Interpreting the cosmic ray composition

The detailed pattern of elemental abundances in the Galactic Cosmic Rays is well determined at energies of a few GeV per nucleon. After correction for propagation effects the inferred source composition shows significant devi ations from the standard pattern of Galactic elemental abundances. These de viations, surprisingly overabundances of the heavy elements relative to Hyd rogen, are clearly a significant clue to the origin of the cosmic rays, but one which has proven very difficult to interpret. We have recently shown that the "standard" model for the origin of the bulk of the Galactic cosmic rays, namely acceleration by the diffusive shock ac celeration process at the strong shocks associated with supernova remnants, can quantitatively explain all features of the source composition if the a cceleration occurs from a dusty interstellar medium. This success must be r egarded as one of the stronger pieces of evidence in favour of the standard model.