We wish to thank U. Schühle, K. Wilhelm, K. Bocchialini and Espen Sollum for useful discussions and comments. We wish to thank also the referee, Dr. K.P.Dere, for his very helpful comments on the submitted manuscript. The SUMER project is financially supported by DLR, CNES, NASA and ESA PRODEX programme (Swiss contribution). SOHO is a mission of international cooperation between ESA and NASA. HEM acknowledges the financial support of PPARC. NSO/Kitt Peak data used here are produced cooperatively by NSF/NOAO, NASA/GSFC, and NOAA/SEL.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000