THE SOURCES OF COSMIC-RAYS

Recent studies of the mass composition of cosmic rays as a function of energy by the MACRO collaboration support our view that the well known steepening of the overall cosmic ray spectrum at 3 x 10(15) eV is not in fact due to Galactic modulation, as has been commonly assumed. Instead, we argue that the Galactic modulation-a reduction in trapping efficiency for the Galaxy as a whole due to the Galactic magnetic field being insufficient-does not become important until a rigidity of about 3 x 10(17) eV. This rigidity is in much better accord with our recently improved knowledge of the properties of the Galactic magnetic field-its large-scale topology and its scale length for irregularities (this important scale length having been found to a similar value for other galaxies, too). The consequences are that there must be at least two main Galactic sources for the cosmic radiation in the energy range 10(9) eV to 10(19) eV.