  With increasing solar activity since 2010, many flares from the backside of
the Sun have been observed by the Extreme Ultraviolet Imager (EUVI) on either
of the twin STEREO spacecraft. Our objective is to estimate their X-ray peak
fluxes from EUVI data by finding a relation of the EUVI with GOES X-ray fluxes.
Because of the presence of the Fe xxiv line at 192 A, the response of the EUVI
195 A channel has a secondary broad peak around 15 MK, and its fluxes closely
trace X-ray fluxes during the rise phase of flares. If the flare plasma is
isothermal, the EUVI flux should be directly proportional to the GOES flux. In
reality, the multithermal nature of the flare and other factors complicate the
estimation of the X-ray fluxes from EUVI observations. We discuss the uncer-
tainties, by comparing GOES fluxes with the high cadence EUV data from the
Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory
(SDO). We conclude that the EUVI 195 A data can provide estimates of the X-ray
peak fluxes of intense flares (e.g., above M4 in the GOES scale) with
uncertainties of a factor of a few. Lastly we show examples of intense flares
from regions far behind the limb, some of which show eruptive signatures in AIA
images.
