Upper left panel: example of a full sun magnetogram during a low activity period, with PIL pixels superimposed (solid line) and filaments (red patches). The dotted circle indicates a distance of 0.85 R_⊙. Upper right panel: for the same magnetogram, positive polarity (white) and negative polarity (black) unipolar areas (see text for more details) are superimposed on the PIL (dotted line). The gray circle indicates the position of the limb. Lower panels: same as upper panels but for a high activity period.