Efficient resonant fast-Alfvén wave coupling as a minimization principle

Abstract

The resonant coupling of the fast magnetosonic wave to the Alfvén wave is considered in the ideal magnetohydrodynamic limit in a 3D equilibrium. It has previously been shown that the most efficient coupling occurs on particular paths that satisfy the “tangential alignment condition” (Wright et al., 2022, https://doi.org/10.1029/2022ja030294). In this article we show how this criterion is equivalent to a minimization principle which may lead to a deeper understanding of the physics of the wave coupling process.