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

The Earth's magnetic field effect on the azimuthal distribution of extensive air showers (EAS) of cosmic rays has been evaluated using a bulk of the Yakutsk array data. The uniform azimuthal distribution of the EAS event rate is rejected at the significance level 10 -14 . Amplitude of the first harmonics of observed distribution depends on zenith angle as A 1 ≈ 0.2×sin 2 θ and is almost independent of the primary energy; the phase coincides with the magnetic meridian. Basing upon the value of measured effect, the correction factor has been derived for the particle density depending on a geomagnetic parameter of a shower. . In [4 a, b] it was shown that the showers initiated by extremely high energy photons have to show the north-south asymmetry. But the last effect has a threshold for the primary energy: E γ > 3⋅10 19 eV. In this paper we draw attention to the fact that due to the LDF asymmetry depending on χ, one can select the showers of equal energy and zenith angle in order to obtain the azimuthal modulation of the EAS event rate because of the different LDF stretch which causes the different observed primary cosmic ray energy and intensity.

In the work the data of the Yakutsk complex EAS array and their comparison with calculation in the case of primary nuclei of different chemical elements are presented. The calculation by QGSJET model have been used interpreting experimental data.

There are some discrepancies in the results on energy spectrum from Yakutsk, AGASA, and HiRes experiments. In this work differential energy spectrum of primary cosmic rays based on the Yakutsk EAS Array data is presented. For the largest events values of S600 and axes coordinates have been obtained using revised lateral distribution function. Simulation of converter's response at large distances showed no considerable underestimation of particle density. Complex shape of spectrum in region of E0 > 10 17 eV is confirmed. After adjustment of parameters and additional exposition at the Yakutsk array there are three events with energy E0 > 10 20 eV.

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