Left panels: 2D scatter plots of B vs. v_LOS for all points within the sunspot boundary at the three optical depth nodes. The horizontal dashed line in each panel indicates the zero velocity level. Right panels: the same, but only showing pixels from the penumbra of the sunspot. The two horizontal solid lines in panels e) and f) indicate thresholds of the velocity used to identify, in an approximative manner, the heads and tails of the penumbral filaments. The two vertical dashed lines in panel f) bound the field strength range 1000 <B< 2500 G, a condition used to identify heads of penumbral filaments in Fig. 12d.