CHARGE-EXCHANGE IN THE SOLAR WIND-COMET INTERACTION

The interaction of the solar wind with the neutral environment of come ts takes place by means of a number of microscopic processes. Photoion ization of the neutral particles by solar UV radiation allows a relati vely simple analytical treatment and has been well described in theore tical models. However, it is well known that the charge exchange of pr otons with heavy cometary molecules is an even more efficient physical mechanism. At the time of the observed spacecraft encounters with the comets Halley and Giacobini-Zinner the charge exchange rate was of th e same order as the photoionization rate. We develop an analytical mod el of the interaction of the supersonic solar wind with a cometary env ironment to include charge exchange, and we discuss the relative impor tance of photoionization and charge exchange in slowing down the plasm a flow. It is shown that the main effect of charge exchange near comet s can be accounted for by a simple increase in the total effective ion ization rate and a corresponding rescaling of the interaction pattern. As a result of the small proton to pickup ion mass ratio at comets, t he dynamical differences in the pickup process by photoionization and charge exchange are small. For realistic solar wind and cometary envir onment parameters the rescaling factor for the interaction pattern is quite large because charge exchange is twice as efficient as photoioni zation. However, as a result of the nonmonotonic dependence of the dis tance to the sonic point on the ionization rate, the shock position (f or M = 2) is only shifted by a factor 1.14. The corresponding correcti on to the neutral density is shown to be small, less than 7%, and is f ound to be almost constant at large distances from the comet.