ON THE ELECTRON-TO-NEUTRAL NUMBER DENSITY RATIO IN THE COMA OF COMET 67P/CHURYUMOV-GERASIMENKO: GUIDING EXPRESSION AND SOURCES FOR DEVIATIONS

We compute partial photoionization frequencies of H2O, CO2, and CO, the major molecules in the coma of comet 67P/Churyumov-Gerasimenko, the target comet of the ongoing ESA Rosetta mission. Values are computed from Thermosphere Ionosphere Mesosphere Energy and Dynamics/Solar EUV Experiment solar EUV spectra for 2014 August 1, 2015 March 1, and for perihelion (2015 August, as based on prediction). From the varying total photoionization frequency of H2O, as computed from 2014 August 1 to 2015 May 20, we derive a simple analytical expression for the electron-to-neutral number density ratio as a function of cometocentric. and heliocentric distance. The underlying model assumes radial movement of the coma constituents and does not account for chemical loss or the presence of electric fields. We discuss various effects/processes that can cause deviations between values from the analytical expression and actual electron-to-neutral number density ratios. The analytical expression is thus not strictly meant as predicting the actual electron-to-neutral number density ratio, but is useful in comparisons with observations as an indicator of processes at play in the cometary coma.