Long-Term Evolution of the Sun's magnetic field during Cycles 15--19 based on their proxies from Kodaikanal Solar Observatory

TL;DR

This study reconstructs the Sun's magnetic field over Cycles 15-19 using Ca II K and Hα observations from Kodaikanal Solar Observatory, revealing insights into magnetic evolution, polarity reversals, and flux surges over a century.

Contribution

It introduces a novel method for reconstructing historical solar magnetic fields from Ca II K and Hα data, enabling analysis of magnetic evolution over multiple solar cycles.

Findings

Reconstructed magnetic maps show the evolution of magnetic flux and polarity reversals.

Identified the role of flux surges in polar field reversals.

Established physical links between flux surges and cycle transitions.

Abstract

The regular observation of the solar magnetic field is available only for about last five cycles. Thus, to understand the origin of the variation of the solar magnetic field, it is essential to reconstruct the magnetic field for the past cycles, utilizing other datasets. Long-term uniform observations for the past 100 years as recorded at the Kodaikanal Solar Observatory (KoSO) provide such opportunity. We develop a method for the reconstruction of the solar magnetic field using the synoptic observations of the Sun's emission in the Ca II K and H$\alpha$ lines from KoSO for the first time. The reconstruction method is based on the facts that the Ca II K intensity correlates well with the unsigned magnetic flux, while the sign of the flux is derived from the corresponding H$\alpha$ map which provides the information of the dominant polarities. Based on this reconstructed magnetic map, we study the evolution of the magnetic field in Cycles 15--19. We also study bipolar magnetic regions (BMRs) and their remnant flux surges in their causal relation. Time-latitude analysis of the reconstructed magnetic flux provides an overall view of magnetic field evolution: emergent magnetic flux, its further transformations with the formation of unipolar magnetic regions (UMRs) and remnant flux surges. We identify the reversals of the polar field and critical surges of following and leading polarities. We found that the poleward transport of opposite polarities led to multiple changes of the dominant magnetic polarities in poles. Furthermore, the remnant flux surges that occur between adjacent 11-year cycles reveal physical connections between them.