NONLINEAR PLANETARY DYNAMOS IN A ROTATING SPHERICAL-SHELL .3. ALPHA-2-OMEGA MODELS AND THE GEODYNAMO

The observed westward drift of some patches of the Earth's magnetic fi eld suggests that the geodynamo is of the alphaomega kind. However sol utions of the alphaomega dynamo equations tend to oscillate on a very rapid time scale, in disagreement with the observed long times between reversals. It is suggested that the problem can be cured by taking in to account the generation of toroidal field by the alpha effect. The r esulting alpha2omega dynamo equations are solved in the geometry of a spherical shell in both the linear and the nonlinear regime. Steady an d oscillatory Taylor state solutions are found. The slow time dependen ce of the oscillatory solutions compares well with the observed freque ncy of geomagnetic reversals.