Magnetic topology just before the occurrence of flares. (a1)–(a4) Top view of the skeletons of the fan-spine represented by green curves, together with the null points represented by the brown dot symbols. The red curves represent the magnetic flux ropes within the fan structures, and the backgrounds are vertical magnetograms taken at different times. (b1)–(b4) Similar to (a1)–(a4), but from a side perspective.