INTRODUCTION

In the last few years there has been a renewed interest in dynamos in rapidly rotating systems, of which the Geodynamo is the most important example. In these systems the inertial and viscous terms in the fluid momentum equations can be considered asymptotically small and one is led to consider the role played by Taylor's constraint (Jones 1991; Soward 1992). Numerical calculations of such dynamos have been carried out by solving the axisymmetric magnetic induction equations for the toroidal and poloidal magnetic field components under prescribed α and ω effects. A variety of models and geometries have been explored. The studies which are more closely related to the present work are the calculations of Abdel-Aziz & Jones (1988) and Jones & Wallace (1992) in planar geometry, of Hollerbach h Ierley (1991) and Hollerbach, Barenghi & Jones (1992) in a sphere, and of Barenghi & Jones (1991) and Barenghi (1992a,b) in a spherical shell.

The observed westward drift of some patches of the Earth's magnetic field suggests that in order to model the Geodynamo one should study the magnetic induction equation in the ato limit (Roberts 1988).