Adaptive critical balance and firehose instability in an expanding, turbulent, collisionless plasma

TL;DR

This study uses hybrid-kinetic simulations to explore how expanding, turbulent, collisionless plasmas develop temperature anisotropies, maintain critical balance, and become unstable to firehose instabilities, affecting plasma dynamics and spectra.

Contribution

It introduces a model describing the evolution of turbulence and instabilities in expanding collisionless plasmas, linking macro- and micro-scale physics with new simulation insights.

Findings

Plasma maintains critical balance during expansion.

Firehose instability excites magnetic fluctuations at ion-Larmor scales.

Temperature anisotropy is regulated by rapid pitch-angle scattering.

Abstract

Using hybrid-kinetic particle-in-cell simulation, we study the evolution of an expanding, collisionless, magnetized plasma in which strong Alfv\'enic turbulence is persistently driven. Temperature anisotropy generated adiabatically by the plasma expansion (and consequent decrease in the mean magnetic-field strength) gradually reduces the effective elasticity of the field lines, causing reductions in the linear frequency and residual energy of the Alfv\'{e}nic fluctuations. In response, these fluctuations modify their interactions and spatial anisotropy to maintain a scale-by-scale "critical balance" between their characteristic linear and nonlinear frequencies. Eventually the plasma becomes unstable to kinetic firehose instabilities, which excite rapidly growing magnetic fluctuations at ion-Larmor scales. The consequent pitch-angle scattering of particles maintains the temperature anisotropy near marginal stability, even as the turbulent plasma continues to expand. The resulting evolution of parallel and perpendicular temperatures does not satisfy double-adiabatic conservation laws, but is described accurately by a simple model that includes anomalous scattering. Our results have implications for understanding the complex interplay between macro- and micro-scale physics in various hot, dilute, astrophysical plasmas, and offer predictions concerning power spectra, residual energy, ion-Larmor-scale spectral breaks, and non-Maxwellian features in ion distribution functions that may be tested by measurements taken in high-beta regions of the solar wind.