Composition of cosmic rays at ultra high energies

► Measurements of CR composition above 1015eV by Yakutsk EASA are presented. ► Interpretation of these results in terms of CR mass composition is given. ► Mean logarithm of the CR atomic number has the peak value ∼2.5 achieved at ∼1017eV. ► Then it substantialy decrease within the energy interval 1017−1018eV to ∼1. ► This behavior indicate for the transition from galactic CR component to extragalactic at 1017−1018eV. We present measurements of the cosmic ray (CR) composition above 1015eV, performed with the Yakutsk extensive air shower array. Almost 42,000 events above 1017eV 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∼1017eV and its substantial decrease within the energy interval 1017−1018eV to 〈lnA〉≈1 at ϵ>1018eV. 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=1017−1018eV.