One-hour averaged continuum image (top-left) and magnetogram (bottom-left) of active region NOAA 12673. The next four panels (top and bottom) covering an area of 120 Mm × 120 Mm show difference maps scaled between ± 0.2 I₀ (continuum images) and ± 500 G (magnetograms). The red and blue contours mark the boundaries of negative and positive polarities comprising the δ-spot. The intersection of these two colored lines demarcates the PIL. The boxes labeled “a”, “b”, “c”, and “d” mark locations of persistent changes.