Space-time slices showing proper motions in the LB section shown in the upper left inset of Fig. 1, corrected for foreshortening; a and b from the Ca ii K images, c and d from broadband images; a and c from the LB part below the dark lane seen in Ca; b and d from the part above the dark lane. The abscissa x lies parallel to the LB. Perpendicular to x, widths of the slices of 092 for Ca ii K and 035 for broadband were chosen, and the slices were compressed appropriately. Discontinuous lines (1 and 2) in panel a mark the slope of the trajectory of bright features moving (starting point marked with “x”) with speeds of 100 m s^-1 and 400 m s^-1, respectively. For comparison, the slope of the continuous lines refers to 250 m s^-1. Ordinate is elapsed time since start of observation.