Dependence of the Sunspot-group Size on the Level of Solar Activity and its Influence on the Calibration of Solar Observers

Abstract

The distribution of the sunspot group size (area) and its dependence on the level of solar activity is studied. It is shown that the fraction of small groups is not constant but decreases with the level of solar activity so that high solar activity is largely defined by big groups. We study the possible influence of solar activity on the ability of a realistic observer to see and report the daily number of sunspot groups. It is shown that the relation between the number of sunspot groups as seen by different observers with different observational acuity thresholds is strongly non-linear and cannot be approximated by the traditionally used linear scaling ($k-$factors). The observational acuity threshold [$A_{\mathrm{th}}$] is considered to quantify the quality of each observer, instead of the traditional relative $k-$factor. A nonlinear $c-$factor based on $A_{\mathrm{th}}$ is proposed, which can be used to correct each observer to the reference conditions. The method is tested on a pair of principal solar observers, Wolf and Wolfer, and it is shown that the traditional linear correction, with the constant $k-$factor of 1.66 to scale Wolf to Wolfer, leads to an overestimate of solar activity around solar maxima.

arXiv commentSolar Physics, accepted

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Cited by in corpus (6)

• New reconstruction of the sunspot group number since 1739 using the direct calibration and "backbone" methods
• Re-calibration of the Sunspot Number: Status Report
• Updated sunspot group number reconstruction for 1749-1996 using the active day fraction method
• Reconstructing solar irradiance from historical Ca II K observations. I. Method and its validation
• Scrutinizing the relationship between plage areas and sunspot areas and numbers
• Stephan Prantner's Sunspot Observations during the Dalton Minimum