The structure of cometary ionospheres. I. H₂O dominated comets

A hydrodynamic description of the density distributions of the ionic products (H₂O⁺, H₃O⁺ CO⁺, and HCO⁺) of H₂O and CO in the atmosphere of an H₂O-dominated comet having 10% CO is given. Two models are discussed. In the first, the effects of photodissociation, photoionization and gas phase reactions are considered. In the second, the effect of an internal ionization source is also included. It is found that while the observed morphology of the H₂O⁺ coma of Comet Kohoutek (1973f) cannot be explained by gas phase reactions alone, it is consistent with the presence of a strong internal source of ionization, presumably an electric current flowing through the inner coma. In the latter case the number densities n(H₂O⁺) and n(CO⁺) remain more or less contant within the the scale lengths against collisional dissociation by the electron flux of their neutrals (H₂O and CO), which may be {less-than or approximate}10³ km. Furthermore, while the abundance ratio of CO⁺ to H₂O⁺ remains considerably less than unity when the ionization source is neglected, it is significantly larger than unity when the internal ionization source is included. Consequently, even the so-called "CO-rich" comets may in reality be H₂O dominated, provided a strong internal source of ionization is operative.