First Experimental Evidence of large-scale wave modes in rotating magnetoconvection

Abstract:

Present day dynamo models simulate thermally-driven convection in fluids differently from liquid metals that exist in planetary cores. In such models, quasi-steady, columnar convection structures drive dynamo action. Here we present the results of an idealized model of core-style convection first by studying rotating convection in liquid gallium and then by including the effect of magnetic fields. In rotating convection in liquid metal, we find that the convection occurs via oscillatory motions occurring throughout the bulk of the fluid. These liquid metal inertial motions are fundamentally different than the quasi-steady modes in present day dynamo models and, further, are unlikely to be efficient at generating quasi-steady planetary magnetic fields. Withthe addition of a magnetic field, the bulk oscillatory convection mode is suppressed and replaced by a previously unobserved flow: a magneto-Coriolis sidewall-attached mode slowly precessing around the rim of the container. This slow wall mode is similar to a rapidly-rotating convection mode found in non-metals. Non-intuitively, then, the effect of the magnetic field is to dampen the inertial aspects of the the liquid metal flow while allowing for large-scale slow modes that can develop in rapidly rotating systems. Overall, our experiments show that the convection driven MC wave modes that can develop in liquid metals are remarkably different from the canonical flows that develop in the fluids used in present day dynamo models.