract

Fast, counter-streaming plasma outflows drive magnetic field amplification, plasma heating, and particle acceleration in numerous astrophysical environments, from supernova remnant shocks to active galactic nuclei jets. Understanding how, in the absence of Coulomb collisions, energy is redistributed between the different plasma species remains a fundamental open question. We use 3D fully-kinetic simulations to investigate energy partition in weakly magnetized counter-propagating plasmas. Our results reveal a complex interplay between different processes, where at early times the Weibel instability drives a first stage of magnetic field amplification and at late times the kinking of current filaments drives a second amplification stage via a dynamo-type mechanism. Electrons are heated primarily during the latter phase through magnetic pumping. By the time the flows thermalize, we observe that the final temperature ratio $T_e/T_i$ and energy partition depend on the ion-to-electron mass ratio. For electron-proton flows, the electron thermal energy only reaches up to a few percent of the initial ion kinetic energy.