The lateral motion of quasi circular flare ribbon to the quasi separatrix line

We report the lateral motion of quasi circular flare ribbon found in an impulsive M5.6 flare occurred 24-Aug-2015. The flare occurred around a positive polarity pore above which negative field lines of neighboring umbra overly. There are two peaks in the flare light curves of coronal X-ray/EUV intensity as well as flare ribbon intensity observed in UV. During the flare, a semi circular flare ribbon found west side of the pore, and the ribbon expands laterally at the first peak, and then shrinks at the later peak. The apparent motion in both expansion and shrinkage is about arc sec in a time scale of a few minutes. We investigate the coronal magnetic field structure around the flare by using the non-linear force free field extrapolation, and find that there exists a fan-spine structure with 3 D null point. A possible scenario of the lateral motion of the quasi circular ribbon is as follows: magnetic reconnect ion between the east(west) side of the field lines inside the fan dome and the west(east) side of the field line overlying the dome is responsible for the first (second) peak.