Nonstationary phenomena in cosmic rays with $E_0\leq10^{18}$ eV according to the data of the Yakutsk EAS array

The energy spectrum of cosmic rays and the fraction of muons with the threshold 1.0sec$\theta$ GeV in the total number of charged particles in extensive air showers with energy $E_0\geq10^{17}$ eV according to Yakutsk array data collected during 35 years of its continuous operation in 1978–2012 have been analyzed. It has been shown that these characteristics are noticeably different in different time periods. Before 1996, the integral intensity of the spectrum at $E_0=10^{17}$ eV varied near one stable position and then began to increase. It increased by $45\pm5\%$ in seven years and, then, began to decrease. This phenomenon was accompanied a similar change in the fraction of muons and was caused by a significant increase in the average weight of the chemical composition of cosmic rays after 1996 as compared to preceding years.