MHD mode coupling in the neighbourhood of a 2D null point

Abstract

At this time there does not exist a robust set of rules connecting low and high $\beta$ waves across the $\beta \approx 1$ layer. The work here contributes specifically to what happens when a low $\beta$ fast wave crosses the $\beta \approx 1$ layer and transforms into high $\beta$ fast and slow waves. The nature of fast and slow magnetoacoustic waves is investigated in a finite $\beta$ plasma in the neighbourhood of a two-dimensional null point. The linearised equations are solved in both polar and cartesian forms with a two-step Lax-Wendroff numerical scheme. Analytical work (e.g. small $\beta$ expansion and WKB approximation) also complement the work. It is found that when a finite gas pressure is included in magnetic equilibrium containing an X-type null point, a fast wave is attracted towards the null by a refraction effect and that a slow wave is generated as the wave crosses the $\beta \approx 1$ layer. Current accumulation occurs close to the null and along nearby separatrices. The fast wave can now \emph{pass through the origin} due to the non-zero sound speed, an effect not previously seen in related papers but clear seen for larger values of $\beta$. Some of the energy can now leave the region of the null point and there is again generation of a slow wave component (we find that the fraction of the incident wave converted to a slow wave is proportional to $\beta$). We conclude that there are two competing phenomena; the refraction effect (due to the variable Alfv\'en speed) and the contribution from the non-zero sound speed. These experiments illustrate the importance of the magnetic topology and of the location of the $\beta \approx 1$ layer in the system.