Generalized phase mixing: Turbulence-like behaviour from unidirectionally propagating MHD waves

Norbert Magyar,Tom Van Doorsselaere,Marcel Goossens

doi:10.1038/s41598-017-13660-1

Norbert Magyar, Tom Van Doorsselaere + Show 1 more

Open Access

https://doi.org/10.1038/s41598-017-13660-1

Copy DOI

Journal: Scientific Reports	Publication Date: Nov 1, 2017
Citations: 45	License type: open-access

Affiliation: KU Leuven

Abstract

We present the results of three-dimensional (3D) ideal magnetohydrodynamics (MHD) simulations on the dynamics of a perpendicularly inhomogeneous plasma disturbed by propagating Alfvénic waves. Simpler versions of this scenario have been extensively studied as the phenomenon of phase mixing. We show that, by generalizing the textbook version of phase mixing, interesting phenomena are obtained, such as turbulence-like behavior and complex current-sheet structure, a novelty in longitudinally homogeneous plasma excited by unidirectionally propagating waves. This study is in the setting of a coronal hole. However, it constitutes an important finding for turbulence-related phenomena in astrophysics in general, relaxing the conditions that have to be fulfilled in order to generate turbulent behavior.

Highlights

We present the results of three-dimensional (3D) ideal magnetohydrodynamics (MHD) simulations on the dynamics of a perpendicularly inhomogeneous plasma disturbed by propagating Alfvénic waves
The initial transverse structure is quickly destroyed by the propagating Alfvénic waves, and transformed into a cross-section presenting structures on a large range of scales, reminiscent of turbulence
This observation was at the basis of a previous study which doubts the existence of packed thin individual and dynamically independent magnetic elements in the solar corona, so-called ‘coronal strands’ or filaments, when perturbed by propagating Alfvénic waves[31]

Summary

Introduction

We present the results of three-dimensional (3D) ideal magnetohydrodynamics (MHD) simulations on the dynamics of a perpendicularly inhomogeneous plasma disturbed by propagating Alfvénic waves. Simpler versions of this scenario have been extensively studied as the phenomenon of phase mixing. In a plasma with an inhomogeneous background or equilibrium, the MHD waves are linearly coupled, or more descriptively, they have mixed properties[17,18] This implies that it is not possible to decompose[19] the perturbations into Alfvén, fast, and slow waves, as there are no pure MHD wave modes. Phenomenology for isothermal compressible MHD turbulence[20], but needs generalization This implies that the generation of turbulence is no longer restricted to counterpropagating Alfvén waves in compressible MHD. By using compressible Elsässer variables (defined as above but with varying density) we can still keep the Elsässer formalism, leading to different equations[16] than in Eqs. 1:

Objectives

Methods

Results

Conclusion