  Atmospheres play a crucial role in planetary habitability. Around M dwarfs
and young Sun-like stars, planets receiving the same insolation as the
present-day Earth are exposed to intense stellar X-rays and extreme-ultraviolet
(XUV) radiation. This study explores the fundamental question of whether the
atmosphere of present-day Earth could survive in such harsh XUV environments.
Previous theoretical studies suggest that stellar XUV irradiation is
sufficiently intense to remove such atmospheres completely on short timescales.
In this study, we develop a new upper-atmospheric model and re-examine the
thermal and hydrodynamic responses of the thermospheric structure of an
Earth-like N2-O2 atmosphere, on an Earth-mass planet, to an increase in the XUV
irradiation. Our model includes the effects of radiative cooling via electronic
transitions of atoms and ions, known as atomic line cooling, in addition to the
processes accounted for by previous models. We demonstrate that atomic line
cooling dominates over the hydrodynamic effect at XUV irradiation levels
greater than several times the present level of the Earth. Consequentially, the
atmosphere's structure is kept almost hydrostatic, and its escape remains
sluggish even at XUV irradiation levels up to a thousand times that of the
Earth at present. Our estimates for the Jeans escape rates of N2-O2 atmospheres
suggest that these 1 bar atmospheres survive in early active phases of Sun-like
stars. Even around active late M dwarfs, N2-O2 atmospheres could escape
significant thermal loss on timescales of gigayears. These results give new
insights into the habitability of terrestrial exoplanets and the Earth's
climate history.
