PREDICTIONS OF ION PRODUCTION RATES AND ION NUMBER DENSITIES WITHIN THE DIAMAGNETIC CAVITY OF COMET 67P/CHURYUMOV-GERASIMENKO AT PERIHELION

We present a one-dimensional ion chemistry model of the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko, the target comet for the ESA Rosetta mission. We solve the continuity equations for ionospheric species and predict number densities of electrons and selected ions considering only gas-phase reactions.We apply the model to the subsolar direction and consider conditions expected to be encountered by Rosetta at perihelion (1.29 AU) in 2015 August. Our default simulation predicts a maximum electron number density of ~8 x 10 super(4) cm super(-3) near the surface of the comet, while the electron number densities for cometocentric distances r > 10 km are approximately proportional to 1/r super(1.23) assuming that the electron temperature is equal to the neutral temperature. We show that even a small mixing ratio (~0.3%-1%) of molecules having higher proton affinity than water is sufficient for the proton transfer from H sub(3)O super(+) to occur so readily that other ions than H sub(3)O super(+), such as NH sub(4) super(+) or CH sub(3)OH sub(2) super(+), become dominant in terms of volume mixing ratio in part of, if not throughout, the diamagnetic cavity. Finally, we test how the predicted electron and ion densities are influenced by changes of model input parameters, including the neutral background, the impinging EUV solar spectrum, the solar zenith angle, the cross sections for photo-and electron-impact processes, the electron temperature profile, and the temperature dependence of ion-neutral reactions.