Electron and ion energization in relativistic plasma turbulence

TL;DR

This study uses particle-in-cell simulations to investigate how electrons and ions gain energy in relativistic plasma turbulence, revealing ion heating dominance and variable nonthermal acceleration affecting astrophysical models.

Contribution

It provides new empirical formulas for electron-ion energy partition and insights into nonthermal acceleration differences in relativistic plasma turbulence.

Findings

Ions are preferentially heated, gaining up to ten times more energy than electrons.

Nonthermal electron populations decrease with lower temperatures, while ion populations remain stable.

The results inform models of hot accretion flows in astrophysics.

Abstract

Electron and ion energization (i.e., heating and nonthermal acceleration) is a fundamental, but poorly understood, outcome of plasma turbulence. In this work, we present new results on this topic from particle-in-cell simulations of driven turbulence in collisionless, relativistic electron-ion plasma. We focus on temperatures such that ions (protons) are sub-relativistic and electrons are ultra-relativistic, a regime relevant for high-energy astrophysical systems such as hot accretion flows onto black holes. We find that ions tend to be preferentially heated, gaining up to an order of magnitude more energy than electrons, and propose a simple empirical formula to describe the electron-ion energy partition as a function of the ratio of electron-to-ion gyroradii (which in turn is a function of initial temperatures and plasma beta). We also find that while efficient nonthermal particle acceleration occurs for both species in the ultra-relativistic regime, nonthermal electron populations are diminished with decreasing temperature whereas nonthermal ion populations are essentially unchanged. These results have implications for modeling and interpreting observations of hot accretion flows.