Modelling Uranus' Global GCR Ionization Profile: Unveiling Geomagnetic Latitude Variations

The importance of studying the Ice Giants is highlighted by NASA’s recent designation of a mission to Uranus as its top priority in upcoming space exploration initiatives [1]. Modelling Galactic Cosmic Ray (GCR) ionization, along with the resulting chemical and electrical profiles, is crucial for interpreting data from a descent probe, as it will provide a detailed characterization of the descent region [2]. A comprehensive global model could influence mission planning by identifying optimal descent locations for maximum scientific return and by guiding recommendations for the necessary instrumentation. In this work, we used CORSIKA8 [3] to model how GCR air showers deposit energy to ionization at incremental pressures. The input parameters such as the pressure profile, as well as energies and fluxes of incident primary particles were scrutinized for robust results. The energy deposited by GCRs was used to calculate the ionization rate in the lower stratosphere and upper troposphere of Uranus. Our results show that the peak of ionization, known as the Regener-Pfotzer (RP) maximum – a universal parameter across planetary atmospheres, occurs at approximately 10⁴ Pa, which is consistent with other planets and existing literature [4], [5].In addition to geomagnetic cut-off rigidity, which determines the minimum GCR energies based on the magnetic field, we examined the impact of Uranus' asymmetric and complex magnetic field on air shower evolution. A key focus was the parameter RP maximum, representing the pressure at which the ionization rate peaks. Although characterizing secondary particle deflections under varying magnetic fields is challenging due to numerous sources of randomness, sensitivity analysis revealed that RP maxima are significantly influenced by magnetic field variations. This prompted a global investigation into RP maxima variations, resulting in a pioneering ionization rate profile. Our analysis showed positively correlating trends between RP maxima and horizontal magnetic field strength. RP maxima were observed to occur at deeper pressures near the poles, with notable hemispheric differences driven by the stronger magnetic field at the southern pole compared to the northern. Given Uranus' large scale height, these pressure differences translate to altitude variations exceeding 25%. These findings have important implications for Uranus' atmospheric chemistry, cloud formation, and electrical conductivity, particularly with respect to geomagnetic latitude variations.    [1] Choi, C. Q. (February 2023). Uranus up close: What proposed NASA 'ice giant' mission could teach us. Space.com. Retrieved from https://www.space.com/nasa-uranus-orbiter-and-probe-mission-objectives[2] Hueso, R., & Sánchez-Lavega, A. (2019). Atmospheric Dynamics and Vertical Structure of Uranus and Neptune’s Weather Layers. Space Science Reviews, 215:52. https://doi.org/10.1007/s11214-019-0618-6[3] Engel, R., Heck, D., Huege, T., et al. (2019). Towards a Next Generation of CORSIKA: A Framework for the Simulation of Particle Cascades in Astroparticle Physics. Computing and Software for Big Science, 3, 2. https://doi.org/10.1007/s41781-018-0013-0[4] Molina-Cuberos, G., et al. (2023). The Low-Altitude Ionosphere of the Ice Giant Planets. Journal of Geophysical Research: Planets. https://doi.org/10.1029/2022JE007568[5] Nordheim, T., et al. (2020). Cosmic ray ionization of Ice Giant atmospheres. 22nd EGU General Assembly, held online 4–8 May, 2020, id.6977 [poster].