Geodynamo simulations of internal dynamics and magnetic fields of ice giants

Uranus and Neptune are the most underexplored planetary bodies of our Solar System , with in-situ observations limited to the Voyagers' fly-bys over three decades ago. This is to be contrasted with the existence of past and present dedicated orbital missions around Jupiter and Saturn.

In contrast with this paucity of observations, a number of studies have attempted at placing constraints on the internal composition, structure and long-term evolution of Uranus and Neptune. In particular, various internal structure models have been proposed to explain the difference in luminosity between the two ice giants and their markedly non-dipolar magnetic fields. Interior structures and compositions vary significantly among these models. This is particularly true for Uranus, for which various scenarios have been proposed to explain its surprisingly low luminosity, in near equilibrium with the solar input.

The internal state of a planet shapes its convective dynamics, including the dynamo processes that generate planetary magnetic fields. For example, some scenarios proposed to explain Uranus low luminosity, invoke stable stratification that would hinder convective dynamics in some regions of the planet’s interior. It is therefore of great interest to explore how the choice of internal structure and composition, influences the internal dynamics of the ice giants. One way of achieving this is by making use of numerical dynamo simulations, which are widely used to characterise the internal dynamics, magnetic fields and surface winds of Jupiter and Saturn. However, a limited number of studies is dedicated to the study of the convective dynamics and dynamo mechanism of ice giants. Therefore, very few internal structure models have been tested in a dynamical framework. One reason for this is the difficulty in performing realistic simulations of the ice giants’ interiors. The turbulent flows that are expected to develop in the extreme parameters regime that characterise the rapidly rotating interiors of Uranus and Neptune are indeed highly multi-scale in nature. The computational cost of performing dynamo simulations in these regimes is enormous, and only becomes higher when complex internal structure models are considered.

In this study we present results from a numerical study targeted at exploring the effect that different internal structure models have on the generation of magnetic fields, on the surface winds and on the luminosity of Uranus and Neptune. We performed highly turbulent numerical simulations with the MagIC numerical code. Banking on the capabilities of modern supercomputer architectures, we considered various background states in which we have varied the electrical conductivity, density and entropy profiles.

Preliminary results indicate that the dynamo processes and radial variations in the electrical conductivity have a limited effect on the dynamical state determined by the internal convective processes. Ongoing and future work involves estimating the effect of density, composition and entropy background profiles, including the effect of internal stratification. Based on previous studies, these are expected to have an important impact on the resulting dynamical state, but the exact impact on the highly turbulent regimes we are focussed on, is currently unclear.

The suite of numerical simulations computed in the course of our investigation will be helpful in establishing a reference for the interpretation of the result of future satellite missions to the ice giants.