Plasma turbulence and kinetic instabilities at ion scales in the expanding solar wind

TL;DR

This study uses 2-D hybrid simulations to explore how plasma turbulence and kinetic instabilities interact at ion scales in the expanding solar wind, revealing complex heating and instability dynamics.

Contribution

It demonstrates that kinetic instabilities can coexist with strong turbulence in a 2-D plasma expansion model, highlighting their role in temperature anisotropy regulation.

Findings

Turbulent cascade develops a Kolmogorov-like spectrum at large scales.

Protons are heated anisotropically, mainly perpendicularly.

Oblique firehose instability reduces parallel temperature anisotropy.

Abstract

The relationship between a decaying strong turbulence and kinetic instabilities in a slowly expanding plasma is investigated using two-dimensional (2-D) 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 Alfv\'enic 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 ion scales. The turbulent cascade leads to an overall anisotropic proton heating, protons are heated in the perpendicular direction, and, initially, also in the parallel direction. The imposed expansion leads to generation of a large parallel proton temperature anisotropy which is at later stages partly reduced by turbulence. The turbulent heating is not sufficient to overcome the expansion-driven perpendicular cooling and the system eventually drives the oblique firehose instability in a form of localized nonlinear wave packets which efficiently reduce the parallel temperature anisotropy. This work demonstrates that kinetic instabilities may coexist with strong plasma turbulence even in a constrained 2-D regime.