Footpoint drift in the SES case and FES case. Panel (a) presents the negative footpoints of the axis in SES, where the white circle locates the footpoint position at t = 11 t₀. Panel (b) is similar to (a), but in FES, and the white circle locates the footpoint position at t = 34 t₀. In both panels, rainbow colors represent different simulation times. Panel (c) shows the axis morphology evolution from t = 24 t₀ to t = 32 t₀ in FES, and the purple contour shows the QSL at the bottom at t = 24 t₀.