This award provides instrumentation and support for a northern hemisphere survey of the faint interstellar H-alpha (6563 Angstroms) hydrogen recombination line with a high throughput Fabry-Perot spectrometer. These observations will provide for the first time a detailed picture of the distribution and kinematics of the warm, ionized component of the diffuse interstellar medium. In many respects this survey of the diffuse ionized hydrogen through the optical H-alpha line will be analogous to earlier all- sky surveys of the neutral hydrogen made through the radio 21 cm line. The scientific return should be substantial, since this ionized gas is a major, but poorly understood component of the interstellar medium, which has a mass approximately one-third that of the neutral atomic hydrogen and accounts for a large fraction of the interstellar power budget; it also appears to be the dominant state of interstellar matter in the lower Galactic halo. This survey therefore will have a significant impact on our understanding of the composition and structure of the interstellar medium and the principal processes of ionization and heating within the Galactic disk and halo. Furthermore, a survey of this component is needed for accurate interpretations of sky survey data obtained at radio, microwave, infrared, ultraviolet, and even gamma-ray wavelengths.
