We thank Claus Fr"ohlich and Thierry Toutain for providing access to IPHIR data cleaned of satellite pointing noise. We are grateful to Michel Tagger and Romain Teyssier for many useful discussions, and Thierry Appourchaux for constructive comments about the manuscript. SOHO is a mission of international cooperation between ESA and NASA.
© European Southern Observatory (ESO) 1998
Online publication: January 8, 1998