Charge exchange in the solar wind‐comet interaction

Abstract

The interaction of the solar wind with the neutral environment of comets takes place by means of a number of microscopic processes, Photoionization of the neutral particles by solar UV radiation allows a relatively simple analytical treatment and has been well described in theoretical models. However, it is well known that the charge exchange of protons 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 the same order as the photoionization rate. We develop an analytical model of the interaction of the supersonic solar wind with a cometary environment to include charge exchange, and we discuss the relative importance of photoionization and charge exchange in slowing down the plasma flow. It is shown that the main effect of charge exchange near comets can be accounted for by a simple increase in the total effective ionization rate and a corresponding rescaling of the interaction pattern. As a result of the small proton to pickup ion mass ratio at comets, the dynamical differences in the pickup process by photoionization and charge exchange are small. For realistic solar wind and cometary environment parameters the rescaling factor for the interaction pattern is quite large because charge exchange is twice as efficient as photoionization. However, as a result of the nonmonotonic dependence of the distance to the sonic point on the ionization rate, the shock position (for M = 2) is only shifted by a factor 1.14. The corresponding correction to the neutral density is shown to be small, less than 7%, and is found to be almost constant at large distances from the comet.