We have explored the possibility that the diffuse ionized gas of the Galaxy is kept ionized by O stars, and that such gas consists of the dilute portions of O star H II regions in a multiple component interstellar medium. To make this exploration, we calculated the H II region structures for the known Galactic O stars and compared the integral properties of the ensemble (dispersion and emission measures) with the observational data. Our model assumes a smooth intercloud (or more precisely extracloud) medium whose density depends only on z, and a statistical distribution of small, opaque clouds. The latter have line of sight mean free path lambda(z) between encounters and are assumed to attenuate the radiation field much as dust grains do, with differential fractional loss dr/lambda in path dr on all sight lines, rather than forming distinctions between shadowed and unshadowed lines of sight. With the simplified assumptions of our standard cloud model, good quantitative agreement is found for the electron density as a function of z and the dispersion measures to pulsars at high and low latitudes as well as quantitative agreement for summary measures such as the mean vertical column density of electrons, the average Halpha surface brightness perpendicular to the Galactic plane, and its decrease at high latitude. The implications of our model to the diffuse interstellar medium and to the distribution of Lyman continuum absorbers in the Galaxy are discussed in detail. We present Aitoff projections of the predicted Halpha surface brightness, the corresponding electron column density, and the residual neutral column density over the sky. We also include views of the ionized gas structure in the solar vicinity, an Halpha view of the Galaxy from above the plane, and two edge views (with and without dust extinction) of the Galaxy.
