Reconstruction of a Large-scale Pre-flare Coronal Current Sheet Associated with an Homologous X-shaped Flare

As a fundamental magnetic structure in the solar corona, electric current sheets (CSs) can form either prior to or during solar flare, and they are essential for magnetic energy dissipation in the solar corona by enabling magnetic reconnection. However static reconstruction of CS is rare, possibly due to limitation inherent in available coronal field extrapolation codes. Here we present the reconstruction of a large-scale pre-flare CS in solar active region 11967 using an MHD-relaxation model constrained by SDO/HMI vector magnetogram. The CS is found to be associated with a set of peculiar homologous flares that exhibit unique X-shaped ribbons and loops occurring in a quadrupolar magnetic configuration. This is evidenced by that the field lines traced from the CS to the photosphere form an X shape which nearly precisely reproduces the shape of the observed flare ribbons, suggesting that the flare is a product of the dissipation of the CS through reconnection. The CS forms in a hyperbolic flux tube, which is an intersection of two quasi-separatrix layers. The recurrence of the X-shaped flares might be attributed to the repetitive formation and dissipation of the CS, as driven by the photospheric footpoint motions. These results demonstrate the power of data-constrained MHD model in reproducing CS in the corona as well as providing insight into the magnetic mechanism of solar flares.