  We report evolution of an atypical X-shaped flare ribbon which provides novel
observational evidence of three-dimensional (3D) magnetic reconnection at a
separator. The flare occurred on 2014 November 9. High-resolution slit-jaw 1330
A images from the Interface Region Imaging Spectrograph reveal four
chromospheric flare ribbons that converge and form an X-shape. Flare
brightening in the upper chromosphere spreads along the ribbons toward the
center of the "X" (the X-point), and then spreads outward in a direction more
perpendicular to the ribbons. These four ribbons are located in a quadrupolar
magnetic field. Reconstruction of magnetic topology in the active region
suggests the presence of a separator connecting to the X-point outlined by the
ribbons. The inward motion of flare ribbons in the early stage therefore
indicates 3D magnetic reconnection between two sets of non-coplanar loops that
approach laterally, and reconnection proceeds downward along a section of
vertical current sheet. Coronal loops are also observed by the Atmospheric
Imaging Assembly on board the Solar Dynamics Observatory confirming the
reconnection morphology illustrated by ribbon evolution.
