Title: HAZMAT II: Ultraviolet Variability of Low-Mass Stars in the GALEX Archive,
Abstract: The ultraviolet (UV) light from a host star influences a planet's atmospheric
photochemistry and will affect interpretations of exoplanetary spectra from
future missions like the James Webb Space Telescope. These effects will be
particularly critical in the study of planetary atmospheres around M dwarfs,
including Earth-sized planets in the habitable zone. Given the higher activity
levels of M dwarfs compared to Sun-like stars, time resolved UV data are needed
for more accurate input conditions for exoplanet atmospheric modeling. The
Galaxy Evolution Explorer (\emph{GALEX}) provides multi-epoch photometric
observations in two UV bands: near-ultraviolet (NUV; 1771 -- 2831 \AA) and
far-ultraviolet (FUV; 1344 -- 1786 \AA). Within 30 pc of Earth, there are 357
and 303 M dwarfs in the NUV and FUV bands, respectively, with multiple\GALEX
observations. Simultaneous NUV and FUV detections exist for 145 stars in
both\GALEX bands. Our analyses of these data show that low-mass stars are
typically more variable in the FUV than the NUV. Median variability increases
with later spectral types in the NUV with no clear trend in the FUV. We find
evidence that flares increase the FUV flux density far more than the NUV flux
density, leading to variable FUV to NUV flux density ratios in the \GALEX\
bandpasses.The ratio of FUV to NUV flux is important for interpreting the
presence of atmospheric molecules in planetary atmospheres such as oxygen and
methane as a high FUV to NUV ratio may cause false-positive biosignature
detections. This ratio of flux density in the\GALEX\ bands spans three orders
of magnitude in our sample, from 0.008 to 4.6, and is 1 to 2 orders of
magnitude higher than for G dwarfs like the Sun. These results characterize the
UV behavior for the largest set of low-mass stars to date.