Saturn’s high degree magnetic moments: Evidence for a unique planetary dynamo

Abstract

Saturn’s intrinsic magnetic field has long been known to be highly axisymmetric. None of the existing field models goes beyond spherical harmonic degree 3. In this paper, we assume that the axisymmetry extends to higher degrees. This allows us to derive the degree 4 and 5 axial moments by searching their parameter space based on the Cassini Saturn Orbital Insertion (SOI) measurements and on our apriori knowledge of the first three zonal moments for Saturn. Gauss coefficients g40 and g50 are found to be 65 ± 70 nT and 185 ± 100 nT, respectively. The Lowes spectrum of Saturn’s field takes a zig-zag shape up to degree 5 with pronounced odd degree moments. All first three odd axial moments share the same sign. This reveals that the magnetic flux inside Saturn is strongly concentrated near the spin-poles, in contrast to the field geometry inside the Earth and convection-driven geodynamo models. The mechanically driven spherical Couette dynamo is considered as an alternative which shows intriguingly similar field properties to those observed at Saturn.