I thank L. Culhane, H. Hudson, J. Lemen, T. Watanabe, and J.-P. Wuelser for fruitful discussions, and the referee for useful suggestions. I acknowledge support from the Naval Research Laboratory/Office of Naval Research basic research program. Yohkoh is a mission of the Institute of Space and Astronautical Sciences (Japan), with participation from the U.S. and U.K.
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999