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

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