ISSN 0016�7932, Geomagnetism and Aeronomy, 2013, Vol. 53, No. 7, pp. 891–895. © Pleiades Publishing, Ltd., 2013.

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1. INTRODUCTION

Solar polar magnetic fields have historically occu�pied a central place in discussions on the nature of thesolar cycle; as a result, these fields are important forpredicting solar activity since it was discovered that thepolar magnetic field reversed at a maximum of the 11�year solar cycle (cycle 19) (Babcock and Livingston,1958; Babcock, 1959). The polarity of the high�lati�tude solar magnetic field was opposite to that of theEarth’s dipole magnetic field from 1953 to 1957. In themiddle of 1957, the magnetic field polarity near theheliographic south pole of the Sun reversed. However,the north polar field reversed after November 1958.Researchers used the mean field dynamo theory inorder to understand the nature of the polar magneticfield (Dikpati, 2011). Standard transport models of thesolar cycle explain that a reversal of the solar polarmagnetic fields results from (1) turbulent diffusion,(2) differential rotation, and (3) meridional circula�tion (Leighton, 1964, 1969). Figures 1e and 1f presentthe evolution of the polar solar magnetic field near theheliographic north and south poles of the Sun forcycles 21–23 and for the current cycle (cycle 24)according to the Solar Wilcox Observatory (SWO)data. The distribution of the magnetic field zonal oraxisymmetric structures is shown in Figs. 1b and 1c.The zones of alternating polarity are colored white(positive polarity, from the Sun) and black (negativepolarity, toward the Sun). These zones extend frommiddle to high latitudes. The polar magnetic fieldreverses when the averaged zonal line between positiveand negative polarities (the neutral line) approaches the

pole. This motion is accompanied by the appearance ofprominences and filaments near the neutral line.

2. NATURE OF THE POLAR MAGNETIC FIELDS

Large�scale solar polar magnetic fields includeclusters of smaller�scale magnetic fields and magneticelements of positive and negative polarities (Severnyi,1965; Lin et al., 1994; Benevolenskaya, 2004, 2010).The predominant polarity of the polar magnetic fieldmanifests itself in compact unipolar magnetic regionsof a strong magnetic field (Okunev and Kneer, 2004).At a solar minimum, these regions are clearly definedin the Ca K line and continuum, since they coincidewith polar faculae. The lifetime of magnetic elementsvaries from several hours to several days. Then, theseelements are completely replaced by new magneticregions emerging from subphotospheric layers. BrightX�ray points related to polar jets are also observed inthe polar regions (Pucci et al., 2012; Savcheva et al.,2007). The Hinode space laboratory data on the vectormagnetic field in the solar polar regions indicated thatpolar faculae have strong fields (stronger than 103 G,1 μT = 10–6 T, 1 T = 104 G) oriented vertically (Shiotaet al., 2012). However, the discussion about the valueof this field is still continuing since it is difficult toobserve the polar field owing to its closeness to thesolar limb. Moreover, it is impossible to simulta�neously observe both solar poles from the Earth’s orbitduring the entire year because the Earth is ±7°15′ outof the helioequator plane in autumn and spring. Thus,the northern and southern polar regions are clearly

Solar Polar Magnetic FieldE. E. Benevolenskaya

Main Astronomical (Pulkovo) Observatory, Russian Academy of Sciences, Pulkovskoe sh. 65, St. Petersburg, 196140 Russia

Received April 25, 2013

Abstract—The solar polar magnetic field has attracted the attention of researchers since the polar magneticfield reversal was revealed in the middle of the last century (Babcock and Livingston, 1958). The polar mag�netic field has regularly reversed because the magnetic flux is transported from the sunspot formation zoneowing to differential rotation, meridional circulation, and turbulent diffusion. However, modeling of theseprocesses leads to ambiguous conclusions, as a result of which it is sometimes unclear whether a transportmodel is actual. Thus, according to the last Hinode data, the problem of a standard transport model (Shiotaet al., 2012) consists in that a decrease in the polar magnetic flux in the Southern Hemisphere lags behindsuch a decrease in the flux in the Northern Hemisphere (from 2008 to June 2012). On the other hand, Sval�gaard and Kamide (2012) consider that the asymmetry in the sign reversal simply results from the asymmetryin the emerging flux in the sunspot formation region. A detailed study of the polar magnetic flux evolutionaccording to the Solar Dynamics Observatory (SDO) data for May 2010–December 2012 is illustrated in thepresent work. Helioseismic & Magnetic Imager (HMI) magnetic data in the form of a magnetic field com�ponent along the line of sight (the time resolution is 720 s) are used here. The magnetic fluxes in sunspot for�mation regions and at high latitudes have been compared.

