A cosmic-ray supported galactic corona

The density of gas perpendicular to the galactic plane is examined in a model in which support is provided by cosmic rays which are diffusing out of the galactic disk. A characteristic density which applies to distances several kpc from the plane is found. This gas is exposed to background radiation both from the Galaxy and from outside the Galaxy. The extragalactic background dominates at far-ultraviolet and greater frequencies. The gas is heated by photoionization and cosmic rays and is cooled by line radiation; the equilibrium temperature is about 10,000 K. The gas is primarily ionized by the radiation field, which ionizes the hydrogen throughout the region from 1 to 5 kpc from the plane. The ionization balance of He, C, N, O, and Si in the corona is solved. The column densities of C IV and Si IV are in approximate agreement with the observed values. C IV is expected to become abundant at distances greater than about 1 kpc from the galactic plane and N(C IV)/N(Si IV) is expected to be similar along different lines of sight, in accord with observations. The density of O VI in the C IV region is expected to be small.