247 20190211112425.0 SIDCimport-272 Berghmans, D. 2006 History of the Sunspot Index: 25 years SIDC Van der Linden, R.A.M. Vanlommel, P. Clette, F. Robbrecht, E. Proc. IAGA, Ed. W. Schröder, Beitrage zur Geschichte der Geophysik und Kosmischen Physik 7 1 2006 288--299 2400236 http://publi2-as.oma.be/record/247/files/paper_revised.pdf 2939 http://publi2-as.oma.be/record/247/files/paper_revised.jpg?subformat=icon-180 icon-180 3920 http://publi2-as.oma.be/record/247/files/paper_revised.gif?subformat=icon icon published in NONREF