The Wisconsin Ha Mapper facility was used to obtain spectra of [O i] l6300 in three directions that sample the warm ionized component of the interstellar medium at the Galactic midplane and at pc. Weak z Ӎ Ϫ300 interstellar [O i] emission was clearly detected toward all three directions, with [O i] l6300/Ha 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]/Ha ratios suggest that most of the Ha 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.

We have used the Wisconsin Hα Mapper (WHAM) facility to measure the interstellar Hα emission toward the high Galactic latitude O star HD 93521 (ℓ = 183. • 1, b = + 62. • 2). Three emission components were detected along this line of sight. These components have radial velocities of -10 km s −1 , -51 km s −1 , and -90 km s −1 with respect to the local standard of rest (LSR) and have Hα intensities of 0.20 R, 0.15 R, and 0.023 R, respectively, corresponding to emission measures of 0.55 cm −6 pc, 0.42 cm −6 pc, and 0.06 cm −6 pc. We have also measured the Hα emission toward the direction ℓ = 148. • 5, b = + 53. • 0, which lies in the region of exceptionally low H I column density known as the Lockman Window. An emission component is detected in this direction at -1 km s −1 (LSR) with an intensity of 0.20 R (0.55 cm −6 pc). In addition, we studied the direction ℓ = 163. • 5, b = + 53. • 5. No Galactic emission was detected along this line of sight, and upper limits on the possible intensity of Galactic emission toward this direction are 0.11 R at the LSR and 0.06 R at -50 km s −1 . As part of the process of separating the interstellar from the terrestrial emission, we also detected and characterized twelve faint (∼0.03-0.15 R), unidentified atmospheric lines present in WHAM Hα spectra. Lastly, we have used WHAM to obtain [O I] λ6300 spectra along the line of sight toward HD 93521. We do not conclusively detect

We have used the Wisconsin H Mapper facility to measure the [O i] 6300/H line intensity ratios for four O star H ii regions: S27 (observation coordinates l = 6=3, b = +23=6), S252 (l = 190=1, b = +0=6), S261 (l = 194=1, b = À1=9), and S264 (l = 195=1, b = À12=0). We find that the ratios range from 0.0015 to 0.0053. These results are roughly a factor of 10 lower than measured [O i]/H ratios in directions that sample the warm ionized component of the interstellar medium. This difference implies a significantly lower hydrogen ionization ratio n(H +)/n(H 0) or higher electron temperature in the diffuse ionized gas compared with that in the bright discrete O star H ii regions.