Cosmic ray ionization of Ice Giant atmospheres

Galactic cosmic rays (GCRs) represent a major ionization source in planetary atmospheres, particularly within deeper layers that are largely unaffected by solar UV and charged particle precipitation. When GCR particles undergo inelastic collisions with atmospheric nuclei they create large numbers of secondary interactions, resulting in extensive nuclear and electromagnetic particle cascades. In thick atmospheres, such as those of the giant planets, these cascades can develop much more extensively than what is the case on Mars and Earth. Furthermore, GCRs are strongly modulated by the heliosphere, and therefore GCR fluxes are significantly higher at the Ice Giants than in the inner Solar System. Intriguingly, observations of Uranus and Neptune show brightness variations that appear to be associated with known variability in the background GCR flux (Aplin and Harrison 2016;2017).

Using a full 3D Monte Carlo particle physics code, we have carried out the first detailed study of cosmic ray ionization within the atmospheres of Uranus and Neptune. We will show preliminary results of this study and discuss the possible importance of GCR ionization to atmospheric chemistry and atmospheric electricity. We will also discuss GCR shielding by the planetary magnetic fields of Uranus and Neptune, and what effect this has on predicted GCR ionization rates at different locations. 

References

Aplin K.L. and Harrison R.G. (2016), Determining solar effects in Neptune's atmosphere, Nature Communications, 7, 11976 doi:10.1038/ncomms11976

Aplin K.L. and Harrison R.G (2017), Solar-driven variability in the atmosphere of Uranus, Geophys. Res. Letts. 44, doi: 10.1002/2017GL07374