DOI: 10.1134/S0016793213070037

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Fig. 1. (a) Relative number of sunspots; (b) black�and�white zonal structure of B||, [–100 µT +100 µT]; (c) |B||| [0 200 µT]. TheWSO data (1976.4–2012.8). Dots show the magnetic field component along the line of sight (B||), averaged over one Carringtonrotation, for the (e) north and (f) south polar fields (at 70° latitude) as a function of time; the thick solid line demonstrates thevalues smoothed for 25 Carrington rotations. The vertical dipole magnetic moment (g) according to the spherical representationof the magnetic field measured at WSO.

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Fig. 2. (a) Sunspot number from May 2010 to December 2012 (CR2097–CR2131); (b) HMI black�and�white zonal structure ofB||, [–1 G + 1 G]. A burst of negative polarity in the Northern Hemisphere is marked with “A”. The latitudinal dependence of theB|| field averaged over longitude in the (c) Northern and (d) Southern hemispheres for three Carrington rotations: CR2114 (August 26,2011–September 22, 2011), CR2120 (February 6, 2012–March 4, 2012), and CR2127 (August 15, 2012–September 11, 2012).

defined in autumn and spring, respectively. The aim ofthe Solar Orbiter and Intergeliozond future space mis�sions is to complexly study the solar polar regions. Theorbits of these missions will approach the Sun, whichwill make it possible to observe solar magnetic fieldswith a high spatial resolution.

The polar magnetic field has decreased during thelast solar cycles (Fig. 1). Livshits and Obridko (2005)studied in detail the behavior of the dipole magneticmoment during the solar cycle. These authors indi�cated that the vertical dipole magnetic moment, cal�culated based on the SWO observations, decreased(Fig. 1g). According to the solar cycle dynamo theory,the polar magnetic field is poloidal (Bp field) and istransformed into a toroidal field (Bt field) owing todifferential rotation. A toroidal magnetic field isobserved at the photospheric level as bipolar com�plexes of solar activity. According to Svalgaard et al.(2005), an observed decrease in the magnetic fieldshould result in a decrease in solar activity and in a rel�atively short cycle 24. In this case, it becomes interest�ing again whether transport models can be used toexplain the weakening of the polar magnetic field.Jiang et al. (2011) possibly answered this question.Weakening of the polar magnetic field could beexplained on the assumption that the meridional flux

velocity in the past cycle increased by 55% or the incli�nation of bipolar structures with respect to the solarequator changed by 28%. The inclination of bipolarstructures is a specific case of the alpha effect in themean field dynamo theory (Leighton, 1969). Variationsin the meridional circulation and alpha effect can resultin changes in the behavior of high�latitude solar activity.However, it is still unclear why the meridional flux oralpha effect vary in one way or another.

3. POLAR MAGNETIC FIELD ACCORDING TO SDO/HMI DATA

The SDO space observatory (Pesnell et al., 2012)observes the Sun at different wavelengths and mea�sures magnetic fields on the entire disk with a resolu�tion of 1′′. Data on the magnetic field componentalong the line of sight (B||), obtained at an interval of720 s (Scherrer at al., 2012), were used to study thepolar magnetic field evolution. Solar images weretransformed into the Carrington coordinate system,and 3600 × 2001 synoptic maps with resolutions of 0.1°along longitude and 0.001 with respect to sine latitudewere subsequently constructed for CR2097–CR2131.

Figure 2b presents the B|| values, averaged over lon�gitude, depending on sine latitude and time, i.e., the

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so�called zonal structure of the solar magnetic field.The sunspot numbers (Wolf numbers) are presented inFig. 2a for comparison. Figure 2b indicates that asurge of negative polarity (“A”, colored black) resultsin a delay in the polarity reversal of the north magneticfield. The latitudinal dependence of the zonal mag�netic field, i.e., the B|| component of the magnetic fieldaveraged over all longitudes, is presented in Figs. 1cand 1d. In the Northern Hemisphere, the polar mag�netic field fluctuates about zero and reaches small neg�ative values in September 2012 (CR2127). The situa�tion is even more complex in the Southern Hemi�sphere: the south pole remains positive during theentire studied period from May 2010 to December2012 (Fig. 2c). To compare the dynamics of magneticfluxes in the sunspot formation zone and in the polarregions, we calculated the total radial component ofthe solar magnetic field (Br) at different latitudes from

