Dynamo models for magnetic field generation in Uranus and Neptune

Abstract

The magnetic fields of the ice giants are multipolar and non-axisymmetric. Voyager-II- data and HST aurorae-observations suggest magnetic power spectra with similar power in the first three spherical harmonic degrees and a peak in the order m=1. Multipolar, non- axisymmetric fields can be modeled with several different approaches including a high density stratification in the dynamo region, strongly turbulent convection, a dynamo generated by fast zonal jets and a geometrical setup with a deep stably stratified fluid layer below the dynamo region. Earlier studies with this geometry found multipolar fields and in a few cases reproduced the peak in the magnetic power spectra at order m=1 (Stanley and Bloxham, 2006). Here we explore the robustness of the multipolarity (similar power for l=1,2,3) and the m=1-peak for a range of parameters and geometrical setups using 3D numerical dynamo models. We compare our results to internal structure models of the ice giants in order to constrain the parameters and geometrical setups that are in accordance with the magnetic field observations