MHD Turbulence, Turbulent Dynamo and Applications

TL;DR

This paper reviews properties of incompressible MHD turbulence, reports on numerical measurements of small-scale dynamo efficiency, and discusses the dynamics of different MHD turbulence modes relevant to space physics.

Contribution

It provides new measurements of dynamo efficiency and anisotropy constants in high-resolution simulations, and clarifies the interactions of Alfvén, slow, and fast modes in compressible MHD turbulence.

Findings

Measured small-scale dynamo efficiency $C_E=0.05$

Determined Kolmogorov constant $C_K=4.2$

Analyzed mode interactions in compressible MHD turbulence

Abstract

MHD Turbulence is common in many space physics and astrophysics environments. We first discuss the properties of incompressible MHD turbulence. A well-conductive fluid amplifies initial magnetic fields in a process called small-scale dynamo. Below equipartition scale for kinetic and magnetic energies the spectrum is steep (Kolmogorov -5/3) and is represented by critically balanced strong MHD turbulence. In this paper we report the basic reasoning behind universal nonlinear small-scale dynamo and the inertial range of MHD turbulence. We measured the efficiency of the small-scale dynamo $C_E=0.05$, Kolmogorov constant $C_K=4.2$ and anisotropy constant $C_A=0.63$ for MHD turbulence in high-resolution direct numerical simulations. We also discuss so-called imbalanced or cross-helical MHD turbulence which is relevant for in many objects, most prominently in the solar wind. We show that properties of incompressible MHD turbulence are similar to the properties of Alfv\'enic part of MHD cascade in compressible turbulence. The other parts of the cascade evolve according to their own dynamics. The slow modes are being cascaded by Alfv\'enic modes, while fast modes create an independent cascade. We show that different ways of decomposing compressible MHD turbulence into Alfv\'en, slow and fast modes provide consistent results and are useful in understanding not only turbulent cascade, but its interaction with fast particles.