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Astron. Astrophys. 338, 1109-1117 (1998)


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

M. Gabriel 1, G. Grec 2, C. Renaud 2, A.H. Gabriel 3, J.M. Robillot 4, T. Roca Cortés 5, S. Turck-Chièze 6 and R.K. Ulrich 7

1 Institut d'Astrophysique de l'Université de Liège, 5, avenue de Cointe, B-4000 Liège, Belgique
2 Département Cassini, UMR 6529 du CNRS, Observatoire de la Côte d'Azur, F-06304 Nice, France
3 Institut d'Astrophysique Spatiale, Unité Mixte CNRS et Université Paris XI, F-91405 Orsay, France
4 Observatoire de l'Université Bordeaux 1, BP 89, F-33270 Floirac, France
5 Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
6 Service d'Astrophysique, DSM/DAPNIA, CE Saclay, F-91191 Gif-sur-Yvette, France
7 Astronomy Department, University of California, Los Angeles, USA

Received 11 May 1998 / Accepted 16 July 1998

Abstract

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 secondly it does not 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.

Key words: Sun: oscillations

Send offprint requests to: M.G.

© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998

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