  Seismic maps of the Sun's far hemisphere, computed from Doppler data from the
Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory
(SDO) are now being used routinely to detect strong magnetic regions on the far
side of the Sun (http:/jsoc.stanford.edu/data/farside/). To test the
reliability of this technique, the helioseismically inferred active region
detections are compared with far-side observation of solar activity from the
Solar TErrestrial RElations Observatory (STEREO), using brightness in extreme
ultraviolet light (EUV) as a proxy for magnetic fields. Two approaches are used
to analyze nine months of STEREO and HMI data. In the first approach, we
determine whether or not new large east-limb active regions are detected
seismically on the far side before they appear Earth side and study how the
detectability of these regions relates to their EUV intensity. We find that,
while there is a range of EUV intensities for which far-side regions may or may
not be detected seismically, there appears to be an intensity level above which
they are almost always detected and an intensity level below which they are
never detected. In the second approach, we analyze concurrent extreme
ultraviolet and helioseismic far-side observations. We find that 100% (22) of
the far-side seismic regions correspond to an extreme ultraviolet plage; 95% of
these either became a NOAA-designated magnetic region when reaching the east
limb or were one before crossing to the far side. A low but significant
correlation is found between the seismic signature strength and the EUV
intensity of a farside region.
