Recent H I absorption surveys are used to derive H I temperatures T(H I) and their variation within a number of H I shells. In these shells T(H I) is in the domain of cold, neutral gas, and ranges down to 35 K; densities are about 2/cu cm. Outside the relatively dense shells, H I temperatures are much higher. It is considered whether temperatures increase toward the edges of H I shells; no systematic trend is apparent. Low-energy cosmic rays are probably powerful enough to account for the observed H I temperatures in shells associated with regions of enhanced synchrotron emission. Low-energy X-rays are probably not powerful enough to heat the gas in the Eridanus H I shell.
Heiles, C.
1982, The Astrophysical Journal, 262, 135-141
http://adsabs.harvard.edu/abs/1982ApJ…262..135H