We thank Dr. J. Hirzberger for helpful criticism. The comments of the unknown referee helped to improve substantially the presentation of this work. Financial support by the Deutsche Forschungsgemeinschaft through grants KN 152/17-1,2 and KN 152/19-1,2 and by the Graduiertenkolleg "Strömungsinstabilitäten und Turbulenz" is gratefully acknowledged. The Vacuum Tower Telescope at the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias/Tenerife is operated by the Kiepenheuer-Institut für Sonnenphysik in Freiburg, Germany.
© European Southern Observatory (ESO) 2000
Online publication: August 23, 2000