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Imaging solar coronal magnetic structures in 3D
Pergamon
Phys. Chem. Earth, Vol. 22, No. 5, pp. 411-418, 1997 © 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0079-1946/97 $17.00 + 0.00 PII: S0079-1946(97)00168-7
Imaging Solar Coronal Magnetic Structures in 3D N. P. Cartledge Department of Applied Mathematics, University of St. Andrews, St. A n d r e w s KY16 9SS, U.K.
Received 10 May 1996; accepted 17 February 1997
Abstract. The study of solar coronal structures and, in particular prominences, is a key part of understandingthe highly complex physical mechanisms occurring in the Sun's atmosphere. Solar prominences are important in their own right and some of the most puzzling questions in solar theory have arisen through their study. For example, how do they form and how is their mass continuously replenished? How can the magnetic field provide their continuous support against gravity over time periods of several months? How can such cool, dense material exist in thermal equilibrium in the surrounding coronal environment? Why do they erupt? A study of their structure and that of the surrounding medium is important in determining the nature of the coronal plasma and magnetic field. Also, prominences are closely associated with other key phenomena such as coronal mass ejections and eruptive solar flares which occur as a prominence loses equilibrium and rises from the solar surface. Our current understanding of these fascinating structures is extremely limited and we know very little about their basic global structure. In fact, recent prominence observations have caused our basic paradigms to be challenged (Priest, 1996) and so we must set up new models in order to gain even a fundamental understanding. Prominences are highly nonlinear, three-dimensionalstructures. Large feet (or barbs) reach out from the main body of a prominence and reach down to the photosphere where the dense material continuously drains away. These provide a real clue to the threedimensional nature of the coronal field and its relation to the photospheric field. It is important, therefore, to make stereographic observations of prominences in order to gain a basic understanding of their essentially three-dimensional nature and attempt to formulate new paradigms for their structure and evolution. There is no doubt that the study of prominences in three dimensions is a crucial exercise if we are to develop a better understanding of the solar magnetic field and the physical processes occurring m the corona. This will only be possible
with the aid of stereographic observations. © 1997 Published by Elsevier Science Ltd
1 Introduction 1.1
Basic Properties
Solar prominences are cool, dense bodies of plasma located in the lower regions of the solar corona and are amongst the most important and yet least understood of the Sun' s coronal structures. Typically, they possess temperatures which are 100 times lower than the surrounding atmosphere and densities 100 times higher, which immediately raises the questions: how do they remain cool despite the surrounding coronal environment which has been heated to an incredible 3 million degrees? How are they supported against the Sun's huge gravitational pull for such long time-scales? The lifetime of a mature quiescent prominence may be as extended as several months. Prominences are best observed in the Ha line where they appear dark against the disc, due to the absorption of the underlying photospheric emission (Figure 1, top), and bright at the limb, due to their relatively high densities (Figure 1, bottom). Prominences are enormous features! A typical quiescent prominence extends for 200 Mm, has a height of 50 Mm, and yet is just 6 Mm wide. Thus, they are generally observed as thin, elongated sheets of plasma appearing as ribbon-like structures against the disc (Figure 1, top). Seen side on at the limb, they display many interesting features such as fine structure (inhomogeneouslydistributed material, clumped in the form of thin, vertical threads) and huge feet which reach down to the photosphere, typically spaced at quasi-periodic intervals of 30 Mm. They are located in many parts of the corona ranging from quiet regions, which are areas of reduced activity and relatively weak field strength (typically I0 Gauss), to active regions where the magnetic field is much stronger (about 50-100 G) and often highly non-potential, such that large quantities of magnetic energy can be stored. Although quiescent prominences display little global
Correspondence to: N. P. Cartledge 411
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N. P. Cartledge There are many other fascinating structural aspects to solar prominences (Tandberg-Hanssen, 1974; Priest, 1989) but it is now worthwhile outlining their main magnetic properties. The magnetic field is of fundamental importance for their very existence and its presence produces many of the physical processes responsible for their various curious properties. 1.2
Magnetic Field
Solar prominences are always observed to overlie a magnetic polarity inversion line (Babcock and Babcock, 1955). This is a curve on the photosphere which separates regions of opposite magnetic polarity. Therefore, the magnetic field is an essential ingredient and of fundamental importance for the formation, existence, evolution and eruption of prominences. Figure 2 depicts an active-region prominence against the disc, showing its Ha structure and its close relation to the photospheric magnetic field. In the magnetogram, black areas represent positive polarity and white areas are negative polarity. Thus, prominences are thought to be contained in large-scale magnetic flux tubes and this field plays an essential role in supporting the dense material over long timescales.
