On the Onset of the X1.3-Class Flare in NOAA Active Region 12975

Understanding the initiation mechanisms of major solar flares remains a central problem in solar physics, particularly when multiple complex magnetic topologies such as null points and flux ropes are involved. This study aims to investigate the magnetic configuration that led to the X1.3-class flare on 2022 March 30 in NOAA Active Region (AR) 12975 and to understand the role of the magnetic topology in triggering the flare. For this purpose, we employ a non-force-free-field (NFFF) extrapolation technique on vector magnetogram data from Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamic Observatory (SDO) to reconstruct a temporal sequence of the three-dimensional (3D) coronal magnetic-field configurations during the flare. In the flaring region, we identify key magnetic features such as a 3D magnetic null, sheared arcades, a magnetic flux rope (MFR), and quasi-separatrix layers (QSLs). The sequence of the extrapolation shows the development of a magnetic-flux rope from an initially sheared arcade structure. The development is suggested to be facilitated by the magnetic reconnections in the magnetic field lines of the sheared arcade. In addition, reconnection at the 3D null is also found to occur. The footpoints of the null point coincide with observed pre-flare brightenings and the some part of the flare ribbons, indicating null-point reconnection as a key flare trigger. Furthermore, there is also indication of the onset of the slipping reconnection at the footpoints of the null which may further contribute to the ribbon brightenings. Overall, based on the extrapolation sequence, a plausible scenario can be proposed in which the reconnection between sheared arcades may lead to the formation and subsequent rise of the magnetic flux rope, which becomes unstable by the removal of the overlying flux through 3D null-point reconnection leading to the flaring event.