Spatio-temporal behavior of magnetohydrodynamic fluctuations with cross-helicity and background magnetic field

TL;DR

This study investigates the spatio-temporal dynamics of MHD turbulence, revealing how decorrelation times are influenced by sweeping and Alfvénic effects depending on magnetic field strength and cross-helicity, with notable wave reflection phenomena.

Contribution

It provides a detailed analysis of decorrelation times in MHD turbulence, highlighting the dominance of different physical effects under varying magnetic and cross-helicity conditions, and reports on wave reflection impacts.

Findings

Decorrelation times are dominated by sweeping effects at low magnetic field and cross-helicity.

Alfvénic effects control decorrelation times at high magnetic field or cross-helicity.

Reflections cause counter-propagation of Alfvénic fluctuations, especially in high cross-helicity flows.

Abstract

We study the spatio-temporal behavior of the Els\"asser variables describing magnetic and velocity field fluctuations, using direct numerical simulations of three-dimensional magnetohydrodynamic turbulence. We consider cases with relatively small, intermediate, and large values of a mean background magnetic field, and with null, small, and high cross-helicity (correlations between the velocity and the magnetic field). Wavenumber-dependent time correlation functions are computed for the different simulations. From these correlation functions, the decorrelation time is computed and compared with different theoretical characteristic times: the local non-linear time, the random-sweeping time, and the Alfv\'enic time. It is found that decorrelation times are dominated by sweeping effects for low values of the mean magnetic field and for low values of the cross-helicity, while for large values of the background field or of the cross-helicity and for wave vectors sufficiently aligned with the guide field, decorrelation times are controlled by Alfv\'enic effects. Finally, we observe counter-propagation of Alfv\'enic fluctuations due to reflections produced by inhomogeneities in the total magnetic field. This effect becomes more prominent in flows with large cross-helicity, strongly modifying the propagation of waves in turbulent magnetohydrodynamic flows.