Quantitative Analysis of Measured H_2O(+) Column Densities from Comet P/Halley

Details Quantitative Analysis of Measured H_2O(+) Column Densities from Comet P/Halley Quantitative Analysis of Measured H_2O(+) Column Densities from Comet P/Halley

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0811h&link_type=abstract

American Astronomical Society, DPS meeting #28, #08.11; Bulletin of the American Astronomical Society, Vol. 28, p.1085

Astronomy and Astrophysics

Astronomy

2

Scientific paper

Spectroscopic measurements of ions, mainly H_2O(+) \ and CO(+) , in cometary comae have been obtained for more than two decades. By the use of long-slit spectrometers or narrow-band filters together with imaging devices, it is possible to measure the two-dimensional distribution of ions in cometary comae. Since the ions have a very different dynamics compared to the neutrals and because their densities are affected by chemical reactions, one needs a completely different model to analyze ion distributions than for neutral gas species. The quantitative analysis of ion distributions has so far been hindered by the lack of suitable models. Using the vector-flow field as computed by our new fully three-dimensional MHD-model of cometary plasma environments [Gombosi et al., 1996] we solved the coupled continuity equations for the major cometary ion species and some interesting trace ions. Our ion-chemical network contains the following processes for a total of about 20 ion species: production of ions by photo- or electron impact ionization of neutral gas, recombination of molecular ions with electrons, and ion-molecule reactions, including charge exchange. This allows us to compute the global distribution of ions in the coma using the boundary conditions (solar wind parameters, production rate) appropriate for the encounter of Vega 1 with comet P/Halley (March 6, 1986). We will show a comparison of computed profiles of H_2O(+) \ column densities with ground based measurements from March 5, as published by DiSanti et al. [1990]. Our computed profiles are similar in shape as the measured ones, especially, we are able to reproduce the pronounced double-peak structures in the cross-tail profiles. If we use the suggested, reduced g factors for H_2O(+) \ excitation by DiSanti et al. [1990] we also obtain good agreement with the measured absolute column densities. In addition to the H_2O(+) \ data we will also show computed column densities for CO(+) .

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