Solar Physics

, Volume 291, Issue 9–10, pp 2967–2979 | Cite as

Correlation Between Sunspot Number and Ca ii K Emission Index

  • Luca BertelloEmail author
  • Alexei Pevtsov
  • Andrey Tlatov
  • Jagdev Singh
Sunspot Number Recalibration


Long-term synoptic observations in the resonance line of Ca ii K constitute a fundamental database for a variety of retrospective analyses of the state of the solar magnetism. Synoptic Ca ii K observations began in late 1904 at the Kodaikanal Observatory in India. In the early 1970s, the National Solar Observatory (NSO) at Sacramento Peak (USA) started a new program of daily Sun-as-a-star observations in the Ca ii K line. Today the NSO is continuing these observations through its Synoptic Optical Long-term Investigations of the Sun (SOLIS) facility. These different data sets can be combined into a single disk-integrated Ca ii K index time series that describes the average properties of the chromospheric emission over several solar cycles. We present such a Ca ii K composite and discuss its correlation with the new entirely revised sunspot number data series. For this preliminary investigation, the scaling factor between pairs of time series was determined assuming a simple linear model for the relationship between the monthly mean values during the duration of overlapping observations.


Chromosphere, Active Magnetic fields, Chromosphere Solar Irradiance 



This work uses SOLIS/ISS data obtained by the NSO Integrated Synoptic Program (NISP), managed by the National Solar Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under a cooperative agreement with the National Science Foundation. The sunspot number data are provided by WDC-SILSO, Royal Observatory of Belgium, Brussels. We thank the anonymous referee for insightful comments that led to a significant improvement of the overall quality of the article.


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Copyright information

© Springer Science+Business Media Dordrecht 2016

Authors and Affiliations

  • Luca Bertello
    • 1
    Email author
  • Alexei Pevtsov
    • 2
  • Andrey Tlatov
    • 3
  • Jagdev Singh
    • 4
  1. 1.National Solar ObservatoryBoulderUSA
  2. 2.National Solar ObservatorySunspotUSA
  3. 3.Kislovodsk Solar Station of Pulkovo ObservatoryKislovodskRussia
  4. 4.Indian Institute of AstrophysicsBangaloreIndia

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