A Detailed Examination of Anisotropy and Timescales in Three-dimensional Incompressible Magnetohydrodynamic Turbulence

TL;DR

This paper investigates anisotropy and timescales in 3D incompressible MHD turbulence using high-resolution simulations, revealing how anisotropy affects spectral transfer and the significance of quasi two-dimensional fluctuations.

Contribution

It provides a quantitative analysis of timescales and spectral transfer in anisotropic MHD turbulence, emphasizing the role of quasi two-dimensional fluctuations and critical balance regions.

Findings

Anisotropy leads to stronger gradients perpendicular to the magnetic field.

Quasi two-dimensional fluctuations significantly influence spectral transfer.

Special spectral regions like the critical balance are important in turbulence dynamics.

Abstract

When magnetohydrodynamic turbulence evolves in the presence of a large-scale mean magnetic field, an anisotropy develops relative to that preferred direction. The well-known tendency is to develop stronger gradients perpendicular to the magnetic field, relative to the direction along the field. This anisotropy of the spectrum is deeply connected with anisotropy of estimated timescales for dynamical processes, and requires reconsideration of basic issues such as scale locality and spectral transfer. Here analysis of high-resolution three-dimensional simulations of unforced magnetohydrodynamic turbulence permits quantitative assessment of the behavior of theoretically relevant timescales in Fourier wavevector space. We discuss the distribution of nonlinear times, Alfv\'en times, and estimated spectral transfer rates. Attention is called to the potential significance of special regions of the spectrum, such as the two-dimensional limit and the "critical balance" region. A formulation of estimated spectral transfer in terms of a suppression factor supports a conclusion that the quasi two-dimensional fluctuations (characterized by strong nonlinearities) are not a singular limit, but may be in general expected to make important contributions.