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

Abstract

The nature of fast magnetoacoustic and Alfvén waves is investigated in a zero $β$ plasma. This gives an indication of wave propagation in the low $β$ 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 Alfvén wave behaves in a different manner in that the wave energy is dissipated along the separatrices. For Alfvén waves that are decoupled from fast waves, the value of the plasma $β$ is unimportant. However, the phenomenon of dissipating the majority of the wave energy at a specific place is a feature of both wave types.