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A.A. Reinard (U. Michigan), M. R. Combi (U. Michigan), P.D. Feldman (Johns Hopkins U.)
Observations of hydrogen Lyman-\alpha were made of comets Bradfield (C/1979 Y1) and Austin (1989 X1) with the short wavelength primary (SWP) vidicon camera of the International Ultraviolet Explorer (IUE) satellite during times of maximum solar activity. See, for example, Weaver et al. (1981, Icarus 47, 449) and Budzien et al. (1994, Icarus 107, 164) for a discussion of the data acquisition and reduction procedures. These measurements were made both with the image of the nucleus centered on the IUE large aperture as well as displaced at various offsets. We have analyzed these data using a combination of the H coma Monte Carlo model and the spherical radiative transfer calculation which had been used already to analyze observations of solar minimum comets 1P/Halley and 21P/Giacobini-Zinner. The photochemical lifetime of water and the ratios for various dissociation and ionization branches are known to depend on the solar far UV spectral flux which varies with solar activity. The H coma model accounts for the time-variation of the gas production, the 3D orbital dynamics with heliocentric velocity dependent radiation pressure acceleration, full water and OH photochemistry, and explicit partial thermalization of H atoms by the collisionally thick water coma. The spherical radiative transfer model accounts for the axially symmetric distribution of the scattering of the solar Lyman-\alpha radiation in the optically thick coma. We find that the analysis of H measurements is much less sensitive to the assumption of solar activity variations in the photochemistry than the OH measurements, and thus Lyman-\alpha measurements might be used to provide a less model dependent method for extracting water production rates in comets compared with OH. A comparison between water production rates from the IUE Lyman-\alpha measurements and those from other measurements is made.
The author(s) of this abstract have provided an email address for comments about the abstract: alysha@umich.edu