  Observations of H I Lyman alpha, the brightest UV emission line of late-type
stars, are critical for understanding stellar chromospheres and transition
regions, modeling photochemistry in exoplanet atmospheres, and measuring the
abundances of neutral hydrogen and deuterium in the interstellar medium. Yet,
Lyman alpha observations are notoriously challenging due to severe attenuation
from interstellar gas, hindering our understanding of this important emission
line's basic morphology. We present high-resolution far- and near-UV
spectroscopy of five G, K, and M dwarfs with radial velocities large enough to
Doppler shift the stellar Lyman alpha emission line away from much of the
interstellar attenuation, allowing the line core to be directly observed. We
detect self-reversal in the Lyman alpha emission line core for all targets, and
we show that the self-reversal depth decreases with increasing surface gravity.
Mg II self-reversed emission line profiles provide some useful information to
constrain the Lyman alpha line core, but the differences are significant enough
that Mg II cannot be used directly as an intrinsic Lyman alpha template during
reconstructions. We show that reconstructions that neglect self-reversal could
overestimate intrinsic Lyman alpha fluxes by as much as 60%-100% for G and K
dwarfs and 40%-170% for M dwarfs. The five stars of our sample have low
magnetic activity and sub-solar metallicity; a larger sample size is needed to
determine how sensitive these results are to these factors.