Fig. 1. Hc~ imagesof the Sun showingribbon-likeprominencesagainst the disc (top) and a quiescentprominenceat the limb(bottom). Courtesy MeudonObservatoryand Big BearSolarObservatory.
change over time-scales of a few hours the fine structure is continuously evolving over a few minutes. On longer timescales, the feet mysteriously evolve over a few days and the overall structure tends to change over periods of several months. A prominence may experience several repeated eruptions during its restless lifetime, the material incredibly reforming after each eruption over the course of a few days. Sometimes the eruptions are so violent that the whole surrounding magnetic structure is blown apart, the release of vast quantities of magnetic energy somehow driving material way up into the corona. In the most extreme cases enough energy is released to drive a coronal mass ejection (Hundhausen et al., 1984) in which a huge quantity of overlying material may be accelerated to speeds of several hundred kilometres per second. This process represents a significant percentage of the total mass lost from the Sun.
Fig.2. The relationbetweena prominence,filamentchannelandthe photospheric magneticfield. Courtesy S. Martin, Big Bear Solar Observatory. The magnetic field also appears to play other roles. Overlying many prominences is a large-scale helmet streamer which can be seen during a total eclipse, and surrounding the prominence is a closed region of reduced density, often with a near circular geometry. This is a coronal cavity (see Engvold (1989) for details) and may be considered to be the outline of a surrounding helical magnetic field. The magnetic field through the prominence is highly sheared and typically makes an angle of 200 with the long-axis of the prominence (Tandberg-Hanssen and Anzer, 1970; Leroy et al., 1983; Kim, 1990) and so may not necessarily be potential. This field is observed to pass through a prominence in
Imaging Solar Coronal Magnetic Structures in 3D either the same direction or the opposite direction when compared with the underlying photospheric field. Priest (1989) suggested that these two cases can be categorised as either normalpolarity or inverse polarity, respectively. It turns out that most prominences are inverse polarity, especially highlatitude or quiet-region prominences. Some prominences are observed to be normal polarity: these are generally the lower latitude active-region prominences which are also located at lower altitudes. Another interesting feature of the prominence field is observed in the underlying chromosphere. There, the magnetic field is strongly aligned with both the axis of the prominence and the polarity inversion line such that the horizontal component of the field is parallel to these features. This is called afilament channel and is characterised, in active regions, by the strong alignment of chromospheric Ha fibrils (Figure 2). These fibrils tend to lie along the field and so enable a reasonable determination of the field direction in the chromosphere. Thus, the field below prominences does not appear to cross over the polarity inversion line, as was thought previously. This has invalidated many of the classical prominence models (see Section 2.2) and has caused our basic paradigms to be reviewed.
Fig. 3. A hugeeruptingprominenceat the limbof the Sun. The twist and three-dimensionalnatureof the magneticfieldis clearlyevidentin this image. CourtesyNavalResearchLaboratory. Occasionally, during a prominence eruption the magnetic structure is outlined, as shown in Figure 3. Here the erupting magnetic field is highlighted by the plasma which clearly shows the twisted, three-dimensional nature of the configuration. It is certainly apparent that the magnetic field in prominences and the surrounding corona varies strongly in all three directions. Its complex topology is far from obvious but at least the material in prominences is sufficiently dense to enable direct field measurements there, thus providing a foundation for understanding the nature of the coronal magnetic field and its evolution.
2 2.1
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Theoretical Approach Important Puzzles
In the past several theoretical models have been proposed in an attempt to gain some understanding of the puzzling qualifies of prominences. They are tremendously complicated creatures and there are several different questions which must be separately tackled before a more general understanding can be attained. One of the most important questions refers to their support. The dense body of a prominence appears to hang magically in the corona for weeks on end despite the huge gravitational pull to which it is subject. The solution appears to lie within the magnetic field, which is somehow organised to provide a continuous upward Lorentz force on the plasma, countering its weight. But how can the field locally provide such support and how does this field match to the background coronal field? Another obvious question relates to the thermal equilibrium and the prominence-coronainterface. How is it possible for prominence material at 10,000K to exist in thermal balance with the surrounding corona, heated to several million degrees? Again, the magnetic field must play an important role. Thermal conduction is very weak a~ross magnetic field lines and so the field may be arranged to thermally shield the prominence material from the background environment. The problem is again three-dimensional. It is important to understand the surrounding field in order to evaluate the energy balance and gain some feeling as to how cool, dense material may remain in equilibrium within the much hotter and more tenuous coronal atmosphere. What is the three-dimensional structure of prominences and how does this relate to the magnetic field? Prominence feet represent large variations in the global structure along the prominence and are locations at which material may drain out of the main body of a prominence. There are no satisfactory explanations for these feet, or, indeed, the many other related features, such as the fine structure. Why is the material not uniformly distributed throughout the main body of the prominence but rather clumped into thin vertical threads of diameter, typically 200km? Another bewildering puzzle relates to the mass supply. Continuous slow, downward flows of 1-2 k m s -1 are observed in prominences which implies that the whole structure should drain away within the course of a few days, yet the prominence lifetime is far greater. How is the mass resupplied to the prominence? Is it lifted up by the reconnecting actions of the underlying magnetic field? The eruption of solar prominences is yet another topic about which very little is understood. What causes the magnetic field to lose equilibrium and how is its energy released so as to drive the prominence material into the high reaches of the corona? Priest and Forbes (1990) presented a good two-dimensional model for such an eruption in which no neighbouringequilibriumof their considered magnetic structure is possible when the shear is increased beyond a critical
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value. The problem is yet to be solved in a three-dimensional geometry, however, and so this presents another important theoretical challenge. Quite clearly, there are many puzzles still to be unravelled if we are to develop a fundamental understanding of these structures and the corona as a whole. 2.2
A unique position for the prominence can then be found from the balance of forces. Figure 5 shows the final equilibrium.
