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. 2019 Sep 23;377(2154):20180398.
doi: 10.1098/rsta.2018.0398. Epub 2019 Aug 5.

Lightning and charge processes in brown dwarf and exoplanet atmospheres

Christiane Helling  1   2 , Paul B Rimmer  3   4   5
Affiliations

Lightning and charge processes in brown dwarf and exoplanet atmospheres

Christiane Helling et al. Philos Trans A Math Phys Eng Sci. .

Abstract

The study of the composition of brown dwarf atmospheres helped to understand their formation and evolution. Similarly, the study of exoplanet atmospheres is expected to constrain their formation and evolutionary states. We use results from three-dimensional simulations, kinetic cloud formation and kinetic ion-neutral chemistry to investigate ionization processes that will affect their atmosphere chemistry: the dayside of super-hot Jupiters is dominated by atomic hydrogen, and not H2O. Such planetary atmospheres exhibit a substantial degree of thermal ionization and clouds only form on the nightside where lightning leaves chemical tracers (e.g. HCN) for possibly long enough to be detectable. External radiation may cause exoplanets to be enshrouded in a shell of highly ionized, H3+-forming gas and a weather-driven aurora may emerge. Brown dwarfs enable us to study the role of electron beams for the emergence of an extrasolar, weather system-driven aurora-like chemistry, and the effect of strong magnetic fields on cold atmospheric gases. Electron beams trigger the formation of H3+ in the upper atmosphere of a brown dwarf (e.g. LSR-J1835), which may react with it to form hydronium, H3O+, as a longer lived chemical tracer. Brown dwarfs and super-hot gas giants may be excellent candidates to search for H3O+ as an H3+ product. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'.

Keywords: aurora; brown dwarf; charges; exoplanet; lightning.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
Highly irradiated, ultra-hot Jupiters develop extreme temperature differences of 2500 K between dayside (ϕ = − 45°, 0°, 45°) and nightside (ϕ = 135°, −180°, − 135°). An ionosphere may emerge on the dayside, and mineral clouds form on the nightside [39]. The terminator regions (ϕ = 90°, −90°) are transition regions between the two extreme atmosphere conditions. The one-dimensional profiles are from a cloud-free three-dimensional GCM simulation [40]. (Online version in colour.)
Figure 2.
Figure 2.
The changing number densities (nx [cm−3], left axis) of hydrogen-binding gas species from the dayside (top) to the nightside (bottom). C/O is shown as a black dashed line (right axis) to demonstrate where cloud formation takes place. The dayside is made of a cloud-free, H-dominated gas and the nightside is made of a H2-dominated gas with vivid cloud formation (C/O > C/Osolar). The extension of the cloud in pressure space (x-axis) changes between equator (θ = 0) and northern hemisphere (θ = 45°) [39,40]. (Online version in colour.)
Figure 3.
Figure 3.
(a) Temperature [K] and (b) eddy diffusion Kzz [cm2 s−1] as a function of pressure for a PHOENIX model M 8.5 dwarf, with an effective temperature of Teff = 2600 K, surface gravity of logg = 5 and the mean molecular mass of 2.33 amu.
Figure 4.
Figure 4.
Mixing ratios versus pressure [pgas, bar] for the ions H3+ and H3O+, as well as the species involved in their destruction (e−, CO and H2O). Solid lines represent the results when accounting for photochemistry and cosmic ray chemistry, and dashed lines represent the results when including an auroral electron beam, as described in §§3d. (Online version in colour.)
Figure 5.
Figure 5.
Gas-phase number density (n [cm−3]) versus pressure (pgas [bar]) for the ions H3+ and H3O+, as well as the species involved in their destruction (e−, CO and H2O). Solid lines represent the results when accounting for photochemistry and cosmic ray chemistry, and dashed lines represent the results when including an auroral electron beam, as described in §§3d. (Online version in colour.)
Figure 6.
Figure 6.
Chemical time scales for H3+ and H3O+, ranging from ≲1 ms where pgas > 10−1 bar to 10–100 s where pgas < 10−4 bar. Solid lines represent the results when accounting for photochemistry and cosmic ray chemistry, and dashed lines represent the results when including an auroral electron beam, as described in §§3d. The black solid line represents the dynamical time scale, tdyn [s], corresponding to a constant Kzz = 1010 cm2 s−1. (Online version in colour.)
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  • Hydrogen molecular ions: H3+, H5+ and beyond.
    Tennyson J, Miller S. Tennyson J, et al. Philos Trans A Math Phys Eng Sci. 2019 Sep 23;377(2154):20180395. doi: 10.1098/rsta.2018.0395. Epub 2019 Aug 5. Philos Trans A Math Phys Eng Sci. 2019. PMID: 31378175 Free PMC article.

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