Amplitude spectra for the EW of the He i 5875 Å line for the three individual spectrographs with sufficient data. In the upper panels, the red dashed lines indicate four times the average noise level computed over the range [0,10] d^-1. The lower panel shows the result of multiplying the periodograms in the upper panels after weighing them according to the S/N of the dominant frequency and placing the dominant frequency after this multiplication at value 1.