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 $\gg B_{\mathrm{Earth}}$, 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 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.

arXiv comment(9 pages, accepted for publication by Plasma Physics and Controlled Fusion, IoP)

References in corpus (31)

• A Non-Gray Theory of Extrasolar Giant Planets and Brown Dwarfs
• The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? I. Mapping the zoo of laboratory collision experiments
• Clouds and Chemistry: Ultracool Dwarf Atmospheric Properties from Optical and Infrared Colors
• Synthetic Spectra and Colors of Young Giant Planet Atmospheres: Effects of Initial Conditions and Atmospheric Metallicity
• Activity in Very Cool Stars: Magnetic Dissipation in late-M and L Dwarf Atmospheres
• Atmospheric Circulation of Hot Jupiters: Three-dimensional circulation models of HD 209458b and HD 189733b with Simplified Forcing
• Dust in Brown Dwarfs and Extra-solar Planets I. Chemical composition and spectral appearance of quasi-static cloud layers
• Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence
• Consistent simulations of substellar atmospheres and non-equilibrium dust-cloud formation
• Three Dimensional Radiative Hydrodynamical Simulations of the Highly Irradiated Short Period Exoplanet HD189733b
• Dust in brown dwarfs and extra-solar planets II. Cloud formation for cosmologically evolving abundances
• BOSS Ultracool Dwarfs I: Colors and Magnetic Activity of M and L dwarfs
• Radiative Hydrodynamic Simulations of HD209458b: Temporal Variability
• A Model for the Scattered Light Contribution and Polarization of the Diffuse H$\alpha$ Galactic Background
• Atmospheres of Brown Dwarfs
• Atmospheric Circulation of Brown Dwarfs: Jets, Vortices, and Time Variability
• Origin of electron cyclotron maser-induced radio emissions at ultra-cool dwarfs: magnetosphere-ionosphere coupling currents
• Ionisation in atmospheres of Brown Dwarfs and extrasolar planets I The role of electron avalanche
• Ionisation in atmospheres of Brown Dwarfs and extrasolar planets III. Breakdown conditions for mineral clouds
• Ionisation in atmospheres of Brown Dwarfs and extrasolar planets II Dust-induced collisional ionization
• Ionization in atmospheres of Brown Dwarfs and extrasolar planets IV. The Effect of Cosmic Rays
• A Fast and Accurate Calculation Scheme for Ionization Degrees in Protoplanetary and Circumplanetary Disks with Charged Dust Grains
• A Signature of Chromospheric Activity in Brown Dwarfs Revealed by 2.5-5.0 Micron AKARI Spectra
• Reference study to characterise plasma and magnetic properties of ultra-cool atmospheres
• The First Millimeter Detection of a Non-Accreting Ultracool Dwarf
• Ionisation in atmospheres of brown dwarfs and extrasolar planets VI: Properties of large-scale discharge events
• The Influence of Galactic Cosmic Rays on Ion-Neutral Hydrocarbon Chemistry in the Upper Atmospheres of Free-Floating Exoplanets
• Ionization in Atmospheres of Brown Dwarfs and Extrasolar Planets V: Alfvén Ionization
• Kepler Monitoring of an L Dwarf II. Clouds with Multiyear Lifetimes
• Grain charging in protoplanetary discs
• Jupiter as a Giant Cosmic Ray Detector

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• Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b: III. Changing ionisation and the emergence of an ionosphere
• Triboelectrification of KCl and ZnS particles in approximated exoplanet environments
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• Galactic cosmic ray propagation through M dwarf planetary systems
• Exploring terrestrial lightning parameterisations for exoplanets and brown dwarfs
• Evolution of spheroidal dust in electrically active sub-stellar atmospheres
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