# Amplitude limits and nonlinear damping of shear-Alfv\'en waves in high-beta low-collisionality plasmas

## TL;DR

This paper investigates how pressure anisotropy limits shear-Alfvén wave amplitudes in high-beta, low-collisionality plasmas, revealing a self-regulating mechanism via the firehose instability that impacts plasma turbulence.

## Contribution

It extends previous work by analyzing the nonlinear damping and amplitude limits of shear-Alfvén waves due to pressure anisotropy in high-beta plasmas, including collisionless and weakly collisional regimes.

## Key findings

- Pressure anisotropy imposes strict amplitude limits on shear-Alfvén waves.
- Above a critical amplitude, waves trigger the firehose instability, halting oscillations.
- Implications for turbulence in astrophysical high-beta plasmas.

## Abstract

This work, which extends Squire et al. [ApJL, 830 L25 (2016)], explores the effect of self-generated pressure anisotropy on linearly polarized shear-Alfv\'en fluctuations in low-collisionality plasmas. Such anisotropies lead to stringent limits on the amplitude of magnetic perturbations in high-beta plasmas, above which a fluctuation can destabilize itself through the parallel firehose instability. This causes the wave frequency to approach zero, "interrupting" the wave and stopping its oscillation. These effects are explored in detail in the collisionless and weakly collisional "Braginskii" regime, for both standing and traveling waves. The focus is on simplified models in one dimension, on scales much larger than the ion gyroradius. The effect has interesting implications for the physics of magnetized turbulence in the high-beta conditions that are prevalent in many astrophysical plasmas.