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

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.

6 pages including figures; accepted for publication in ApJL

References in corpus (8)

• Magnetic Flux Transport at the Solar Surface
• Calibrating 100 Years of Polar Faculae Measurements: Implications for the Evolution of the Heliospheric Magnetic Field
• Solar Cycle Variability Induced by Tilt Angle Scatter in a Babcock--Leighton Solar Dynamo Model
• Polar Network Index as a magnetic proxy for the solar cycle studies
• Recovering the unsigned photospheric magnetic field from Ca II K observations
• Long term study of the solar filaments from the Synoptic Maps as derived from $H_{\alpha }$ Spectroheliograms of Kodaikanal Observatory
• Evolution of the Sun's Polar-fields and the Poleward Transport of Remnant Magnetic Flux
• Association of Plages With Sunspots: A multi wavelength Study Using Kodaikanal Ca $\text{II}$ K and Greenwich sunspot area Data

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• Analysis of full-disc H alpha observations: Carrington maps and filament properties in 1909-2022
• Detection of Solar Filaments using Suncharts from Kodaikanal Solar Observatory Archive Employing a Clustering Approach
• Solar Cycle Prediction: Challenges, Progress, and Future Perspectives
• Evolution of the Sun's activity and the poleward transport of remnant magnetic flux in Cycles 21--24