Combined Fourier power spectra from the 3 data sets. The Fourier power of a data set is defined in Sect. 3. The dashed lines in all the panels mark the 3-σ level above which the random probability is lower than 0.3%, considering the number of independent trials of about 60 000. Left panels are plotted in a log-log scale to show the high power at low frequencies, which are due to atmospheric variations. Right panels show the power spectra in 1–10 Hz.