Nonlinear Fast Magnetoacoustic Wave Propagation in the Neighbourhood of a 2D magnetic X-point: Oscillatory Reconnection

TL;DR

This study models nonlinear fast magnetoacoustic waves near a 2D magnetic X-point, revealing shock formation, oscillatory reconnection, and asymmetric heating, extending previous linear models with new nonlinear effects.

Contribution

It introduces nonlinear effects into models of wave propagation near magnetic X-points, showing shock formation and oscillatory reconnection not seen in linear regimes.

Findings

Formation of fast and slow magnetic shocks.

Deformation of the X-point into a cusp-like structure.

Oscillatory reconnection with current sheet evolution.

Abstract

This paper extends the models of Craig & McClymont (1991) and McLaughlin & Hood (2004) to include finite $\beta$ and nonlinear effects. We investigate the nature of nonlinear fast magnetoacoustic waves about a 2D magnetic X-point. We solve the compressible and resistive MHD equations using a Lagrangian remap, shock capturing code (Arber et al. 2001) and consider an initial condition in $ {\bf{v}}\times{\bf{B}} \cdot {\hat{\bf{z}}}$ (a natural variable of the system). We observe the formation of both fast and slow oblique magnetic shocks. The nonlinear wave deforms the X-point into a 'cusp-like' point which in turn collapses to a current sheet. The system then evolves through a series of horizontal and vertical current sheets, with associated changes in connectivity, i.e. the system exhibits oscillatory reconnection. Our final state is non-potential (but in force balance) due to asymmetric heating from the shocks. Larger amplitudes in our initial condition correspond to larger values of the final current density left in the system. The inclusion of nonlinear terms introduces several new features to the system that were absent from the linear regime.