Suprathermal Particles in XUV-Heated and Extended Exoplanetary Upper Atmospheres

Abstract

The photolysis of hydrogen-rich atmosphere of a close-in exoplanet by the extreme ultraviolet radiation of the parent star leads to the formation of the suprathermal particles (i.e., particles with an excess of kinetic energy), primary photoelectrons in the $H_{2}/H/\mathit{He}$ ionization and hydrogen atoms in the H ₂ dissociation and dissociative ionization processes. These particles with excess kinetic energies are an important source of thermal energy in the upper atmosphere of the hydrogen-rich exoplanets. In the contemporary aeronomical models the kinetics and transfer of hot hydrogen atoms and fresh photoelectrons were not calculated in detail, because they require solving of the Boltzmann equation for a non-thermal population of these particles. This chapter estimates the effect of the XUV radiation of the parent star on the production of the suprathermals in the $H_{2} \rightarrow H$ transition region in the upper atmosphere of a hydrogen-rich exoplanet. Partial deposition rates of the stellar XUV radiation due to the photolytic processes in the $H_{2} \rightarrow H$ transition region in the upper atmosphere of HD 209458b were calculated. The Monte Carlo model developed by authors was used to calculate the collisional kinetics and the transport of photoelectrons in the atmosphere of HD209458b. Using this model the partial deposition rates of the stellar XUV radiation due to the electron impact processes in the $H_{2} \rightarrow H$ transition region in the upper atmosphere of HD209458b were calculated. This allowed us to estimate the heating rate of the atmospheric gas by photoelectrons in the upper atmosphere of exoplanet. For the first time the heating efficiency η with and without taking into account the photoelectron impact processes in the $H_{2} \rightarrow H$ transition region in the hydrogen-rich atmosphere of exoplanet was calculated. Using the numerical stochastic model for a hot planetary corona the kinetics and transfer of suprathermal hydrogen atoms in the upper atmosphere and the emergent flux of atoms evaporating from the atmosphere were investigated. The latter is estimated as $5.8 \times 10^{12}\,\text{cm}^{-2}\text{s}^{-1}$ for a moderate stellar activity level of UV radiation, which leads to a planetary atmosphere evaporation rate of 5. 8 × 10⁹ g/s due to the process of the dissociation of H ₂. This estimate shows that suprathermal hydrogen atoms provide a significant contribution to the observational estimate of ∼ 10¹⁰ g/s for the atmospheric loss rate of HD 209458b.