On the secular variation of Saturn's magnetic field

Saturn's nearly axisymmetric magnetic field poses a problem for dynamo theory, requiring that a special mechanism operates to axisymmetrize the field. The leading explanation is that zonal flows in a stably stratified layer surrounding the dynamo source region attenuate the non-axisymmetric components of the field. Here we demonstrate an important theoretical consequence of this axisymmetrization process: the secular variation of the field components must be extremely slow. Observational evidence suggests this may indeed be the case for Saturn. In addition, we present numerical dynamo simulations that reproduce the observed axisymmetry of Saturn's field and confirm the extremely slow secular variation rates in highly axisymmetric models. A consequence of this result is that we can use time variation of the axisymmetric field to learn about the non-axisymmetric field components which are not observed in present data from Cassini.