Mixed Properties of MHD Waves in Non-uniform Plasmas

Marcel L Goossens,Inigo Arregui,Tom Van Doorsselaere

doi:10.3389/fspas.2019.00020

Abstract

This paper investigates the mixed properties of MHD waves in a non-uniform plasma. It starts with a short revision of MHD waves in a uniform plasma of infinite extent. In that case the MHD waves do not have mixed properties. They can be separated in Alfv\'{e}n waves and magneto-sonic waves. The Alfv\'{e}n waves propagate parallel vorticity and are incompressible. In addition they have no parallel displacement component. The magneto-sonic waves are compressible and in general do have a parallel component of displacement but do not propagate parallel vorticity. This clear separation has been the reason why there has been a strong inclination in the literature to use this classification in the study of MHD waves in non-uniform plasmas. The main part of this paper is concerned with MHD waves in a non-uniform plasma. It is shown that the MHD waves in that situation in general propagate both vorticity and compression and hence have mixed properties. Finally, the close connection between resonant absorption and MHD waves with mixed properties is discussed.

Highlights

Most textbooks on Magnetohydrodynamics (MHD) and plasma physics contain at least an elementary discussion of MHD waves in a uniform plasma of infinite extent
Pure Alfvén waves and pure magneto-acoustic waves exist in a uniform plasma of infinite extent
In a non-uniform plasma the MHD waves combine the properties of the classic Alfvén waves and of magneto-sonic waves in a uniform plasma of infinite extent

Summary

INTRODUCTION

Most textbooks on Magnetohydrodynamics (MHD) and plasma physics contain at least an elementary discussion of MHD waves in a uniform plasma of infinite extent (see e.g., Thompson, 1964; Mestel and Weiss, 1974; Goedbloed, 1983; Goossens, 2003; Goedbloed and Poedts, 2004; Walker, 2004). The magneto-sonic waves are compressible and have a parallel component of displacement They do not propagate parallel vorticity and are driven by pressure and magnetic tension. Mixed properties mean that the general rule is that MHD waves propagate both parallel vorticity as in classic Alfvén waves and compression as in classic magneto-sonic waves. This strong shear causes violent KH-instabilities (Terradas et al, 2008; Antolin et al, 2018) that accelerate the damping of the MHD waves and facilitate heating of plasma (Antolin et al, 2015; Arregui, 2015; Terradas and Arregui, 2018)

LINEAR MHD WAVES OF A UNIFORM PLASMA OF INFINITE EXTENT

MIXED PROPERTIES IN NON-UNIFORM PLASMAS

RESONANT ABSORPTION OF MHD WAVES

CONCLUSIONS