Left: line-of-sight magnetic field B_z over the central 23.5° region, imaged with SDO/HMI 12 h after the beginning of the observation. Right: zoomed-in centre-disc region of 10° ×10° (frame in left plot) showing attractive LCS calculated for 12 h negative integration time (ridges of the negative-time FTLE field, red colormap), superimposed with passive tracer locations after 12 h positive integration time (black dots), and corresponding local B_z magnetograms (green/purple colormap). The coloured markers and lines tag 12 h (positive time) trajectories of a few selected tracers, see discussion in Sect. 5 and Fig. 7 below (visualisation continued for longer times in Fig. 4).