Space-time slices of line-of-sight component of magnetic field from rectangle “3” in Fig. 1. The abscissa x is parallel to the umbral border, and its scale is corrected for foreshortening. The width of the stripes corresponds to 713. The slices were compressed appropriately. The slope of the white line refers to 250 m s^-1. The indicated “x” show the starting position of the features discussed in the text. Ordinate is elapsed time since start of observation.