Ionization fraction in the thermosphere of the exoplanet HD 209458b

Dissociation and ionization of hydrogen molecules and ionization of hydrogen atoms due to extreme UV radiation from the parent star are accompanied by the formation of a concurrent photoelectron flux with excess kinetic energy. These dissociation and ionization processes are the main source of atomic and molecular ions in the thermospheres of extrasolar planets, such as the “hot Jupiter” HD 209458b. The ionization processes are the most important part of contemporary aeronomic models of planetary atmospheres in the Solar System and extrasolar systems (Johnson et al., 2008; Yelle et al., 2008). We estimate the contribution of the dissociation and ionization processes due to the stellar UV radiation and the concurrent photoelectron flux to the formation of extended ionospheres around extrasolar giant planets. As opposed to models of other researchers, we calculated the ionization rates due to the concurrent photo-electron flux for the first time. It is established that, in contrast to a widely used parametrization of the photoelectron contribution (Cecchi-Pestellini et al., 2006; 2009), the rate of secondary ionization due to the photoelectrons depends appreciably on the altitude, approaching the photoionization rate in the lower layers of the thermosphere. The calculated ionization rate in the thermosphere of the extrasolar giant planet (EGP) orbiting close to its parent star is a necessary link when modeling an aeronomic model and estimating the rate of the EGP atmospheric loss.