Propagation and Chemical Composition of Ultra High Energy Cosmic Rays

Extragalactic cosmic ray protons with an injection spectrum of the typeE show a spectrum on earth with a dip due to the Bethe-Heitler pair production against the photons of the cosmic microwave background. The dip is produced in the energy region 10 − 4 × 10 eV with position and shape that reproduce with high accuracy the spectrum observed experimentally. This interpretation of the observed data predicts the existence of an energy scale that signals a possible transition from galactic to extragalactic cosmic rays. In fact, at energies lower than a characteristic value Ec ≈ 1×10 18 eV, determined by the equality between the rate of energy losses due to pair production and adiabatic losses, the spectrum of cosmic rays flattens in all cases of interest. In this model, the transition from galactic to extragalactic cosmic rays occurs at some energy below Ec, corresponding to the position of the so-called second knee. Another viable explanation of the observed data is based on a completely different approach assuming a mixed composition with protons and nuclei at energies E ≥ 10 eV. This scenario implies a transition from galactic to extragalactic cosmic rays at the ankle energies (∼ 10 eV). In the present paper we will review the main features of the dip model comparing it with the model of transition at the ankle.