  Stellar wind and photon radiation interactions with a planet can cause
atmospheric depletion, which may have a potentially catastrophic impact on a
planet's habitability. While the implications of photoevaporation on
atmospheric erosion have been researched to some degree, studies of the
influence of the stellar wind on atmospheric loss are in their infancy. Here,
we use three-dimensional magnetohydrodynamic simulations to model the effect of
the stellar wind on the magnetosphere and outflow of a hypothetical planet,
modeled to have an H-rich evaporating envelope with a pre-defined mass loss
rate, orbiting in the habitable zone close to a low-mass M dwarf. We take the
TRAPPIST-1 system as a prototype, with our simulated planet situated at the
orbit of TRAPPIST-1e. We show that the atmospheric outflow is dragged and
accelerated upon interaction with the wind, resulting in a diverse range of
planetary magnetosphere morphologies and plasma distributions as local stellar
wind conditions change. We consider the implications of the wind-outflow
interaction on potential hydrogen Lyman-alpha (Lya) observations of the
planetary atmosphere during transits. The Lya observational signatures depend
strongly on the local wind conditions at the time of the observation and can be
subject to considerable variation on timescales as short as an hour. Our
results indicate that observed variations in exoplanet Lya transit signatures
could be explained by wind-outflow interaction.