Classical Theoretical Models
Traditionally, prominence models have focussed on their transverse structure, concentrating on the supporting mechanisms and the topology of the surrounding magnetic field. With the limited aid of two-dimensionalobservations this has led to a great deal of misinterpretation. This section briefly describes two of the classical theoretical models and points out some of the inevitable ambiguities which have arisen.
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One of the first advances was due to Kippenhahn and Schltiter (1957) (hereafter abbreviated to K-S) who gave a simple model for the support of a prominence in a normalpolarity configuration. The basic topology is shown in Figure 4 where the prominence is represented as a sheet with current I at a height h above the photosphere. The current is directed out of the plane and results from a discontinuity in the vertical field component. If the photospheric footpoints are linetied during the formation of a prominence, the preservation of the footpoint position can be modelled by adding an image current ( - 1 ) at a distance h below the photosphere to the original arcade and prominence sheet. Thus, the prominence mass (m) is supported against gravity both by the line tying (the repulsion p I 2 /(47rh) between I and ( - I ) ) and also by the Lorentz force 1B acting on I in the original background field B at height h. An alternative magnetic topology (inverse-polarity) was proposed by Kuperus and Raadu (1974) (K-R) where the magnetic field passes through the prominence in the opposite direction. They represented the prominence by a line current embedded in the background field of a vertical neutral sheet. Again, to simulate the line-tying during the formation, an image current is required and this provides the upward repulsive force pI2/(4rh) to balance the weight of the filament.
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Fig. 5. The Kuperus-Raadumodel. A linecurrent,representingthe prominence,is supportedin equilibriumin an inverse-polarityconfiguration. It is obvious, even from the two models described, that several different interpretations are possible with 2-D observations. In the K-S model, the field passes directly from the photosphere through the prominence, whereas in K-R models the prominence field is detached. The field in the KR model reverses its sign beneath the prominence and possesses an X-type neutral point. There is also an O-type neutral point present. These are two very contrasting topologies, but which type is correct, if either? Many subsequent models have presented further interpretations, which really fuels the need for 3-D observations to pinpoint the correct structure. To date, most models have assumed invariance in the third, axial direction. This has provided a basic understanding of their two-dimensional properties and has certainly been invaluable in that sense. However, it is clear that prominences are very much three-dimensional structures with substantial variations in the axial direction such as kinks, bends and prominence feet reaching down to the photosphere. Both new models and observations are required to take these important aspects into account. 2.3
Recent Advances
2.3.1 Observations A recent detailed study by Martin et al. (1994) has uncovered several new interesting observational facts about prominences. The main results are as follows: (1) Prominences lie along polarity inversion lines, with fibrils in the filament channel running parallel to the prominence rather than crossing it. Many of the classical models have required that the field crosses through the prominence from one side to the other, such as the K-S and K-R models. (2) If an imaginary observer is standing in the photosphere at the positive side of the prominence and sees that the field is directed to the right, then the field is said to be dextral.
