Source: WDC-SILSO, Royal Observatory of Belgium, Brussels, 
DOI: https://doi.org/10.24414/qnza-ac80”.

https://www.sidc.be/SILSO/home
https://www.sidc.be/SILSO/datafiles

https://www.sidc.be/SILSO/DATA/SN_d_tot_V2.0.txt
https://www.sidc.be/SILSO/DATA/SN_m_tot_V2.0.txt
https://www.sidc.be/SILSO/DATA/SN_ms_tot_V2.0.txt
https://www.sidc.be/SILSO/DATA/SN_y_tot_V2.0.txt
https://www.sidc.be/SILSO/ssngraphics

https://lasp.colorado.edu/lisird/data/international_sunspot_number/