Kodaikanal calcium images: Detection of plages, Fixing the heliographic coordinates and Estimation of Area

TL;DR

This paper presents a method for detecting and accurately locating plages in nearly a century of Ca II K images from Kodaikanal Observatory, validating the results against sunspot and magnetic plage data, and providing a publicly available dataset.

Contribution

The authors developed a novel Python-based method to detect plages, fix their heliographic coordinates, and estimate their areas from historical solar images, with validation against other solar features.

Findings

Heliographic coordinates of plages match well with sunspots and magnetic plages.

The method provides accurate positional data for nearly a century of solar images.

A publicly accessible dataset of plage areas and coordinates is created.

Abstract

Kodaikanal Observatory is a veritable treasure trove of data, with the data repository covering almost 100 years of observations. For the years 1909-2007, we use calibrated Ca II K spectroheliograms from the Kodaikanal Observatory to detect the plages, fix their heliographic coordinates and also estimate the plage areas. We adopt the following procedure. After ensuring that, for all the years, Kodai calcium images have very negligible ellipcity, a circle is fitted and two central coordinates and radius of calcium images are determined uniquely. For each pixel of the calcium image, we then fix heliographic coordinates and extract plages along with their weighted average coordinates. The heliographic coordinates of these extracted plages are then compared with the heliographic coordinates of photospheric sunspots from the Greenwich sunspot database and chromospheric magnetic plages detected from the SOHO/MDI magnetograms. We find that the heliographic coordinates of calcium plages match very well with the heliographic coordinates of sunspots and magnetic plages authenticating our method of detection of plages and computation of positional coordinates. A code is developed in Python and all the nearly century scale plages data, with accurately estimated heliographic coordinates and areas, is available to the public.