Imaging Solar Coronal Magnetic Structures in 3D
Conversely, if the field points to the left, it is said to be sinistral. In addition, there are two structural classes for prominences, corresponding to a directional asymmetry in the angle the legs (also known as appendages or barbs) make with the main spline of the prominence. For some prominences, the legs bear off the spline to the right and for others the legs bear off to the left. These two structural classes of prominence are said to be either right-bearing or left-bearing, respectively. This brings us to the second result which states that there is a near-perfect correlation between the magnetic class and the structural class: dextral prominences have rightbearing structure and sinistral prominences are left-bearing. This is a very significant result that provides a major clue to the relation between the magnetic field and the basic prominence structure. It also emphasises the importance of understanding prominence barbs. Figure 6 shows examples of dextral and sinistral prominences and their associated magnetic structures. The next result is possibly the most intriguing, however, and relates to the discovery of a global pattern in the magnetic field and structural patterns of quiescent prominences. (3) There is a clear hemispherical ordering of high-latitude and quiet-region prominences: dextral prominences dominate in the northern hemisphere while sinistral prominences dominate in the southern hemisphere. Moreover, this global pattern seems to be independent of the solar cycle, i.e. dextral prominences always dominate in the northern hemisphere. This is a previously undiscovered fact about prominences and has definite consequences for the global organisation of the magnetic field on the Sun including possible clues for the nature of the solar dynamo. Finally, there is a fourth fact which has been established by Bothmer and Schwenn (1994), namely (4) Magnetic clouds in interplanetary space have lefthanded twist in the northern hemisphere and right-handed in the southern hemisphere. This appears to be consistent with the magnetic ordering of quiescent prominences described above and so it is possible that some of the helicity of the prominence field is conserved through an eruption and transferred into interplanetary space. These results, along with other recent observations, are dramatically altering our traditional ideas about prominences, particularly the details of their structure and the physical processes responsible for their formation, evolution and eruption. In fact, the main paradigms of classical prominence theory are currently being challenged in an attempt to reconcile their newly discovered properties with advanced physical ideas. 2.3.2
Theory
The main existing paradigms Priest (1996) are that: (a) prominence formation is by radiative instability when the length of a flux tube exceeds a critical value such that the conduction time exceeds the radiation time; (b) the basic structure of the magnetic field around a prominence is that of a highly sheared force-free arcade with
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DEXTRAL
Fig. 6. Examples of dextral and sinistral prominences (Courtesy S. Martin, Big Bear Solar Observatory).
the field lines crossing the axis at a small angle and so being highly non-potential; the shear is produced by photospheric motions parallel to the prominence; in a plane normal to the prominence axis it may be of normal or inverse polarity; (c) prominence material is static; (d) eruption occurs when the twist in a prominence is too great. These are being challenged, respectively, as follows: (A) theoretical approaches now suggest that, as the length of a hot coronal loop increases, there is no onset of radiative instability or lack of a hot equilibrium: rather, the hot equilibrium remains in stable existence and it is rather difficult to transfer it to a cool equilibrium. (B) The observations of Martin (1990) and Martin et al. (1994) suggest that some prominences form without evidence of appreciable shear flow; also the fact that there is a strong field component along the filament does not imply the field has to be non-potential, since the distribution of flux sources along the filament channel could produce such a strong component. (C) Prominence material is definitely not static! Many complex motions have been observed in and around prominences. (D) Although twist and twisting motions are observed in some prominences, and there are theoretical grounds for believing paradigm (d), it has yet to be carefully shown from observations. In a recent paper Priest et al. (1996) have suggested a new theory for the formation and structure of a filament channel and prominence. In their paper, they addressed the above paradigms and proposed a new model which accounts for some of the key points: • Their model is dynamic with a continual input and out-
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put of mass and magnetic flux, driven by flux reconnection; emergence of flux builds up a filament channel with flux parallel to the polarity inversion line, and then flux reconnection forms and maintains a prominence in the filament channel. In this way, cool mass may be added to a prominence by a second mechanism in addition to the radiative condensation referred to in paradigm (a), namely the lifting up of photospheric and chromospheric mass in response to flux reconnections beneath the prominence. This mechanism would then account for both prominence formation and the continued supply of material to the prominence. n They consider the prominence field to be that of a threedimensional magnetic flux tube which may be untwisted or twisted, but the twist is not an essential ingredient. It is not a sheared force-free arcade field and the field lines are basically parallel to the polarity inversion line as opposed to crossing it. It may happen that the field becomes twisted as part of the eruptive process, resulting in the type of event shown in Figure 3. • They propose several local models for prominence feet. This is the first serious attempt to understand feet and it provides a preliminary explanation as to why dextral prominences have right-bearing magnetic structures and viceversa. • They suggest a process by which effects of sub-surface velocity shear (due to differential rotation), flux emergence and flux reconnection combine to produce the global hemispheric patterns outlined in observation (3). The main features of this are shown in Figure 7. The hemispherical ordering of prominence chirality is possibly the most important of the recent findings and so any reasonable future prominence model should take this into account. These are the main points in their model which appears to be very promising in the light of the latest observations. However, many questions remain and a detailed programme of prominence observations is required to confirm and supplement these theoretical ideas. From their studies and the recent observations of Martin et al. (1994), it is quite clear that a good knowledge of the three-dimensional structure of prominences and, in particular, the nature of the feet is an essential step towards understanding their basic properties and overall behaviour.
