The author gratefully acknowledges the useful help in the map-making work from Dr. Zhang Xiaoyu of Yunnan Observatory. A part of the data used in this paper is provided by the staff of the sunspot observation group of the Yunnan Observatory, the author is deeply indebted to their support. The author is grateful to Dr. Whl for his useful comments on this paper.
© European Southern Observatory (ESO) 1999
Online publication: April 28, 1999