A dynamo model for axisymmetrizing Saturn's magnetic field

Abstract

Magnetic field measurements demonstrate that Saturn's internally generated magnetic field has an extremely small dipole tilt. The nearly-perfect axisymmetry of Saturn's dipole is troubling because of Cowling's theorem which states that an axisymmetric magnetic field cannot be maintained by a dynamo. A possible mechanism to axisymmetrize the observed field involves differential rotation in a stably-stratified electrically conducting layer surrounding the dynamo. Here we use numerical dynamo models to study the axisymmetrizing effects of stably stratified layers surrounding the dynamo. We find that a thin stably-stratified layer which undergoes differential rotation due to thermal winds as a result of pole to equator temperature differences can produce a more axisymmetrized field. Surprisingly, we find that the direction of the zonal flows and their equatorial symmetry is a crucial factor for magnetic field axisymmetry since some zonal flows act to destabilize the dynamo producing non-axisymmetric fields.