THE INTERACTION OF COSMIC RAYS WITH DIFFUSE CLOUDS

Abstract

We study the change in cosmic-ray pressure, the change in cosmic-ray density,\nand the level of cosmic-ray induced heating via Alfven-wave damping when cosmic\nrays move from a hot ionized plasma to a cool cloud embedded in that plasma.\nThe general analysis method outlined here can apply to diffuse clouds in either\nthe ionized interstellar medium or in galactic winds. We introduce a\ngeneral-purpose model of cosmic-ray diffusion building upon the hydrodynamic\napproximation for cosmic rays (from McKenzie & Voelk and Breitschwerdt and\ncollaborators). Our improved method self-consistently derives the cosmic-ray\nflux and diffusivity under the assumption that the streaming instability is the\ndominant mechanism for setting the cosmic-ray flux and diffusion. We find that,\nas expected, cosmic rays do not couple to gas within cool clouds (cosmic rays\nexert no forces inside of cool clouds), that the cosmic-ray density does not\nincrease within clouds (it may slightly decrease in general, and decrease by an\norder of magnitude in some cases), and that cosmic-ray heating (via Alfven-wave\ndamping and not collisional effects as for ~10 MeV cosmic rays) is only\nimportant under the conditions of relatively strong (10 micro-Gauss) magnetic\nfields or high cosmic-ray pressure (~10^{-11} ergs cm^{-3}).\n