## 5 Elemental Composition in the \(10^{14}\) to \(10^{18}\) eV Region
At slightly higher energies (\(E\) = \(10^{17.08\pm 0.09}\) eV), the light component shows a hardening of the slope, with the spectral index changing from \(-\)3.25 \(\pm\) 0.06 below this ankle-like structure to \(-\)2.79 \(\pm\) 0.09 above. The positions of the changes of the slope as well as the intensities of the different components depend on the interaction model adopted to interpret the data. The knee in the heavy component seems visible also in the all-particle spectrum, as it is the dominant component. The results obtained by Tunka-133, measuring the Cherenkov light deep in the atmosphere, suggest that the mass composition becomes heavier in the energy range \(10^{16}\)-\(3\times 10^{16}\) eV, then stays heavy till \(10^{17}\) eV, where the composition starts becoming lighter. IceTop/IceCube, exploiting the high-energy muons underground, indicates an increase of \(\langle\ln A\rangle\) in the energy range \(10^{16}\)-3 \(\times\)\(10^{17}\) eV [65]. However, as discussed in the previous sections, the average logarithmic mass of CR \(\langle\ln A\rangle\) is used to describe the evolution of the composition as a function of energy when the mass resolution of the experiments do not allow a discrimination between different mass groups. Figure 7: The energy spectra shown in Figure 6 compared with the results obtained by HAWC in the 10–100 TeV region [63] and by KASCADE–Grande [27] and IceTop/IceCube [29] combined above the PeV. The parametrization of the light component provided by [PERSON] [62] is also shown. In Figure 8 recent measurements of the all-particle energy spectrum down to about 100 PeV by the Pierre Auger Observatory are also reported. These observations suggest that the second knee is not a sharp feature, but a softening that extends in the interval 100-200 PeV [66]. Preliminary results based on the distribution of the depth of the shower maximum are consistent with a spectrum dominated by heavy nuclei in the \(10^{17}\) eV range becoming lighter with increasing energy [67]. The findings of Tunka-133, IceTop/IceCube and Auger are qualitatively in agreement with KASCADE-Grande. Despite the large uncertainty in the absolute composition, a common general trend is reported, composition gets heavier through the knee region and becomes lighter approaching the ankle. In conclusion, the observations of the different ground-based arrays show two conflicting results regarding the maximum energy at which the light component is accelerated in CR sources. This disagreement is summarized in Figure 9 where the ARGO-YB] results are compared with the KASCADE-Grande light and heavy spectra. These results cannot be reconciled and show the existence of a still unknown systematic uncertainty that, as discussed in previous sections, could be due to the different array characteristics (altitude, coverage), the observables used (muons or shower core characteristics), and the dependence on hadronic interaction models. These are certainly among the major sources of systematic errors that affect the interpretation of shower array measurements (for a recent discussion see, for instance, refs. [68; 69]), although recent re-analyses of the KASCADE-Grande data with the latest versions of the post-LHC codes confirm previous results [70]. Important information could be deduced, in principle, by the measurement of the flux of atmospheric neutrinos, sensitive to the spectrum of parent CRs. In particular, the tail of the spectrum of atmospheric neutrinos is mainly shaped by the parent protons rather than by heavier element. As a consequence, we expect different predictions for the flux of atmospheric neutrinos according to ARGO-YB] and KASCADE proton energy spectra, predictions that, in principle, can be checked at energies \(E_{v}\geq 100\) TeV if the atmospheric neutrinos could be properly identified. Unfortunately, in this energy region the total neutrino flux detected by IceCube departs from the existing predictions for atmospheric neutrinos suggesting the onset of an astrophysical component.