3
Wider Relevance of Solar Prominences
The study of prominences is an important branch of solar physics as it contributes significantly to the overall understanding of the Sun and its atmosphere. One only needs to be presented with the illuminating fact that there is more mass contained in these bodies than in the remainder of the entire corona to be convinced of their importance. The study of prominences provides essential clues to the nature of the coronal magnetic field. Measurements of the magnetic field in prominences is possible by means of Zeeman and Hanle methods, thus giving direct information about the field in the corona. The stunning X-ray videos from
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(f) Dextral fieki before eruption
Fig. 7. The main features of the new model of Priest et ai. (1996). Subsurfacedifferentialrotation acts on the field in the preparation phase to produce the observedfielddirectionin dextral (a) and sinistral (e) prominences. Emergence of flux builds up the field in the filamentchannel (b) and recon-
nection and flux cancellationacts to raise cool material up into the corona, buildingup the prominencematerial (c). The field produced has the correct sense of twist before eruption (f) to match with observationsof magnetic clouds in interplanetaryspace.
Yohkoh highlight the large-scale coronal loops and arcades that overlie prominences and provide details of their orientation. The close association of prominences with overlying helmet streamers is evident from eclipse photographs which highlight the global structure of the field and often reveal a prominence lying at the base of the configuration. Also, the chromospheric fibril patterns in H a give more information about the field structure at lower heights. The hemispheric patterns observed by Martin et al. (1994) now provide even more incentive for studying prominences since this may lead to a better understanding of the global organisation of the field on the Sun. This in turn has obvious benefits as it leads to a better understanding of the origins of the field and details of the solar dynamo. The Yohkoh videos also capture many associated dynamic events and emphasise the dramatic effects a prominence eruption can have on the coronal field. During such an event the overlying structures can be broken entirely open, resulting in a large-scale reorganisation of the field. Following the eruption the field immediately reconnects back to a new closed configuration and unexpected shear angles often appear in the reformed arcades which have been strongly heated by the vast release of magnetic energy. This process often
Imaging Solar Coronal Magnetic Structures in 3D leads to a two-ribbon flare (see below). The prominence is often seen to reform again over the course of a few days after the eruption. Dynamical prominence eruptions are often associated with coronal mass ejections (or CME's). In this event, a huge body of overlying material is driven outwards and accelerated to tremendous velocities, often higher than 1000 km s -1 . The initiation of a CME takes place before the onset of the prominence eruption and the whole process occurs over a period of a few hours. Coronal mass ejections are hugely important phenomena as they represent a substantial proportion of the mass lost from the Sun. Prominences are intimately associated also with tworibbon solar flares which are observed when a prominence loses equilibrium. An enormous amount of heating occurs and temperatures can soar to 10rK - 10SK, again due to the release of stored magnetic energy. In this highly complex burst of activity two distinct, bright Ha ribbons (heated photospheric material) on either side of the polarity inversion line are observed. These ribbons correspond to footpoints of the arcade field and are simultaneously heated by reconnection processes at the top of the arcade. As the field closes back down the hot loops appear to move outwards and the two ribbons correspondingly move apart. The hot loops can reach an altitude of 100,000 km. This is another very significant type of coronal event as it provides a large contribution of heat and fast particles to the corona. Solar prominences are clearly of great interest in their own right and there are many solar phenomena with which they are closely associated, but they are also of great importance in a wider, astrophysical context. They provide a natural laboratory for studying fundamental magnetohydrodynamical processes, particularly the subtle and nonlineareffects of gravitational, magnetic and pressure forces on the equilibrium and stability of a plasma. They also provide an ideal opportunity for studying the thermal properties of a plasma in a magnetic field to understand how the field can allow such material to exist in neighbouring cool and hot states.
