Solar Wind at 6.8 Solar Radii from UVCS Observation of Comet C/1996Y1

The comet C/1996Y1, a member of the Kreutz family of Sun-grazing comets, was observed with the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO) satellite. The Lyα line profile and spatial distribution are interpreted in terms of the theory of bow shocks driven by mass-loading. At the time of the observation, the comet was 6.8 R☉ from the Sun in a region of high-speed wind, a region difficult to observe directly with the SOHO instruments but an important region for testing models of solar wind acceleration and heating. We find a solar wind speed below 640 km s-1 and a constraint on the combination of solar wind speed and proton temperature. The total energy per proton at 6.8 R☉ is 50%-75% of the energy at 1 AU, indicating that significant heating occurs at larger radii. The centroid and width of the Lyα line generally confirm the predictions of models of the cometary bow shock driven by mass-loading as cometary molecules are ionized and swept up in the solar wind. We estimate an outgassing rate of 20 kg s-1, which implies an active area of the nucleus only about 6.7 m in diameter at 6.8 R☉. This is likely to be the size of the nucleus, because any inert mantle would have probably been blown off during the approach to the Sun.