Galactic Basins of Helium and Iron around the Knee Energy

Abstract

The differential energy spectrum of cosmic rays exhibits a change of slope, called "knee" of the spectrum, around the nominal energy of 3 x 10^15 eV, and individual "knees" for single ions, at different energies. The present work reports a detailed account of the characteristics and the origin of the knees for Helium and Iron. Current observational data regarding the magnetic field, the insterstellar matter density, the size of the Galaxy and the galactic wind, are incorporated in appropriate algorithms which allow to simulate millions of cosmic-ion trajectories in the disk. Bundles of ion trajectories define galactic regions called basins utilized in the present analysis of the knees. The fundamental role of the nuclear cross sections in the origin of the helium and iron knees is demonstrated and highlighted. The results of the calculation are compared with the experimental data in the energy interval 10^12 eV - 5 x 10^17 eV. There is a fair agreement between the computed and measured energy spectra of Helium and Iron; rather surprisingly their relative intensities are also in accord with those computed here. The results suggest that acceleration mechanisms in the disk are extraneous to the origin of the "knees".