The origin of the ankle

The differential intensity of cosmic radiation shows a sequence of depressions referred to as "knees" in a large energy band above 10^15 eV. The global depression entailed in the complete spectrum with respect to the extrapolated intensity based on low energy data amounts to a maximum factor of 8, occurring at 5 x 10^18 eV, where flux measurements exhibit a relative minimum, referred to as the "ankle". It is demonstrated by a full simulation of cosmic ray trajectories in the Galaxy that the intensity minimum around the ankle energy is primarily due to the nuclear interactions of the cosmic ions with the interstellar matter and to the galactic magnetic field. "Ankles" signal the onset energies of the rectilinear propagation in the Milky Way at the Earth, being for example, 4 x 10^18 eV for helium and 6 x 10^19 eV for iron. The ankle, in spite of its notable importance at the Earth, is a local perturbation of the universal spectrum which, between the knee and the ankle, decreases by a round factor 10^9, regaining its unperturbed status above 10^19 eV.