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Sunspot and Group Number: Recent advances from historical data

Published online by Cambridge University Press:  03 March 2020

Frédéric Clette
Royal Observatory of Belgium, 3, avenue Circulaire, 1180 Brussels, Belgium email:
José M. Vaquero
Departamento de Fsica, Universidad de Extremadura, 06071 Mérida, Spain
María Cruz Gallego
Departamento de Fsica, Universidad de Extremadura, 06071 Badajoz, Spain
Laure Lefèvre
Royal Observatory of Belgium, 3, avenue Circulaire, 1180 Brussels, Belgium email:
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Due to its unique 400-year duration, the sunspot number is a central reference for understanding the long-term evolution of solar activity and its influence on the Earth environment and climate. Here, we outline current data recovery work. For the sunspot number, we find historical evidence of a disruption in the source observers occurring in 1947–48. For the sunpot group number, recent data confirm the clear southern predominance of sunspots during the Maunder Minimum, while the umbra-penumbra ratio is similar to other epochs. For the Dalton minimum, newly recovered historical observations confirm a higher activity level than in a true Grand Minimum.

Contributed Papers
© International Astronomical Union 2020


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