Ion-Neutral Collisions and the Propagation Distance of Interstellar Turbulence

Plasma turbulence exists throughout the pervasive ``Diffuse Ionized Gas'' (DIG) phase of the interstellar medium. The generators of this turbulence are unknown, but are widely believed to be supernova remnants. The DIG is a partially ionized plasma, since at least half of the helium is neutral, and perhaps a small fraction of the hydrogen. The damping rate of MHD waves on neutral helium in the DIG is of order $(3-6) \times 10^{-12}$ Hz. With a corresponding Alfven speed of 23 kilometers per sec, interstellar turbulence should be strongly damped within a parsec of its sources. This result is strongly discordant with the observation that turbulence in the DIG seems relatively uniformly distributed and is found far from obvious turbulence ``generators'' such as supernova remnants and star formation regions. I discuss possible ways in which turbulence could be generated or propagate through the highly lossy interstellar medium.