Observations recently published by Reynolds show that there is diffuse ionized gas (DIG) both in the Galactic plane and at high Galactic latitudes. Relative to normal H II regions, this gas shows strong [S II]/Hα, normal [N II]/Hα, and very weak [O III]/Hα. Models are presented for this gas, assuming that it is photoionized by O stars in the Galactic plane, that the ionized zone is in a steady state as regards both ionization and thermal balance, and that the DIG is also responsible for the pulsar dispersion measures. It is possible to obtain an excellent fit to the observations with a plausible mixture of spectral types for the ionizing stars. It is also shown that shocks are unlikely to be producing the diffuse emission lines.
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J. S. Mathis (1986) studied this question.