Time-correlation of the solar p-mode velocity signal from GOLF

Since the launch of SOHO, the Solar Heliospheric Orbital Observatory, the helioseismic observations are nearly uninterrupted. The GOLF instrument (A. Gabriel et al., 1997) measures the mean velocity integrated over the disk. The autocorrelation function of this velocity shows two main features: Firstly, the initial decrease of the peak amplitudes is much faster than expected from the width of the most powerful lines and %!second secondly it does not %!go decrease to zero for large times. These two features have been studied using the model of stochastically excited oscillators.The second one can also be understood on the basis of a completely general discussion. We show that the fast initial decrease of the peak amplitudes results from the departure of the mode frequencies from the values predicted by the first order asymptotic theory (the modes are not equidistant) and that the damping time of the modes has only a small influence. The non vanishing amplitudes at large times result either from the presence of a periodic non-stochastic component in the signal or from the stochastic nature of the excitation. Further tests have shown that the second possibility is the right one. This result gives a new argument in favor of the stochastic excitation of solar p-modes. The use of the ACF also suggests a new method to study line profiles which has been tested for radial modes and Lorentz profiles.