  Exoplanets orbiting M-dwarfs within habitable zones are exposed to stellar
environments more extreme than that terrestrial planets experience in our Solar
System, which can significantly impact the atmospheres of the exoplanets and
affect their habitability and sustainability. This study provides the first
prediction of hot oxygen corona structure and the associated photochemical loss
from a 1 bar CO2-dominated atmosphere of a Venus-like rocky exoplanet, where
dissociative recombination of O2+ ions is assumed to be the major source
reaction for the escape of neutral O atoms and formation of the hot O corona
(or exospheres) as on Mars and Venus. We employ a 3D Monte Carlo code to
simulate the exosphere of Proxima Centauri b (PCb) based on the ionosphere
simulated by a 3D magnetohydrodynamic model. Our simulation results show that
variability of the stellar wind dynamic pressure over one orbital period of PCb
does not affect the overall spatial structure of the hot O corona but
contributes to the change in the global hot O escape rate that varies by an
order of magnitude. The escape increases dramatically when the planet possesses
its intrinsic magnetic fields as the ionosphere becomes more extended with the
presence of a global magnetic field. The extended hot O corona may lead to a
more extended H exosphere through collisions between thermal H and hot O, which
exemplifies the importance of considering nonthermal populations in exospheres
to interpret future observations.
