Inertial-Range Kinetic Turbulence in Pressure-Anisotropic Astrophysical Plasmas

TL;DR

This paper develops a theoretical framework for low-frequency turbulence in pressure-anisotropic, collisionless plasmas, extending RMHD to include species drifts and non-Maxwellian distributions, with implications for astrophysical plasmas like the solar wind.

Contribution

It generalizes reduced magnetohydrodynamics to non-Maxwellian plasmas with species drifts, providing a quantitative theory for turbulence in astrophysical environments.

Findings

Alfvenic and compressive fluctuations are decoupled in energy.

Pressure anisotropy modifies the Alfven ratio and cascade dynamics.

Approaching instability thresholds alters plasma energy partitioning.

Abstract

A theoretical framework for low-frequency electromagnetic (drift-)kinetic turbulence in a collisionless, multi-species plasma is presented. The result generalises reduced magnetohydrodynamics (RMHD) and kinetic RMHD (Schekochihin et al. 2009) for pressure-anisotropic plasmas, allowing for species drifts---a situation routinely encountered in the solar wind and presumably ubiquitous in hot dilute astrophysical plasmas (e.g. intracluster medium). Two main objectives are achieved. First, in a non-Maxwellian plasma, the relationships between fluctuating fields (e.g., the Alfven ratio) are order-unity modified compared to the more commonly considered Maxwellian case, and so a quantitative theory is developed to support quantitative measurements now possible in the solar wind. The main physical feature of low-frequency plasma turbulence survives the generalisation to non-Maxwellian distributions: Alfvenic and compressive fluctuations are energetically decoupled, with the latter passively advected by the former; the Alfvenic cascade is fluid, satisfying RMHD equations (with the Alfven speed modified by pressure anisotropy and species drifts), whereas the compressive cascade is kinetic and subject to collisionless damping. Secondly, the organising principle of this turbulence is elucidated in the form of a generalised kinetic free-energy invariant. It is shown that non-Maxwellian features in the distribution function reduce the rate of phase mixing and the efficacy of magnetic stresses; these changes influence the partitioning of free energy amongst the various cascade channels. As the firehose or mirror instability thresholds are approached, the dynamics of the plasma are modified so as to reduce the energetic cost of bending magnetic-field lines or of compressing/rarefying them. Finally, it is shown that this theory can be derived as a long-wavelength limit of non-Maxwellian slab gyrokinetics.