4
Stereographic Observations of Prominences
It is clear from current observations that solar prominences and their associated magnetic fields are fully threedimensional in nature: Ha images reveal their highly complex forms and structures; NIXT and Yohkoh X-ray data show their close association with overlying coronal arcades; white light eclipse and coronagraph observations inform us of the surrounding coronal cavities and vast overlying helmet streamers; Helium 10830 images highlight the underlyingfilament channels which change spatially in width and kink; and photospheric magnetograms allow us to see their link with the global polarity inversion lines that weave their way across the Sun. Coronal instruments on SOHO are currently engaged in observing programmes to try and determine their local thermodynamic properties, but without the benefit of a stereoscopic view. At present it is not possible to obtain a three-dimensional
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or stereographic view of prominences. It is currently possible to observe a prominence against the disc and observe it again when it has appeared at the limb of the Sun, some 7 days later. Typical quiescent prominence lifetimes permit this, and a rough estimate of their global structure and magnetic fields may be achieved with this method. However, there are problems: the technique of using solar rotation does not work well for east-west aligned prominences located more than 30 degrees north or south of the equator because the barbs on the poleward side are never visible; also, many of the important features of prominences are changing on much shorter timescales. For example, the feet can appear and disappear over the course of two or three days, and the internal fine structure is continuouslyevolving over tens of minutes or an hour. Plasma flows of 1-2 k m s - 1 are persistently acting to change the basic structure, and important magnetic fluctuations are occurring all the time: flux emergence, cancellation and reconnection events are particularly important as they provide essential clues to the overall stability and masssupply to the prominence. A precise interpretation of such variations is only possible with stereographic observations, the availability of which would allow vital important physical questions to be addressed: • What is the true three-dimensional structure of prominences and what is the structure of the associated magnetic field? • Where are the feet located: how do they relate to the photospheric magnetic field and what are the corresponding plasma flows? • What is the overlying structure and how does this vary along the axis of the prominence? • What are the dominant flows in prominences: in limb videos vertical and horizontal motions are present but what would a simultaneous disc video or dopplergram reveal? • Is there a correspondence between cancelling or emerging photospheric flux and mass supply to the prominence? • Is the global equilibrium due to magnetic support, continuous plasma injection or a combination of these effects? This is merely a sample of the many important questions which could be answered with stereographic observations in order to provide a better understanding of prominences and the surrounding medium. Indeed, many other coronal events and structures could be studied with such technology, resulting in a vastly improved knowledge of the complex physical mechanisms occurring in the Sun's rich atmosphere.
5
Summary
Solar prominences are of great importance, both in their own right and also in a much wider context. Many of the corona' s intriguing features and dynamical events are closely associated with prominences and so a fundamental knowledge of their basic properties is essential if we are to understand the principles behind other key phenomena. In particular, highly complex events, such as solar flares and coronal mass ejections, are closely linked with prominence eruptions; the con-
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cept o f cool, dense plasma in magnetohydrostatic and thermal equilibrium is o f direct relevance; and the very nature o f the coronal magnetic field and it's origins may be better understood through p r o m i n e n c e observations and models. Over the last few years, new observations have caused the main paradigms o f classical prominence theory to be challenged. It is clear that prominences are fully threedimensional in nature and the role o f prominence barbs in determining many o f their key features is becoming evermore apparent. The recently discovered hemispherical patterns are stimulating many new questions about their formation and structure and also the global organisation o f the magnetic field on the Sun. At present, crude techniques are available to estimate the basic global structure o f prominences but these are far from ideal and we are always left to guess the complete three-dimensional structure. If we could observe prominences from different vantage points (stereoscopically), we could much more readily establish the geometry o f any given prominence. Such technology would provide an essential step towards a fundamental understanding o f these fascinating structures and, indeed, o f the corona as a whole. Acknowledgements. I would like to acknowledge financial support from the UK Particle Physics and Astronomy Research Council. I have benefitted by working with members of the PROM research team funded by NASA grant NAGW-4300 and would like to acknowledge fruitful discussions with
Sara Martin, Jack Zirker and Eric Priest and thank them for their helpful comments.
References Babcock, H. and Babcock, H., Astrophys. J., 121,349, 1955. Bothmer, V. and Schwenn, R., Space Sci. Rev., 70. 215, 1994. Engvold, O., in Dynamics and Structure of Quiescent Solar Prominences, edited by E. R. Priest, Kluwer Academic Publishers, Dordrecht, Holland, 1989. Hundhausen, A. J., Sawyer, C. R., House, L., Illing, R. M. E., and Wagner, W. J., J. Geophys. Res., 89, 2639, 1984. Kim, I. S., in Dynamics of Quiescent Prominences, Proc. IAU Conf., edited by V. Ruzdjak and E. Tandberg-Hansen,vol. 117, p. 49, 1990. Kippenhabn, R. and SchlUter,A., Z. Astrophys., 43, 36, 1957. Kuperus, M. and Raadu, M., Astron. Astrophys., 31, 189, 1974. Leroy, J. L., Bommier, V., and Sabal-Br~chot,S., Solar Phys., 83, 135, 1983. Martin, S. E, in Dynamics of Quiescent Prominences, Proc. IAU Conf., edited by V. Ruzdjak and E. Tandberg-Hanssen, vol. 117, p. 1, 1990. Martin, S. E, Bilimoria, R., and Tracadas, P. W., in Solar Surface Magnetism, edited by R. J. Rutten and C. J. Schrijver, p. 303, SpringerVerlang, New York, 1994. Priest, E. R., Dynamics and Structure of Quiescent Solar Prominences, Kluwer, Holland, 1989. Priest, E. R., in Solar Drivers of Interplanetary Disturbances, edited by K. Balasabramariar, S. Keil, and R. Smartt, Astron. Soc. Pacific, 1996. Priest, E. R. and Forbes, T. G., Solar Phys., 126, 319, 1990. Priest, E. R., VanBallegooijen, A. A., and MacKay, D. H., Astrophys. J., 460, 530, 1996. Tandberg-Hanssen,E., Solar Prominences, D. Reidel, Dordrecht, 1974. Tandberg-Hanssen,E. and Anzer, U., Solar Phys., 15, 158, 1970.
