Sunspot Number V2.0 Through Solar Cycle 25: A Long-term Multi-Proxy Stability Analysis.

The International Sunspot Number (SN V2.0) is one of the longest and most detailed available series in astrophysics and its accuracy and stability is important for a large variety of scientific domains, not the least of which is the evolution of the Earth Climate. 
Since its recalibration and release in 2015, SN V2.0 has been the subject of sustained scrutiny within the scientific community yet no community-wide audit has covered the first full decade of that recalibration through the rise of Solar Cycle 25. A systematic assessment of the long-term stability of SN V2.0 is thus in order. In parallel, the American Sunspot Number, which has been computed continuously since the mid-20th century, experienced documented inconsistencies in the 1990s, as highlighted in previous studies (Schaefer, 1997). However, a comprehensive evaluation of its long-term behavior in the subsequent decades is still lacking.

In this work, we analyze the temporal stability of SN V2.0 over multi-decadal timescales. We compare SILSO SN V2.0 with AAVSO Ra, and independent proxies such as Sunspot areas, F10.7, Nobeyama microwave fluxes, Mg II, ISGI aa, and SDO/HMI unsigned field to diagnose the long-term behavior of both indices. We examine their mutual consistency, sensitivity to calibration changes, and suitability for long-term comparative studies. This analysis allows us to assess the relative robustness of each index, identify potential residual biases, and evaluate their reliability for studies of long-term solar variability. We conclude by discussing implications for future sunspot number reconstructions and by outlining perspectives for maintaining stable, homogeneous solar activity indices over extended timescales.