Long term study of the solar filaments from the Synoptic Maps as derived from $H_{\alpha}$ Spectroheliograms of Kodaikanal Observatory

TL;DR

This study analyzes a century-long dataset of solar H-alpha spectroheliograms from Kodaikanal Observatory to investigate filament evolution, their relation to magnetic activity, and their role in polar reversal, using innovative digitization and mapping techniques.

Contribution

It presents the oldest uniform digitized dataset of solar filaments and introduces a semi-automated segmentation method, linking filament behavior to solar magnetic cycles.

Findings

Filaments exhibit poleward migration in butterfly diagrams.

Polar filament cycles lag behind sunspot cycles.

Polar filaments are linked to polar magnetic reversal timing.

Abstract

The century long (1914-2007) $H_{\alpha}$ (656.28 nm) spectroheliograms from Kodaikanal Solar Observatory (KSO) have been recently digitised. Using these newly calibrated, processed images we study the evolution of dark elongated on disk structures called filaments, potential representatives of magnetic activities on the Sun. To our knowledge this is the oldest uniform digitised dataset with daily images available today in $H_{\alpha}$. We generate Carrington maps for entire time duration and try to find the correspondences with maps of same rotation from Ca II K KSO data. Filaments are segmented from Carrington maps using a semi-automated technique and are studied individually to extract their centroids and tilts. We plot the time-latitude distribution of filament centroids producing Butterfly diagram, which clearly shows presence of poleward migration. We separate polar filaments for each cycle and try to estimate the delay between the polar filament number cycle and sunspot number cycle peaks. We correlate this delay with the same between polar reversal and sunspot number maxima. This provides new insight on the role of polar filaments on polar reversal.