Rigozo, N. R.; Echer, E.; Vieira, L. E. A.; Nordemann, D. J. R. Reconstruction of Wolf Sunspot Numbers on the Basis of Spectral Characteristics and Estimates of Associated Radio Flux and Solar Wind Parameters for the Last Millennium. Solar Physics. 2001, 203 (1): 179–191. Bibcode:2001SoPh..203..179R. doi:10.1023/A:1012745612022.
Rigozo, N. R.; Echer, E.; Vieira, L. E. A.; Nordemann, D. J. R. Reconstruction of Wolf Sunspot Numbers on the Basis of Spectral Characteristics and Estimates of Associated Radio Flux and Solar Wind Parameters for the Last Millennium. Solar Physics. 2001, 203 (1): 179–191. Bibcode:2001SoPh..203..179R. doi:10.1023/A:1012745612022.