Ionisation and discharge in cloud-forming atmospheres of brown dwarfs and extrasolar planets

Abstract

Brown dwarfs and giant gas extrasolar planets have cold atmospheres with a rich chemical compositions from which mineral cloud particles form. Their properties, like particle sizes and material composition, vary with height, and the mineral cloud particles are charged due to triboelectric processes in such dynamic atmospheres. The dynamics of the atmospheric gas is driven by the irradiating host star and/or by the rotation of the objects that changes during its lifetime. Thermal gas ionisation in these ultra-cool but dense atmospheres allows electrostatic interactions and magnetic coupling of a substantial atmosphere volume. Combined with a strong magnetic field B Earth , a chromosphere and aurorae might form as suggested by radio and X-ray observations of brown dwarfs. Non-equilibrium processes like cosmic ray ionisation and discharge processes in clouds will increase the local pool of free electrons in the gas. Cosmic rays and lighting discharges also alter the composition of the local atmospheric gas such that tracer molecules might be identified. Cosmic rays affect the atmosphere through air showers in a certain volume which was modelled with a 3D Monte Carlo radiative transfer code to be able to visualise their spacial extent. Given a certain degree of thermal ionisation of the atmospheric gas, we suggest that electron attachment to charge mineral cloud particles is too inefficient to cause an electrostatic disruption of the cloud particles. Cloud particles will therefore not be destroyed by Coulomb explosion for the local temperature in the collisional dominated brown dwarf and giant gas planet atmospheres. However, the cloud particles are destroyed electrostatically in regions with strong gas ionisation. The potential size of such cloud holes would, however, be too small and might occur too far inside the cloud to mimic the effect of, e.g., magnetic field induced star spots.

Exoplanet Clouds

“…Glawe et al 2015). Turbulent, small-scale fluctuations will extent the cloud formation regime for exoplanets and amplifies the intermittency of the cloud coverage (Helling et al 2004), but turbulence also facilitate cloud particle charging in exoplanet atmospheres (Helling et al 2011(Helling et al , 2016c.…”

Dynamic mineral clouds on HD 189733b

“…Garcia Munoz & Isaak (2015) used a Preconditioned Backward Monte Carlo method to constrain the cloud particle scattering properties of . Recently, Monte Carlo transport methods have also been used to model the path and decay of cosmic rays through a Jupiter-like atmosphere (Helling et al 2016b), important to the ion chemistry in exoplanet atmospheres (Rimmer & Helling 2016).…”

Galactic cosmic ray propagation through M dwarf planetary systems

“…Depending on the temperature of the planet, GJ 411 c may be a good candidate to study the impact of Galactic cosmic rays on atmospheric chemistry. Spectroscopic observations of molecular features from ions, such as H 3 O + and NH + 4 (Helling et al 2016;Barth et al 2021), with the James Webb Space Telescope (JWST, Gardner et al 2006) and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (Ariel, Tinetti et al 2021) could possibly constrain the incident cosmic ray spectrum and detect the existence of a possible excess of low-energy particles.…”