Phys. Chem. Earth, Vol. 22, No. 5, pp. 411-418, 1997 © 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0079-1946/97 $17.00 + 0.00 PII: S0079-1946(97)00168-7
Imaging Solar Coronal Magnetic Structures in 3D N. P. Cartledge Department of Applied Mathematics, University of St. Andrews, St. A n d r e w s KY16 9SS, U.K.
Received 10 May 1996; accepted 17 February 1997
Abstract. The study of solar coronal structures and, in particular prominences, is a key part of understandingthe highly complex physical mechanisms occurring in the Sun's atmosphere. Solar prominences are important in their own right and some of the most puzzling questions in solar theory have arisen through their study. For example, how do they form and how is their mass continuously replenished? How can the magnetic field provide their continuous support against gravity over time periods of several months? How can such cool, dense material exist in thermal equilibrium in the surrounding coronal environment? Why do they erupt? A study of their structure and that of the surrounding medium is important in determining the nature of the coronal plasma and magnetic field. Also, prominences are closely associated with other key phenomena such as coronal mass ejections and eruptive solar flares which occur as a prominence loses equilibrium and rises from the solar surface. Our current understanding of these fascinating structures is extremely limited and we know very little about their basic global structure. In fact, recent prominence observations have caused our basic paradigms to be challenged (Priest, 1996) and so we must set up new models in order to gain even a fundamental understanding. Prominences are highly nonlinear, three-dimensionalstructures. Large feet (or barbs) reach out from the main body of a prominence and reach down to the photosphere where the dense material continuously drains away. These provide a real clue to the threedimensional nature of the coronal field and its relation to the photospheric field. It is important, therefore, to make stereographic observations of prominences in order to gain a basic understanding of their essentially three-dimensional nature and attempt to formulate new paradigms for their structure and evolution. There is no doubt that the study of prominences in three dimensions is a crucial exercise if we are to develop a better understanding of the solar magnetic field and the physical processes occurring m the corona. This will only be possible
with the aid of stereographic observations. © 1997 Published by Elsevier Science Ltd
1 Introduction 1.1
Basic Properties
Solar prominences are cool, dense bodies of plasma located in the lower regions of the solar corona and are amongst the most important and yet least understood of the Sun' s coronal structures. Typically, they possess temperatures which are 100 times lower than the surrounding atmosphere and densities 100 times higher, which immediately raises the questions: how do they remain cool despite the surrounding coronal environment which has been heated to an incredible 3 million degrees? How are they supported against the Sun's huge gravitational pull for such long time-scales? The lifetime of a mature quiescent prominence may be as extended as several months. Prominences are best observed in the Ha line where they appear dark against the disc, due to the absorption of the underlying photospheric emission (Figure 1, top), and bright at the limb, due to their relatively high densities (Figure 1, bottom). Prominences are enormous features! A typical quiescent prominence extends for 200 Mm, has a height of 50 Mm, and yet is just 6 Mm wide. Thus, they are generally observed as thin, elongated sheets of plasma appearing as ribbon-like structures against the disc (Figure 1, top). Seen side on at the limb, they display many interesting features such as fine structure (inhomogeneouslydistributed material, clumped in the form of thin, vertical threads) and huge feet which reach down to the photosphere, typically spaced at quasi-periodic intervals of 30 Mm. They are located in many parts of the corona ranging from quiet regions, which are areas of reduced activity and relatively weak field strength (typically I0 Gauss), to active regions where the magnetic field is much stronger (about 50-100 G) and often highly non-potential, such that large quantities of magnetic energy can be stored. Although quiescent prominences display little global
Correspondence to: N. P. Cartledge 411
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N. P. Cartledge There are many other fascinating structural aspects to solar prominences (Tandberg-Hanssen, 1974; Priest, 1989) but it is now worthwhile outlining their main magnetic properties. The magnetic field is of fundamental importance for their very existence and its presence produces many of the physical processes responsible for their various curious properties. 1.2
Magnetic Field
Solar prominences are always observed to overlie a magnetic polarity inversion line (Babcock and Babcock, 1955). This is a curve on the photosphere which separates regions of opposite magnetic polarity. Therefore, the magnetic field is an essential ingredient and of fundamental importance for the formation, existence, evolution and eruption of prominences. Figure 2 depicts an active-region prominence against the disc, showing its Ha structure and its close relation to the photospheric magnetic field. In the magnetogram, black areas represent positive polarity and white areas are negative polarity. Thus, prominences are thought to be contained in large-scale magnetic flux tubes and this field plays an essential role in supporting the dense material over long timescales.
