Same as Fig. 1 but for 21 yr [1996.01.28–2017.05.23] of TSI data from SoHO/VIRGO. In total, 7787 days are considered. This corresponds to about 186 889 data points at an hourly cadence. The GLS, ACF, and PS methods do not show a clear signal of the rotation period. GPS shows a prominent peak at 4.15 days, resulting in a solar rotation period value of about 26.3 days (see text for details). Orange vertical lines and marks in panel a represent a splitting of the VIRGO dataset into Kepler-like timespan K_n, each one representing the full lifetimes of the Kepler mission, around 4 yr (see Sect. 3.5).