Solar Phys (2014) 289:1971–1981DOI 10.1007/s11207-013-0456-8

Reversals of the Sun’s Polar Magnetic Fields in Relationto Activity Complexes and Coronal Holes

A.V. Mordvinov · S.A. Yazev

Received: 17 June 2013 / Accepted: 21 November 2013 / Published online: 10 December 2013© Springer Science+Business Media Dordrecht 2013

Abstract A spatiotemporal analysis of long-term measurements of the Sun’s magnetic fieldwas carried out to study changes in its zonal structure and reversals of the polar fields inCycles 21 – 24. A causal relationship between activity complexes, their remnant magneticfields, and high-latitude magnetic fields has been demonstrated in the current cycle. Theappearance of unipolar magnetic regions near the poles is largely determined by the decayof long-lived activity complexes. The nonuniform distribution of sunspot activity and itsnorth–south asymmetry result in the asymmetry of remnant fields that are transported pole-ward due to meridional circulation. The asymmetry of high-latitude magnetic fields leads toan asynchrony of polar-field reversals in both hemispheres. The interaction of high-latitudeunipolar magnetic regions with the polar fields affects the embedded coronal holes. The evo-lution of large-scale magnetic fields was also studied in a time–latitude aspect. It is shownthat regular reversals of the Sun’s polar fields resulted from cyclic changes in high-latitudemagnetic fields. A triple polarity reversal of the polar fields in Cycle 21 and short-term po-larity alternations at the poles were interpreted taking into account the interaction of theremnant fields with the Sun’s polar fields.

Keywords Active regions, magnetic fields · Magnetic fields, photosphere · Coronal holes ·Solar cycle, observations

1. Introduction

The Sun’s magnetic field exhibits a multiscale and hierarchical organization. The poloidaland toroidal components of the global magnetic field of the Sun vary within an 11/22-year

A.V. Mordvinov (B) · S.A. YazevInstitute of Solar-Terrestrial Physics of Siberian Branch of Russian Academy of Sciences,Lermontov st., 126a, Irkutsk 664033, Russiae-mail: avm@iszf.irk.ru

S.A. YazevIrkutsk State University, Karl Marx st., 1, Irkutsk 664003, Russiae-mail: syazev@gmail.com

1972 A.V. Mordvinov, S.A. Yazev

cycle and interact with magnetic fields of active regions. The reversals of the Sun’s polarfields are of crucial importance for understanding the origin of solar magnetism and itscyclic changes. The mean-field dynamo infers that reversals of the polar magnetic fieldsresult from the interaction of low-order modes, which essentially determine cyclic changesof photospheric magnetic fields (DeRosa, Brun, and Hoeksema, 2012).

It is also appropriate to consider the polar-field reversals in causal relation to the decayof long-lived activity complexes and their remnant fields during the recent cycles. Earlystudies of solar magnetograms with a moderate resolution revealed weak, diffuse magneticfields that arise from the decay of active regions and migrate poleward (Babcock and Bab-cock, 1955). Based on these observations, an empirical concept has been developed thatqualitatively explains the origin of solar magnetic fields and their cyclic changes (Leighton,1969). The Babcock–Leighton concept suggests that the following polarities of magneticbipoles drift poleward and form the polar field of a new cycle.

A further development of this concept revealed an important role of the differential rota-tion of the Sun, meridional circulation, and diffusion in the convection zone (DeVore, Boris,and Sheeley, 1984; Sheeley, 1992; Sheeley, 2005). Numerical models adequately describedthe cyclic evolution of the background fields, which are shaped by the differential rotationand meridional circulation (Wang, Nash, and Sheeley, 1989; Wilson, McIntosh, and Snod-grass, 1990; Worden and Harvey, 2000; Schrijver, de Rosa, and Title, 2002). The inclusionof these processes in the dynamo theory has led to its further development (Choudhuri,Schüssler, and Dikpati, 1995; Dikpati and Charbonneau, 1999). The flux-transport dynamomodels satisfactorily reproduce major solar cycle features, including the evolution of thelarge-scale magnetic field of the Sun and its global reversals and the unusual behavior of so-lar activity in the current cycle (Dikpati et al., 2004; Nandy, Munoz-Jaramillo, and Martens,2011).

