Turbulence Mode Decomposition and Anisotropy in Magnetically Dominated Collisionless Plasmas

TL;DR

This study uses 3D kinetic simulations to analyze turbulence modes and anisotropy in relativistic, magnetically dominated collisionless plasmas, extending existing decomposition methods to this regime.

Contribution

It extends the mode decomposition technique to relativistic collisionless plasmas and reveals differences in turbulence anisotropy and mode coupling compared to non-relativistic MHD.

Findings

Alfvén and slow modes are anisotropic, following Goldreich & Sridhar scaling.

Fast modes are isotropic and have a larger kinetic energy fraction.

Weaker scale dependence of dynamic alignment than previously thought.

Abstract

We use the 3D fully kinetic simulation to study different turbulence modes and turbulence anisotropy of relativistic turbulence in magnetically dominated collisionless plasmas. We extend the method developed by Cho & Lazarian (2002) for decomposing non-relativistic magnetohydrodynamic (MHD) turbulence into Alfv\'en, fast, and slow modes to the regime of collisionless plasmas. We find that Alfv\'en and slow modes are anisotropic, following the Goldreich & Sridhar (1995) scaling, while fast modes are isotropic. We observe a larger kinetic energy fraction of fast modes compared to that in the non-relativistic MHD turbulence, suggesting a stronger coupling of Alfv\'en and fast modes in relativistic magnetized turbulence in collisionless plasmas. We further examine the dynamic alignment and find a weaker scale dependence of the alignment angle than previously proposed. The dominant thermal fluctuations in the kinetic range can cause flattening of the turbulent velocity structure function and weakening of the turbulence anisotropy and dynamic alignment near the kinetic scales.