Galactic and Extragalactic Propagation of Cosmic Rays

Abstract

A model for the origins of cosmic radiation is examined in which nonsolar primaries below ${10}^{17}$ eV are assumed to come from supernovae within the spiral arms of our galaxy, and those at higher energies are attributed to extragalactic sources. Supernova-produced particles are taken as diffusing first in the spiral arm region, then leaking into the galactic halo where they travel with a larger diffusion mean free path, and eventually diffusing into extragalactic space. These particles would be observed with the characteristic energy and mass spectra with which they are injected into the spiral arms, except in the energy range ${10}^{15}$-${10}^{17}$ eV where they begin to see longer effective mean free paths and to escape more easily from the spiral arms. The energy spectrum is consequently steepened in this range and the abundance of primaries shifted towards heavier nuclei. Above approximately ${10}^{17}$ eV the flux from supernovae falls below that of another cosmic-ray population originating in extragalactic sources and taken as diffusing throughout the local supercluster of galaxies. The lifetimes of these primaries are sufficiently long that most of the higher energy particles will have photodecayed into protons. Parameters in the computations are chosen to fit cosmic-ray observations, to minimize the total cosmic-ray energy required, and to conform reasonably with current astronomical speculation. They result in a flux in the galactic halo almost one order of magnitude less intense than in the spiral arms, and that in the supercluster almost another two orders of magnitude lower. The galactic sources are required to furnish an average of over ${10}^{48}$ ergs per year in cosmic-ray energy.