The 13-DAY Period Oscillation and the Solar Cycle

From the analysis of radial velocity measurements of the Sun, obtained at Izaña from 1976 to 1985, stable periods longer than 1 day, have been found in the observed signal. The appearance of an oscillation with a 13 day period has been confirmed. The comparison, for 1981 - 84, of the observations with a calibrated numerical model of the passage of inhomogeneities (spots and plages) on the solar surface, shows that the signal is not only due to this effect. It is believed that a velocity field, probably related to the surface inhomogeneities, contributes to the observed signal.