Figure 1
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Charged cosmic ray spectrum measured from 
eV to few 1020 eV by cosmic ray observatories. The spectrum features corresponding to inflections points around 1016 eV, 1017 eV, and 1019 eV are highlighted with the “knee”, “2nd knee”, and the “ankle”. UHECRs are in the range of energies above 1018 eV. Taken from [44], see also [45] and references therein.