Transition from XUV Driven Ion Escape to Hydrodynamic Escape: Implications For Habitability of TRAPPIST1 b-h and TOI-700d Rocky Exoplanets

Recent data suggest that most of rocky exoplanets around active G-M planet hosts should be exposed to high ionizing radiation fluxes from stellar coronae, flares, magnetized winds and coronal mass ejections. This raises the question of the atmospheric plasma dynamics and subsequent plasma escape driven by the energy deposited via X-ray and Extreme UV driven (XUV) photoionization. The atmospheric escape is one of the central issues to exoplanetary habitability, because the presence of a thick high molecular weight secondary atmosphere over sufficiently long timescales is a crucial factor associated with planetary surface pressure due to the exposure to stellar UV and particle irradiation. Here, we extend our previous results of Airapetian et al. (2017) by applying the Exo-Global Ionosphere-Thermosphere (exo-GITM) model of thermodynamics and atmospheric dynamics of an Earth-like exoplanet controlled by the coronal XUV emission from TRAPPIST-1 and TOI-700 rocky exoplanets. We show that at relatively low XUV energy deposition in the atmosphere (<60 times of the solar flux), the atmospheric escape is mostly driven by the ion escape, which is characteristic of conditions for a recently discovered rocky exoplanet TOI-700d. Our models suggest that the transition from the XUV driven ion escape to hydrodynamic atmospheric scape occurs at > 60 times of the XUV solar flux. We discuss the implications of the atmospheric escape due to XUV photoionization driven heating and Joule heating rates for unmagnetized and a magnetized planet and habitability conditions for TRAPPIST b-h rocky exoplanets.