2 Observations and data analysis

The data used in this work come from the observations carried out at the Observatorio del Teide between 1984 and 1999. The observations consist of daily measurements of the solar radial velocity obtained with the Mark-I resonant scattering spectrophotometer. This instrument has been sited at the Observatorio del Teide since 1975. After some hardware updates in 1984, the experiment has been running without interruption other than bad weather and instrumental failures. The data reduction process is explained in more detail elsewhere (van der Raay et al. 1985; Pallé et al. 1986, 1993). Briefly, the data are corrected from the annual scan (Earth's orbit around the Sun) of the non-linear solar line shape, and calibrated by fitting the known daily velocity of the observatory; only two parameters are fitted, taken to be the same over the all 15 years analyzed here. Then, the daily residuals are joined in consecutive 360 days leading to a total of 30 time-series with 6 months in common between consecutive series. Although all series are not independent, they show very similar duty cycles (around $25\%$). Finally, the corresponding power spectra were calculated for every time-series using a traditional Fourier analysis. All the spectra show the peaks of the low-degree p-modes with $\ell\leq3$. The typical sideband structure appears in the spectra at $k_D = 11.57~\mu$Hz as a direct consequence of the observing window achieved from just one station.