Detection of [O [CSC]i[/CSC]] λ6300 Emission from the Diffuse Interstellar Medium

The Wisconsin Hα Mapper facility was used to obtain spectra of [O I] λ6300 in three directions that sample the warm ionized component of the interstellar medium at the Galactic midplane and at z ≃-300 pc. Weak interstellar [O I] emission was clearly detected toward all three directions, with [O I] λ6300/Hα intensity ratios for individual radial velocity components that range from less than 0.01 to approximately 0.04. According to photoionization models of the warm ionized medium, these [O I]/Hα ratios suggest that most of the Hα originates from density-bounded, nearly fully ionized regions along the lines of sight rather than from partially ionized H I clouds or layers of H II on the surfaces of H I clouds.