The generation of nonaxisymmetric magnetic fields in the giant planets

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Abstract
We consider mean-field dynamo models in uniformly rotating spheres and spherical shells, with anisotropic alpha and magnetic diffusivity tensors which are functions of the inverse Rossby number, Ω*. When we include an a-quenching nonlinearity we show that, for all values of Ω* considered, nonaxisymmetric magnetic fields are stable to axisymmetric perturbations. However the stability of nonaxisymmetric magnetic fields is weakened for large values of Ω*, which would make the generation of axisymmetric magnetic fields more probable. When a small 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 diffusi vity tensors. The more nearly axisymmetric magnetic fields of Jupiter and Saturn could result from their more rapid rotation, or possibly because of internal differential rotation.

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