Far‐Ultraviolet Observations of Comet 2P/Encke at Perihelion

Comet 2P/Encke was observed with UVCS/SOHO near perihelion (2000 September 9 and 11) in the Lyman lines of hydrogen. We present a Lyalpha image reconstructed from a series of long-slit spectra, along with the intensity ratios of Lyalpha, Lybeta, and Lygamma. The narrow Lyalpha profile indicates that the observed photons are scattered from hydrogen atoms produced by dissociation of H2O and OH, though a broader profile far from the coma suggests a contribution from hydrogen atoms produced by charge transfer with solar wind protons. The outgassing rate derived from the Lyalpha intensity distribution is in excellent agreement with Sekanina's model of two active vents. We also present upper limits for the outgassing rates of the noble gases helium, neon, and argon.