May 2010 to December 28, 2012, for CR2097–CR2031 independently for either hemisphere. Thecalculated values correspond to the radial magneticflux since the Br values were summed for pixels ofidentical areas. The time variations in these values arepresented as shaded regions in Figs. 3b–3e for four lat�itudinal zones of the Sun. We divided the sunspot forma�tion zone into two areas: 0°–20° and 20°–40°. Negativemagnetic flux “A” (Fig. 2b), which resulted in a delay ofthe polarity reversal of the north polar magnetic field inthe middle of 2012, is shown in Figs. 3 and 1d. It is inter�esting that the peak in the sunspot areas in the NorthernHemisphere in 2011 is bimodal (I, II).

The first peak corresponds to the magnetic flux weak�ening at low latitudes, where the magnetic flux is mostlynegative (from the Sun). The second peak coincides withthe impulse of negative polarity at low latitudes (Figs. 3,1b) and positive polarity at midlatitudes (Figs. 3, 1c).

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Fig. 3. Sunspot area in the Northern (1a) and Southern (2a) solar hemispheres from 2010 to 2012 (CR2097–CR2031) in mil�lionths of the solar hemisphere area. The total values of the magnetic field radial component for the Northern (1) and Southern(2) hemispheres at latitudes of (b) 0°–20°, (c) 20°–40°, (d) 40°–60°, and (e) 60°–80°. The burst of activity in the sunspot numberin the Northern Hemisphere is marked with I (September 2011) and II (November 2011).

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Thus, the second impulse corresponds to the topology ofa large�scale field of a new polarity according to the Haleand Joe laws. As a result, an extensive zone of positivepolarity (2012) is formed at low latitudes (40°–60°)(Figs. 3, 1d), which is followed by a decrease in the nega�tive magnetic flux of the old polarity at high latitudes(60°–80°) in the Northern Hemisphere (Figs. 3, 1e). Inthe Southern Hemisphere, the pattern of the magneticfield polarity reversal is more complex than in the North�ern Hemisphere. In the Southern Hemisphere, activityincreased abruptly several times during the consideredperiod. The first surge (in 2010) corresponds to the polar�ity in the previous solar cycle (cycle 23) (Figs. 3, 2d, 2c):the magnetic flux was negative and positive at low andmiddle latitudes, respectively. The next magnetic fluxbursts correspond to the topology of the current solarcycle (positive and negative polarities are registered atsmaller and larger distances from the equator, respec�tively). Nevertheless, the first burst substantially affectedthe polarity reversal of the south polar magnetic field: thefield polarity remained positive (from the Sun) during theentire considered period up to 2013.

The sunspot area (Figs. 3, 2a) became maximal inJuly 2012. This event coincided with the weakening ofthe positive (0°–20°) and negative (20°–40°) mag�netic fluxes. The main burst of new�polarity solaractivity, which was registered in the magnetic flux vari�ations, was observed in November 2012 and causedpolar magnetic field weakening near the south pole ofthe Sun (Figs. 3, 2e). Thus, the current solar cycle(cycle 24) is characterized by a north–south asymmetryin the polarity reversal of the surface magnetic fields inthe solar polar regions. A polarity reversal is naturallyrelated to the development of solar activity in the sunspotformation region, i.e., to the north–south asymmetry inthe emerging magnetic flux (Svalgaard et al., 2012).

4. CONCLUSIONS

Summarizing the analyzed data on the solar polarfields, we should note that solar activity impulses(Benevolenskaya, 2003) play a key role in the forma�tion of alternating�polarity zones (Fig. 1b) and polarmagnetic fields. The nature of the polar magnetic fieldis related to that of solar activity and the solar cycle.Therefore, to understand this nature, it is necessary tostudy the subphotospheric dynamics of the magneticfield and convection, internal rotation, and meridi�onal circulation with high time and spatial resolutions.It is also necessary to study coronal processes. Theseproblems are of current interest for present�day andfuture space missions, such as Hinode, Solar Dynam�ics Observatory, Solar Orbiter, and Intergeliozond.

ACKNOWLEDGMENTS

I am grateful to the SDO scientific teams for thepresented data.

This work was partially supported by the Presidiumof the Russian Academy of Sciences, program no. 22.

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Translated by Yu. Safronov