We acknowledge GOLF, SOI/MDI and GONG teams for allowing the use of data, and C. Rabello-Soares & T. Appourchaux who provided us their analysis of GONG data for comparison. SoHO is a project of international cooperation between ESA and NASA. GONG is a project managed the NSO, a division of the NOAO, wich is operated by AURA, Inc. under a cooperative agreement with the NSF. This work has been performed using the computing facilities provided by the program ``Simulations Interactives et Visualisation en Astronomie et Mécanique'' (SIVAM, OCA, Nice) and by the ``Institut du Développement et des Ressources en Informatique Scientifique'' (IDRIS, Orsay). Thanks to the conference organizers for financial support.