A formula for the start of a new sunspot cycle

Relating the adjusted 10.7 cm solar flux to the start of a new sunspot cycle


It is known that the solar radio flux is strongly correlated to the sunspot cycle and that the flux is nearly the same at every minimum. If 64 sfu is taken as the baseline for all cycles, then we can calculate the proxy sunspot number from the flux, and vice versa. Furthermore, we find that the fitted adjusted flux divided by the fitted sunspot number gives a strong marker for the start of Solar Cycle 25 in October 2019. The high resolution 2K (based on 2048×2048 pixels SDO images) sunspot number currently indicates a start in June 2019.

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Data Sources

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  2. Source: WDC-SILSO, Royal Observatory of Belgium, Brussels. SILSO, World Data Center - Sunspot Number and Long-term Solar Observations, Royal Observatory of Belgium, online Sunspot Number catalogue: http://www.sidc.be/SILSO/, ‘year(s)-of-data 1947-2019’

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I thank the anonymous first and second reviewer and members of the editorial board for their useful comments and suggestions. I also thank the Chinese astronomer for his multiple comments.

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Correspondence to P. P. A. Geryl.

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Geryl, P.P.A., Alvestad, J. A formula for the start of a new sunspot cycle. Astrophys Space Sci 365, 113 (2020). https://doi.org/10.1007/s10509-020-03800-x

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  • Sunspot Cycle 25
  • Low sunspot activity
  • Sunspot cycle
  • Polarity sunspots
  • Polar fields Sun
  • 10.7 cm Solar flux
  • Reversed polarity sunspots