Azimuthal winds, convection and dynamo action in the polar regions of planetary cores

Abstract

We investigate azimuthal winds in planetary cores using a thermal convection-driven dynamo. When inertial forces are not negligible in the equation of motion, the inertially driven thermal winds are cyclonic. When the Lorentz forces are strong enough, we find anticyclonic thermal winds as observed in the Earth’s polar region from secular variation data. Under strong thermal convection, the azimuthal flow is created by the magnetic mode with one or more coherent, strong upwellings inside the tangent cylinder (TC), offset from the polar axis. We also find that, as the convection in the TC becomes stronger, these vortex plumes shrink in size, consistent with the convection being controlled by the magnetic field. In addition, strong upwellings in the TC could expel magnetic field in its path, creating regions of weak or even reverse flux patches. These patches drift westward, but at a significantly slower angular speed than the rotation about the vortex itself. Calculations with electrically conducting and stress-free boundaries reveal that the mechanism of generation of polar thermal winds is fairly independent of the boundary conditions imposed, provided the Rayleigh number is high enough to excite the magnetic mode.