Interplanetary and interstellar plasma turbulence

TL;DR

This paper reviews theoretical models of low-frequency plasma turbulence in astrophysical environments, emphasizing kinetic theory and anisotropy, and discusses cascades from large to electron scales in solar wind and interstellar media.

Contribution

It systematically derives reduced analytical models for plasma turbulence, including hybrid fluid-kinetic descriptions and cascade processes across different scales.

Findings

Alfven waves decouple from density and magnetic fluctuations above ion gyroscale.

Hybrid fluid-kinetic model describes turbulence independently of collisionality.

Kinetic Alfven wave cascade occurs below ion gyroscale, damped at electron gyroscale.

Abstract

Theoretical approaches to low-frequency magnetized turbulence in collisionless and weakly collisional astrophysical plasmas are reviewed. The proper starting point for an analytical description of these plasmas is kinetic theory, not fluid equations. The anisotropy of the turbulence is used to systematically derive a series of reduced analytical models. Above the ion gyroscale, it is shown rigourously that the Alfven waves decouple from the electron-density and magnetic-field-strength fluctuations and satisfy the Reduced MHD equations. The density and field-strength fluctuations (slow waves and the entropy mode in the fluid limit), determined kinetically, are passively mixed by the Alfven waves. The resulting hybrid fluid-kinetic description of the low-frequency turbulence is valid independently of collisionality. Below the ion gyroscale, the turbulent cascade is partially converted into a cascade of kinetic Alfven waves, damped at the electron gyroscale. This cascade is described by a pair of fluid-like equations, which are a reduced version of the Electron MHD. The development of these theoretical models is motivated by observations of the turbulence in the solar wind and interstellar medium. In the latter case, the turbulence is spatially inhomogeneous and the anisotropic Alfvenic turbulence in the presence of a strong mean field may coexist with isotropic MHD turbulence that has no mean field.