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Based on a joint analysis of the data from 10 midlatitude decameter radarsthe effects are investigated during 80 x-ray flares in the period 2013-2017.For the investigation nine mid-latitude SuperDARN radars of the northernhemisphere and Ekaterinburg coherent decameter radar of ISTP SB RAS are used.All the radars work in the same 8-20MHz frequency band and have similarhardware and software. During the analysis the temporal dynamics of noise fromeach of the radar direction and for each flare is investigated separately. As aresult, on the basis of about 13000 daily measurements we found a stronganticorrelation between noise power and x-ray flare intensity, that allows touse short-wave sky noise to diagnose the ionospheric effects of x-ray solarflares. It is shown that in 88.3\% of cases an attenuation of daytime decameterradio noise is observed during solar flare, correlating with the temporaldynamics of the solar flare. The intensity of decameter noise anticorrelateswell (the Pearson correlation coefficient better than -0.5) with the shape ofthe X-ray flare in the daytime (for solar elevation angle $>0$) in 33\% ofcases, the average Pearson correlation over the daytime is about -0.34. Medianregression coefficient between GOES 0.1-0.8nm x-ray intensity and daytimesky-noise attenuation is about $-4.4\cdot10^{4}\ [dB\cdot m^{2}/Wt]$. Thus, ithas been shown that measurements of the decameter sky noise level atmidlatitude decameter radars can be used to study the ionospheric absorption ofhigh-frequency waves in the lower ionosphere during x-ray solar flares. Thiscan be explained by the assumption that the most part of decameter sky noisedetected by the radars can be interpreted as produced by ground sources atdistances of the first propagation hop (\textasciitilde{} 3000 km).

Attenuation of decameter sky noise during x-ray solar flares in 2013-2017 based on the observations at midlatitude radars