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A.J. Lovell, A.L. Iler, F.P. Schloerb (FCRAO), K.C. Hansen, M.R. Combi, R.M H\"aberli, T.I. Gombosi (Univ. of Michigan)
Millimeter-wave images of the HCO+ J=1-0 rotational transition in Comet C/1995 O1 Hale-Bopp revealed both the spatial distribution and the radial velocity structure of this ion in the coma of this bright comet. Near-perihelion observations, taken at the Five College Radio Astronomy Observatory (FCRAO), revealed extended emission, a peak offset ~100,000 km anti-sunward from the nucleus, and broad, redshifted spectral lines (Lovell et al. ApJ 497, L117). Based on the results of rather simple models, we originally attributed these features to the formation of HCO+ via ion-molecule reactions in the coma, and acceleration of the ions in the anti-sunward direction. In this paper, we compare the observations to the results of model calculations which combine a magneto-hydrodynamic description of the comet/solar wind interaction with an ion-chemistry calculation that accounts for HCO+ formation. The model of Häberli et al. (1997, Icarus, 130, 373) has been used to simulate the behavior of HCO+ during the times of the FCRAO observations, allowing a direct comparison with the data. The model has been computed assuming both fast and slow solar wind conditions. Many aspects of the general behavior of the HCO+ ion appear to be consistent with the initial results of the MHD model; a more detailed comparison, including the effects of both collisional and radiative excitation of HCO+ will be presented in this paper.