The ionospheres and plasma tails of comets

The paper reviews the current state of knowledge about cometary plasma (type I) tails and ionospheres. Observational statistics for type I tails are examined along with spectroscopic observations of plasma tails, identified ion species in such tails, and the morphology of cometary plasma tails and ionospheres. Evidence for a strong interaction between comets and the solar wind is evaluated on the basis of observations of plasma-tail orientations, large accelerations of tail structures, and correlations between disturbances in type I tails and solar-wind or geomagnetic disturbances. The use of comets as solar-wind probes is discussed, the nature of comet-solar-wind interactions is investigated, and ionization sources for cometary gases are considered. Hydrodynamic models of comet-solar-wind interaction are summarized, and the structure and ion chemistry of cometary ionospheres are studied. Observations suggesting that significant magnetic fields are associated with comets are briefly reviewed and interpreted.