  Observations of sungrazing comets can be used to probe the solar corona, to
study the composition of the comets, and to investigate the plasma processes
that govern the interaction between the coronal plasma and cometary gas. UVCS
observations of the intensities and line profiles of \hi\ Ly$\alpha$ trace the
density, temperature and outflow speed of the corona. Analysis of \hi\
Ly$\alpha$ observations of comet C/2002 S2 showed a surprising split in the
comet's Ly$\alpha$ tail and an asymmetry of red-shifted and blue-shifted
emission across the tail axis. It was suggested that the velocity structure
might result from a population of neutrals produced by charge transfer between
pickup ions and cometary neutrals. Here we present numerical simulations of the
\hi\ Ly$\alpha$ intensity and velocity centroid for sungrazing comets under the
assumption that the magnetic field and solar wind are radial. The models
qualitatively reproduce the observations of Comet C/2002 S2 and potentially
explain the split tail morphology that was seen in C/2002 S2 and also C/2001
C2. They also match the observed red- and blue-shifts, though the solar wind
velocity needed to explain the blue-shift implies strong Doppler dimming and
requires a higher outgassing rate to match the light curve. However, the models
do not match the observations in detail, and we discuss the remaining
discrepancies and the uncertainties in the model. We briefly discuss the
implications for other UVCS comet observations and sungrazing comet
observations with the Metis coronagraph.