This approach was also supported experimentally. According to direct measurementsand helioseismology estimates, a typical velocity of the meridional flow is approximately10 – 20 m s−1 (Hathaway, 1996; Basu and Antia, 2010). Meridional transport concentratesmagnetic fields of mixed polarities in the Sun’s polar zones. High-resolution measurementsrevealed that the Sun’s polar fields are composed of small-scale magnetic elements reach-ing above 1 kG (Tsuneta et al., 2008). As the activity cycle progresses, magnetic flux ofpredominantly following polarity is transported to the poles of the Sun (Benevolenskaya,Kosovichev, and Scherrer, 2001). Magnetic elements of predominantly the same polaritiescover the polar regions of the Sun and contribute to its poloidal field.

Magnetic activity is distributed nonuniformly across the heliographic longitude andappears asynchronously in the northern and southern hemispheres of the Sun (Mursulaand Hiltula, 2004). Sunspot activity varies nonmonotonically and progresses in impulses.Sunspots tend to recur in sunspot nests (Gaizauskas et al., 1983; Obridko, 1985). As a re-sult, long-lived activity complexes (ACs) appear. Strong magnetic fields of ACs are thoughtto be the manifestation of the toroidal magnetic field from the solar interior. As an 11-yearcycle progresses, sunspot activity zones drift equatorward according to Spörer’s law. Anal-yses of the east-west tilt angle of surface magnetic fields and flows also revealed the spa-tiotemporal evolution of the toroidal magnetic fields (Ulrich and Boyden, 2005; Lo, Hoek-sema, and Scherrer, 2010) and their relation to sunspot activity (Mordvinov, Grigoryev, andPeshcherov, 2012). Segments of the toroidal magnetic field emerge on the surface of theSun as �-shaped bipolar magnetic structures. After the decay of ACs, their magnetic fieldsdissipate and, later, spread over vast areas, forming large-scale unipolar magnetic regions(UMRs) in the photosphere (van Driel-Gesztelyi, 1998).

In the present study the Sun’s magnetic fields were analyzed to examine cyclic changesin their zonal structure. Reversals of the polar fields were studied in relation to long-lived

Reversals of the Sun’s Polar Magnetic Fields 1973

ACs, their remnant fields, and high-latitude UMRs. Special attention is paid to the evolutionof coronal holes (CHs) in the current solar cycle.

2. Evolution of Large-Scale Magnetic Fields and CHs in the Current Cycle

The current Cycle 24 started after a deep and prolonged minimum, and so far, magneticactivity of the Sun has been somewhat lower than in recent cycles. In the beginning of thecurrent cycle the background fields of the Sun were very weak and fragmentary. It becamepossible to detect the remnant magnetic fields that formed after the decay of the first activitycomplexes (Mordvinov and Yazev, 2013). To study the evolution of the Sun’s magneticfields we analyzed synoptic maps of the radial field from the Kitt Peak observatory (Harveyand Worden, 1998). The synoptic maps and their zonal averages adequately describe cyclicrearrangements of solar magnetic fields (Ulrich and Boyden, 2005; Hoeksema, 2010).

The rearrangements of solar magnetic fields are also exhibited in the spatiotemporal be-havior of coronal holes (Wang, 2009; Karachik, Pevtsov, and Abramenko, 2010). To studythe evolution of UMRs in their relationship to ACs and CHs, we analyzed synoptic maps ofthe EUV emission in Fe XII 19.5 nm, based on the EUVI/STEREO measurements (Howardet al., 2008). In these synoptic maps long-lived ACs appear as bright features that persistover several Carrington rotations. Low-emission patterns are also visible in the EUV syn-optic maps. Mid-latitude CHs, which are embedded in UMRs, are also involved in theirpoleward migrations (Harvey and Recely, 2002; Wang, 2009). Thus, UMRs and CHs areclosely related because UMRs indicate the probable location of CHs, their polarity, andpoleward migration.

In this study we analyzed the synoptic maps of the logarithm of the Fe XII 19.5 nm emis-sion (log(E)). These synoptic maps represent low-emission patterns better than its originalview. By analogy with previous studies (Krista and Gallagher, 2009; de Toma, 2011), CHswere identified from the local intensity thresholding. The threshold values were chosen nearthe local maxima at the low-end intensity distributions of the EUV synoptic maps.

