The chemical composition of the cosmic radiation around the ankle and the related spectral indices

Antonio Codino, Francois Plouin

Published 2009-11-24, updated 2010-02-28Version 2

Some recent measurements of the chemical composition of the cosmic radiation indicate that at the energy of 3 x 10 **18 eV, around the ankle, light cosmic ions dominate the spectrum as it occurs in the preknee energy region. Taking advantage of a recent theory of cosmic radiation which provides a quantitative explanation of the knee, the second knee and the ankle, the chemical composition of cosmic radiation is explicitly calculated giving individual ion spectra and ion fractions from 10 ** 12 eV to 5 x 10 ** 19 eV. The calculation assumes two components of the cosmic radiation feeding the ion flux at Earth: one originated in the disc volume and another one, called extradisc component, which from the disc boundaries traverses the Galaxy reaching the solar system. Data above 10 ** 17 eV collected during half century of experimentation by Auger, HiRes, Agasa, Akeno, Fly' s Eye, Yakutsk, Haverah Park and Volcano Ranch experiments are reviewed, examined and compared with the theoretical <ln(A)>. The comparison between computed and measured <ln(A)> exhibits a good global accord up to 2 x 10 ** 19 eV except with the HiRes experiment and an excellent agreement in the range 10 ** 15 - 10 ** 17 eV with Kascade, Eas-top, Tunka and other experiments. The accord requires a flux of the extradisc component of 1.8 x 10 ** 14 particles / m ** 2 sr s eV **(1.5) at 10 ** 19 eV, twice that generated by disc sources.