Full disk image of the Sun measured with the japanaise satellite Yohkoh together with the corresponding full-disk lightcurve (bottom panel) that reports how the integrated X-ray solar luminosity changes with time. This movie shows the Sun during five rotations, corrisponding to about five months. The light curve lowers when the visible side of the Sun is dark, while it is high when active region appear on the disk. The resulting light curve show a periodical modulation, although quite irregular.
Courtesy of S. Orlando
