Sep 21 – 23, 2015
Royal Observatory of Belgium
Europe/Brussels timezone

The new and improved Sunspot Number SN

Sep 23, 2015, 2:30 PM
Meridian Room (Royal Observatory of Belgium)

Meridian Room

Royal Observatory of Belgium

Avenue Circulaire - 3 - Ringlaan 1180 Brussels
4-Space era data versus historical records


Dr Laure Lefevre (Royal Observatory of Belgium)


We present the first end-to-end revision of the Sunspot Number since the creation of this reference index of solar activity by Rudolf Wolf in 1849. Most corrections rely entirely on original sunspot data, using various approaches. Newly recovered historical sunspot records were added and a critical data selection was applied for the 17th and 18th century, confirming the low solar activity during the Maunder Minimum. Over the 19th century, the k scaling coefficients of individual observers were recomputed using new statistical methodologies, like the "backbone" method resting on a chain of long-duration observers. In particular, we corrected a major inhomogeneity in 1947 in the Sunspot Number (~20% overestimate) caused by a weighting introduced by Pr. Waldmeier during the Zurich era. Finally, a full re-computation of the sunspot number was done over the last 50 years, using all original data from the 270 stations archived by the World Data Center - SILSO in Brussels. We conclude on the implications for solar cycle and Earth climate studies and on important new conventions adopted for the new series: new unit scales (constant "heritage" factors 0.6 and 12.08 dropped for the Sunspot and Group Numbers respectively), new SN and GN symbols and a new version-tracking scheme implemented at the WDC-SILSO, as a framework open to future improvements of those unique data series.


On July 1st 2015, we published an improved version of the Sunspot Number. Here, we will present and explain the corrections.

Primary author

Dr Laure Lefevre (Royal Observatory of Belgium)

Presentation materials

There are no materials yet.