GENERATION OF MAGNETIC-FIELDS BY CONVECTION IN A ROTATING SPHERICAL FLUID SHELL OF INFINITE PRANDTL NUMBER

The equations of motion for buoyancy-driven convection and the equation of induction for the magnetic field are solved for a fluid of infinite Prandtl number in a rotating spherical shell of radius ratio η = 0.4. The analysis is motivated by the problem of the geodynamo in the Earth's core which is likely to be driven by chemical buoyancy. To make the results comparable with previous computations for moderate Prandtl number fluids, the boundary conditions have not been changed. The results show subcritical finite amplitude onset of dynamo action, at least in the case of the azimuthal wavenumber m = 2. The magnetic field may either enhance the amplitude of convection by counteracting the Lorentz force or diminish it by adding an additional sink of energy, depending on which of the two processes dominates. In contrast to the computations for low Prandtl number fluids the phase velocity shows a strong dependence on the amplitude of convection and on the strength of the magnetic field. Changes from prograde to retrograde drift of the convection columns are observed. © 1990.