3D WKB solution for fast magnetoacoustic wave behaviour around an X-line

TL;DR

This paper develops a 3D WKB analytical method to study fast magnetoacoustic wave propagation around an X-line in a magnetic field, revealing wave trapping and refraction effects that influence energy deposition and heating in plasma systems.

Contribution

It introduces a novel 3D WKB solution for magnetoacoustic waves in complex magnetic topologies, specifically around X-lines, and demonstrates its application to wave trapping and energy localization.

Findings

15.5% of wave energy is trapped by the X-line.

Waves experience refraction influenced by local Alfvén speed.

X-lines act as focal points for wave energy accumulation.

Abstract

We study the propagation of a fast magnetoacoustic wave in a 3D magnetic field created from two magnetic dipoles. The magnetic topology contains an X-line. We aim to contribute to the overall understanding of MHD wave propagation within inhomogeneous media, specifically around X-lines. We investigate the linearised, 3D MHD equations under the assumptions of ideal and cold plasma. We utilise the WKB approximation and Charpit's method during our investigation. It is found that the behaviour of the fast magnetoacoustic wave is entirely dictated by the local, inhomogeneous, equilibrium Alfv\'en speed profile. All parts of the wave experience refraction during propagation, where the magnitude of the refraction effect depends on the location of an individual wave element within the inhomogeneous magnetic field. The X-line, along which the Alfv\'en speed is identically zero, acts as a focus for the refraction effect. There are two main types of wave behaviour: part of the wave is either trapped by the X-line or escapes the system, and there exists a critical starting region around the X-line that divides these two types of behaviour. For the set-up investigated, it is found that $15.5\%$ of the fast wave energy is trapped by the X-line. We conclude that linear, $\beta=0$ fast magnetoacoustic waves can accumulate along X-lines and thus these will be specific locations of fast wave energy deposition and thus preferential heating. The work here highlights the importance of understanding the magnetic topology of a system. We also demonstrate how the 3D WKB technique described in this paper can be applied to other magnetic configurations.