A Comparison of Diffuse Ionized and Neutral Hydrogen Away from the Galactic Plane: H alpha -emitting H i Clouds

We compare velocity-resolved maps of the interstellar Hα emission at 1° angular resolution within a 10° × 12° region of the sky, and 3' resolution within a 1° × 1° region, with corresponding maps of 21 cm emission. The results indicate that at least 30% of the Hα background and 10%-30% of the 21 cm emission are spatially and kinematically associated with "clouds" containing both neutral and ionized hydrogen. These Hα-emitting H I clouds, identified as prominent Hα and 21 cm emission enhancements of several degrees extent on narrow (12 km s⁻¹) velocity interval maps, have H I column densities that range from about 2 × 10¹⁹ cm⁻² to 2 × 10²⁰ cm⁻² and emission measures from 2 cm⁻⁶ pc to 10 cm⁻⁶ pc. Their radial velocities suggest distances |z| from the Galactic midplane that range from |z| ≲ 100 pc to |z| ≈ 1 kpc, with a |z|-distribution that is much thicker than that of the non-Hα-emitting H I. The high |z| Hα-emitting H I clouds identified on these maps are approximately 40% ionized, have densities n ≈ 0.2-0.3 cm⁻³, and are associated with large filament and loop structures. The relationship between the Ho and the H⁺ within these clouds is not clear; however, the available data suggest that the neutral and ionized components of each cloud are spatially separated. If the clouds are photoionized, the Hα surface brightnesses imply that at |z| ≈ 1 kpc the strength of the incident Lyman continuum flux 4πJ ≈ 2 × 10⁶ photons cm⁻² s⁻¹.