Butterfly Diagram and Carrington Maps for Century-Long Ca II K Spectroheliograms from Kodaikanal Observatory

TL;DR

This study analyzes a century-long dataset of Ca II K spectroheliograms from Kodaikanal Observatory to track solar plages, revealing their evolution, distribution, and correlation with magnetic activity over 9.5 solar cycles.

Contribution

It provides the longest continuous analysis of Ca II K data, including the creation of butterfly diagrams and magnetic activity proxies from historical observations.

Findings

High correlation (95.6%) with Mount Wilson plage areas.

Clear butterfly diagram showing plage evolution over cycles.

Long-term Ca II K data can serve as magnetic activity proxies.

Abstract

The century-long (1907-2007) Ca II K spectroheliograms from Kodaikanal Solar Observatory (KSO) are calibrated, processed and analysed in the present study to follow the evolution of bright on disc structures called plages, the possible representatives of magnetic activity on the Sun. This has been the longest dataset studied in Ca II K till date covering about 9.5 cycles of 11 year periods. Plages were segmented with area $\geq 1\:\textrm {arcmin}^2$ using global thresholds for individual full disc images and subsequent application of morphological closing operation. Plage index was calculated and seen to have close positive correlation with fractional disc area covered by plages. The newly generated plage area cycle (from KSO) was compared with the same from Mount Wilson observatory (Correlation~$95.6\%$) for the overlapping years i.e. 1915-2000. Study illustrated time-latitude distribution of plage centroids rendering butterfly diagram (as observed for sunspots). The 3D visualisation of the diagram showed one to one mapping between plage location, time and area. This work further delineated positional correlation between magnetic patches and plage regions through comparison of synoptic maps derived from both Kodaikanal Ca II K images and space based full disc LOS (line of sight) magnetograms. Regular synoptic magnetograms from ground based observatories are available only after 1970s. Thus the long term Ca II K data from KSO can be used as a proxy for estimating magnetic activity locations and their strengths at earlier times.