Collisional quenching of cometary emission in the 18 centimeter OH transitions

Abstract

A model of collisional quenching of the OH 2Pi(3/2) J = 3/2 Lambda doublet in cometary comae is presented. It is found that collisions with ions and electrons in the outer coma have a strong quenching effect on the Swings-effect inversion of the Lambda doublet that is responsible for the OH radio emission at 18 cm wavelength. For the conditions of Halley's comet, collisional quenching should lead radio observers to systematically underestimate the OH parent production rate by a factor of approximately 3 relative to its actual value, and in general, radio-derived production rates should always be less than or equal to UV-derived production rates, which are relatively unaffected by this process. The observation that UV production rates exceed those derived by radio techniques is well known; the direct measurement of this ratio, using a consistent coma model, should provide information about the ion and electron content of the cometary coma.