Upper left: 2D histogram of normalized intensities computed for each Carrington rotation. The upper part of the plot corresponds to intensities of 0.9, the lower part to a normalized intensity of 1.5. Upper right: distribution of normalized intensities for all data sets. For reference, the lower panel shows the monthly international sunspot number. The horizontal axis for the upper right panel is the same as in the lower panel. The sharp peaks in the distribution of normalized intensities (upper right panel) at about 1 and about 1.3 are artifacts caused by missing data (whose intensity was set to unity) and erroneous data. The color bar in the lower right corner shows the scaling used for the 2D histogram in the upper left corner.