M. Dumberry and J. Bloxham
Department of Earth and Planetary Sciences, Harvard University, Cambridge, Mass., USA.
dumberry@geophysics.harvard.edu
We present solutions of a numerical model of the geodynamo that exhibit oscillatory behavior akin to torsional oscillations in the Earth's core. For the regime of parameters used in the simulations, which specify stress-free conditions on the velocity at the inner and outer core boundaries, the force balance involves the Lorentz force and inertia, as expected for torsional oscillations. Additionally, the Reynolds stresses and viscous stresses also play important roles in the dynamics of the observed wave motion. From these results, we speculate on the role of these forces in the case of Earth's core. The viscous forces are important mainly as a result of the enhanced viscosity used in the simulation to facilitate numerical convergence and its effects should be weaker in the Earth's core. The Reynolds stresses on the other hand could be important if the typical length scale of the eddies in the flow is small enough. Moreover, because of the natural ability of the Reynolds stresses to transfer energy from the small scale eddies into the large scale flow, they provide a mechanism to continually excite the torsional oscillations.