Ionization mechanisms of the intercloud medium

A generalized treatment of the ionization equilibrium of the intercloud medium is developed, which is model-independent in the steady-state approximation, and which is suited to an interpretation of ultraviolet absorption data. It is found that the most satisfactory models require the presence of density inhomogeneities along the line of sight. In all models considered for lambda Sco, 2-MeV cosmic rays and X rays are found to lead to large disagreements with the observations, and data on upsilon Sco, alpha Leo, and other stars seem to support this conclusion. Therefore, it would seem that these cannot be the physical agents responsible for the bulk of the ionization of the gas. An alternative model is developed, in which the ionization below 24.58 eV is provided by ultraviolet photons. This model appears to satisfy theoretical requirements, and leads to good agreement with the data.