Galactic Cosmic Ray Ionization on Uranus; Geomagnetic Latitude Dependencies

Galactic Cosmic Rays (GCRs) are a major source of atmospheric ionization, influencing ion abundance, aerosol formation, and electrical processes. GCR-induced effects are expected to be more pronounced on Uranus than planets closer to the Sun for two reasons; reduced solar irradiance, and weaker solar modulation of incident GCR. In this study, we model GCR-induced ionization using CORSIKA8, including the Uranian magnetic field. The maximum ionization rate and its corresponding altitude are two key parameters for understanding the vertical distribution of ions, which may be linked to cloud microphysics and atmospheric chemistry. In a comparison between a dominant geomagnetic polar region and a displaced, near-equatorial point, the maximum ionization rate differs by up to two orders of magnitude, and the altitude of the maximum by a factor of two. These geomagnetic latitude dependencies are mapped to produce the first detailed profile of GCR ionization in Uranus. The spatial variability of GCR ionization could aid in identifying regions of interest for a future descent probe. Our results show that the dipole model of Uranus's magnetic field hides important anomalies, like increased ionization at a secondary dipole, as a more detailed geomagnetic model is included. On a global average, the ionization peak occurs around 32 km, within the lower stratosphere. Comparisons with ionospheric ionization rates suggest that GCRs exceed ionospheric sources by two orders of magnitude, making GCRs the dominant source and potentially shaping Uranus' global atmospheric electricity.