The effect of electron collisions on rotational excitation of cometary water

Abstract

The e-H2O collisional rate for exciting rotational transitions in cometary water is evaluated for conditions found in Comet Halley. The e-H2O collisional rate exceeds that for excitation by neutral-neutral collisions at distances exceeding 3000 km from the cometary nucleus, in the case of the O sub 00 yields 1 sub 11 transition. The estimates are based on theoretical and experimental studies of e-H2O collisions, on ion and electron parameters acquired in-situ by instruments on the Giotto and Vega spacecraft, and on results obtained from models of the cometary ionosphere. The contribution of electron collisions may explain the need for large water-water cross-sections in models which neglect the effect of electrons. The importance of electron collisions is enhanced for populations of water molecules in regions where their rotational lines are optically thick