Figures 1a – e depict synoptic maps for Carrington rotations (CRs) 2102, 2107, 2121,2130, and 2140, which demonstrate the evolution of solar magnetic fields compared withassociated changes seen in the EUV emission (Figures 2a – e). To demonstrate large-scalepatterns of weak magnetic fields, the synoptic maps from the Kitt Peak observatory weresmoothed using a median filter with a mask of 3×3 pixels. In the smoothed distributions(Figures 1a – e) there are well-defined patterns of both magnetic polarities. These patternsreveal vast regions of dominant magnetic polarities of a few Gauss in strength. The regionsare not entirely unipolar, because they include small-scale magnetic fields of opposite polar-ities. As the cycle progresses, these large-scale patterns are shaped by differential rotationand meridional circulation. Cumulative distributions of magnetic fields that exceed 200 Gin absolute value are shown as black areas and crosses. In each synoptic map (Figures 1a –e) the cumulative distribution summarizes strong magnetic fields across the 12 precedingCRs. For example, the synoptic map for CR 2102 (Figure 1a) also shows contours of strongfields for CRs 2090 until 2102. The map for CR 2107 (Figure 1b) shows strong fields forCRs 2095 until 2107, and so on. Despite the differential rotation, highly concentrated strongmagnetic fields exist in long-lived ACs, which recur near the same heliographic coordinatesfrom rotation to rotation.

Figure 1a depicts at smoothed distribution of solar magnetic fields in CR 2102 (October2010) at the beginning of the current cycle. In the northern and southern hemispheres thereare well-defined UMRs of both magnetic polarities. To relate the UMRs with preceding

1974 A.V. Mordvinov, S.A. Yazev

Figure 1 Synoptic maps of solarmagnetic fields (at values−3 – 3 G, red to blue) and thecumulative distributions of strongfields (> 200 G in modulus,black areas and crosses). Thebackground fields are comparedwith the distributions of strongmagnetic fields during the 12preceding synoptic maps for CRs2102 (a), 2107 (b), 2121 (c),2130 (d), and 2140 (e).

Figure 2 Synoptic maps of theFe XII 19.5 nm emissionlogarithm in CRs 2102 (a),2107 (b), 2121 (c), 2130 (d), and2140 (e). The arrows indicate thelocation of polarity inversionlines near the poles.

ACs, magnetic fields above 200 G in absolute value are shown as black contours. Thesefields are associated with active regions and sunspot groups that have existed during the12 preceding CRs. There are well-defined activity clusters, where strong magnetic fieldsconcentrate from rotation to rotation. These magnetic clusters describe recurrent ACs in

Reversals of the Sun’s Polar Magnetic Fields 1975

both hemispheres. The evolution of solar magnetic fields is also demonstrated in the videosequence of the synoptic maps presented in the supporting material.

Magnetic polarities of the clusters follow Hale’s law. In the northern hemisphere the lead-ing and following polarities of magnetic clusters during Cycle 24 are negative and positive,respectively. In the southern hemisphere the leading and following polarities of magneticclusters are opposite in sign. Negative polarity dominates near the north pole, while positivepolarity dominates in the south polar zone, and its latitude extent exceeds that in the northernhemisphere.

In the northern hemisphere four patterns of following polarity stretched from the mag-netic clusters to higher latitudes in late 2010. A compact magnetic cluster centered at theheliographic longitude λ = 230◦ and latitude ϕ = 30◦ resulted in an extended UMR of pos-itive polarity that stretched up to latitudes of about 60◦. The edges of the UMRs (Figure 1a)are aligned, revealing the profile of the Sun’s differential rotation. It takes about a year for theremnant magnetic fields to migrate to latitudes of about 60◦. In the distribution of magneticfields (Figure 1a) negative polarity dominates at latitudes >60◦. In the southern hemisphere,compact magnetic clusters resulted in two patterns of following (negative) polarity. TheUMRs of leading polarities also exhibit a poleward drift. For example, the magnetic clustercentered at λ = 210◦ and latitude ϕ = −15◦ (southern hemisphere) resulted in the leadingpolarity UMR whose latitudinal extent exceeded that of the following polarity. Thus, therelationship between the UMRs and preceding ACs is clearly visible in the beginning of thecurrent cycle.

Figure 2a shows the distribution of the Fe XII 19.5 nm emission logarithm log(E) for CR2102. The boundaries of CHs were determined using the local thresholding algorithm (Kristaand Gallagher, 2009; de Toma, 2011). In Figures 2a – e these boundaries are marked withblack contours, which slightly increase the edge contrast. In the distribution of log(E), CHsand other low-emission patterns become clearly visible against the background of brighterareas. The polar and low-latitude CHs are surrounded by dark features whose emissionslightly exceeds the intensity threshold. Vast dark areas dominate in the southern hemisphereand near the solar equator. Low-emission areas surround the polar CHs. Narrow featuresseen in low-emission are observed as extensions of CHs at low latitudes. The dark fea-tures sometimes correspond to chromospheric filaments and trace the boundaries of UMRs.At low-latitudes, the narrow dark features are locally aligned to the edges of surroundingUMRs.

