THE GENERATION OF NONAXISYMMETRIC MAGNETIC-FIELDS IN THE GIANT PLANETS

We consider mean-field dynamo models in uniformly rotating spheres and spherical shells, with anisotropic alpha and magnetic diffusivity ten sors which are functions of the inverse Rossby number, Omega. When we include an alpha-quenching nonlinearity we show that, for all values of Omega considered, nonaxisymmetric magnetic fields are stable to ax isymmetric perturbations. However the stability of nonaxisymmetric mag netic fields is weakened for large values of Omega, which would make the generation of axisymmetric magnetic fields more probable. When a s mall amount of differential rotation is introduced, only axisymmetric dipole-like solutions are stable. We draw attention to the possibility that the nonaxisymmetric magnetic fields of Uranus and Neptune could be the result of anisotropic alpha and magnetic diffusivity tensors. T he more nearly axisymmetric magnetic fields of Jupiter and Saturn coul d result from their more rapid rotation, or possibly because of intern al differential rotation.