Figure 6: Magnetic topology evolution.

(a) Sampled field lines traced from topology separatrix at modelling time t=57 (with different colours denoting different connectivity as shown). They represent the field lines undergoing reconnection during the flare. At the bottom is AIA 304 Å image near the flare peak time to show the flare ribbons. It can be seen that the locations of flare ribbons are matched well by those footpoints of the reconnecting field lines. (b) Top view of the field lines with the background image showing the photospheric magnetic flux. Extents shown in a,b are identical. (c) Magnetic squashing degree log(Q) at the bottom surface of the FER (FoV is marked in b by the box) at different times (t=0, 12, 24, 48 and 57). The separatrix is distinctly revealed by the quasi-circular narrow line with log(Q)>5. The black regions represent footpoints of open flux. The yellow lines are PILs. The arrows denote the newly-formed QSL. (d) Squashing degree maps for vertical cross-sections whose locations are denoted by the oblique lines shown in c. The arrows denote the X-like configuration that is formed along with the new QSL.