The birth and evolution of solar active regions

A& SpaceRes.Vol. 13, NO. 9, PP. (9)5-(9)1‘6 1993 Print& in Great Britain.

0273-1177/93 S24.00 1993 COSPAR

THE BIRTH AND EVOLUTION OF SOLAR ACTIVE REGIONS V. Gaizauskas He&erg Institute of Astrophysics, National Research Council of Canada, Ottawa, Canada

ABSTRACT

The growth of solar active regions is a well-observed surface phenomenon with its origins concealed in the solar interior. We review the salient facts about the emergence of active regions and the consequences of their growth on the solar atmosphere. The most powerful flares, the ones which display a range of phenomena that still pose serious challenges for high-energy astrophysics, are associated with regions of high magnetic complexity. How does that degree of complexity arise when the vast majority of active regions are simple bipolar entities? In order to gain some insight into that problem, we compare the emergence of magnetic flux in ordinary regions with an instance when magnetic complexity is apparent from the very Fit appearance of a new region - clearly a subsurface prefabrication of complexity - and with others wherein a new region interacts with a preexisting one to create the complexity in plain view. INTRODUCTION

Studies in modem solar physics, apart from nuclear processes in the solar core, touch upon the reality of active regions whether we are dealing with localized environments in the atmosphere or with gross properties of the Sun as a star. At one extreme - the interior - models for the solar dynamo rely on facts provided by active regions: solar rotation rates; cyclical variation in numbers, magnetic polarities, and latitudes of active regions. At another extreme, active regions are the visible link between the solar dynamo and plasmas reacting to it at remote distances throughout interplanetary space. It is, however, in the lower solar atmosphere where active regions compel our attention and inform us most about their nature. Recent reviews by Zwaan 11.21 of the emergence and development of sunspots, pores, and faculae, provide a synthesis of of the flux-emergence process for magnetically buoyant flux tubes of different sixes. Here the emphasis will be different. We will focus on those aspects of emerging flux regions (EFR) which bear on the production of flares. The position taken here is that emerging flux is less likely to be a trigger than a driver, even a remote one, for the flare process. We will briefly recall the signatures of emerging flux in different levels of the atmosphere for ordinary active regions. Then we will consider how the tendency of active regions to form in clusters affects flare production. Finally, we will examine the role of clustering in the creation of magnetic complexity, especially in the flare-rich 6 spots. SIGNATURES OF EMERGING FLUX IN ORDINARY ACTIVE REGIONS

Unless noted otherwise, the facts listed in point form below about emerging flux are extracted from reviews [ 1. 2, 3,4] where the original sources are cited. The list is not meant to be comprehensive; but it highlights those aspects of emerging flux which could bear on flare activity.

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Anomalous alignments and darkenings of intergranular lanes precede the first pores by several hours. Strong downdrafts, 1 - 2 km s-1, lasting - 1 hr are observed in photospheric lines near protopores. An existing pore grows by adding new flux on the face facing the centre of the swelling active region. Leading @) and trailing (t) umbrae grow by coalescence of pores of the same polarity. Major regions attain a maximum flux of - 3 x 1022 Mx in several days. New groups of spots spread apart in longitude at an average rate of 100 m s-1, sustained for 5 - 6 days. At birth the separation is much faster, - 1 - 2 km s-1; after some hours it drops to - 0.5 km s-t. The typical distance between centroids of opposite polarities is 150 Mm.

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Chromospheric Signatures. • The arch filament x~stt’m(AFS) is an infallible indicator of an EFR: it consists of parallel low-lying arches embedded in bright. compact. bipolar plage (see Figure h. • The arches cross transverse to the magnetic inversion line in a newly forming sunspot group. • The AFS stage of development usually la.sLs for about 2 days, rarely as long as 4 days. • Individual arches have lifetimes of 211 30 mm and bring 101’) Mx to the surface. • The AFS has a unique velocity pattern which distinguishes it conclusively from other systems of parallel arches (e.g. field transition arches): matter streams rapidly ( 50 km s~)down each leg of the arches while the midpoints of the arches rise more slowly ( 10km s1)• • For some EFR. the first manifestation may be surge activity which continues for some hours [5.6]. • Chromospheric faculac brighten in Ha and Call K preceding the AFS stage hut no statistical study has been done to determine the average delay between this phase and the onset of an AFS. —

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Fig. I The dynamic growth 2 1-22 June 19~tlof an EFR. NOAA 2530. from its early AFS stage when pores are barely visible to a typical bipolar sunspot group with rudimentary penumbra forming around 2 with N at top. W at right. the biggest spots. Left:photographs Ha : right: off-hand Ha. Each panel is 108 x 61 Mm Digitally reprocessed from the Ottawa River Solar Observatory ORS( )i.