Figure 1b shows the synoptic map for CR 2107 (February – March 2011). Magnetic pat-terns seen in CR 2102 are still discernible in CR 2107. At high latitudes these are shifted tothe left because their rotation is slower than the Carrington rotation period of 27.2753 d. Inthe northern hemisphere the most extended UMR of positive polarity shifted left by about90◦. Some positive-polarity UMRs dispersed at higher latitudes and reached the north poleof the Sun. In early 2011, positive magnetic polarity was detected at the north pole for a shortperiod (the WSO online database, Hoeksema and Scherrer, 1986). At this time, the highestlatitudes near the north pole of the Sun were, however, invisible because of the inclinationof the Earth’s orbit relative to the solar equator. Small-scale magnetic fields of negative po-larity still remained near the north pole. The cancellation of these fields with approachingUMRs of positive polarity is visible in a bright EUV emission (Figure 2b). The north-southasymmetry of microwave emission also suggests that solar maximum conditions prevailedin the northern hemisphere during this period (Gopalswamy et al., 2012).

The most extensive low-emission features at high latitudes correspond to the polarityinversion lines of the large-scale magnetic fields that are traced by the polar-crown fila-ments. These features trace the boundaries of high-latitude UMRs and move toward the

1976 A.V. Mordvinov, S.A. Yazev

poles of the Sun (Makarov, Fatianov, and Sivaraman, 1983; McIntosh and Wilson, 1985).Low-emission features related to high-latitude polarity inversion lines are marked by arrowsin Figures 2b – d. In the southern hemisphere two UMRs of positive polarity stretched to thepolar zone and extended its area between longitudes 60◦–180◦ (Figure 1b). The fragmentarymagnetic fields of negative polarity approached the subpolar zone of the southern hemi-sphere and appeared near the south pole between longitudes 330◦ – 360◦. These remnantfields resulted from the UMR of negative polarity that stretched from the magnetic clustercentered at ϕ = −30◦, λ = 130◦ in CR 2102 and is discernible also at mid-latitudes in CR2107. The cancellation of high-latitude fields of opposite polarities became visible in theEUV emission in both polar zones (Figures 2b – e).

As sunspot activity increases, many long-lived ACs appear (Figures 1c and d). Theirdecay and dispersal resulted in multiple UMRs seen in CRs 2121 (March 2012) and 2130(November – December 2012). The UMRs of opposite polarities interact at low- and mid-latitudes. At the north pole negative polarity recovered again, while subsequent UMRs ofpositive polarity approached the polar zone. At the south pole positive polarity dominates.However, its latitude extent became smaller in CR 2130 than that in CR 2121. This tendencywas continued in the following CRs due to the poleward migration of the subpolar UMRsof negative polarity. Adjacent magnetic fields of open and closed configurations interactedbecause of the interchange reconnection that resulted in the decrease in the dominant mag-netic polarity area at the pole. The reconnection was accompanied by a release of energythat is observed in the EUV emission that covers most of the polar zones except the CHs(Figures 2c and d). In the southern hemisphere a narrow low-emission feature stretched overthe whole EUV synoptic map for CR 2130 (Figure 2d). This indicates the line of polarityinversion of the large-scale magnetic fields, which are transported toward the Sun’s southpole.

During the phase of activity rise, magnetic clusters appeared at all longitudes in the north-ern hemisphere. The widespread emergence of active regions and their subsequent decayresulted in remnant fields of opposite polarities at high-latitudes in CR 2130 (Figure 1d).Such an alternation led to a mixture of opposite polarities near the north pole. The predom-inance of positive polarity is stronger in CR 2140 (Figure 1e) than in the preceding CRs. Inthe southern hemisphere, the extended UMR of negative polarity partially reached the southpole. This disposition of the subpolar UMRs predetermines the polar-field reversal in thenear future.

There is a large CH centered at ϕ = 29◦, λ = 221◦ (Figure 2e). This CH overlies UMRsof negative polarity in the northern hemisphere (Figure 1e). These UMRs are parts of atransequatorial structure that stretched to the south pole of the Sun. The narrow low-emissionfeatures follow along this magnetic structure from the CH to the polar zone of the southernhemisphere. It is reasonable to suggest that CHs take part in the cyclic rearrangements ofthe Sun’s open magnetic flux.

