  Typical solar flares display two quasi-parallel, bright ribbons on the
chromosphere. In between is the polarity inversion line (PIL) separating
concentrated magnetic fluxes of opposite polarity in active regions (ARs).
Intriguingly a series of flares exhibiting X-shaped ribbons occurred at the
similar location on the outskirts of NOAA AR 11967, where magnetic fluxes were
scattered, yet three of them were alarmingly energetic. The X shape, whose
center coincided with hard X-ray emission, was similar in UV/EUV, which cannot
be accommodated in the standard flare model. Mapping out magnetic
connectivities in potential fields, we found that the X morphology was dictated
by the intersection of two quasi-separatrix layers, i.e., a hyperbolic flux
tube (HFT), within which a separator connecting a double null was embedded.
This topology was not purely local but regulated by fluxes and flows over the
whole AR. The nonlinear force-free field model suggested the formation of a
current layer at the HFT, where the current dissipation can be mapped to the
X-shaped ribbons via field-aligned heat conduction. These results highlight the
critical role of HFTs in 3D magnetic reconnection and have important
implications for astrophysical and laboratory plasmas.
