Did a current sheet form prior to a major flare in solar active region 11158?

The initiation mechanism of coronal mass ejections and solar flares remains a central topic in solar physics. While it is widely accepted that magnetic reconnection plays a crucial role in these phenomena, the exact sequence of events leading to their onset is still debated. This study investigates the possibility of a current sheet (CS) formed prior to the X2.2 flare in solar active region (AR) 11158 on 2011 February 15. Using vector magnetograms from the Helioseismic and Magnetic Imager (HMI) and a magnetohydrodynamic (MHD) relaxation model, we provide evidences for the existence of a pre-flare CS. Observations reveal thin, J-shaped ribbons of strong vertical current density (J_z) along the polarity inversion line before the flare, which we interpret as the photospheric footprints of a coronal CS. Numerical reconstruction of the coronal field constrained by the vector magnetogram confirms the presence of a CS in the corona, with its thickness converging to a true discontinuity at higher resolutions. The synthetic coronal emission from the simulated CS matches the observed sigmoidal structure in the Atmospheric Imaging Assembly (AIA) 131 Å channel, further supporting the existence of a pre-flare CS. Our findings suggest that the X2.2 flare in AR 11158 was likely triggered by reconnection within a CS that formed gradually before the eruption, rather than by the ideal MHD instability of a pre-existing magnetic flux rope. This study provides new insights into the triggering mechanisms of solar eruptions and highlights the importance of pre-eruption CS formation in the initiation of major flares.