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Regular version of the site

Article

When Daily Sunspot Births Become Positively Correlated

Solar Physics. 2015. Vol. 290. No. 10. P. 2709-2717.
Shapoval A., Le Mouël J., Shnirman M., Courtillot V.

This paper studies the first differences w(t) of the International Sunspot Numbers daily series, ISSN, over the 1850-2013 time span. The one-day correlations r1 between w(t) and w(t+1) are computed within 4 year sliding windows and found to shift from negative to positive values near the end of Cycle 17. They remain positive during the last Grand Maximum and until ~2009, when they fall to zero. We test an autoregressive process of order 1 (AR(1)) as a model that can reproduce the high frequency component of ISSN: we compute r1 for this AR(1) process, and find that it is negative. Positive values of r1 are found only if the process involves positive correlation: this leads us to suggest that the births of successive spots are positively correlated. We also show that the two-day correlation r2 of ISSN is, as expected, closer to 0 than r1. Finally, we identify two prominent regime changes in ~1915 and ~2009, strengthening previous evidence of major anomalies of solar activity at these dates.