Review article: MHD wave propagation near coronal null points of magnetic fields

TL;DR

This review comprehensively examines how MHD waves behave near coronal null points, highlighting the influence of magnetic field topology on wave propagation, energy accumulation, and potential heating in the solar corona.

Contribution

It synthesizes existing research on MHD wave dynamics near null points across different geometries, providing new insights into wave focusing, energy dissipation, and implications for solar plasma processes.

Findings

Fast waves focus near null points due to refraction.

Alfven waves propagate along magnetic fieldlines and accumulate at separatrices.

Null points are key sites for wave energy dissipation and plasma heating.

Abstract

We present a comprehensive review of MHD wave behaviour in the neighbourhood of coronal null points: locations where the magnetic field, and hence the local Alfven speed, is zero. The behaviour of all three MHD wave modes, i.e. the Alfven wave and the fast and slow magnetoacoustic waves, has been investigated in the neighbourhood of 2D, 2.5D and (to a certain extent) 3D magnetic null points, for a variety of assumptions, configurations and geometries. In general, it is found that the fast magnetoacoustic wave behaviour is dictated by the Alfven-speed profile. In a $\beta=0$ plasma, the fast wave is focused towards the null point by a refraction effect and all the wave energy, and thus current density, accumulates close to the null point. Thus, null points will be locations for preferential heating by fast waves. Independently, the Alfven wave is found to propagate along magnetic fieldlines and is confined to the fieldlines it is generated on. As the wave approaches the null point, it spreads out due to the diverging fieldlines. Eventually, the Alfven wave accumulates along the separatrices (in 2D) or along the spine or fan-plane (in 3D). Hence, Alfven wave energy will be preferentially dissipated at these locations. It is clear that the magnetic field plays a fundamental role in the propagation and properties of MHD waves in the neighbourhood of coronal null points. This topic is a fundamental plasma process and results so far have also lead to critical insights into reconnection, mode-coupling, quasi-periodic pulsations and phase-mixing.