Research Note

Inconsistency of the Wolf sunspot number series around 1848

¹ Department of Physics, PO Box 3000, University of Oulu, 90014 Oulu, Finland
e-mail: Ilya.Usoskin@oulu.fi
² Sodankylä Geophysical Observatory (Oulu unit), University of Oulu, 90014 Oulu, Finland
³ Leibniz Institute for Astrophysics Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 26 July 2013
Accepted: 23 September 2013

Abstract

Aims. Sunspot numbers form a benchmark series in many studies, but may still contain inhomogeneities and inconsistencies. In particular, an essential discrepancy exists between the two main sunspot number series, Wolf and group sunspot numbers (WSN and GSN, respectively), before 1848. The source of this discrepancy has remained unresolved so far. However, the recently digitized series of solar observations in 1825–1867 by Samuel Heinrich Schwabe, who was the primary observer of the WSN before 1848, makes such an assessment possible.

Methods. We construct sunspot series, similar to WSN and GSN but using only Schwabe’s data. These series, called here WSN-S and GSN-S, respectively, were compared with the original WSN and GSN series for the period 1835–1867 for possible inhomogeneities.

Results. This study supports the earlier conclusions that the GSN series is more consistent and homogeneous in the earlier part than the WSN series. We show that: the GSN series is homogeneous and consistent with the Schwabe data throughout the entire studied period; the WSN series decreases by roughly 20% around 1848, which is caused by the change of the primary observer from Schwabe to Wolf and an inappropriate individual correction factor used for Schwabe in the WSN; this implies a major inhomogeneity in the WSN, which needs to be corrected by reducing its values by 20% before 1848; the corrected WSN series is in good agreement with the GSN series.