3. Polar-Field Reversals in Relation to Remnant Fields of Activity Complexes

After the decay of ACs, their remnant magnetic fields disperse, form facular fields and aresubsequently redistributed in the surrounding photosphere. We study cyclic changes in thelarge-scale magnetic field of the Sun in relation to its sunspot activity. Sunspot areas quantifythe strength and position of strong, local magnetic fields of the Sun. Figures 3a and c showchanges in total sunspot areas in the northern and southern hemispheres (AN, AS); thesevalues are measured in millionths of a solar hemisphere area (µHem). Both AN and AS vary

Reversals of the Sun’s Polar Magnetic Fields 1977

Figure 3 Changes in total sunspot area in the northern hemisphere (a); time-latitude diagram of the aver-aged magnetic fields (at values −7 – 7 G, blue to red) and the distribution of sunspot areas (black contourscorresponding to 100 µHem) for 1975 – 2013 (b); changes in total sunspot area in the southern hemisphere (c).

in a pulse-like fashion. The current Cycle 24 is characterized by a lower activity level thanprevious cycles. There is a significant north–south asymmetry of sunspot activity (Hathaway,2010). In the current cycle, the sunspot activity in the northern hemisphere exceeded that inthe southern one until the middle of 2012.

The evolution of the Sun’s large-scale magnetic fields and reversals of the polar fieldswere studied using long-term synoptic observations from Kitt Peak, Mount Wilson, andStanford observatories (Harvey and Worden, 1998; Ulrich and Boyden, 2005; Hoeksema,2010, respectively). The zonal structure of the Sun’s magnetic field was estimated usinglongitude averages of successive synoptic maps. Following this approach, we estimated thezonal averages of the synoptic maps from the Kitt Peak observatory for CRs 1905 – 2142.A time–latitude diagram was constructed of the latitude profiles obtained from longitude-averaging of successive synoptic maps from rotation to rotation. To reduce the noise thetime–latitude distribution obtained was denoised using its wavelet decomposition.

Figure 3b depicts the time-latitude diagram of solar magnetic fields after wavelet denois-ing. The time-latitude diagram reveals well-defined patterns of opposite magnetic polarities.These magnetic domains are mostly related to vast UMRs seen in the synoptic maps. Thewavelet-denoised averages of zonal magnetic flux adequately characterize the distributionof dominant magnetic polarities, although at any given latitude the two magnetic polaritiesare generally mixed. Bearing this in mind, we denote the patterns of dominant magneticpolarities in the time-latitude diagram as UMRs to avoid redundant terms.

The time-latitude diagram contains a wealth of information about the cyclic behaviorof large-scale magnetic fields of the Sun. Opposite polarities replace each other in polarregions of the Sun, thereby demonstrating the reversals of its polar fields. There were regularreversals of polar fields of the Sun that occurred after sunspot activity maxima. In Cycle 23polar fields reversed at the north and south poles by mid-2001, while in Cycle 22 the polarfield reversed at the north and south poles of the Sun in late 1991 and mid-1990, respectively.

1978 A.V. Mordvinov, S.A. Yazev

A triple reversal of the polar field occurred in Cycle 21 at the north pole. The polar magneticfield changed its sign from positive to negative in mid-1980, then from negative to positivein early 1981, and finally from positive to negative in mid-1982. Despite the differences inthe behavior of the polar magnetic fields in Cycles 21 – 23, the underlying physical causesremain the same.

In the northern and southern hemispheres there are also well-defined cyclic patterns ofdifferent polarities that resemble the time-latitude distribution of sunspot activity. By anal-ogy with the butterfly diagram of sunspots, leading polarities of bipolar magnetic regionsdominate at low latitudes and demonstrate the equatorward drift – Spörer’s law. These pat-terns characterize sunspot zones where ACs appear. Leading-polarity patterns are also com-posed of remnant fields dispersed in the photosphere after decay of the leading sunspots ofmagnetic bipoles. At the phase of activity increase these patterns migrate equatorward alongwith sunspot activity zones. Toroidal magnetic fields, causally related to sunspot activity,also exhibit a similar drift (Mordvinov, Grigoryev, and Peshcherov, 2012). In Figure 3b thinblack contours localize the latitudes of sunspot appearance (at value 5 µHem), the thickblack contours indicate latitudes of intense sunspot activity above 100 µHem. The sunspotarea distribution was smoothed using the median filter to reduce its fluctuations.

