[Paper Review] Ca II K 1-A Emission Index Composites

Luca Bertello, Andrew R. Marble+1 more|arXiv (Cornell University)|Feb 2, 2017

Intermetallics and Advanced Alloy Properties3 citations

TL;DR

This paper presents a calibrated composite of the Ca II K 1-Å emission index using data from the NSO/Sac Peak, SOLIS/ISS, and Kodaikanal Solar Observatory, merging 100+ years of solar chromospheric activity measurements. By applying empirical scaling factors and linear regression to align disparate instruments, the authors produce a continuous, publicly available time series from 1907 to present, enabling long-term studies of solar variability and its climatic impacts.

ABSTRACT

We describe here a procedure to combine measurements in the 393.37 nm Ca II K spectral line taken at different observatories. Measurements from the National Solar Observatory (NSO) Integrated Sunlight Spectrometer (ISS) on the Synoptic Optical Long-term Investigations of the Sun (SOLIS) telescope, the NSO/Sac Peak Ca II K-Line Monitoring Program, and Ca II K filtergrams from Kodaikanal Solar Observatory (KKL) are merged together to create a pair of composites of the Ca II K 1-A emission index. These composites are publicly available from the SOLIS website at http://solis.nso.edu/0/iss/.

Motivation & Objective

• To create a continuous, calibrated time series of the Ca II K 1-Å emission index spanning over a century.
• To resolve discrepancies between historical measurements from the NSO/Sac Peak K-line monitor and the newer SOLIS/ISS instrument.
• To align the Kodaikanal plage index data with the NSO scale to extend the composite back to 1907.
• To produce a publicly accessible, homogeneous dataset for long-term solar activity and irradiance variability studies.

Proposed method

• Inter-calibration of NSO/Sac Peak and SOLIS/ISS data using overlapping observations from December 2006 to October 2015.
• Application of a quadratic fit to ISS data (2007–2011) to model the expected emission index trend during solar cycle 23/24 minimum.
• Empirical determination of a correction factor (0.984) for Sac Peak data between 2007.8 and 2010.0 to minimize residual differences from the ISS model.
• Use of linear regression between quasi-simultaneous daily measurements to derive a scaling equation: SP_ISS = (0.0062 ± 0.0015) + (0.8781 ± 0.0163) × SP_orig.
• Conversion of Kodaikanal plage index to the NSO 1-Å emission index scale using linear regression on annual means (1977–1999), yielding KKL_NS0 = (0.08217 ± 0.00049) + (0.00020 ± 0.00001) × KKL_orig.
• Construction of a final composite by merging rescaled Kodaikanal data (1907–1987) with NSO data (1988–2017), using ISS data as the primary reference for days with dual observations.

Experimental results

Research questions

• RQ1 How can historical Ca II K 1-Å emission index measurements from multiple observatories be inter-calibrated to form a continuous time series?
• RQ2 What scaling factors are required to correct for instrumental discrepancies between the NSO/Sac Peak monitor and the SOLIS/ISS instrument?
• RQ3 To what extent can the Kodaikanal plage index be reliably converted into the NSO 1-Å emission index scale?
• RQ4 How well do the rescaled data from different observatories align in terms of temporal trends and amplitude?
• RQ5 Can a single, homogeneous composite of the Ca II K 1-Å emission index be constructed from 1907 to 2017 with sufficient accuracy for long-term solar variability studies?

Key findings

• The Sac Peak 1-Å emission index values from 2007.8 to 2010.0 were found to be systematically overestimated by approximately 5.5% compared to ISS data, necessitating a correction factor of 0.984.
• The linear regression between Sac Peak and ISS data yielded a scaling equation with a correlation coefficient of r = 0.91, indicating a strong and statistically significant relationship (p < 0.0001).
• The Kodaikanal plage index was successfully converted to the NSO 1-Å emission index scale using a linear transformation with a correlation coefficient of r = 0.999, confirming high consistency between the two datasets.
• The final composite time series, spanning from January 1907 to October 2017, integrates data from Kodaikanal (1907–1987), NSO/Sac Peak (1977–2015), and SOLIS/ISS (2006–2017), ensuring continuity and calibration.
• The composite is publicly available via the SOLIS website at http://solis.nso.edu/0/iss/, enabling long-term studies of solar chromospheric activity and its influence on Earth's climate.
• The merged dataset provides a reliable proxy for solar ultraviolet irradiance variability over more than a century, supporting climate and space weather research.