The energy spectrum of cosmic rays in the range E ∼ 10 15 eV to 6 × 10 19 eV has been studied using the air Cherenkov light detectors of the Yakutsk array. The total flux of photons produced by relativistic electrons (including positrons as well, hereafter) of extensive air showers in the atmosphere is used as the energy estimator of the primary particle initiating a shower. The resultant differential flux of cosmic rays exhibits, in accordance with previous measurements, a knee and ankle features at energies 3 × 10 15 and ∼ 10 19 eV, respectively. A comparison of observational data with simulations is made in the knee and ankle regions in order to choose the models of galactic and extragalactic components of cosmic rays which describe better the energy spectrum measured.

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The arrival direction distribution of the highest-energy cosmic rays with respect to Galactic latitude has been analysed using both the Yakutsk and the available world data from extensive air shower arrays. A north-south asymmetry is indicated in the distribution observed with the Yakutsk array at 10 19 eV in the outer Galaxy direction. A comparison with trajectory calculations for charged particles in the Galactic magnetic field indicates that the observed effect can be explained by a contribution from heavy nuclei originating in the Galactic disk to an extragalactic isotropic primary flux. The mean-mass number of Galactic nuclei estimated using the model is log 10 (A) = 1.37 ± 0.2.

The energy spectrum of UHECRs is discussed on the basis of the Yakutsk array database analysis. In the region E=0.1 to 30 EeV the showers are detected under trigger-500, while at energies above 30 EeV the whole acceptance area for trigger-1000 is used in order to utilize all the data available in the region of GZK cutoff.Comment: Invited talk at CRIS2004: GZK and surroundings, Catania, Italy, 31.05.04. To appear in Nucl. Phys. B Proc. Supp

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