  We study the correlation between abrupt permanent changes of magnetic field
during X-class flares observed by the GONG and HMI instruments, and the hard
X-ray (HXR) emission observed by RHESSI, to relate the photospheric field
changes to the coronal restructuring and investigate the origin of the field
changes. We find that spatially the early RHESSI emission corresponds well to
locations of the strong field changes. The field changes occur predominantly in
the regions of strong magnetic field near the polarity inversion line (PIL).
The later RHESSI emission does not correspond to significant field changes as
the flare footpoints are moving away from the PIL. Most of the field changes
start before or around the start time of the detectable HXR signal, and they
end at about the same time or later than the detectable HXR flare emission.
Some of the field changes propagate with speed close to that of the HXR
footpoint at a later phase of the flare. The propagation of the field changes
often takes place after the strongest peak in the HXR signal when the
footpoints start moving away from the PIL, i.e. the field changes follow the
same trajectory as the HXR footpoint, but at an earlier time. Thus, the field
changes and HXR emission are spatio-temporally related but not co-spatial nor
simultaneous. We also find that in the strongest X-class flares the amplitudes
of the field changes peak a few minutes earlier than the peak of the HXR
signal. We briefly discuss this observed time delay in terms of the formation
of current sheets during eruptions.
