Magnetohydrodynamics wave propagation in the neighbourhood of two dipoles

TL;DR

This study investigates magnetohydrodynamics wave behavior near two dipoles using simulations and analytical methods, revealing wave attraction, escape, and energy accumulation phenomena relevant to plasma physics.

Contribution

It provides new insights into wave propagation, refraction, and energy distribution near magnetic null points in a dipole configuration, including wave escape mechanisms not previously observed.

Findings

40% of wave energy accumulates at the null point

Some waves escape the magnetic configuration, contrary to previous findings

Wave energy concentrates along separatrices, leading to potential wave heating

Abstract

This paper is the third in a series of investigations by the authors. The nature of fast magnetoacoustic and Alfv\'en waves is investigated in a 2D $\beta=0$ plasma in the neighbourhood of two dipoles. We use both numerical simulations (two-step Lax-Wendroff scheme) and analytical techniques (WKB approximation). It is found that the propagation of the linear fast wave is dictated by the Alfv\'en speed profile and that close to the null, the wave is attracted to the neutral point. However, it is also found that in this magnetic configuration some of the wave can escape the refraction effect; this had not been seen in previous investigations by the authors. The wave split occurs near the regions of very high Alfv\'en speed (found near the loci of the two dipoles). Also, for the set-up investigated it was found that 40% of the wave energy accumulates at the null. Ohmic dissipation will then extract the wave energy at this point. The Alfv\'en wave behaves in a different manner in that part of the wave accumulates along the separatrices and part escapes. Hence, the current density will accumulate at this part of the topology and this is where wave heating will occur. The phenomenon of wave accumulation at a specific place is a feature of both wave types, as is the result that a fraction of the wave can now escape the numerical box when propagating in this magnetic configuration.