LAUSR

Abstract The paper presents the results of the longitudinal development of extensive air showers ( X max ) of ultra-high energies and mass composition of cosmic rays. The measurements of X max are based on data from observations of the Cherenkov radiation at the Yakutsk array for the period 1974–2014. The cascade curves of individual showers and the depth of maximum X max were reconstructed over the energy range 1016–5.7 · 1019 eV. It is shown that the displacement rate of the parameter d X max /dE in the atmosphere is nonlinear and depends on the energy. Such a feature indicates a change in mass composition, which is confirmed by fluctuations of X max in this energy region. The composition of cosmic rays was determined by interpolation using the QGSJetII-04 model.