On July 19, 2021, we added a new time series to our data products: extended hemispheric sunspot numbers starting in 1874. Those reconstructed numbers are based on the sunspot areas listed in the Greenwich photographic catalogue, and are calibrated on the SILSO reference hemispheric series, which starts only in 1992. This newly published series results from a collaboration between SILSO, the University of Graz (Austria) and the Skolkovo Institute of Science and Technology (Moscow, Russia).
It can be used as a direct extension of SILSO's hemispheric numbers before 1992 , to retrace the distinct evolution of the North and South solar activity levels, now over 13 solar cycles instead of just the last 3 cycles, and up to the present.
The data and related publication can be found in our Data section
The related press release can be found here : https://www.skoltech.ru/en/2021/07/new-sunspot-catalogue-to-improve-space-weather-predictions/
The published article can be found here : Published article (A&A 652, A56)