Patterns of following polarities dominate at mid-latitudes of the time-latitude diagram(Figure 3b). These typically appear near the upper edges of the sunspot activity zones andthen migrate to the polar zones. The dominant patterns of following polarities clearly demon-strate the poleward migration of the UMRs. These patterns are formed after the appearanceof the first ACs and represent the joint effect of the meridional circulation and diffusionof remnant magnetic fields. The typical speed of meridional circulation is estimated to be20 m s−1 (Basu and Antia, 2010; Hathaway, 1996), although at the beginning of the currentcycle this speed was about 10 m s−1 (Hathaway and Rightmire, 2010).

In the time-latitude diagram of solar magnetic fields thick black contours indicate long-lived ACs that persist for several rotations. After the decay of ACs their strong magneticfields disperse in the surrounding photosphere because of diffusion and meridional circula-tion. When comparing changes in sunspot areas with the time-latitude distribution of mag-netic fields, it becomes evident that most prominent peaks of AN and AS indices are associ-ated with the inclined patterns that are most often composed of following polarities. Thesepatterns are composed of remnant magnetic fields originating from the decay of ACs. Duringperiods of high magnetic activity, zones of intense sunspot activity are typically followed byflows that reach the polar zones of the Sun.

Near the maximum of every 11-year cycle, remnant fields of ACs approach the polarzones. Sometimes remnant fields penetrate through the polar magnetic fields of oppositepolarity and reach the poles of the Sun. These short-term changes in sign of polar fieldsare the precursors of cyclic reversals of polar fields. Such short-term changes in sign of thepolar fields preceded the regular reversals in Cycles 21 – 23. In the current cycle short-termpolarity reversals took place at the north pole in 2010 – 2011. Despite the low activity levelin the current cycle, two UMRs of following (positive) polarity reached the polar zone inthe northern hemisphere. The magnetic field at the north pole consists of mixed polaritiesand its average value depends on the interplay of opposite polarities. Moreover, the annualvariation of the polar field is observed because of the inclination of the Earth’s orbit relativeto the solar equatorial plane (Jin, Harvey, and Pietarila, 2013). This variation contributes tothe observed polar field and leads to short-term polarity alternations, which are observed atthe north pole before the regular reversal.

In Cycle 23, two peaks of solar activity in the southern hemisphere that exceeded the500 µHem threshold in 2000 and 2001 – 2002 (Figure 3c) resulted in well-defined patterns

Reversals of the Sun’s Polar Magnetic Fields 1979

Figure 4 Sunspot areas in the northern and southern hemispheres (a and c); time–latitude diagram of solarmagnetic fields based on the HMI/SDO measurements (at values −1 – 1 G, red to blue) and the distributionof sunspot areas (black contours corresponding to 10 and 100 µHem) for CRs 2096 – 2142 (b).

that reached the polar zones in a year (Figure 3b). A small activity increase in 1998 causedthe UMR of positive polarity, which did not reach the south pole, however. In the northernhemisphere, strong peaks of sunspot activity also led to UMRs of negative polarity thatreached the north pole after some time. The sunspot activity increase in late 2003 resultedin narrow UMRs of positive and negative polarities that reached the polar zones in bothhemispheres.

A regular reversal in Cycle 21 occurred in mid-1980 when the polar fields changed theirsigns synchronously at both poles. At the north pole the polar field switched from positiveto negative, while it switched from negative to positive at the south pole. Two subsequentreversals occurred due to a considerable increase in sunspot activity in 1979 in the northernhemisphere prior to the regular reversal. The decay of preceding ACs in 1979 resulted inremnant fields. The UMR of positive (leading) polarity was first transported to the northpole and led to a second reversal of the polar field. In the time–latitude diagram (Figure 3b)the remnant fields arise from the decay of the zone of intense sunspot activity marked inblack within the contour 100 µHem. After its decay the UMR of positive polarity coveredthe latitudes 0◦ – 30◦ and was then transported to the polar zone. The UMR appeared in1980, and the north pole was positive during most of 1981. Positive polarity dominated atthe north pole for a year. Subsequent UMRs of following (negative) polarity reached thenorth pole and led to a third reversal that changed the polar field from positive to negative.These remnant fields arose due to the next activity increase seen in the time-latitude diagramas the zone of intense sunspot activity (Figures 3a and b).

