Thermodynamics and collisionality in firehose-susceptible high-$\beta$ plasmas

TL;DR

This paper investigates how high-beta collisionless plasmas evolve under firehose instability, identifying three distinct thermodynamic states and focusing on the newly characterized Alfvén-enabling state that supports Alfvén waves and turbulence.

Contribution

It introduces a new Alfvén-enabling state in firehose-susceptible plasmas, expanding understanding of plasma behavior and magnetic turbulence in astrophysical environments.

Findings

Three distinct thermodynamic states identified in firehose-susceptible plasmas.

The Alfvén-enabling state supports Alfvén waves and turbulence.

Collision frequency scales as β/τ, with velocity-dependent scattering rates.

Abstract

We study the evolution of collisionless plasmas that, due to their macroscopic evolution, are susceptible to the firehose instability, using both analytic theory and hybrid-kinetic particle-in-cell simulations. We establish that, depending on the relative magnitude of the plasma $\beta$, the characteristic timescale of macroscopic evolution, and the ion-Larmor frequency, the saturation of the firehose instability in high-$\beta$ plasmas can result in three qualitatively distinct thermodynamic (and electromagnetic) states. By contrast with the previously identified `ultra-high-beta' and `Alfv\'en-inhibiting' states, the newly identified `Alfv\'en-enabling' state, which is realised when the macroscopic evolution time $\tau$ exceeds the ion-Larmor frequency by a $\beta$-dependent parameter, can support linear Alfv\'en waves and Alfv\'enic turbulence because the magnetic tension associated with the plasma's macroscopic magnetic field is never completely negated by anisotropic pressure forces. We characterise these states in detail, including their saturated magnetic-energy spectra. The effective collision operator associated with the firehose fluctuations is also described; we find it to be well approximated in the Alfv\'en-enabling state by a simple quasilinear pitch-angle scattering operator. The box-averaged collision frequency is $\nu_{\rm eff} \sim \beta/\tau$, in agreement with previous results, but certain sub-populations of particles scatter at a much larger (or smaller) rate depending on their velocity in the direction parallel to the magnetic field. Our findings are essential for understanding low-collisionality astrophysical plasmas including the solar wind, the intracluster medium of galaxy clusters and black-hole accretion flows. We show that all three of these plasmas are in the Alfv\'en-enabling regime of firehose saturation and discuss the implications of this result.