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Coronal and Transition-Zone Signatures. Skylab results indicate a strong morphological distinction between the cooler coronal plasma (NeVII, —0.5 x 106 K) and the hotter plasma (FeXV, —2.0 x 106 K) at an EFR. • NeVII images show long thin spikes diverging outwards from the outer edges of an EFR for l0~ l0~km from their footpoints; lifetimes are 30 mm. • The high temperature plasma is confined to relatively diffuse loops or systems of unresolved loops which join opposite poles in an EFR or between adjacent active regions. Individual loops evolve on a time scale of 6 hr while collective patterns endure for several days or more. -

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The only study to be published so far of simultaneous Ha and UV observations of an EFR at high spatial and

temporal resolution was made with the HRTS instrument [7]. • The activity in the transition zone precedes the formation of the chromospheric AFS. • UV spectra of an EFR in CIV (1548 A) show large non-thermal broadening, indicating turbulent velocities 1. l00kins• Features associated with the strongest broadenings are small, rapidly changing surge-like or filament-like —

features seen in the centre, red, or blue wings of Ha; they are attributed to macrospicules or explosive events. The alignments ofthe granulation together with the AFS at an EFR are consistent with the penetration of the rising tops of magnetic loops through the photosphere into the chromosphere. This rising, spreading action has often been invoked as the energy source for flares by creating current sheets between emerging and pre-existing magnetic flux at the solar surface. The observations most supportive of this view are the numerous surges and small explosive events observed in Ha and the UV in the early stages of an EFR growing in isolation [8].But if one looks critically [4] at studies of particular two-ribbon flares, or at the numbers of medium-to-large two-ribbon flares compared to the total number of EFR over periods as short as one solar rotation or as long as several years, one is struck by two facts. There are far more EFR than flares rated as strong as Ml in soft X-rays; the chromospheric AFS, an unmistakable signature for emerging flux lasting hours even for ephemeral regions, is usually conspicuous by its absence at the site of most two-ribbon flares. The circumstances needed to make the conventional Emerging Flux Model [9] work for flares thus seem to be too special for general application to every large, eruptive, two-ribbon event. The sustained spreading of active regions has other ways to influence flare activity long after the initial emergence has ended. To understand this we examine the tendency for active regions to cluster together. FORMATION OF ACTIVE REGIONS IN CLUSTERS

Active regions on the Sun tend to form not at random but in clusters. Persistent injection of new bipolar regions in just a few locations creates and sustains ‘complexes of activity’ for three to six solar rotations [10].The clustering tendency has long been suspected from patterns of alignments seen in time vs. position maps of recurrent solar activity. Objective procedures applied to different data bases now place the tendency on a firm basis. In making a graphical-statistical analysis of the positions of recurrent groups of sunspots, Castenmiller, Zwaan, and van der Zalm [11] coined the term ‘sunspot nests’ for the clusters. Brouwer and Zwaan [12] and Petrovay and Abuzeid [13] applied different versions of cluster analysis to large samples of positions of sunspot groups. Knowledge about patterns of active-region recurrences has thus been extended to more than one solar cycle. The latest work shows that different levels of clustering coexist, with the smallest elements in the hierarchy of clusters occupying spatial dimensions on the order of a couple of heliographic degrees. The arrival of intense magnetic fields in bursts at a few localized sites has been modeled by Parker [14]as a thermal relaxation oscillator. He proposes that an intense azimuthal magnetic field (>3 kG) is pressed down into the lower convective zone by its thermal shadow. The accumulation of heat beneath the field causes the gas heated below to penetrate intermittently through the field, sending thermal plumes of gas with entrained fields to the surface at irregular intervals 1 week. The quasiregular distribution of solar active regions in clusters has also been reproduced by applying percolation theory [15]. In this model all the complicated MHD and turbulent processes are bundled into two dimensionless parameters: one expresses the probability that the release and rise of one flux tube stimulates the subsequent release and rise of its neighbours, while the other measures the lifetime of flux once it arrives at the surface. —