Using a similar technique, we also plotted a time-latitude diagram based on theHMI/SDO synoptic maps. Figure 4b depicts the evolution of the radial magnetic field dur-ing CRs 2096 – 2142. In the two time-latitude diagrams (Figures 3b and 4b) the large-scalemagnetic fields exhibit a similar behavior. The invisible regions near the Sun’s poles areshown in yellow. The UMR of positive polarity approached the north pole by the CR 2136

1980 A.V. Mordvinov, S.A. Yazev

(April – May 2013). Unfortunately, during this period the visibility of the northern polarzone was limited. The sunspot area distribution is superimposed at the time–latitude dia-gram. In Figure 4b the sunspot area distribution is plotted without smoothing.

The UMR of positive polarity originated from the sunspot activity impulse that peakedat CR 2116 (October–November 2011) in the northern hemisphere (Figures 4a and b). The100 µHem contours of large sunspot groups are denoted in black during this activity impulse.The largest AC that was observed at this period included Active Region NOAA 1476. Afterthe decay of these ACs, it took about 1.6 years for the remnant field to reach the northernpole. In the southern hemisphere the negative-polarity UMR reached 60◦ latitude. This UMRoriginated from the activity impulse that peaked in CR 2125 (June – July 2012) seen inFigures 4b and c. During that time large ACs were observed in the southern hemisphere(e.g. NOAA 1504). Their 100 µHem contours are also denoted in black. After the decay ofthese ACs their remnant fields reached 60◦ latitude in about a year.

4. Conclusion

The evolution of the Sun’s large-scale magnetic fields was studied to gain a deeper under-standing of the polar-fields reversal in the past four cycles. We found that regular reversals ofthe Sun’s polar field result from cyclic changes in the high-latitude zonal structure of large-scale magnetic fields. In both hemispheres magnetic fields of predominantly following po-larities are transported poleward by meridional circulation. Background magnetic fields arelargely determined by the evolution and decay of long-lived activity complexes. The nonuni-form distribution of sunspot activity, its north–south asymmetry, and the pulse-like changesresult in the asymmetry of remnant fields that migrate poleward. The north–south asymme-try of sunspot activity and the asymmetry of existing polar flux lead to an asynchrony inpolar-field reversals and short-term polarity reversals. A causal relationship between long-lived activity complexes, their remnant magnetic fields, and high-latitude magnetic fieldswas demonstrated. Within this concept, it became possible to explain the multiple reversalsof the polar fields and short-term polarity alternations that occurred in Cycles 21 – 24. Thetriple reversal of the polar field in Cycle 21 occurred because of the significant activity im-pulse in the northern hemisphere and subsequent UMRs of leading and following polaritiesthat reached the pole and changed the sign of the polar field after its regular reversal.

In the current cycle the remnant fields appeared after the decay of the first activity com-plexes in the northern hemisphere. Subsequent UMRs of following polarity reached thenorth pole of the Sun and resulted in short-term changes in the sign of the polar field. Long-lived magnetic clusters produced remnant fields that were transported to high latitudes. Thesubsequent development of sunspot activity has led to a mixture of opposite polarities athigh-latitudes with a predominance of positive polarity near the north pole. In the southernhemisphere activity complexes have been concentrated in well-defined magnetic clusters.During the phase of activity increase remnant fields have been transported from these clus-ters. The remnant fields of negative polarity reached the south pole by October 2012, butthe polar-field average still remains positive. The cyclic reversal of the polar fields will bereaching completion in both hemispheres in the near future.

Acknowledgements This work used SOLIS data obtained by the NSO Integrated Synoptic Program(NISP), managed by the National Solar Observatory, which is operated by the Association of Universitiesfor Research in Astronomy (AURA), Inc. under a cooperative agreement with the National Science Founda-tion. The authors also employed the sunspot database of MSFC of NASA and synoptic maps of solar magneticfields and the EUV emission prepared by the HMI/SDO and EUVI/SECCHI teams. This work was supportedby RFBR grant 11-02-00333 and by the project II.16.3.1 under the Program of Fundamental Research of SBRAS. The authors are grateful to the referee for the useful suggestions.

Reversals of the Sun’s Polar Magnetic Fields 1981

References

Babcock, H.W., Babcock, H.D.: 1955, Astrophys. J. 121, 349.Basu, S., Antia, H.M.: 2010, Astrophys. J. 717, 488.Benevolenskaya, E.E., Kosovichev, A.G., Scherrer, P.H.: 2001, Astrophys. J. 554, L107.Choudhuri, A.R., Schüssler, M., Dikpati, M.: 1995, Astron. Astrophys. 303, L29.de Toma, G.: 2011, Solar Phys. 274, 195.DeRosa, M.L., Brun, A.S., Hoeksema, J.T.: 2012, Astrophys. J. 757, 96.DeVore, C.R., Boris, J.P., Sheeley, N.R.: 1984, Solar Phys. 92, 1.Dikpati, M., Charbonneau, P.: 1999, Astrophys. J. 518, 508.Dikpati, M., de Toma, G., Gilman, P.A., Arge, C.N., White, O.R.: 2004, Astrophys. J. 601, 1136.Gaizauskas, V., Harvey, K.L., Harvey, J.W., Zwaan, C.: 1983, Astrophys. J. 265, 1056.Gopalswamy, N., Yashiro, S., Makela, P., Michalek, G., Shibasaki, K., Hathaway, D.H.: 2012, Astrophys. J.

