The polar field reversal is a crucial process in the cyclic evolution of the large-scale magnetic field of the Sun.Various important characteristics of a solar cycle, such as its duration and strength, and also the cycle predictability, are determined by the polar field reversal time. While the regular measurements of solar magnetic field have been accumulated for more than half a century, there is no consensus in the heliophysics community concerning the interpretation of the Sun's polar field measurements and especially the determination of polar field reversal time. There exists a severe problem of non-reproducibility in the reported results even from studies of the same observational dataset, and this causes an obstacle to make more accurate forecasts of solar cycle. Here, we analyze the solar magnetograms from four instruments for the last four cycles, to provide a more correct interpretation of the polar field observations and to find more accurate time of the reversals. We show the absence of triple (multipolar) reversals in Cycles 21 - 24, significant variations in the time interval between reversals in the hemispheres and in the time interval between a reversal and a cycle beginning. In order to understand the origin of the reversal time variation, we perform Surface Flux Transport (SFT) simulations and find out that the presence of the 'anomalous' bipolar magnetic regions (BMRs) in different phases of a cycle can cause cycle-to-cycle variations of the reversal time within the similar range found in observations.

We used the elaborated earlier catalog of the magneto-morphological classes (MMC) of active regions (ARs) to study 2046 ARs of the solar cycle (SC) 23 and 1507 ARs of the SC24. According to empiric rules for sunspot groups (Hale's polarity law, Joy's law, etc.) and MMC, all ARs (except for unipolar spots) were sorted out between two categories: A-type - regular bipolar ARs; B-type - all the rest irregular ARs. We found that the number of both regular and irregular ARs follows the cycle with the Pearson's correlation coefficient of 0.92 and 0.78, respectively. The regular ARs are distributed evenly between the two maxima of each cycle. The irregular ARs are also distributed evenly between the two maxima in the SC 23, however their number is enhanced in the second maximum of the SC 24. Both regular and irregular ARs exhibit strong north-south (N-S) asymmetry. The significance of asymmetry is confirmed using the Pearson's v-square test and one more test based on the normal approximation to a binomial distribution. During the two maxima of a cycle, the peaks in two hemispheres for both regular and irregular ARs number do not vary synchronously. This can be explained by the fluctu-ations in Babcock-Leighton mechanism. In general, there are more irregular ARs in the S-hemisphere in both cycles, which might be the result of an additional weakening of the toroidal field due to interplay between the dipole and quadrupole components of the global magnetic field. (c) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

— This study is devoted to the interplay between the global dynamo and small-scale dynamo and their mutual manifestations in the solar cycle. The cyclic variations of active regions violating the Hale’s Polarity Law (anti-Hale ARs) from 1989 to 2020 are investigated. Synthetic cycle data (14 838 sunspot groups from solar cycles 16–18, 23, and 24, including 367 anti-Hale ARs) are also used to improve the statistics. The temporal variations in the number and area of the anti-Hale groups are found to be synchronous with the cycle, which indicates their intrinsic connection to the global dynamo. During the second maximum of the cycle, the number of anti-Hale ARs is greater than during the first maximum. This can be due to the loss of regularity of the toroidal field and the possible interference of the small-scale dynamo. In the ascending phase and during the cycle maximum (when the toroidal field is strong), the relative number of anti-Hale groups is almost constant. During spatiotemporal intervals of the weakened toroidal field on the butterfly diagram, the global dynamo action is less pronounced and the role of the small-scale dynamo in their interplay is more significant. The weak tendency for the fraction of anti-Hale ARs to increase as the cycle progresses and activity is approaching the equator can indicate the presence of obstacles to dynamo-wave propagation and be related to the α-quenching known from mean-field dynamo theory.

The spatial-temporal picture of appear-ance of active regions and the relationship of their ap-pearance with the structure and development of a large-scale magnetic field were studied during the transition from solar cycle 24 to 25. During this period, solar ac-tivity is low, and therefore the dynamics of a large-scale magnetic field in the appearance of new active regions is most noticeable. We have used SDO/HMI data on the longitudinal magnetic field to determine the time and heliographic coordinates of the origin of an active re-gion, as well as daily WSO maps (Wilcox Solar Obser-vatory) to compare with the structure of the large-scale magnetic field. We have obtained the following results. During the transition from one cycle to another, new active regions appeared in half of the cases in the polari-ty inversion line of the large-scale magnetic field, and almost exclusively at the Hale boundaries in the corre-sponding hemispheres and solar cycles. In other cases, the places of appearance were unipolar regions of the large-scale magnetic field without a clear advantage in the location of the field regions according to the Hale law. The formation of active regions is preceded or ac-companied by changes in the structure of the large-scale magnetic field. At the same time, in the fine structure of the magnetic field in the photosphere we can observe an increase in the magnetic field network on a spatial scale of the size of supergranules and larger, as well as the appearance of small regions of a new magnetic field of both polarities. The appearing active regions were con-centrated in two narrow longitudinal zones that covered both hemispheres of the Sun. The new cycle began in the same longitudinal zones, where the activity of the old cycle decayed.

