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M. R. Combi (U. Michigan), A.A. Reinard (U. Michigan), J.-L. Bertaux (CNRS), The SOHO SWAN Team
The SWAN all-sky hydrogen Lyman-\alpha camera on the SOHO spacecraft observed the coma of Comet Hale-Bopp from January through May 1997. The SOHO spacecraft operated at the L1 Lagrange point about 1 million km from the Earth enabling continuous temporal coverage of solar and solar wind events. The hydrogen coma is seen to vary in brightness, shape, and orientation with the comet's position over the 56 separate days throughout the period, giving about 3 images per week. The observations cover heliocentric distances from 1.75 AU before perihelion to 1.29 AU after. On the day of perihelion the hydrogen coma could be seen spanning a region of sky nearly 40 degrees across, corresponding to 150 million km (1 AU). We have reproduced the brightness and shape of the observed H coma using a Monte Carlo model which 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 very thick water coma. Analyses of past wide-field, usually rocket borne, UV camera images of bright comets (Kohoutek, West and Halley) showed clear evidence that a small but noticeable fraction of the suprathermal H atoms are partially thermalized by collisions with the outflowing heavy coma gas. Because of the extremely large gas production rates (up to nearly 1031 s-1), a significantly larger fraction of H atoms undergo more collisions out to much larger distances from the nucleus of Hale-Bopp. This results in a decrease in the average outflow velocity of H atoms in Hale-Bopp compared with more "typical" bright comets. The decrease effects both the water production rate determined from the absolute brightness and the shape of the Lyman-\alpha coma because of the "sorting" effect of between atom velocity and radiation pressure acceleration.
The author(s) of this abstract have provided an email address for comments about the abstract: mcombi@umich.edu