This pulse-like rejuvenation of active regions at a few tightly-defined locations over long periods is thus a basic property of solar activity. Because each rejuvenation is immediately followed by a spreading apart of new bipolar pairs of spots, interplay between regions in the same cluster is inevitable. Some adjacent bipoles clustered in the same nest will eventually collide as they develop. The number of bipolar pairs in a cluster, their orientation and their rate of injection determine the likelihood of collisions and the complexity of the resulting patterns. We examine below specific cases of clusters for clues about their influence on flare productivity. JASR 13:9-B

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Superposed Bipolar Regions. A nesting action is easily recognized in the cluster of bipolar regions arranged in a

regular pattern in the magnetogram of Figure 2. Two bipolar regions, C and D, emerge inside a third, B, in such a way that the three bipolar pairs are nearly colinear. The simplicity of this arrangement and its slow development reveal relationships between the coaligned structures with great clarity. The normal spreading action during the growth of each region leads to the collision of the leading spot of the outermost region B with the leading spot of inner region C (p-p collision); the leading and trailing spots of the two interior regions C and D overlap into a single penumbral enclosure (p-fcollision), thus forming a ~ configuration. Although ~ spots arc often complex and produce strong flares, only very small flares occur in this one until five days after its formation. Then thefspot in C suddenly splits, flies apart in in a few hours [161.and becomes the site of a series of strong flares.

Fig. 2 Magnetogram of a cluster of three nested bipolar regions comprising McMath 153 14 at central meridian passage, 27 May 1978. Outlines of individual bipoles are labelled in chronological order of their emergence. White is positive, leading p polarity. NSO magnetogram (Kili Peak) courtesy of J.W Harvey; outlines by K.L. Harvey. Prior to that fracture the steady convergence of the colliding p-f SpotS did not by itself initiate magnetic reconnection in a current sheet [9] or at a X-type neutral point as originally conjectured in the classic model of Sweet [17]. Not until a shearing motion, unrelated to flux emergence, pulled these interacting current sources past each other were the right conditions created to trigger flares. The cluster’s role in stimulating flares in this example was to confine the f spot in region C between new expanding region D and old region B (in retrograde motion) until it splits. A new regime of local currents set up by the new motions might lead to an instability and thus to flares. In extreme cases, developing spots may actually interpenetrate one another I 8, 191. Adjacent Bipolar Regions. The effect of flux emerging inside a different kind of cluster, one where no bipolar regions overlap, is illustrated in Figure 3. This figure shows how the growth of adjacent bipolar groups in the same cluster affects their boundaries in 6 days. The images have been digitally processed and transformed to eliminate changes in perspective due to solar rotation. The rectangular borders of each panel define a fixed area at the Sun; motions of features with respect to those borders are true proper motions. Regions are identified in Panel I. Two main emergences of flux are evident. The first, already at least a day old in the lower half of panel A, is NOAA 2522 (panel I); itspreads over the 6 days until it spans most of the width of the area (panel K). The second

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one, NOAA 2530 (panel I), is the same EFR (panel B) shown in closer detail in Figure 1. A long chromospheric filament initially aligned almost N-S in panel A is trapped between these two new regions; it is pulled during their expansion into an E-W alignment (panel H).

Fig.3 Cluster of 3 regions with 2 major flux emergences, 19 25 June 1980. Region identities are placed near the 3 leading-polarity sunspots in Panel I. Multiple minor emergences inside NOAA 2522 continue until 24 June as indicated by the changing black patches superposed on the bright plages in the chromospheric images. The Ha images and the overlaid near-continuum images were digitally2. processed and spatially coaligned to give a fixed field, free of perspective, 138 x 69 Mm ORSO photographs. -

Flux does not cease to emerge inside both of these regions as soon as its main p- and f-polarity spots form. An attempt is made to illustrate this point in Figure 3 by superposing on the chromospheric images the shapes of the!polarity spots and most of the intermediate spots of NOAA 2522. When these 11 images are viewed as a movie sequence one senses a continual upwelling of small spots in the middle of NOAA 2522 and a streaming towards

