Evolution of forced magnetohydrodynamic waves in a stratified fluid

Abstract

The evolution of a buoyancy disturbance in a stratified incompressible fluid permeated by a uniform vertical magnetic field is investigated. Two regimes are considered in the absence of background rotation - that of strong stratification, where the internal gravity wave frequency is much higher in magnitude than the magnetic (Alfvén) wave frequency, and that of strong magnetic field, where is dominant. For small but finite magnetic diffusion, perturbations that initially lie in the strong-field regime are shown to cross over to the regime of strong stratification, so that small-scale motions may exist as damped internal gravity waves at large times. The induced magnetic field propagates as damped Alfvén waves for a much longer time than the velocity before undergoing the above transition. With strong rotation, the unstably stratified system that satisfies the inequality |\omega _M| \gg |\omega _A| \gg |\omega _\eta |]]>, where is the inertial wave frequency and is the diffusion frequency, is of relevance to convection-driven dynamos. Here, a parameter space with is found wherein the flow intensity of the slow magnetic-Archimedean-Coriolis (MAC) waves is of the same order of magnitude as that of the fast MAC waves. Slow wave motions at horizontal length scales much smaller than the width of the fluid layer can therefore generate substantial helicity in rapidly rotating dynamos. The excitation of slow MAC waves at scales of 10a km in the Earth's core may play a crucial role in the generation of the axial dipole field.