Variation of the granulation contrast (panel a)) and scale (panel b)) of the selected images in the N-S scan of December 21, 2009 (+), compared to the variations of all the selected images in the 12 N-S scans made in 2009. The blurred images at positions –543″, –444″, +523″, +623″ in Fig. 2 are removed. Here, the selected images are divided in two subfields 1024 × 512 pixels (112″ × 56″), in the direction parallel to the polar limbs, in order to increase the scan resolution. Moreover two subfields extracted from the images containing the southern limb are added, one of size 1024 × 512 pixels at the position –890″, the other one, of smaller size, 1024 × 384 pixels, centered at the position –928″; the outer boundary of this subfield is at 10″ from the limb (no subfields have been added close to the northern limb, because the corresponding images are slightly blurred).