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Observations of sungrazing comets can be used to probe the solar corona, tostudy the composition of the comets, and to investigate the plasma processesthat govern the interaction between the coronal plasma and cometary gas. UVCSobservations of the intensities and line profiles of \hi\ Ly$\alpha$ trace thedensity, temperature and outflow speed of the corona. Analysis of \hi\Ly$\alpha$ observations of comet C/2002 S2 showed a surprising split in thecomet's Ly$\alpha$ tail and an asymmetry of red-shifted and blue-shiftedemission across the tail axis. It was suggested that the velocity structuremight result from a population of neutrals produced by charge transfer betweenpickup ions and cometary neutrals. Here we present numerical simulations of the\hi\ Ly$\alpha$ intensity and velocity centroid for sungrazing comets under theassumption that the magnetic field and solar wind are radial. The modelsqualitatively reproduce the observations of Comet C/2002 S2 and potentiallyexplain the split tail morphology that was seen in C/2002 S2 and also C/2001C2. They also match the observed red- and blue-shifts, though the solar windvelocity needed to explain the blue-shift implies strong Doppler dimming andrequires a higher outgassing rate to match the light curve. However, the modelsdo not match the observations in detail, and we discuss the remainingdiscrepancies and the uncertainties in the model. We briefly discuss theimplications for other UVCS comet observations and sungrazing cometobservations with the Metis coronagraph.

Probing Coronal Magnetic Fields with Sungrazing Comets: \hi\ Ly$\alpha$ from Pickup Ions