Galactic cosmic rays and magnetic fields

An often overlooked component of the ISM is the relativistic phase. Although cosmic rays are collisionless particles they form a gas whose energy density is comparable to that of the other gaseous phases. The magnetic field of the Galaxy, though not a gas (but rather a byproduct of it), falls into a similar category. The cosmic rays and the Galactic magnetic field are in approximate pressure equilibrium with the other phases of the ISM. It is therefore appropriate to treat them as legitimate non-gravitating phases of the ISM. This review will be carried out in the context of the above remarks. The latest observational and theoretical work in these areas will be covered and connections with the overall ISM emphasized.