WEAKLY IONIZED COSMIC GAS - IONIZATION AND CHARACTERIZATION

Since collective plasma behavior may determine important transport pro cesses (e.g., plasma diffusion across a magnetic field) in certain cos mic environments, it is important to delineate the parameter space in which weakly ionized cosmic gases may be characterized as plasmas. In this short note, we do so. First, we use values for the ionization fra ction given in the literature, wherein the ionization is generally ass umed to be due primarily to ionization by cosmic rays. We also discuss an additional mechanism for ionization in such environments, namely, the photoelectric emission of electrons from cosmic dust grains in an interstellar FUV radiation field. Simple estimates suggest that under certain conditions this mechanism may dominate cosmic ray ionization, and possibly also the photoionization of metal atoms by the interstell ar FUV field, and thereby lead to an enhanced ionization level.