On the Secular Variation of Saturn's Magnetic Field

English Abstract:

Saturn's nearly axisymmetric observed magnetic field is problematic for dynamo theory and requires unique processes occurring in its dynamo generation region. 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 axisymmetric field components must be extremely slow. Observational evidence suggests this may 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.