  Photoevaporation is a potential explanation for several features within
exoplanet demographics. Atmospheric escape observed in young Neptune-sized
exoplanets can provide insight into and characterize which mechanisms drive
this evolution and at what times they dominate. AU Mic b is one such exoplanet,
slightly larger than Neptune (4.19 Earth radii). It closely orbits a 23 Myr
pre-Main Sequence M dwarf with a period of 8.46 days. We obtained two visits of
AU Mic b at Lyman-alpha with HST/STIS. One flare within the first HST visit is
characterized and removed from our search for a planetary transit. We present a
non-detection in our first visit followed by the detection of escaping neutral
hydrogen ahead of the planet in our second visit. The outflow absorbed about
30% of the star's Lyman-alpha blue-wing 2.5 hours before the planet's
white-light transit. We estimate the highest velocity escaping material has a
column density of 10^13.96 cm^-2 and is moving 61.26 km/s away from the host
star. AU Mic b's large high energy irradiation could photoionize its escaping
neutral hydrogen in 44 minutes, rendering it temporarily unobservable. Our
time-variable Lyman-alpha transit ahead of AU Mic b could also be explained by
an intermediate stellar wind strength from AU Mic that shapes the escaping
material into a leading tail. Future Lyman-alpha observations of this system
will confirm and characterize the unique variable nature of its Lyman-alpha
transit, which combined with modeling will tune the importance of stellar wind
and photoionization.
