  Based on a joint analysis of the data from 10 midlatitude decameter radars
the effects are investigated during 80 x-ray flares in the period 2013-2017.
For the investigation nine mid-latitude SuperDARN radars of the northern
hemisphere and Ekaterinburg coherent decameter radar of ISTP SB RAS are used.
All the radars work in the same 8-20MHz frequency band and have similar
hardware and software. During the analysis the temporal dynamics of noise from
each of the radar direction and for each flare is investigated separately. As a
result, on the basis of about 13000 daily measurements we found a strong
anticorrelation between noise power and x-ray flare intensity, that allows to
use short-wave sky noise to diagnose the ionospheric effects of x-ray solar
flares. It is shown that in 88.3\% of cases an attenuation of daytime decameter
radio noise is observed during solar flare, correlating with the temporal
dynamics of the solar flare. The intensity of decameter noise anticorrelates
well (the Pearson correlation coefficient better than -0.5) with the shape of
the X-ray flare in the daytime (for solar elevation angle $>0$) in 33\% of
cases, the average Pearson correlation over the daytime is about -0.34. Median
regression coefficient between GOES 0.1-0.8nm x-ray intensity and daytime
sky-noise attenuation is about $-4.4\cdot10^{4}\ [dB\cdot m^{2}/Wt]$. Thus, it
has been shown that measurements of the decameter sky noise level at
midlatitude decameter radars can be used to study the ionospheric absorption of
high-frequency waves in the lower ionosphere during x-ray solar flares. This
can be explained by the assumption that the most part of decameter sky noise
detected by the radars can be interpreted as produced by ground sources at
distances of the first propagation hop (\textasciitilde{} 3000 km).
