Kinetic Modeling of Non-Thermal Processes in the Upper Atmosphere of Sub-Neptune \(\pi\) Men c

Abstract

Observations of exoplanets in the ultraviolet (UV) wavelength range provide data about the structure of the exoplanetary atmosphere, as well as the atmospheric escape rate. Observations by the Hubble Space Telescope (HST) carried out in the spectral H Ly\(\alpha\) line for the exoplanet \(\pi\) Men c did not reveal any absorption in the blue and red wings of the line. However, due to the peculiarities of the HST orbit, these observations do not allow us to speak unambiguously about the presence or absence of hydrogen in the exoplanetary atmosphere; they only provide an estimate of the upper limit of the mass velocity for hydrogen in the atmosphere. The exoplanet \(\pi\) Men c falls on the outer boundary of the Fulton division from the sub-Neptune side, which implies the presence of hydrogen in the exoplanetary atmosphere. Based on the assumption that \(\pi\) Men c has a primary hydrogen-helium envelope, we performed kinetic modeling of non-thermal processes in the upper atmosphere of the exoplanet \(\pi\) Men c. We considered the processes of suprathermal particle production in exothermic photochemistry under the action of hard ultraviolet (UV) radiation from the host star, as well as the effect of the stellar wind proton flux penetrating the atmosphere. The non-thermal atmospheric escape caused by the exposure to the hard radiation of the host star turned out to be comparable with the estimates of the escaping atmospheric flux due to the thermal processes. The energy of the stellar wind protons penetrating into the atmosphere is completely spent on heating the atmosphere.