Observed properties of the diffuse HI

摘要

We review observations of the physical properties of the diffuse ISM HI components, namely the Cold and Warm Neutral Media (CNM and WNM). There is somewhat more WNM than CNM, and at least half of the WNM is not thermally stable. The CNM has typical turbulent Mach number 3. Magnetic energy densities in the CNM considerably exceed thermal energy densities, and are comparable to turbulent energy densities. Magnetic fields in the CNM are comparable to those in the Warm Ionized Medium and the volume-average from synchrotron emissivity. We identify a probable new source of turbulence for the diffuse ISM. We discuss one very cold cloud that has considerable internal turbulence and, because of its extreme thinness similar to 0.05 pc, a turbulent crossing time of only similar to 5 x 10(4) yr.