# Mirror instability in the turbulent solar wind

## TL;DR

This study uses hybrid simulations to explore how turbulence and expansion in the solar wind lead to mirror instability, resulting in magnetic structures that regulate temperature anisotropy.

## Contribution

It demonstrates the development of mirror instability in turbulent, expanding plasma with a perpendicular magnetic field using 2D hybrid simulations.

## Key findings

- Turbulent cascade produces Kolmogorov-like spectra at large scales.
- Expansion induces proton temperature anisotropy.
- Mirror instability creates magnetic humps that reduce anisotropy.

## Abstract

The relationship between a decaying strong turbulence and the mirror instability in a slowly expanding plasma is investigated using two-dimensional hybrid expanding box simulations. We impose an initial ambient magnetic field perpendicular to the simulation box, and we start with a spectrum of large-scale, linearly-polarized, random-phase Alfvenic fluctuations which have energy equipartition between kinetic and magnetic fluctuations and vanishing correlation between the two fields. A turbulent cascade rapidly develops, magnetic field fluctuations exhibit a Kolmogorov-like power-law spectrum at large scales and a steeper spectrum at sub-ion scales. The imposed expansion (taking a strictly transverse ambient magnetic field) leads to generation of an important perpendicular proton temperature anisotropy that eventually drives the mirror instability. This instability generates large-amplitude, nonpropagating, compressible, pressure-balanced magnetic structures in a form of magnetic enhancements/humps that reduce the perpendicular temperature anisotropy.