Linking Uranus’ temperature profile to wind-induced magnetic fields

Abstract

ABSTRACT The low luminosity of Uranus is still a puzzling phenomenon and has key implications for the thermal and compositional gradients within the planet. Recent studies have shown that planetary volatiles become ionically conducting under conditions that are present in the ice giants. Rapidly growing electrical conductivity with increasing depth would couple zonal flows to the background magnetic field in the planets, inducing poloidal and toroidal field perturbations $\boldsymbol {B}^{\omega } = \boldsymbol {B}^{\omega }_\mathrm{ P} + \boldsymbol {B}^{\omega }_\mathrm{ T}$ via the ω-effect. Toroidal perturbations $\boldsymbol {B}^{\omega }_\mathrm{ T}$ are expected to diffuse downwards and produce poloidal fields $\boldsymbol {B}^{\alpha }_\mathrm{ P}$ through turbulent convection via the α-effect, comparable in strength to those of the ω-effect, $\boldsymbol {B}^{\omega }_\mathrm{ P}$. To estimate the strength of poloidal field perturbations for various Uranus models in the literature, we generate wind decay profiles based on Ohmic dissipation constraints assuming an ionically conducting H2–He–H2O interior. Because of the higher metallicities in outer regions of hot Uranus models, zonal winds need to decay to ∼0.1 per cent of their surface values in the outer 1 per cent of Uranus to admit decay solutions in the Ohmic framework. Our estimates suggest that colder Uranus models could potentially have poloidal field perturbations that reach up to $\mathcal {O}(0.1)$ of the background magnetic field in the most extreme case. The possible existence of poloidal field perturbations spatially correlated with Uranus’ zonal flows could be used to constrain Uranus’ interior structure, and presents a further case for the in situ exploration of Uranus.