  High levels of X-ray and UV activity on young M dwarfs may drive rapid
atmospheric escape on temperate, terrestrial planets orbiting within the liquid
water habitable zone. However, secondary atmospheres on planets orbiting older,
less active M dwarfs may be stable and present more promising candidates for
biomarker searches. We present new HST and Chandra observations of Barnard's
Star (GJ 699), a 10 Gyr old M3.5 dwarf, acquired as part of the Mega-MUSCLES
program. Despite the old age and long rotation period of Barnard's star, we
observe two FUV ($\delta_{130}$ $\approx$ 5000s; $E_{130}$ $\approx$
10$^{29.5}$ erg each) and one X-ray ($E_{X}$ $\approx$ 10$^{29.2}$ erg) flares,
and estimate a high-energy flare duty cycle (defined here as the fraction of
the time the star is in a flare state) of $\sim$ 25\%. A 5 A - 10 $\mu$m SED of
GJ 699 is created and used to evaluate the atmospheric stability of a
hypothetical, unmagnetized terrestrial planet in the habitable zone ($r_{HZ}$
$\sim$ 0.1 AU). Both thermal and non-thermal escape modeling indicate (1) the
$quiescent$ stellar XUV flux does not lead to strong atmospheric escape:
atmospheric heating rates are comparable to periods of high solar activity on
modern Earth, and (2) the $flare$ environment could drive the atmosphere into a
hydrodynamic loss regime at the observed flare duty cycle: sustained exposure
to the flare environment of GJ 699 results in the loss of $\approx$ 87 Earth
atmospheres Gyr$^{-1}$ through thermal processes and $\approx$ 3 Earth
atmospheres Gyr$^{-1}$ through ion loss processes, respectively. These results
suggest that if rocky planet atmospheres can survive the initial $\sim$ 5 Gyr
of high stellar activity, or if a second generation atmosphere can be formed or
acquired, the flare duty cycle may be the controlling stellar parameter for the
stability of Earth-like atmospheres around old M stars.
