Mass content of ultrahigh-energy cosmic rays within different time periods

Estimates obtained for the average atomic number 〈lnA〉 of nuclei of primary particles with energies in the region of E ₀ ? 10¹⁵ eV over the past 36 years at the Yakutsk array and other arrays worldwide for studying extensive air showers are presented. It is shown that these estimates are markedly different with in different time periods. Earlier than 1996, the composition of cosmic rays in the energy range of 5 × 10¹⁵–10¹⁸ eV was markedly lighter than in later years. After 2008, there appeared a trend toward a decrease in 〈lnA〉. This is likely to be a manifestation of some explosive process in the Milky Way Galaxy after 1996.