Intermittency in Collisionless Large-Amplitude Turbulence

TL;DR

This study investigates the intermittent nature of collisionless large-amplitude turbulence in electron-positron plasmas using 3D kinetic simulations, revealing how pressure anisotropy influences turbulence structures and potential cosmic ray interactions.

Contribution

It provides the first detailed analysis of intermittency and pressure anisotropy effects in collisionless turbulence beyond MHD approximations.

Findings

Pressure anisotropy steepens the magnetic field-curvature relationship.

The probability density function of field-line curvature hardens with increased turbulence amplitude.

Mirror and firehose instabilities occupy significant volume fractions, affecting plasma dynamics.

Abstract

Large-amplitude turbulence -- characterized by a fluctuating magnetic field component, $\delta B$, that is stronger than the mean component, $B_0$ -- is generically intermittent, populated with intense localized structures such as sharp field-line bends and rapid field reversals. Recent MHD simulations suggest that these structures play an important role in particle transport and acceleration; however, MHD is inapplicable in most of our Universe, where the plasma is so hot or diffuse that Coulomb collisions are negligible. Therefore, in this paper, we analyze the intermittent properties of collisionless large-amplitude turbulence in electron-positron plasmas via fully kinetic 3D simulations, exploring a wide range of $\delta B / B_0$ and scale separations between the turbulence driving scale, $L$, and kinetic scales, $c/\omega_{\rm p}$. The steady-state collisionless turbulence in our simulations broadly resembles that of MHD, but the development of pressure anisotropy steepens the scaling between magnetic field strength, $B$, and scalar field-line curvature, $K_\parallel$ -- yielding $B \propto K_\parallel^{-3/4}$ -- and consequently modifies the power-law slope of the probability density function of $K_\parallel$; this slope hardens from $K_\parallel^{-2.5}$ to $K_\parallel^{-2.0}$ as $\delta B / B_0$ increases from 4 to 140. Pressure anisotropy also triggers mirror and firehose instabilities, with the volume-filling fractions of these fluctuations increasing with $\delta B / B_0$; for our largest $\delta B / B_0$, $20\%$ of the volume is mirror-unstable and $6\%$ is firehose-unstable. Both the curvature and the Larmor-scale fluctuations in collisionless large-amplitude turbulence are expected to significantly influence cosmic ray transport and acceleration in the interstellar medium of our Galaxy and the intracluster medium of galaxy clusters.