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the trailing end of the region. A similar localized upwelling is found when the pro~essis repeated in NOAA 2530. Because the magnetic polarities of the intermediate spots are mixed [20], their continuing emergences must indicate the arrivals of new, small magnetic loops detached from the main loops forming the two regions. The emergence of smaller ioops promotes the formation of additional chromospheric filaments along new magnetic inversion lines between them and pre-existing patterns of magnetic flux. The most conspicuous of these new filaments is best seen as a short arch just above the sunspots superposed on the lower part of the panels, e.g. F-K. As long as new spots keep emerging in the middle of NOAA 2522 that filament is arched concave side towards them (panels F, G, H). When that process ends, the short filament arches concave side outwards (panel K) and fades away completely over the course of the next day (not shown). Between the two times depicted by panels J and K the short filament erupts in conjunction with a two-ribbon flare (Imp. 1BIM4) then quickly reforms. The basic steps for this eruption and flare can be summarized thus: • a continual upwelling of flux inside an already emerged bipolar region inserts an area of mixed polarities thereby creating a new polarity inversion line and compelling the formation of a small filament along it; • when the upwelling stops, the polarity inversion line and filament recede towards the point of emergence; • hours later (<1 day) the filament undergoes strong internal flows, untwists, kinks and erupts, all in 20 mm; • the onset of the flare follows the filament eruption by 2 3 mm; • the filament reforms in —10 mm but gradually dissolves in about a day. The following scenario is proposed for this flare. The main driver for this flare is the buoyancy which keeps propelling magnetic flux ropes to nearly the same location near the surface. Some of this energy is stored in the structure created at the interface between existing and newly arriving flux the Ha filament [21].When flux is no longer being replenished at this location, magnetic cancellation dominates in determining the local magnetic connectivity. The filament begins to lose stability when enough field lines restraining it have been cut. At this point a number of MHD instabilities can take over kinking is observed in this instance [221 and magnetic reconnection of the field lines under the ballooning filament triggers the flare. —

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The two preceding examples, by their very different natures, illustrate the same fundamental point: there is not just a single step but a sequence of steps leading from the stable preflare configuration across the threshold to instability. Various combinations of steps are possible due to the tendency of active regions to form in clusters. This begs questions about the ability of clusters to manufacture the richly flare-active 6 spots. Can the repeated injection of numerous bipoles in a sunspot nest build up over time the required degree of complexity? FORMATION OF DELTA SUNSPOTS The ability of 6 spots to be intensely flare active is long-established [23, 24]. During a single solar cycle only a handful of sunspot groups may consist of a large 6 spot, each producing several Great Flares within a few days. Cycle 22 had two especially impressive examples March 1989 and June 1991. Since chances for observing these structures are so limited, important issues about their formation remain unsettled. A major uncertainty concerns the degree of prefabrication of the complex 8 configuration before it penetrates the surface. If all the complexity is created by superposing bipoles inside the same sunspot nest, there is some hope that special signatures could be discovered early in their development for use in forecasting the degree of complexity and for learning how these spots construct magnetic accelerators for creating the particles with GeV energies observed in Great Flares. -

The evidence to date does not favour clustering as the sole process for building up the complexity of 8 spots. In a study of 21 6 spots, Zirin and Liggett [25] found that they are shorter-lived than normal spots of the same size, seldom lasting more than one passage across the solar disk. This means that their complexity has, on average, to be produced very quickly, at or before their emergence. The likeliest kind to produce Great Hares emerge all at once with opposite polarities intertwined in a compact “island” 8 configuration. Another kind of 8 spot, formed by spots colliding in a sunspot nest (see the preceding section), may produce large flares but not Great Flares. In a rare instance where the early development of a 6 spot could be followed in detail (McMath 13043, July 1974), Tanaka [19] found 7 EFR emerging at one time. Only the central EFR was born as a tight 6 spot; the others, packed around it, quickly coalesced. He explained the convoluted patterns of motion of the developing spot by a magnetic topology consisting of a tightly twisted knot rising through the surface and trailed by a section of magnetic “rope” with a hump in it. The 6 days of development of McMath 13043 to maximum size and the observed translational speeds (— 200 ms-1 ) of its component umbrae are consistent with a continuous outward buoyant motion from the bottom of the convection zone. Figure 4 illustrates another 6 spot which did emerge all at once but was only remotely linked to major flare activity. The closeness of the maturep and f umbrae and their speed of emergence were remarkable. But in other respects alignment of polarities, AFS the pair were normal. A Great Flare did occur one day later, not in the 6 -

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spot but about 80 Mm away among the small spots marked by the arrow. A long Ha filament linked the 6 spot with these small spots and extended far beyond. This Great Flare could be a case where a global field supporting the filament was altered so greatly by a major rapid intrusion of new flux at the 6 spot that an instability was promoted on a separator between interlocked magnetic cells in this multipolar complex of regions [20].

