Alfvén-fast wave coupling in a 3D non-uniform medium

In this paper, we consider the process of Alfvén-fast wave mode coupling, through numerical simulation. We model the process using the ideal, linear magnetohydrodynamic equations on a three-dimensional Cartesian grid; assuming the cold plasma limit, β≪1. We initialize the simulation with a cylindrical Alfvén wave pulse (comprising an azimuthal magnetic field and velocity perturbations) propagating along a uniform magnetic field. The wave starts in a region where the density is uniform. As it propagates, part of the Alfvén wave encounters a change in density, before emerging into a second uniform region. We introduce the natural Helmholtz Hodge decomposition as a method to identify the properties of the Alfvén wave perturbations at the end of the simulation. Our results show that the Alfvén wave propagates efficiently through the non-uniform region, with the wave pulse's final structure sharing strong characteristics of the initial wave pulse structure. More than 69% of the initial energy is carried by the transmitted Alfvén wave. Alfvén-fast wave coupling has potential applications in planetary magnetospheres, such as in the Io-Jupiter Alfvén wave interaction, and the solar corona.