Evolution of the twist number T_w, the winding number 𝒯 (including both their largest value and mean value), the decay index, and height of the MFR axis. Parameters in panels b and c are calculated from different positions of the MFR, where panel b corresponds to region A in Fig. 3a (also to the point of intersection between the magenta line and the MFR on the middle-top panel in Fig. 4), panel c corresponds to region B in Fig. 3a (and the point where the red line intersects with the MFR on the middle-top panel in Fig. 4). The height z is in unit of arcsecond (or 720 km).