Ion and Electron Acceleration in Fully Kinetic Plasma Turbulence

TL;DR

This study uses 3D kinetic simulations to show that turbulence in low-beta plasmas accelerates ions and electrons into nonthermal distributions, with distinct anisotropies and energization mechanisms relevant to space and astrophysical plasmas.

Contribution

It provides the first detailed kinetic simulation analysis of particle acceleration and anisotropy in turbulent low-beta plasmas, highlighting different energization processes for ions and electrons.

Findings

Ions have a harder energy spectrum than electrons.

Both particle distributions steepen with increasing plasma beta.

Electrons show energy-dependent pitch-angle anisotropy, mainly moving parallel to magnetic fields.

Abstract

Turbulence is often invoked to explain the origin of nonthermal particles in space and astrophysical plasmas. By means of 3D fully kinetic particle-in-cell simulations, we demonstrate that turbulence in low-$\beta$ plasmas ($\beta$ is the ratio of plasma pressure to magnetic pressure) accelerates ions and electrons into a nonthermal energy distribution with a power-law energy range. The ion spectrum is harder than the electron one, and both distributions get steeper for higher $\beta$. We show that the energization of electrons is accompanied by a significant energy-dependent pitch-angle anisotropy, with most electrons moving parallel to the local magnetic field, while ions stay roughly isotropic. We demonstrate that particle injection from the thermal pool occurs in regions of high current density. Parallel electric fields associated with magnetic reconnection are responsible for the initial energy gain of electrons, whereas perpendicular electric fields control the overall energization of ions. Our findings have important implications for the origin of nonthermal particles in space and astrophysical plasmas.