  Around the peaks of substantial flares, bright artifact nearly horizontal
saturation streaks (B-streaks) corresponding to the brightest parts of the
flare sources appear in the STEREO/EUVI 195 A images. We show that the length
of such B-streaks can be used for the solution of an actual problem of
evaluating the soft X-ray flux and class of far-side flares registered with
double STEREO spacecraft but invisible from Earth. For this purpose from data
on about 350 flares observed from January 2007 to July 2014 (mainly exceeding
the GOES M1.0 level) both with GOES and STEREO, an empirical relation is
established correlating the GOES 1-8 A peak flux and the B-streak length. This
allowed us for the same years to estimate the soft X-ray classes for
approximately 65 strong far-side flares observed by STEREO. The results of this
simple and prompt method are consistent with the estimations of Nitta et al.
(Solar Phys., 288, 241, 2013) based on the calculations of the EUVI full-disk
digital number output. In addition, we studied some features of the B-streaks
in impulsive and long-duration flares and demonstrated that B-streaks in
several consecutive EUVI images can be used to reconstruct a probable time
history of strong far-side flares.
