The nature of the interstellar medium

The lifetime, isotropy, and composition of cosmic rays are determined to a significant extent by the structure of the Interstellar Medium (ISM). Recent observational and theoretical work has led to a picture of the ISM in which most of the mass is concentrated in cold (T < or approximately 102 K) clouds, whereas most of the volume is occupied by warm (T approximately 104 K) and hot (T approximately 10sup(5.7) K) gas. The gas in the galactic disk, which has a thickness of order 200 pc, exchanges mass and energy with that in the halo relatively efficiently. This three phase model is consistent with much of the existing data on the ISM, but further observational confirmation is required to establish its validity. Here I shall summarize three phase models of the ISM, focussing on the role of supernova remnants (SNRs) in generating hot gas in the galaxy and the mechanisms by which a steady state can be maintained