  Interstellar neutral hydrogen (HI) emission spectra manifest several families
of linewidths whose numerical values (34, 13 & 6 km/s) appear to be related to
the critical ionization velocities (CIVs) of the most abundant interstellar
atomic species. Extended new analysis of HI emission profiles shows that the 34
km/s wide component, probably corresponding to the CIV for helium, is
pervasive. The 34 km/s wide linewidth family is found in low-velocity (local)
neutral hydrogen (HI) profiles as well as in the so-called high-velocity
clouds. In addition, published studies of HI linewidths found in the Magellanic
Stream, Very-High-Velocity Clouds, and Compact High-Velocity Clouds, all of
which are believed to be intergalactic, have noted that typical values are of
the same order. If the critical ionization velocity effect does play a role in
interstellar space it may be expected to produce locally enhanced electron
densities where rapidly moving neutral gas masses interact with surrounding
plasma. Evidence is presented that suggests that this phenomenon is occurring
in interstellar space. It manifests as a spatial association between peaks in
HI structure offset with respect to peaks in high-frequency radio continuum
data obtained with the Wilkinson Microwave Anisotropy Probe.
