EMERGING FLUX AND X-CLASS FLARES IN NOAA-6555

The active region NOAA 6555 had several locations of highly sheared ma gnetic field structure, yet, only one of them was the site for all the five X-class hares during its disk passage in March 1991. The pre-fla re observations of high-resolution Her filtergrams, vector magnetogram s and HP Dopplergrams of the 2B/X5.3 flare on 25 March 1991 show that the flaring site was characterized by a new rising 'emerging flux regi on' (EFR) near the highly sheared magnetic field configuration. The po larity axis of the emerging flux was nearly perpendicular to the pre-e xisting magnetic neutral line. The location of the EFR was the site of initial brightening in Ha. The post-flare magnetograms show higher ma gnetic shear at the flare location compared to the post-flare magnetog rams, which might indicate that the EFR was sheared at the time of its emergence. As the new EFR coincided with the occurrence of the flare, we suggest that it might have triggered the observed flare. Observati ons from Big Bear Solar Observatory and Marshall Space Flight Center a lso show that there was emergence of new flux at the same location pri or to two other X-class flares. We find that out of five observed X-cl ass flares in NOAA 6555, at least in three cases there are clear signa tures of flare-related flux emergence. Therefore, it is concluded that EFRs might play an important role in destabilizing the observed shear ed magnetic structures leading to large X-class flares of NOAA 6555.