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