A View of the Galactic H alpha Background: 208 degrees <=l <=218 degrees ; -2 degrees <=b <= +8

Faint, diffuse H-alpha emission from the interstellar medium has been mapped at 1-deg angular resolution and 12 km/s radial velocity resolution within an 11 x 11 deg region of the Galactic plane. The observations reveal that warm, ionized gas is widespread throughout the Galactic disk and that it has a complex morphology with faint, previously unidentified H-alpha emission regions superposed on a more diffuse H-alpha background. The H-alpha intensities indicate that, in the diffuse interstellar medium outside the discrete H II regions, the mean hydrogen ionization rate is about 3.5 x 10 to the 6th/s per sq cm column perpendicular to the Galactic disk and the mean square electron density of about 0.01 per cm to the 6th near the midplane. This warm, ionized gas appears to contain a substantial fraction (perhaps most) of the H(+) in the interstellar medium. The source of this ionization is not understood. Neither the faint emission regions nor the more diffuse emission appear to be associated with traditional H II regions immediately surrounding early-type stars. The high forbidden S II line 6716/H-alpha intensity ratios in the background clearly rule out scattered light as a primary source of the emission.