The aim of the thesis is the study of properties of solar flares via reconstruction of energy distributions of accelerated/heated electrons, diagnostics of flare plasma based on EUV and X-ray observations, as well as the estimation of the thermal balance within the standard flare model. The X-ray data were obtained from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Russian solar observatory KORONAS-F (IRIS experiment). The EUV data were obtained from the Solar Dynamics Observatory / Atmospheric Imaging Assembly (SDO/AIA). In the first chapter a new technique is presented, which allowed to determine at first the photon spectra based on the KORONAS-F/IRIS observations and then to reconstruct the energy spectra of the emitting electrons using the random search method and the Tikhonov regularization method. In the second chapter the developed technique of simultaneously fitting the model differential emission measure functions (DEM) to RHESSI and SDO/AIA data is introduced, which allowed to infer the electron distribution from X-ray and EUV observations. The proposed method allows to reconstruct of both the spectra of accelerated/heated electrons and the basic parameters of the flare plasma, such as temperature, emission measure, total electron number density, and flare energy. In addition, the analytical function suitable for both DEM analysis and mean electron flux spectra in flares has been developed and applied, as well as kappa-distribution in the form of differential emission measure. The thermal balance and hard X-ray emission of coronal loops from different flare regions within the standard flare model is considered in the third chapter. This thesis is based on the next papers: Motorina et al., TePh, 2012, Motorina & Kontar, Ge&Ae, 2015, Battaglia et al., ApJ, 2015, Motorina et al., TePh, 2016, Tsap et al., Ge&Ae, 2016.
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