Long observational series for bipolar active regions (ARs) provide significant information about the mutual transformation of the poloidal and toroidal components of the global solar magnetic field. The direction of the toroidal field determines the polarity of leading sunspots in ARs in accordance with the Hale's polarity law. The vast majority of bipolar ARs obey this regularity, whereas a few percent of ARs have the opposite sense of polarity (anti-Hale ARs). However, the study of these ARs is hampered by their poor statistics. The data for five 11-year cycles (16-18 and 23,24) were combined here to compile a synthetic cycle of unique time length and latitudinal width. The synthetic cycle comprises data for 14838 ARs and 367 of them are the anti-Hale ARs. A specific routine to compile the synthetic cycle was demonstrated. We found that, in general, anti-Hale ARs follow the solar cycle and are spread throughout the time-latitude diagram evenly, which implies their fundamental connection with the global dynamo mechanism and the toroidal flux system. The increase in their number and percentage occurs in the second part of the cycle, which is in favour of their contribution to the polar field reversal. The excess in the anti-Hale ARs percentage at the edges of the butterfly diagram and near an oncoming solar minimum (where the toroidal field weakens) might be associated with strengthening of the influence of turbulent convection and magnetic field fluctuations on the arising flux tubes. The evidence of the misalignment between the magnetic and heliographic equators is also found.

The polarity of leading sunspots in active regions (ARs) is determined by the direction of the toroidal component of the global solar magnetic field. The vast majority of bipolar ARs have the certain leading sunspot polarity in accordance with the Hale's polarity law, whereas some ARs have the opposite polarity (anti-Hale ARs). The number of such groups is relatively small (about 4%), which complicates some important tests. The data on 14838 ARs (including 367 anti-Hale groups) related to five 11-yr cycles (16-18 and 23, 24) were combined in a synthetic cycle of unique time length and latitudinal width. A specific routine was created for this purpose. Our analysis of the synthetic cycle shown that anti-Hale ARs follow the cycle and their time-latitude distribution quite uniform, which implies their genetic connection with the global dynamo. The higher values of anti-Hale ARs percentage near the equator may be explained by the misalignment between the magnetic and heliographic equators. The increase in the number and percentage of anti-Hale ARs that occurs in the second part of the cycle might indicate the relationship with the process of the polar field reversal. The increase in the percentage that found in the solar minimum and at the edges of the butterfly diagram (zones where the magnetic field is weakened) might indicate facilitation of the processes that distort flux tubes during their rising through the convection zone. In addition, the synthetic cycle compilation method might be used for studying other magnetic activity tracers with poor statistics.

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 behaviour of the critical surges that lead to the polar field reversals is important. We analyse 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 build-up 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-yr 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.

We study the evolution of the photospheric magnetic field at the early stage of active region development. We use data on longitudinal component of the magnetic field and line-of-sight velocities from SOHO/MDI and SDO/HMI. The visual inspection of 48 cases of birth of active regions and the detailed analysis of the magnetic flux dynamics in 4 active regions have shown that at the time of emergence of a new magnetic field, the field of the following polarity is the first to be detected in the photosphere. The flux asymmetry of the leading and following polarities persists for several tens of minutes. The observed asymmetry of magnetic fluxes supports the results of the numerical simulation of emergence of the active region magnetic field in the upper layers of the convective zone, which has been carried out by Rempel and Cheung [2014].

In this paper, 2046 active regions of solar cycle 23 and 1507 active regions of solar cycle 24 observed during the period from May 1996 to December 2018 have been studied. The sunspot groups are distributed according to the recently proposed magneto-morphological classification. Regular active regions (obeing Hale’s polarity law, Joy’s law, and having a leading sunspot that prevails over the main tail sunspot), irregular sunspot groups, and unipolar spots have been identified. It is shown that regular active regions make the major contribution to the development of the cycle, which is consistent with the magnetic cycle models. The contribution of irregular sunspot groups is about two to five times smaller (at the cycle maxima) and comparable with the contribution of regular active regions in the cycle minima, which may indicate the joint action of the global mean field dynamo and the fluctuation dynamo. The increase in the number of irregular active regions in the southern hemisphere at the second maximum of each of the studied cycles can be explained by weakening of the toroidal field (produced by the global dynamo) and an increase in the contribution of the fluctuation dynamo to their competitive interaction. Comparison of the curves describing the time dependence of the sunspot group asymmetry index of regular and irregular active regions showed that, when the activity moves to the southern hemisphere, regular active regions are ahead irregular active regions by ~1.5–2 years. The application of the magneto-morphological classification made it possible to detect the alternating activity of the northern and southern hemispheres in both studied cycles; the order of a response of the hemispheres changed from cycle to cycle; the opposite priority with respect to each other was observed for regular and irregular active regions in the given cycle. Comparison of our results with the results on the cyclic variations of sunspot groups of simple and complex magnetic configuration in different hemispheres obtained earlier by other authors showed the following. An increase of the toroidal field produced by the global dynamo makes it difficult to detect asymmetry manifestations and to reveal the effect of the fluctuation dynamo on the magnetic tubes of the active regions.

