Composition of cosmic rays at ultra high energies

We present measurements of the cosmic ray (CR) composition above 1015 eV, performed with the Yakutsk extensive air shower array. Almost 42,000 events above 1017 eV observed by Cherenkov detectors are selected for the analysis of the depth of maximum of the longitudinal development of air showers induced by CRs. The interpretation of these results in terms of CR mass composition is given. It is shown that mean logarithm of the CR atomic number 〈lnA(ϵ)〉 is characterized by the peak value 〈lnA〉 ≈ 2.5 achieved at ϵ ∼ 1017 eV and its substantial decrease within the energy interval 1017 − 1018 eV to 〈lnA〉 ≈ 1 at ϵ > 1018 eV. Such a behavior can be considered as indication for the transition from galactic CR component, which is produced in galactic supernova remnants, to extragalactic CR component at ϵ = 1017 − 1018 eV.