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

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 -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 H 2 is not substantially dissociated (at large orbital distances) have a layer in which H + 3 is the major ion at the base of the ionosphere. For fast-rotating planets, densities of short-lived H + 3 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 H + 3 peak in EGPs with upper atmospheres where H 2 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 H + 3 , and while, in an H/H 2 /He atmosphere, these are larger from planets orbiting close to more active stars, they still appear too low to be detected with current observatories.

How expanded ionospheres of Hot Jupiters can prevent escape of radio emission generated by the cyclotron maser instability

“…Shaikhislamov et al (2014) and Khodachenko et al (2015) simulated the densities at HD 209458b up to a distance of 20 planetary radii along the equator and 10 planetary radii along the pole. Other authors studied the ionospheres and upper atmospheres of HD 209458b and HD 189733b and obtained similar results (Yelle 2004;García Muñoz 2007;Koskinen et al 2010Koskinen et al , 2013aKoskinen et al ,b, 2014bGuo 2011;Lavvas et al 2014;Salz et al 2015;Chadney et al 2015Chadney et al , 2016Erkaev et al 2016).…”

“…Over the past decade, several studies applied hydrodynamical upper atmosphere models which included photochemistry, ionization and dissociation to Hot Jupiters (Yelle 2004;García Muñoz 2007;Koskinen et al 2010Koskinen et al , 2013aKoskinen et al ,b, 2014bGuo 2011;Shaikhislamov et al 2014;Khodachenko et al 2015;Salz et al 2015;Chadney et al 2015Chadney et al , 2016Erkaev et al 2016). In our study we use ionospheric profiles for Hot Jupiters modelled in previous studies of Shaikhislamov et al (2014) and Khodachenko et al (2015) for HD 209458b, and Guo (2011) for HD 189733b.…”

Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b

“…Horizontal circulation is neglected and it is assumed that vertical gradients are dominant over horizontal gradients. More details about these models and their couplings can be found in Chadney et al (2016).…”

“…We have previously studied the high-energy spectrum of Eri and its effect on planetary upper atmospheres (Chadney et al 2015(Chadney et al , 2016. However, the Eri irradiances used in these earlier publications represent a quiescent (i.e., non-flaring) period.…”

“…In model A with a cool lower boundary (see Sect. 2.4), this region is dominated by the ion H + 3 , which is a short-lived ion and as such responds quickly to changes in stellar irradiance (Chadney et al 2016). Furthermore, given the composition of the atmospheres that we consider in our model, photons of shorter wavelength are absorbed at lower altitudes.…”