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ADF (active-day fraction) method of sunspot number calibration

This paper introduces a new method for calibrating sunspot numbers using the Active Day Fraction (ADF) and a reference data set. The proposed method eliminates the need for daisy-chaining, providing independent calibrations for each observer. The method includes finding the observational threshold for each observer and applying a correction matrix. The new sunspot group series constructed using this method confirms the Grand Modern maximum but may slightly underestimate high cycles and overestimate low cycles.

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ADF (active-day fraction) method of sunspot number calibration

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  1. ADF (active-day fraction) method of sunspot number calibration I. Usoskin 1, G. Kovaltsov 2, M. Lockwood 3, K. Mursula 1, M. Owens 2, S. Solanki 4, T. Willamo 5 1 University of Oulu, Finland 2 Ioffe Phys-Tech. Inst., Russia 3 University of Reading, UK 4 Max Planck Institute for Solar System Research, Germany 5 University of Helsinki, Finland

  2. Calibration Daisy chain linear regression (Ri, Rg) ’Backbone’ daisy chain (SS15) Can we propose something better?

  3. New independent calibration Reference data set Active day fraction  No daisy-chain. All calibrations independent. * Another daisy-chain-free method by Andres Munos-Jaramillo

  4. 1. Calibration curve Reference data set (RGO, 1900-1976), sunspot groups with area. For each month  ADF: A=Nactive/Nobs Apply a threshold Ss  Cumulative pdf P(A, SS)

  5. 2. Finding SS For each observer we find P and compared with the calibration curves to find the observational threshold SS

  6. 3. Correction matrix Using the reference data set (RGO, 1900-1976) we build a correction matrix for each observer, using SS

  7. Individual corrections Each observer is calibrated independently to the reference data set.

  8. Summary series with 95% c.i.

  9. Final series

  10. Test

  11. Conclusions A new sunspot group series is constructed using the direct calibration method since 1749, normalized to the reference data set (RGO, 1900-1976). It lies between low GSN (Hoyt & Schatten, 1998) and high (Svalgaard & Schatten, 2016) series. The Grand Modern maximum is confirmed, but the high period in 18th century is not. The method may sightly understimate high cycles and strongly overestimate low cycles, and is not applicable to very low cycles (DM or MM).

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