# Proton fire hose instabilities in the expanding solar wind

## TL;DR

This study uses hybrid simulations to explore how proton temperature anisotropies and fire hose instabilities evolve in the expanding solar wind, revealing the roles of magnetic field orientation in plasma stability.

## Contribution

It demonstrates the dynamic interplay between anisotropy-driven instabilities and magnetic field orientation in collisionless plasma expansion.

## Key findings

- Oblique fire hose instability effectively reduces anisotropy.
- Magnetic field orientation determines plasma stability.
- Electromagnetic fluctuations are damped to protons.

## Abstract

Using two-dimensional hybrid expanding box simulations we study the competition between the continuously driven parallel proton temperature anisotropy and fire hose instabilities in collisionless homogeneous plasmas. For quasi radial ambient magnetic field the expansion drives $T_{\mathrm{p}\|}>T_{\mathrm{p}\perp}$ and the system becomes eventually unstable with respect to the dominant parallel fire hose instability. This instability is generally unable to counteract the induced anisotropization and the system typically becomes unstable with respect to the oblique fire hose instability later on. The oblique instability efficiently reduces the anisotropy and the system rapidly stabilizes while a significant part of the generated electromagnetic fluctuations is damped to protons. As long as the magnetic field is in the quasi radial direction, this evolution repeats itself and the electromagnetic fluctuations accumulate. For sufficiently oblique magnetic field the expansion drives $T_{\mathrm{p}\perp}>T_{\mathrm{p}\|}$ and brings the system to the stable region with respect to the fire hose instabilities.