Time evolution during the slip-running motion of field lines A and B. Side a) and top b) views of the line A defined from Fig. 4 with fixed F₋(x = 0.035,y = −2.1). The set of colors varies from black to white in a rainbow fashion, and corresponds to line A at every Δt = 0.06  t_A from t = 20  t_A (in black) to t = 20.54  t_A (in white). The gray-scale image and the cyan/pink contourplots correspond to J_z(z = 0) and B_z(z = 0) at t = 20  t_A with the same color coding as in Fig. 3. c) A top view of the QSLs at t = 20  t_A with the same color–coding as in Fig. 3b (but shaded) is presented with the locations of the moving F₊ at different times. The field line at t = 20  t_A and at t = 20.6  t_A is represented in black and red. d), e), f): same views for line B from Fig. 4 with its fixed footpoint F₋(x = 0.32,y = −3.4). The color-coding is the same as for a), b), c).