Ionization by Cosmic Rays of the Ice Giant Atmospheres

Some of the atmospheric processes occurring in the Ice Giant planets Uranus and Neptune critically depend on the electrical properties of their respective atmospheres. Therefore, the modelling of the electromagnetic properties of the atmosphere provides valuable information to characterize several aspects of the atmosphere. For example, lightning phenomena is an indicator of strong convective movements, and the number densities of electrons and ions affect the growth of aerosols and cloud formation. In the pressure range where such process take place the solar radiation has been mainly absorbed and the main ionization source is galactic cosmic rays. Furthermore, the long distance to the Sun decreases the role of solar radiation on the atmospheric electrical properties, while the cosmic ray flux is similar or even larger compared to that at the inner solar system. This implies a proportionally greater role of the galactic cosmic rays induced ionosphere for the ice giant planets than for the rest of solar system planets.

Protons and alpha particles, which mainly compound the cosmic rays, can reach the Uranus and Neptune lower stratosphere to ionize the atmospheric constituents and thus produce a low altitude ionospheric layer. The flux and composition of cosmic ray incident on a planetary atmosphere depend on the distance to the Sun and varies over the 11-year solar activity cycle. Furthermore, the planetary magnetic field deflects the incoming cosmic rays depending on their rigidity and angle of incidence. Therefore, both factors solar cycle and magnetic latitude must be considered to calculate the ionization rate by cosmic rays.

The presence of aerosols also affects the ion-neutral chemistry by capturing electrons and positive ions depending on the aerosol size and concentration. Here we present the first results obtained with a numerical model, which is based on previous studies for  Mars and Titan [1-2], capable to calculate the number density of electrons and positive ions, as well the aerosol charging between 0.1 and 15 bars in the atmosphere of Neptune and Uranus. We have found that, for the particle density and size vertical profiles obtained in previous works [3-4], aerosols are negatively charged, and the number density of electrons is lower than that of positive ions for pressures lower than around 1 bar.

References

[1] Cardnell, S., et al. (2016), A photochemical model of the dust-loaded ionosphere of Mars, J. Geophys. Res. Planets, 121, doi:10.1002/2016JE005077.

[2] Molina-Cuberos, G.J., et al. (2018) Aerosols: The key to understanding Titan's lower ionosphere, Planet. Space Sci, 153, 157 – 162, doi: 10.1016/j.pss.2018.02.007.

[3] Toledo et al. (2019), Constraints on Uranus's haze structure, formation and transport, Icarus 0019-1035, doi: 10.1016/j.icarus.2019.05.018

[4] Toledo et al. (2020) Constraints on Neptune’s haze structure and formation from VLT observations in the H-band, Icarus, doi: 10.1016/j.icarus.2020.113808