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Fig. 4 Emergence of a 6 spot (lower right corner) in McMath 15266 27 April 1978, at 15:18 UT. The p and f umbrae in the 6 spot are still coalescing from smaller fragments at the two ends of the group. The image was recorded 13 hr after the emerging pores were first sited at the Yunnan Observatory, and 10 hr after penumbral strands were first seen linking the two umbrae of opposite polarity. The arrow marks the initial site of a Great Flare. Field of view 134 x 187 Mm2. N at top, W at right. ORSO photograph. ,

The case for promoting growth of 6 spots through clustering receives only ambiguous support from the superactive region, NOAA 6659, which produced six Great Flares in June 1991. It was already a compact 6 spot with an unusual N-S orientation on its previous rotation (NOAA 6619). It remained a tight 6 configuration during that disk passage, showed little evolution, and produced some large, long-duration flares but no Great Flares. Unlike more open clusters of sunspots, compact 6 spots provide few clues for differentiating ‘old’ from ‘new’ flux if they survive their 2-week transit of the back of the Sun. With that long hiatus in observations we cannot tell whether NOAA 6619 disappeared before NOAA 6659 formed in its place, or if it was rejuvenated while still thriving as a 6 spot on the back side. We do know that it produced a Great Flare (June 1) while still behind the east limb, and that its main p umbra (Figure 5) continued to expand for about a week after it crossed the east limb. In a close study of the proper motions of all umbral components belonging to this huge 8 spot, Kalman and GyOri [26] draw attention to the great imbalance of flux in favour of the p polarity. The imbalance increased as the huge central p umbra swelled. Just as puzzling was the continual appearance of small umbral fragments ofjust p polarity [26, 27] which steadily streamed along a narrow channel as elongated features squeezed between the main p and f umbrae (marked in Figure 5 as ‘magnetic channel’). This cannot be attributed to the ordinary process of flux emergence. The origin of this seemingly unipolar flux in the magnetic channel poses an intriguing problem because it is cospatial with the five Great Flares observed during the disk transit of NOAA 6659 [26].The existence of similar magnetic dynamism in channels was discovered recently by Tang and Wang [28] in other superactive 8 spots. They propose two alternatives to account for the ‘unipolar emergences’ in narrow channels: the new umbrae are bits which have been stripped away from one of the large umbrae; or the new umbrae have ‘condensed’ out of penumbral fields. Puzzling hydrodynamic effects seem to operate in the narrow channels where the opposite poles in a 6 spot are squeezing together.

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1991 June 10, 16:33 UT 240

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Fig. 5 An ‘island 6 spot’, NOAA 6659. Left: 07 June 1991, showing approximate of magnetic channel of 2); right: 10 location June 1991, map in heliographic streaming p polarity magnetic flux (ORSO photograph, 184x147 Mm coordinates showing splitting of P2 from pi and its fragmentation. Lines with arrows show motions of main umbral fragments (heavier outlines) over 10 days. Note minor movements of P1 and f compared to P2. Courtesy of Debrecen Heliophysical Observatory. DEFORMATION AND DECAY OF DELTA SPOTS

