MHD waves in homogeneous and continuously stratified atmospheres

TL;DR

This paper reviews the fundamental equations and modes of linear ideal MHD waves in stratified atmospheres, emphasizing mode conversion processes and their implications for solar atmospheric heating, supported by exact solutions and eikonal methods.

Contribution

It generalizes the theory of MHD waves to gravitationally-stratified atmospheres and analyzes mode conversion with detailed theoretical and mathematical approaches.

Findings

Mode conversion occurs at specific atmospheric layers.

Exact solutions are provided for certain wave modes.

Implications for solar atmospheric heating are discussed.

Abstract

The basic equations, concepts, and modes of linear, ideal, MHD waves -- slow, Alfv\'en and fast -- are set out and generalised to gravitationally-stratified atmospheres. Particular attention is devoted to mode conversion, wherein the local behavior of a global wave changes from one mode to another in passing through particular atmospheric layers. Exact solutions are explored where available. Eikonal methods -- WKBJ and ray theory -- are described. Although our emphasis is on the theoretical underpinning of the subject, the solar atmospheric heating implications of fast/slow and fast/Alfv\'en conversions are discussed in detail.