The Origin of Cosmic Rays

The isotropy and composition of the primary cosmic radiation suggest that cosmic rays are trapped within the galaxy for an average time of the order of 10⁶ years,-a long time compared with the time of escape along straight-line paths, but short compared with the mean life against nuclear collisions with interstellar matter. If one accepts this conclusion, it appears possible to account for the observed properties of cosmic rays under the assumption that cosmic rays acquire their large energies through a gradual acceleration in space, such as suggested by Fermi. In contrast to the original Fermi theory (which denied any possibility of escape from the galaxy), we now find that the energy spectra of protons and heavier nuclei are approximately the same, and that the required injection energies are very modest for all components. We are obliged, however, to assume a much faster rate of acceleration than the original theory required. In this paper we develop in some detail the consequences of the above assumptions on the basis of a specific model, describing the motion of cosmic rays through the galaxy as a random motion between scattering centers represented by moving magnetized clouds. We briefly discuss the astrophysical implications of our assumptions and the plausibility of the model.