Linking Uranus' temperature profile to wind-induced magnetic fields

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 ${\mathbf{B}}^{\omega }={\mathbf{B}}_P^{\omega }+{\mathbf{B}}_T^{\omega }$ via the $\omega$-effect. Toroidal perturbations ${\mathbf{B}}_T^{\omega }$ are expected to diffuse downwards and produce poloidal fields ${\mathbf{B}}_P^{\alpha }$ through turbulent convection via the $\alpha$-effect, comparable in strength to those of the $\omega$-effect; ${\mathbf{B}}_P^{\omega }$. 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 H${}_2$-He-H${}_2$O interior. Due to the higher metallicities in outer regions of hot Uranus models, zonal winds need to decay to $\sim$0.1% of their surface values in the outer 1% 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 $\text{in situ}$ exploration of Uranus.

Accepted for publication in MNRAS. 6 pages, 2 figures. Typos corrected (v2). Typos corrected (v3)

References in corpus (10)

• Constraints on Deep-seated Zonal Winds Inside Jupiter and Saturn
• A new equation of state for dense hydrogen-helium mixtures
• Effect of Non-Adiabatic Thermal Profiles on the Inferred Compositions of Uranus and Neptune
• Combined magnetic and gravity measurements probe the deep zonal flows of the gas giants
• Anelastic spherical dynamos with radially variable electrical conductivity
• Metallization of Shock-Compressed Liquid Ammonia
• Constraining the Depth of the Winds on Uranus and Neptune via Ohmic Dissipation
• Internal water storage capacity of terrestrial planets and the effect of hydration on the M-R relation
• Searching for gravitational waves via Doppler tracking by future missions to Uranus and Neptune
• Exploration of the Ice Giant Systems: A White Paper for NASA's Planetary Science and Astrobiology Decadal Survey 2023-2032