The Global Electrical Structure of Uranus: 3D Conductivity Mapping via GCR Ionisation in the Haze Layer

Characterising the electrical environment on Uranus is crucial for the understanding of atmospheric chemical and physical processes such as chemical composition, cloud microphysics, and aerosol charging mechanisms. Due to its complex and highly asymmetric magnetic field, Uranus exhibits strong variations in Galactic Cosmic Ray (GCR) cutoff rigidities. As a result, the GCR-induced ionisation is highly non-uniform across the planet. Previous work modelled the atmospheric conductivity of the Ice Giants by assuming simple 1D ionisation rate averages and used ion-aerosol attachment calculations based on classical theory that fail in the unique thermodynamic conditions of Uranian stratosphere.

This work offers the first 3D model of conductivities of Uranus based on its global GCR ionisation profiles (computed with CORSIKA 8) and a newly developed first-principles solution for the kinetic attachment of ions to aerosols. Unlike the previous classical models, based Fuchs-Hoppel framework, our model accounts for the full Maxwellian distribution of ions' velocities instead of assuming an average ion velocity. The inclusion of Maxwellian distribution leads to the introduction of a new capture surface at the repulsive Coulomb barrier. This approach provides a more rigorous prediction of the electron ‘bite-out’; a region within the stratospheric hazes where aerosol surfaces rapidly scavenge free electrons.

Using the steady-state ion and electron concentrations, we show how the vertical range and altitude of this bite-out depend on the local balance between the GCR ionisation intensity and the static aerosol sink. Moreover, the deeper-atmospheric conductivity profile resulting from GCRs is compared with the upper-atmospheric conductivity profile resulting from solar radiation. This comparison provides a more holistic view of the planet's electrical structure, providing a baseline for future studies of lightning generation, haze stability, and local electrical potentials in the Uranian atmosphere.