MHD wave propagation in the neighbourhood of a two-dimensional null point

TL;DR

This paper investigates how fast magnetoacoustic and Alfvén waves behave near a two-dimensional null point in a low-beta plasma, revealing their dissipation mechanisms relevant to solar corona heating.

Contribution

It demonstrates that null points cause fast waves to focus and dissipate energy rapidly, highlighting their role in coronal wave heating, and distinguishes the behavior of Alfvén waves along separatrices.

Findings

Fast waves are attracted to null points and dissipate energy there.

Alfvén waves dissipate energy along separatrices.

Wave dissipation occurs regardless of plasma beta for decoupled Alfvén waves.

Abstract

The nature of fast magnetoacoustic and Alfv\'en waves is investigated in a zero $\beta$ plasma. This gives an indication of wave propagation in the low $\beta$ solar corona. It is found that for a two-dimensional null point, the fast wave is attracted to that point and the front of the wave slows down as it approaches the null point, causing the current density to accumulate there and rise rapidly. Ohmic dissipation will extract the energy in the wave at this point. This illustrates that null points play an important role in the rapid dissipation of fast magnetoacoustic waves and suggests the location where wave heating will occur in the corona. The Alfv\'en wave behaves in a different manner in that the wave energy is dissipated along the separatrices. For Alfv\'en waves that are decoupled from fast waves, the value of the plasma $\beta$ is unimportant. However, the phenomenon of dissipating the majority of the wave energy at a specific place is a feature of both wave types.