Correlation Between Sunspot Number and ca II K Emission Index

TL;DR

This study constructs a long-term composite Ca II K emission index from multiple observatories and analyzes its correlation with the revised sunspot number series, revealing insights into solar magnetic activity over several cycles.

Contribution

It provides a unified Ca II K index time series and explores its correlation with sunspot numbers, combining historical and modern data sets.

Findings

Strong correlation between Ca II K index and sunspot number.

The composite index effectively captures solar activity variations.

Revised sunspot data enhances the analysis accuracy.

Abstract

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 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.