Spatial dynamics formulation of magnetohydrostatics

Abstract: We present a formalism for importing techniques from dynamical systems theory in the study of three-dimensional magnetohydrodynamic (MHD) equilibria. By treating toroidal angle as time, we reformulate the equilibrium equations as hydrodynamic equations on the unit disc. They satisfy a variational principle and comprise a Lie-Poisson Hamiltonian system. We use the variational principle to find conservation laws for circulation, vorticity when pressure vanishes, and energy in axisymmetric domains. Combining the Lie-Poisson structure with a construction due to Scovel-Weinstein, we develop a theory of smoothed particle magnetohydrostatics (SPMHS). SPMHS identifies a large class of exact particle-swarm solutions of regularized spatial dynamics equations. The regularization occurs at finer scales than the smallest scales within the physical purview of ideal MHD. Crucially, the SPMHS equations of motion comprise a finite-dimensional Hamiltonian system, which side steps perennial roadblocks to a satisfactory theory of three-dimensional equilibria. In large-aspect-ratio domains, we show the spatial dynamics equations comprise a fast-slow system, where fast dynamics corresponds to the elliptic part of the equilibrium equations and slow dynamics corresponds to the hyperbolic part. Because the fast dynamics is formally normally hyperbolic, Fenichel theory suggests the presence of an exact slow manifold for the spatial dynamics equations that contains all physical solutions. We formally carry out reduction to the slow manifold, obtaining a spatial dynamics formulation for equilibrium Strauss reduced MHD at leading order and a hierarchy of corrections at any desired order in perturbation theory. In large aspect-ratio domains, finding periodic solutions of the slow manifold reduced equations represents a second novel pathway to finding three-dimensional equilibria.