Ground-based measurements of O1D and the H2O production rate from comets

Abstract

Ground-based high spectral resolution measurements of Comet Halley in the 6300.3 A spectral region establish this technique as a powerful and low cost method for determining cometary H2O production rates. A model on the line profile to be expected from the O1D coma emission indicates that ground-based observations with fields of view on the comet of about 1 x 105 km measure profiles characteristic of O1D from the H2O parent. As the field of view increases, the signature of more energetic O1D ejected by CO2 and CO photolysis may become evident in the wings of the 6300.3 A line profile. The width of the 6300.3 A O1D profile is found to be equal to the ejection speed of O1D following H2O photolysis for fields of view to 1 x 106 km. Halley observations show that cometary NH2 (0,8,0) emissions do not contaminate the cometary O1D signature when the spectral resolution is 2O production rates determined by scaling the measured O1D production rates according to the H2O photolysis branching ratio are in good agreement with other measurements. Minor upgrades in the tracking and spectral resolution capabilities of observatories now dedicated to measurements of the terrestrial airglow would be useful for expansion of the high resolution cometary O1D data base - to include comets that come with less fanfare than did Halley.