Predicted Impacts of Proton Temperature Anisotropy on Solar Wind Turbulence

TL;DR

This paper develops a framework to understand how proton temperature anisotropy affects solar wind turbulence, finding that large-scale turbulence remains unaffected by anisotropy and instability-driven fluctuations do not significantly alter the cascade.

Contribution

It introduces a unified linear dispersion relation framework for proton anisotropy instabilities and analyzes their impact on nonlinear solar wind turbulence.

Findings

Large-scale turbulence is insensitive to proton temperature anisotropy.

Instability-driven fluctuations do not cause significant nonlinear evolution.

The framework identifies stable and unstable modes in wavevector space.

Abstract

Particle velocity distributions measured in the weakly collisional solar wind are frequently found to be non-Maxwellian, but how these non-Maxwellian distributions impact the physics of plasma turbulence in the solar wind remains unanswered. Using numerical solutions of the linear dispersion relation for a collisionless plasma with a bi-Maxwellian proton velocity distribution, we present a unified framework for the four proton temperature anisotropy instabilities, identifying the associated stable eigenmodes, highlighting the unstable region of wavevector space, and presenting the properties of the growing eigenfunctions. Based on physical intuition gained from this framework, we address how the proton temperature anisotropy impacts the nonlinear dynamics of the \Alfvenic fluctuations underlying the dominant cascade of energy from large to small scales and how the fluctuations driven by proton temperature anisotropy instabilities interact nonlinearly with each other and with the fluctuations of the large-scale cascade. We find that the nonlinear dynamics of the large-scale cascade is insensitive to the proton temperature anisotropy, and that the instability-driven fluctuations are unlikely to cause significant nonlinear evolution of either the instability-driven fluctuations or the turbulent fluctuations of the large-scale cascade.