# Kinetic simulations of the interruption of large-amplitude shear-Alfv\'en waves in a high-beta plasma

## TL;DR

This study uses hybrid-kinetic simulations to show that large-amplitude shear-Alfvén waves in high-beta collisionless plasmas are rapidly damped due to pressure anisotropy and firehose instabilities, preventing their support above a certain amplitude.

## Contribution

It demonstrates the nonlinear interruption mechanism of shear-Alfvén waves in high-beta plasmas and highlights the role of velocity-space instabilities in plasma dynamics.

## Key findings

- Large-amplitude waves decay rapidly above the amplitude limit.
- Firehose modes generate long-lived fluctuations causing particle scattering.
- Collisionless plasmas cannot support linearly polarized Alfvén waves beyond a certain amplitude.

## Abstract

Using two-dimensional hybrid-kinetic simulations, we explore the nonlinear "interruption" of standing and traveling shear-Alfv\'en waves in collisionless plasmas. Interruption involves a self-generated pressure anisotropy removing the restoring force of a linearly polarized Alfv\'enic perturbation, and occurs for wave amplitudes $\delta B_{\perp}/B_{0}\gtrsim \beta^{\,-1/2}$ (where $\beta$ is the ratio of thermal to magnetic pressure). We use highly elongated domains to obtain maximal scale separation between the wave and the ion gyroscale. For standing waves above the amplitude limit, we find that the large-scale magnetic field of the wave decays rapidly. The dynamics are strongly affected by the excitation of oblique firehose modes, which transition into long-lived parallel fluctuations at the ion gyroscale and cause significant particle scattering. Traveling waves are damped more slowly, but are also influenced by small-scale parallel fluctuations created by the decay of firehose modes. Our results demonstrate that collisionless plasmas cannot support linearly polarized Alfv\'en waves above $\delta B_{\perp}/B_{0}\sim \beta^{\,-1/2}$. They also provide a vivid illustration of two key aspects of low-collisionality plasma dynamics: (i) the importance of velocity-space instabilities in regulating plasma dynamics at high $\beta$, and (ii) how nonlinear collisionless processes can transfer mechanical energy directly from the largest scales into thermal energy and microscale fluctuations, without the need for a scale-by-scale turbulent cascade.