On the propagation of cosmic rays in the galaxy

Abstract

An important problem in the study of cosmic rays, under the assumption of a Galactic origin for energies SlO^^eV, is their propagation in the Galaxy, The experimental observations of the cosmic rays, i.e. the energy spectrum, anisotropy, composition, y-rays and long term variations, are reviewed. Parameters of the interstellar medium relevant to the propagation of cosmic rays are discussed. Theories about Galactic cosmic ray sources and cosmic ray acceleration are also discussed. Calculations are presented which indicate the improbability of the compound diffusion model as a reasonable theory for the galactic propagation of cosmic rays. The effect of turbulence in the interstellar medium on the rate of separation of neighbouring magnetic field lines is investigated. It is concluded that cosmic ray propagation is better described by an isotropic three-dimensional diffusion than by a compound diffusion. Plasma effects of the cosmic rays with respect to the generation of Alfven waves are discussed. Cosmic ray generated Alfven waves are able to confine cosmic rays with energies ≤100 GeV in the Galaxy, Model calculations are presented for the propagation of cosmic rays with energy l0(^9)-10(^13) eV in the Galaxy using idealised parameters for the cosmic rays and interstellar medium and random discrete sources at a frequency corresponding to supernova explosions. The most encouraging model suggests that cosmic rays with energy ≤10 GeV are confined to spiral arms "by self generated waves, cosmic rays with energy > 10 GeV and <300 GeV are confined within a "leaky box" of Galactic dimensions "by self generated waves and cosmic rays with energy ≥300 GeV are confined to the spiral arms "by interstellar turbulence.