An interesting paper has been published end 2025: 'An independent assessment of the International Sunspot Number since 1996' by Peter Meadows. Peter is an observer of the SILSO network. SILSO is the World Data Centre for the International Sunspot Number and takes care of the production, validation, distribution and preservation of this index of solar activity. This index goes back to early 1600!
Laure Lefevre, Director of the WDC-SILSO: 'This is actually a great paper written by one of our own observers supporting our evidence.'
The paper reads: "These findings support the overall consistency and reliability of the International Sunspot Number. They also highlight the value of space-based white-light observations for sunspot number derivation, as they are unaffected by atmospheric seeing and provide a stable reference for long-term solar activity monitoring. Independent cross-checks of this kind remain vital for maintaining confidence in the datasets that underpin solar-terrestrial and space-weather research."
From the paper, figure 1:
Monthly International Sunspot Number Ri compared with (a) SOHO/MDI and SDO/HMI image derived sunspot number, (b) USAF/NOAA SRS sunspot number and (c) the author's own sunspot number. Differences are also shown.
Laure Lefevre: 'The independent cross-checks are indeed what we should focus on. Space-based observations to derive the Sunspot Number have been tested: they give similar results but different - as any proxy does - and the differences need to be characterised. So they are interesting for comparison.'
From this it is clear that space-based data should and could not replace ground based data into the computation of the International Sunspot Number. Not until we have studied the relationship between the two for sufficiently long. This is more than 2 solar cycles.
Laure Lefevre continues: 'We only take projection, drawings or eye-based observations in SILSO. Even CCD observations are not common so as to keep the homogeneity of the series. We are preparing a paper on this subject. But as said, this is a very interesting paper.'
The paper: https://iopscience.iop.org/article/10.3847/2515-5172/ae1cbb
