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The Yakutsk Array is designed to study cosmic rays at energy 10 15 -10 20 eV. It consists several independent arrays that register charged particles, muons with energy E≥1 GeV, Cherenkov light and radio emission. The paper presents a technical description of the Yakutsk Radio Array and some preliminary results obtained from measurements of radio emission at 30-35 MHz frequency induced by air shower particles with energy ε ≥ 1·10 17 eV. The data obtained at the Yakutsk array in 1986-1989 (first set of measurements) and 2009-2014 (new set of measurements). Based on the obtained results we determined:Lateral distribution function (LDF) of air showers radio emission with energy ≥ 10 17 eV. Radio emission amplitude empirical connection with air shower energy. Determination of depth of maximum by the ratio of amplitude at different distances from the shower axis. For the first time, at the Yakutsk array, radio emission from the air shower with energy > 10 19 eV was registered including the shower with the highest energy ever registered at the Yakutsk array with energy ∼2 · 10 20 eV.

The experimental distributions of Xmax obtained with the Yakutsk EAS array at fixed energies of 5 × 10 17 , 1 × 10 18 and 5 × 10 18 eV are analysed. A recent version of the QGSJET model is used as a tool of our analysis. In the framework of this model, the most adequate mass composition of primary particles satisfying the experimental data on Xmax is selected.

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The Yakutsk array group is developing a wide field-of-view Cherenkov telescope to be operated in coincidence with the surface detectors of the extensive air shower array. Currently, the engineering prototype of the reflecting telescope with the front-end electronics is designed, assembled, and tested to demonstrate the feasibility of the conceived instrument. The status and specifications of the prototype telescope are presented, as well as the modernization program of the already existing Cherenkov light detectors subset of the array measuring ultra-high energy cosmic rays.

Energy portion Eem/E 0 transferred to EAS electron-photon component at E 0 = 10 15 ± 10 19 eV was estimated by using ofČerenkov radiation data and charged particles data obtained at the Yakutsk array. The results are compared with the models for different energy dissipation into EAS electronphoton component and with calculations for different primary nuclei. In the energy intervals 10 15 ± 10 16 eV and 10 18 ± 10 19 eV Eem/E 0 is equal to (77 ± 2) and (88 ± 2)% respectively, that doesn't contradict with mixed composition of primary particles for the first energy interval and the proton composition for the second one.

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The aim of the Yakutsk array enhancement project is to create an instrument to study the highest-energy galactic cosmic rays (CRs) – their sources, energy spectrum, and mass composition. Additionally, there will be unique capabilities for investigations in the transition region between galactic and extragalactic components of CRs.

Using the well-developed imaging atmospheric Cherenkov telescope technique adapted to the energy region E>10¹⁵ eV, we plan to measure the longitudinal structure parameters of the shower, e.g. angular and temporal distributions of the Cherenkov signal related to X_max and the mass composition of CRs.

The main advantages of the Yakutsk array, such as its multi-component measurements of extensive air showers, and model-independent CR energy estimation based on Cherenkov light measurements, will be inherited by the instrument to be created