THE INTERACTION OF COSMIC RAYS WITH DIFFUSE CLOUDS

We study the change in cosmic-ray pressure, the change in cosmic-ray density, and the level of cosmic-ray-induced heating via Alfvén-wave damping when cosmic rays move from a hot ionized plasma to a cool cloud embedded in that plasma. The general analysis method outlined here can apply to diffuse clouds in either the ionized interstellar medium or in galactic winds. We introduce a general-purpose model of cosmic-ray diffusion building upon the hydrodynamic approximation for cosmic rays (from McKenzie & Völk and Breitschwerdt and collaborators). Our improved method self-consistently derives the cosmic-ray flux and diffusivity under the assumption that the streaming instability is the dominant mechanism for setting the cosmic-ray flux and diffusion. We find that, as expected, cosmic rays do not couple to gas within cool clouds (cosmic rays exert no forces inside of cool clouds), that the cosmic-ray density does not increase within clouds (it may decrease slightly in general, and decrease by an order of magnitude in some cases), and that cosmic-ray heating (via Alfvén-wave damping and not collisional effects as for ∼10 MeV cosmic rays) is only important under the conditions of relatively strong (10 μG) magnetic fields or high cosmic-ray pressure (∼10 −11 erg cm −3 ).