A two-dimensional approach to modelling the short timescale zonal flow in Earth's core

Abstract:

Reconstructions of flow in Earth's outer core based on surface magnetic data predict mean zonal accelerations on several timescales. Since accelerations in the core couple to the angular momentum of the mantle, their existence has been confirmed by length-of-day observations. Recent studies suggest that free modes of torsional oscillations are responsible for relatively weak signals with a 5-6 year period. The mechanisms responsible for stronger decadal signals are less well understood.

To address the problem, we construct a quasi-geostrophic model of magnetoconvection, with thermally-driven flows perturbing a steady, imposed background magnetic field. This approach is justified by the Taylor-Proudman theorem, in which velocities in a rapidly rotating system vary little parallel to the rotational axis. Using only two dimensions allows a much more rapid exploration of parameter space than traditional three-dimensional approaches.

Our model is capable of producing mean zonal accelerations similar to those predicted by the geomagnetic reconstructions of Earth. In particular, we see a clear separation in period between the free modes (short) and forced modes (long) of torsional oscillations. We then systematically run the model with a variety of parameters, attempting to extrapolate our results to the conditions found in Earth's core.