When some 6 spots are in a highly flare-active mode they exhibit peculiar umbral motions or umbral deformations, such as interpenetration of one spot by another of opposite polarity [18,19],or a sudden splitting of an umbra [16]. The component umbrae of 6 spots may pull apart with age but seldom leave the encircling penumbra. According to Zirin and Liggett [25] 6 spots do not separate but die out locked together. On the other hand, cases have been reported where 6 spots disappearthrough a process of flux cancellation [29],sometimes accompanied by flares. Refemng again to the superactive region, NOAA 6659, the huge central p polarity umbra distorted and grew during its disk transit in June until it splintered into many fragments (Figure 5). In the subsequent decay, the I umbra and pi, a core piece of the p umbra, changed hardly at all in size or position. The huge excess ofp polarity flux broke up and disappeared almost in situ. Other fragments such as P2 moved (Figure 5) towards the narrow channel separating p andf polarities, thereby sustaining the steep magnetic gradient at the channel where the Great Hares were concentrated. When the region returned for a fourth rotation in July 1991, the compactness of the second and third rotations was lost; the magnetic flux was distributed between many medium-sized spots. A huge yet compact bipolar spot in which the magnetic polarities are widely imbalanced is not likely to be the accidental outcome of a clustering process where one bipolar group succeeds another in exact coincidence. The compactness, complexity, magnetic imbalance, and steady streaming of unipolar flux in narrow channels are unusual effects related to some as yet obscure hydrodynamic process. NOAA 6659 might originate in a tightly twisted flux rope without knots, unlike the 6 spots described by Tanaka [19].In its break-up we may be witnessing the untwisting of a flux rope through the sudden release of an instability instead of the unravelling of a knot. The strange behaviour of NOAA 6659 may have its origin in the spot’s high latitude (30°N)which is exceptional for the maximum phase of the solar cycle and its great latitudinal width (— 10 heliographic degrees). Differential rotation between the extreme latitudes in this spot amounts to 2% of the rotation at its central latitude based on rotation laws for the photosphere. SUMMARY AND FUTURE OUTLOOK

The emergence of normal active regions is a process with well-defined signatures in different atmospheric regimes as the regions evolve. The process can be understood in terms of magnetic buoyancy acting on flux ropes initially compressed by convective motions near the bottom of the convection zone and then controlled dynamically by convection after the tops of the flux ropes penetrate the surface. Isolated bipolar regions exhibit minor flare and surge activity when the newly emerging magnetic fields encounter only weak overlying fields. But at a few localized sites in each hemisphere, a pulse-like injection of magnetic flux is repeated at intervals of days to a few weeks. A multipolar magnetic topology created at those locations has greatly enhanced possibilities at each emergence or disappearance of flux, to stress and eventually to destabilize structures supported within a global

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topology. Specific structures which are vulnerable to these evolutionary stresses include sunspots colliding in tight clusters and filaments formed along the boundaries of each bipolar unit comprising a cluster. Destabilization can proceed by many different paths to cause reconnection and the release of non-potential energy as flares. Abnormal active regions, ofwhich 6 spots are the supreme example, present a more difficult challenge. Some may be understood as the emergence of buoyant flux ropes that have been twisted into tight knots. Others somehow maintain a large magnetic flux imbalance as they evolve by processes which remain obscure. The recent discovery that some of these scarce, dual polarity sunspots have unusual flow patterns along narrow channels separating the umbrae complicates attempts to explain flares as the result of excessive shear built up by umbral motions. Many interesting questions remain for future investigations of the role of emerging/evolving magnetic flux in producing solar flares: • Can coronal mass ejections be linked to specific emergences or cancellations of magnetic flux inside clusters of active regions? • What proportion of active-region filaments are destabilized by emergences of flux as compared to cancellations of flux? • How are large imbalances of magnetic flux in favour of one polarity created in active regions? How can they be sustained for many days? • Does evidence favour the existence of anomalous large-scale flow patterns in the photosphere preceding the formation of large 6 spots, such as those of March 1989 or June 1991? • How are ‘island 6 spots’ created? What sustains them for days or even weeks? What are the dominant processes responsible for their disintegration? • How are the recently discovered magnetic channels in 6 spots created? Do they have any role in the energy storage process for flares? There has been substantial progress in the past decade for understanding the part played by the evolution of active regions, including interactions between active regions, in the flare process. A large part of that progress has been the acceptance of the importance of the decay of active regions, not just their growth. Now the biggest new challenges lie in the domain of the ‘island 6 spots’, home to anomalous sunspot behaviourand to Great Flares. ACKNOWLEDGMENTS The generosity is gratefully acknowledged of J. W. Harvey, National Solar Observatory, for a magnetogram of colliding sunspots, and of B. Kalman, Debrecen Heliophysical Observatory, for maps and a detailed report of the daily evolution of NOAA 6659. REFERENCES

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