1999AJ....118.1252V

Analysis of Galactic neutral hydrogen emission profiles that have been corrected for sidelobe radiation confirm the existence of three distinct component line width regimes identified by Verschuur & Magnani in 1994. In addition, a fourth becomes recognizable in the data in directions of low total column density. The line width regimes are around 50 km.s^–1 (component 1a), 31 km.s^–1 (component 1b), 13 km.s^–1 (component 2), and 5.2 km.s^–1 for the narrow lines arising from cool H I (component 3). In this paper, the new data are presented and compared with previously published results. The possible origin of the distinct line width regimes is briefly examined, and it is concluded that a new interpretation is needed, one that involves a plasma phenomenon known as the critical ionization velocity, which will be fully discussed in a subsequent paper.