There is no list of bipolar active regions (ARs) with reverse polarity (anti-Hale regions), although statistical investigations of such ARs (bearing the imprint of deep subphotospheric processes) are important for understanding solar-cycle mechanisms. We studied 8606 ARs from 1 January 1989 to 31 December 2018 to detect anti-Hale regions and to compile a catalog. The Solar and Heliospheric Observatory (SOHO) and the Solar Dynamics Observatory (SDO) data, as well as the Debrecen Photoheliographic Data, the Mount Wilson Observatory catalog and drawings, and the USAF/NOAA Solar Region Summary were used. Complex, ambiguous cases related to anti-Hale region identification were analyzed. Two basic and four additional criteria to identify an AR as an anti-Hale region were formulated. The basic criteria assume that: i) dominant features of an AR have to form a bipole of reverse polarity with sunspots/pores of both polarities being present; ii) magnetic connections between the opposite polarities have to be observed. A catalog of anti-Hale regions (275 ARs) is compiled. The catalog contains: NOAA number, date of the greatest total area of sunspots, coordinates, and corrected sunspot area for this date. The tilt and the most complex achieved Mount Wilson magnetic class are also provided. The percentage of anti-Hale groups meeting the proposed criteria is $\approx3.0\%$ from all studied ARs, which is close to early estimations by authors who had examined each AR individually: $\approx2.4\%$ by Hale and Nicholson (Astrophys. J. 62, 270, 1925) and $\approx3.1\%$ by Richardson (Astrophys. J. 107, 78, 1948). The enhancement of the anti-Hale percentage in later research might be related to: i) increasing sensitivity of instruments (considering smaller and smaller bipoles); ii) the ambiguities in the anti-Hale region identification. The catalog is available as the Supplementary Information and at the CrAO website ( sun.crao.ru/databases/catalog-anti-hale/ ).

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 H alpha 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 H alpha 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.

We study the plasma flows in the solar photosphere during the emergence of two small active regions, NOAA 9021 and 10768. Using Solar and Heliospheric Observatory/Michelson Doppler Imager data, we find that the strong plasma upflows appear at the initial stage of active region formation, with maximum upflow velocities of -1650 and -1320 m s(-1). The structures with enhanced upflows have size similar to 8 Mm in diameter, and they exist for 1-2 hr. The parameters of the enhanced upflows are consistent with those of the large active region NOAA 10488, which may suggest the possibility that the elementary emerging magnetic loops that appear at the earliest phase of active region formation have similar properties, irrespective of scales of active regions. Comparison between the observations and a numerical simulation of magnetic flux emergence shows a striking consistency. We find that the driving force of the plasma upflow is at first the gas pressure gradient and later the magnetic pressure gradient.

In order to clarify a possible role of small-scale dynamo in formation of solar magnetic field, we suggest an observational test for small-scale dynamo action based on statistics of anti-Hale sunspot groups. As we have shown, according to theoretical expectations the small-scale dynamo action has to provide a population of sunspot groups which do not follow the Hale polarity law, and the density of such groups on the time-latitude diagram is expected to be independent on the phase of the solar cycle. Correspondingly, a percentage of the anti-Hale groups is expected to reach its maximum values during solar minima. For several solar cycles, we considered statistics of anti-Hale groups obtained by several scientific teams, including ours, to find that the percentage of anti-Hale groups becomes indeed maximal during a solar minimum. Our interpretation is that this fact may be explained by the small-scale dynamo action inside the solar convective zone.

An explanation is suggested for the north-south asymmetry of the polar magnetic field reversal in the current cycle of solar activity. The contribution of the Babcock-Leighton mechanism to the poloidal field generation is estimated using sunspot data for the current activity cycle. Estimations are performed separately for the northern and southern hemispheres. The contribution of the northern hemisphere exceeded considerably that of the southern hemisphere during the initial stage of the cycle. This is the probable reason for the earlier reversal of the northern polar field. The estimated contributions of the Babcock-Leighton mechanism are considerably smaller than similar estimations for the previous activity cycles. A relatively weak (<1 G) large-scale polar field can be expected for the next activity minimum.