The cometary atmosphere and its interaction with the solar wind

The present state of knowledge of cometary atmospheres and their interaction with the solar winds is assessed. Current models of the magnetospheres and ionospheres of comets are based on either an atmospheric chemistry approach, minimizing the effects of atmospheric dynamics and thermodynamics, while the other method takes the exact opposite viewpoint and roles and significance are reversed. A major difficulty in completing the models is a lack of data on the chemical composition of the cometary nucleus, although UV observations have revealed the H, C, O, and S elemental abundance ratios. It is suggested that further studies of collisional ionization by energetic electrons be performed to characterize processes in the cometary atmosphere. Hydronium may be the dominat molecule in the inner coma, if the assumption that the ionosphere is a magnetic field-free cavity separated from the solar wind by an unyielding tangential discontinuity surface is accurate. Particular observations and theoretical attention are recommended for the solar wind interaction in the intermediate range, i.e., 2.5-5 AU, when the cometary atmosphere develops.