ION ABUNDANCES AND IMPLICATIONS FOR PHOTOCHEMISTRY IN COMETS HALLEY (1986 III) AND BRADFIELD (1987 XXIX)

Spectra of the plasma tails of comets P/Halley (1986 III) and P/Bradfield (1987 XXIX, also 1987s) were recorded using the Ohio State University Image Dissector Scanner (IDS) on the Perkins 1.8 m telescope at the Lowell Observatory. The ionic species CO+, N2+, CH+, and H2O+ were identified in these spectra and column densities for them were calculated from measured fluxes. The observed abundance ratios of CO+/H2O+ in comets Halley and Bradfield are consistent with predictions by photochemical models, provided that CO+ and H2O+ are produced primarily from photoionization of CO and H2O, respectively. However, the observed N2+/H2O+ ratios are at least an order of magnitude lower and the observed CH+/H2O+ ratios are a factor of 100 higher, than theoretical results. The abundance ratio N2/CO was derived in the plasma tail of Bradfield from N2+ and CO+ data, and found to be an order of magnitude higher than a value measured in Halley. The relative ion abundances of CH+, N2+, CO+ and H2O+ in Halley are consistent with in situ measurements obtained from the Giotto spacecraft. Fluorescence efficiencies have been recalculated for optical transitions of N2+, CH+, NH, CH, OH+, and CO and are also presented.