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EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars

EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars
EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars
The composition and structure of the upper atmospheres of extrasolar giant planets (EGPs) are affected by the high-energy spectrum of their host stars from soft X-rays to the extreme ultraviolet (EUV). This emission depends on the activity level of the star, which is primarily determined by its age. In this study, we focus upon EGPs orbiting K- and M-dwarf stars of different ages – ϵ Eridani, AD Leonis, AU Microscopii – and the Sun. X-ray and EUV (XUV) spectra for these stars are constructed using a coronal model. These spectra are used to drive both a thermospheric model and an ionospheric model, providing densities of neutral and ion species. Ionisation – as a result of stellar radiation deposition – is included through photo-ionisation and electron-impact processes. The former is calculated by solving the Lambert-Beer law, while the latter is calculated from a supra-thermal electron transport model. We find that EGP ionospheres at all orbital distances considered (0.1−1 AU) and around all stars selected are dominated by the long-lived H+ ion. In addition, planets with upper atmospheres where H2 is not substantially dissociated (at large orbital distances) have a layer in which H3+ is the major ion at the base of the ionosphere. For fast-rotating planets, densities of short-lived H3+ undergo significant diurnal variations, with the maximum value being driven by the stellar X-ray flux. In contrast, densities of longer-lived H+ show very little day/night variability and the magnitude is driven by the level of stellar EUV flux. The H3+ peak in EGPs with upper atmospheres where H2 is dissociated (orbiting close to their star) under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species (e.g. hydrocarbons, water). The inclusion of secondary ionisation processes produces significantly enhanced ion and electron densities at altitudes below the main EUV ionisation peak, as compared to models that do not include electron-impact ionisation. We estimate infrared emissions from H3+, and while, in an H/H2/He atmosphere, these are larger from planets orbiting close to more active stars, they still appear too low to be detected with current observatories.
0004-6361
Chadney, Joshua
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Galand, Marina
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Koskinen, Tommi
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Miller, Steve
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Sanz-Forcada, Jorge
ad1239b8-b31b-418b-a96c-23b44922cfc4
Unruh, Yvonne
c8e151aa-e9d2-43e1-bcde-c12a25f31368
Yelle, Roger
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Chadney, Joshua
4209e45f-6092-4c9d-b46b-4b598269a0f3
Galand, Marina
083b2513-57ab-4b08-88e4-ba759f290c98
Koskinen, Tommi
fd4b7af2-f84b-46ba-9dee-223eb741cf05
Miller, Steve
9a2767e5-d7ce-483b-804d-6804c3d0a031
Sanz-Forcada, Jorge
ad1239b8-b31b-418b-a96c-23b44922cfc4
Unruh, Yvonne
c8e151aa-e9d2-43e1-bcde-c12a25f31368
Yelle, Roger
b40713f4-6d42-4670-8fe0-31ac54a623e2

Chadney, Joshua, Galand, Marina, Koskinen, Tommi, Miller, Steve, Sanz-Forcada, Jorge, Unruh, Yvonne and Yelle, Roger (2016) EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars. Astronomy & Astrophysics, 587. (doi:10.1051/0004-6361/201527442).

Record type: Article

Abstract

The composition and structure of the upper atmospheres of extrasolar giant planets (EGPs) are affected by the high-energy spectrum of their host stars from soft X-rays to the extreme ultraviolet (EUV). This emission depends on the activity level of the star, which is primarily determined by its age. In this study, we focus upon EGPs orbiting K- and M-dwarf stars of different ages – ϵ Eridani, AD Leonis, AU Microscopii – and the Sun. X-ray and EUV (XUV) spectra for these stars are constructed using a coronal model. These spectra are used to drive both a thermospheric model and an ionospheric model, providing densities of neutral and ion species. Ionisation – as a result of stellar radiation deposition – is included through photo-ionisation and electron-impact processes. The former is calculated by solving the Lambert-Beer law, while the latter is calculated from a supra-thermal electron transport model. We find that EGP ionospheres at all orbital distances considered (0.1−1 AU) and around all stars selected are dominated by the long-lived H+ ion. In addition, planets with upper atmospheres where H2 is not substantially dissociated (at large orbital distances) have a layer in which H3+ is the major ion at the base of the ionosphere. For fast-rotating planets, densities of short-lived H3+ undergo significant diurnal variations, with the maximum value being driven by the stellar X-ray flux. In contrast, densities of longer-lived H+ show very little day/night variability and the magnitude is driven by the level of stellar EUV flux. The H3+ peak in EGPs with upper atmospheres where H2 is dissociated (orbiting close to their star) under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species (e.g. hydrocarbons, water). The inclusion of secondary ionisation processes produces significantly enhanced ion and electron densities at altitudes below the main EUV ionisation peak, as compared to models that do not include electron-impact ionisation. We estimate infrared emissions from H3+, and while, in an H/H2/He atmosphere, these are larger from planets orbiting close to more active stars, they still appear too low to be detected with current observatories.

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Published date: February 2016

Identifiers

Local EPrints ID: 415036
URI: https://eprints.soton.ac.uk/id/eprint/415036
ISSN: 0004-6361
PURE UUID: 52fac88a-b65e-4de8-99dd-578eb9724cb6
ORCID for Joshua Chadney: ORCID iD orcid.org/0000-0002-5174-2114

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Date deposited: 23 Oct 2017 16:30
Last modified: 14 Mar 2019 01:31

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