ON THE NATURE OF 21 CM EMISSION PROFILE STRUCTURE AT HIGH GALACTIC LATITUDE - IMPLICATIONS FOR THE WARM NEUTRAL MEDIUM

Gaussian analysis of neutral hydrogen emission profiles obtained with the Arecibo 309 m, the NRAO 140 ft, and the Hat Creek 25 m radio telescopes reveal three distinct linewidth regimes. Component 1 is typically from 25-50 km/s wide and may be due to Galactic emission in far sidelobes, although this remains to be proven. The bulk of this study concerns Component 2, a pervasive feature approximately 12 km/s wide (full width at half maximum brightness), although somewhat narrower toward the North Polar Spur. If this component is due to a warm neutral medium, its linewidth implies a temperature of approximately 3000 K, substantially lower than many of the canonical values for this medium in the literature. On the other hand, there is evidence to suggest that at least some of this component may be due to the superposition of unresolved cool features in the beam. Component 3 encompasses narrow features with linewidths in the 2-6 km s-1 range with the majority around 3 km s-1.