In this paper we analyze the distribution of magnetic strength ratios (MSR) across the solar disk using magnetograms in different spectral lines from the same observatory (Mount Wilson Observatory (MWO) and Sayan Observatory (SO)), magnetograms in the same line from different observatories (MWO, SO, Wilcox Solar Observatory (WSO)), and in different spectral lines from different observatories (the three observatories mentioned above, the National Solar Observatory/Kitt Peak (KP) and Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SoHO)). We find peculiarities in some combinations of data sets. Besides the expected MSR center-to-limb variations, there is an equatorto-pole asymmetry, especially in the near-limb areas. Therefore, it is generally necessary to use 2D matrices of correction coefficients to reduce one kind of observation into another one.

Detailed study of the solar magnetic field is crucial to understand its generation, transport and reversals. The timing of the reversals may have implications on space weather and thus identification of the temporal behavior of the critical surges that lead to the polar field reversals is important. We analyze the evolution of solar activity and magnetic flux transport in Cycles 21–24. We identify critical surges of remnant flux that reach the Sun’s poles and lead to the polar field reversals. We reexamine the polar field buildup and reversals in their causal relation to the Sun’s low-latitude activity. We further identify the major remnant flux surges and their sources in the time-latitude aspect. We find that special characteristics of individual 11-year cycles are generally determined by the spatiotemporal organization of emergent magnetic flux and its unusual properties. We find a complicated restructuring of high-latitude magnetic fields in Cycle 21. The global rearrangements of solar magnetic fields were caused by surges of trailing and leading polarities that occurred near the activity maximum. The decay of non-Joy and anti-Hale active regions resulted in the remnant flux surges that disturbed the usual order in magnetic flux transport. We finally show that the leading-polarity surges during cycle minima sometimes link the following cycle and a collective effect of these surges may lead to secular changes in the solar activity. The magnetic field from a Babcock–Leighton dynamo model generally agrees with these observations.

A method of synoptic map assimilation has been developed to study global rearrangements of open magnetic flux and formation of polar coronal holes (PCHs) in the current cycle. The analysis reveals ensembles of coronal holes (ECHs) which appear within unipolar magnetic regions associated with decaying activity complexes (ACs). The cause-effect relations between them explain asynchronous PCH formation observed at the northern and southern hemispheres of the Sun. Thus, the decay of large ACs, that were observed in 2014, led to formation of an extensive ECH, which then became the south PCH in mid-2015. Intricate structure of magnetic fields in the northern polar zone has impeded the north PCH formation, despite the fact that the dominant polarity at the North Pole reversed two years earlier than at the South Pole. The north PCH formed only by mid-2016 as the result of gradual merger of high-latitudinal ECHs associated with several decaying ACs.

The regular observation of the solar magnetic field is available only for about the 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 data sets. Long-term uniform observations for the past 100 yr as recorded at the Kodaikanal Solar Observatory (KoSO) provide such an 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 Hα lines from KoSO for the first time. The reconstruction method is based on the fact that the Ca II K intensity correlates well with the unsigned magnetic flux, while the sign of the flux is derived from the corresponding Hα map that 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 yr cycles reveal physical connections between them.

North-south asymmetry of sunspot activity resulted in an asynchronous reversal of the Sun's polar fields in the current cycle. The asymmetry is also observed in the formation of polar coronal holes. A stable coronal hole was first formed at the South Pole, despite the later polar-field reversal there. The aim of this study is to understand processes making this situation possible. Synoptic magnetic maps from the Global Oscillation Network Group and corresponding coronal-hole maps from the Extreme ultraviolet Imaging Telescope aboard the Solar and Heliospheric Observatory and the Atmospheric Imaging Assembly aboard the Solar Dynamics Observatory are analyzed here to study a causal relationship between the decay of activity complexes, evolution of largescale magnetic fields, and formation of coronal holes. Ensembles of coronal holes associated with decaying active regions and activity complexes are presented. These ensembles take part in global rearrangements of the Sun's open magnetic flux. In particular, the south polar coronal hole was formed from an ensemble of coronal holes that originated after the decay of multiple activity complexes observed during 2014.