The acceleration of particles in the vicinity of comets

The various processes capable of accelerating ions in the vicinity of comets as part of the comet/ solar wind interaction are considered. It is concluded that the most important are ion pick-up, adiabatic compression and second order Fermi acceleration in the turbulence associated with the interaction. Model particle spectra are calculated on the assumption that the turbulence is dominated by Alfven waves, and it is shown that the peak associated with ion pick-up remains a distinct feature of the spectrum despite spreading to higher and lower energies. Diffusive shock acceleration and magnetic field reconnection are shown to be relatively unimportant in producing energetic ions.