Dip in UHECR and Transition from Galactic to Extragalactic Cosmic Rays

The dip is a feature in the diffuse spectrum of UHE protons in energy range 1 × 10 − 4 × 10 eV, which is caused by electron-positron pair production on CMB photons. Calculated for power-law generation spectrum with index γg = 2.7, the shape of the dip is confirmed with high accuracy by data of Akeno AGASA, HiRes, Yakutsk and Fly’s Eye detectors. The predicted shape of the dip is robust: it is valid for the rectilinear and diffusive propagation, for different discretenesses in the source distribution, for local source overdensity and deficit, for source inhomogeneities on scale l < ∼ 100 Mpc etc. Below the characteristic energy Ec ≈ 1× 10 18 eV the spectrum of the dip flattens for both diffusive and rectilinear propagation, and more steep galactic spectrum becomes dominant at E < Ec. The energy of transition Etr < Ec approximately coincides with the position of the second knee E2kn observed in the cosmic ray spectrum. The critical energy Ec is determined by the energy Eeq = 2.3× 10 eV, where adiabatic and pair-production energy losses are equal. Thus, position of the second knee is explained by proton energy losses on CMB photons.