750, L42.Harvey, K.L., Recely, F.: 2002, Solar Phys. 211, 31.Harvey, J., Worden, J.: 1998, In: Balasubramaniam, K.S., Harvey, J., Rabin, D. (eds.) Synoptic Solar Physics,

ASP Conf. Ser. 140, 161.Hathaway, D.H.: 1996, Astrophys. J. 460, 1027.Hathaway, D.H.: 2010, Living Rev. Solar Phys. 7, 1.Hathaway, D.H., Rightmire, L.: 2010, Science 327, 1350.Hoeksema, J.T.: 2010, In: Kosovichev, A.G., Andrei, A.H., Rozelot, J.-P. (eds.) Solar and Stellar Variability:

Impact on Earth and Planets, Proc. IAU Symp. 264, 222.Hoeksema, J.T., Scherrer, P.H.: 1986, Solar Magnetic Fields 1976 Through 1985, UAG Report, Boulder,

USA, 36. http://sun.stanford.edu.Howard, R.A., Moses, J.D., Vourlidas, A., Newmark, J.S., Socker, D.G., Plunkett, S.P., et al.: 2008, Space

Sci. Rev. 136, 67.Jin, C.L., Harvey, J.W., Pietarila, A.: 2013, Astrophys. J. 765, 79.Karachik, N.V., Pevtsov, A.A., Abramenko, V.I.: 2010, Astrophys. J. 714, 1672.Krista, L.D., Gallagher, P.T.: 2009, Solar Phys. 256, 87.Leighton, R.B.: 1969, Astrophys. J. 166, 1.Lo, L., Hoeksema, J.T., Scherrer, P.H.: 2010, In: Cranmer, S., Hoeksema, J.T., Kohl, J. (eds.) SOHO-23:

Understanding a Peculiar Solar Minimum, ASP Conf. Ser. 428, 109.Makarov, V.I., Fatianov, M.P., Sivaraman, K.R.: 1983, Solar Phys. 85, 215.McIntosh, P.S., Wilson, P.R.: 1985, Solar Phys. 97, 59.Mordvinov, A.V., Grigoryev, V.M., Peshcherov, V.S.: 2012, Solar Phys. 280, 379.Mordvinov, A.V., Yazev, S.A.: 2013, Astron. Rep. 57, 448.Mursula, K., Hiltula, T.: 2004, Solar Phys. 224, 133.Nandy, D., Munoz-Jaramillo, A., Martens, P.C.H.: 2011, Nature 471, 80.Obridko, V.N.: 1985, Sunspots and Activity Complexes, Nauka, Moscow (in Russian).Schrijver, C.J., de Rosa, M.L., Title, A.M.: 2002, Astrophys. J. 577, 1006.Sheeley, N.R. Jr.: 1992, In: Harvey, K.L. (ed.) The Solar Cycle, ASP Conf. Ser. 27, 1.Sheeley, N.R. Jr.: 2005, Living Rev. Solar Phys. 2, 5.Tsuneta, S., Ichimoto, K., Katsukawa, Y., Lites, B.W., Matsuzaki, K., Nagata, S., et al.: 2008, Astrophys. J.

688, 1374.Ulrich, R.K., Boyden, J.E.: 2005, Astrophys. J. 620, L123.van Driel-Gesztelyi, L.: 1998, In: Alissandrakis, C.E., Schmieder, B. (eds.) Three-Dimensional Structure of

Solar Active Regions, ASP Conf. Ser. 155, 202.Wang, Y.-M.: 2009, Space Sci. Rev. 144, 383.Wang, Y.M., Nash, A.G., Sheeley, N.R.: 1989, Science 245, 712.Wilson, P.R., McIntosh, P.S., Snodgrass, H.B.: 1990, Solar Phys. 127, 1.Worden, J., Harvey, J.: 2000, Solar Phys. 195, 247.