EMERGING FLUX AND X-CLASS FLARES IN NOAA 6555 DEBI PRASAD CHOUDHARY 1 , ASHOK AMBASTHA 1 and G. AI 2 1 Udaipur Solar Observatory, Physical Research Laboratory, Off Badi Road, Udaipur 313001, India 2 Beijing Astronomical Observatory, Chinese Academy of Sciences Beijing, 100080, China (Received 9 December 1996; accepted 23 September 1997) Abstract. The active region NOAA 6555 had several locations of highly sheared magnetic field structure, yet, only one of them was the site for all the five X-class flares during its disk passage in March 1991. The pre-flare observations of high-resolution H filtergrams, vector magnetograms and H Dopplergrams of the 2B/X5.3 flare on 25 March 1991 show that the flaring site was characterized by a new rising ‘emerging flux region’ (EFR) near the highly sheared magnetic field configuration. The polarity axis of the emerging flux was nearly perpendicular to the pre-existing magnetic neutral line. The location of the EFR was the site of initial brightening in H . The post-flare magnetograms show higher magnetic shear at the flare location compared to the post-flare magnetograms, 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. Observations from Big Bear Solar Observatory and Marshall Space Flight Center also show that there was emergence of new flux at the same location prior to two other X-class flares. We find that out of five observed X-class flares in NOAA 6555, at least in three cases there are clear signatures of flare-related flux emergence. Therefore, it is concluded that EFRs might play an important role in destabilizing the observed sheared magnetic structures leading to large X-class flares of NOAA 6555. 1. Introduction Solar flares are generally observed near the location of an active region where the magnetic field is sheared (Hagyard, West, and Smith, 1993; Athay, Jones, and Zirin, 1985). However, the magnitude of the shear alone does not appear to be a clear indicator of the flare productivity of any location. For example, two regions with the same amount of shear need not be equally flare productive. The extensive observations of the vector magnetic fields of two large super active regions, namely NOAA 6555 and NOAA 6659, during the solar activity cycle 22 clearly show that additional parameters may be required to explain the occurrence of large flares (Ambastha, Hagyard, and West, 1993; Debi Prasad et al., 1997; Schmieder et al., 1994). Further, it was both observationally and theoretically demonstrated that sheared magnetic field configurations can be stable under several situations, and it is necessary to destabilize the field configuration in order to produce a flare (Hagyard and Rabin, 1986). The role of emerging flux for destabilizing the magnetic field configuration and hence triggering the solar flares has long been recognized. A new flux tube emerging from below the photosphere, by virtue of magnetic buoyancy, can interact with the existing flux tubes, leading to the formation of a neutral sheet at their common boundary (Heyvaerts, Priest, and Rust, 1977). The reconnection mechanism at these neutral sheets may lead to the observed Solar Physics 179: 133–140, 1998. c 1998 Kluwer Academic Publishers. Printed in Belgium.