Development of Anisotropy in Incompressible Magnetohydrodynamic Turbulence

TL;DR

This study uses 3D simulations to explore how an external magnetic field influences anisotropy development in incompressible MHD turbulence, revealing new filament formation phenomena and spectral behaviors related to wave turbulence.

Contribution

It provides the first detailed analysis of filament formation within current sheets in strongly magnetized MHD turbulence and compares spectral characteristics under different averaging methods.

Findings

Wider inertial range with steep power law in 2D spectra

Filament formation within current and vorticity sheets

Averaging over k_// may obscure wave turbulence transition

Abstract

We present a set of three-dimensional (3D) direct numerical simulations of incompressible decaying magnetohydrodynamic turbulence in which we investigate the influence of an external uniform magnetic field B_0. A parametric study in terms of B_0 intensity is made where, in particular, we distinguish the shear- from the pseudo-Alfven waves dynamics. The initial kinetic and magnetic energies are equal with a negligible cross-correlation. Both the temporal and spectral effects of B_0 are discussed. A sub-critical balance is found between the Alfven and nonlinear times with both a global and a spectral definition. The nonlinear dynamics of strongly magnetized flows is characterized by a different k_perp-spectrum (where B_0 defines the parallel direction) if it is plotted at a fixed k_// (2D spectrum) or if it is integrated (averaged) over all k_// (1D spectrum). In the former case a much wider inertial range is found with a steep power law, closer to the wave turbulence prediction than the Kolmogorov one like in the latter case. It is believed that the averaging effect may be a source of difficulty to detect the transition towards wave turbulence in natural plasmas. For the first time, the formation of filaments is reported within current and vorticity sheets in strongly magnetized flows which modifies our classical picture of dissipative sheets in conductive flows.