  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.
