Ion pickup at comets Original Research Article

The interaction of a comet with the solar wind starts with ion pickup. This key process is also important in other solar system contexts wherever neutral particles become ionized and injected into a flowing plasma such as at Mars, Venus, Io, Titan and interstellar neutrals in the solar wind. At comets, ion pickup removes energy from the solar wind and puts it into cometary particles, via plasma waves. Here we review what comets have shown us about how this process operates, and briefly look at how this can be applied in other contexts. We use in-situ measurements from the four comets visited by spacecraft so far- Giacobini-Zinner, Halley, Grigg-Skjellerup and Borrelly, to illustrate the process in action.