MHD waves with mixed properties / Alfv\'{e}n waves with pressure variations: a review

TL;DR

This review discusses how non-uniform plasmas alter the properties of MHD waves, causing a blending of Alfvén and magneto-sonic wave characteristics and leading to pressure variations in waves traditionally considered incompressible.

Contribution

It provides a comprehensive overview of how non-uniformity affects MHD wave properties, challenging the traditional clear separation between Alfvén and magneto-sonic waves.

Findings

Non-uniform plasmas cause MHD waves to exhibit mixed properties.

Alfvén waves can show pressure variations in non-uniform conditions.

Wave characteristics depend on local plasma equilibrium.

Abstract

Non-uniformity plays an important role for MHD waves. For a uniform plasma of infinite extent the MHD waves can be subdivided in two classes with distinct properties. The first class contains the Alfv\'en waves. The Alfv\'en waves are incompressible and propagate parallel vorticity. They do not have a parallel component of displacement, they do not cause variations in pressure and are driven by magnetic tension only. The second class contains the magneto-sonic waves. They are compressible and have a parallel component of displacement. They do not propagate parallel vorticity and are driven by pressure and magnetic tension. In non-uniform plasmas the situation can be very different. The clear division between Alfv\'en waves and magneto-sonic waves is no longer present. In a given part of the equilibrium an MHD wave can strongly resemble a magneto-sonic wave with little or no resemblance to Alfv\'en waves; while in another part of the equilibrium the MHD wave is practically an Alfv\'en wave, which has the amazing property of being accompanied by variations in pressure.