Fig. 1. Hc~ imagesof the Sun showingribbon-likeprominencesagainst the disc (top) and a quiescentprominenceat the limb(bottom). Courtesy MeudonObservatoryand Big BearSolarObservatory.
change over time-scales of a few hours the fine structure is continuously evolving over a few minutes. On longer timescales, the feet mysteriously evolve over a few days and the overall structure tends to change over periods of several months. A prominence may experience several repeated eruptions during its restless lifetime, the material incredibly reforming after each eruption over the course of a few days. Sometimes the eruptions are so violent that the whole surrounding magnetic structure is blown apart, the release of vast quantities of magnetic energy somehow driving material way up into the corona. In the most extreme cases enough energy is released to drive a coronal mass ejection (Hundhausen et al., 1984) in which a huge quantity of overlying material may be accelerated to speeds of several hundred kilometres per second. This process represents a significant percentage of the total mass lost from the Sun.
Fig.2. The relationbetweena prominence,filamentchannelandthe photospheric magneticfield. Courtesy S. Martin, Big Bear Solar Observatory. The magnetic field also appears to play other roles. Overlying many prominences is a large-scale helmet streamer which can be seen during a total eclipse, and surrounding the prominence is a closed region of reduced density, often with a near circular geometry. This is a coronal cavity (see Engvold (1989) for details) and may be considered to be the outline of a surrounding helical magnetic field. The magnetic field through the prominence is highly sheared and typically makes an angle of 200 with the long-axis of the prominence (Tandberg-Hanssen and Anzer, 1970; Leroy et al., 1983; Kim, 1990) and so may not necessarily be potential. This field is observed to pass through a prominence in
Imaging Solar Coronal Magnetic Structures in 3D either the same direction or the opposite direction when compared with the underlying photospheric field. Priest (1989) suggested that these two cases can be categorised as either normalpolarity or inverse polarity, respectively. It turns out that most prominences are inverse polarity, especially highlatitude or quiet-region prominences. Some prominences are observed to be normal polarity: these are generally the lower latitude active-region prominences which are also located at lower altitudes. Another interesting feature of the prominence field is observed in the underlying chromosphere. There, the magnetic field is strongly aligned with both the axis of the prominence and the polarity inversion line such that the horizontal component of the field is parallel to these features. This is called afilament channel and is characterised, in active regions, by the strong alignment of chromospheric Ha fibrils (Figure 2). These fibrils tend to lie along the field and so enable a reasonable determination of the field direction in the chromosphere. Thus, the field below prominences does not appear to cross over the polarity inversion line, as was thought previously. This has invalidated many of the classical prominence models (see Section 2.2) and has caused our basic paradigms to be reviewed.
Fig. 3. A hugeeruptingprominenceat the limbof the Sun. The twist and three-dimensionalnatureof the magneticfieldis clearlyevidentin this image. CourtesyNavalResearchLaboratory. Occasionally, during a prominence eruption the magnetic structure is outlined, as shown in Figure 3. Here the erupting magnetic field is highlighted by the plasma which clearly shows the twisted, three-dimensional nature of the configuration. It is certainly apparent that the magnetic field in prominences and the surrounding corona varies strongly in all three directions. Its complex topology is far from obvious but at least the material in prominences is sufficiently dense to enable direct field measurements there, thus providing a foundation for understanding the nature of the coronal magnetic field and its evolution.
2 2.1
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Theoretical Approach Important Puzzles
In the past several theoretical models have been proposed in an attempt to gain some understanding of the puzzling qualifies of prominences. They are tremendously complicated creatures and there are several different questions which must be separately tackled before a more general understanding can be attained. One of the most important questions refers to their support. The dense body of a prominence appears to hang magically in the corona for weeks on end despite the huge gravitational pull to which it is subject. The solution appears to lie within the magnetic field, which is somehow organised to provide a continuous upward Lorentz force on the plasma, countering its weight. But how can the field locally provide such support and how does this field match to the background coronal field? Another obvious question relates to the thermal equilibrium and the prominence-coronainterface. How is it possible for prominence material at 10,000K to exist in thermal balance with the surrounding corona, heated to several million degrees? Again, the magnetic field must play an important role. Thermal conduction is very weak a~ross magnetic field lines and so the field may be arranged to thermally shield the prominence material from the background environment. The problem is again three-dimensional. It is important to understand the surrounding field in order to evaluate the energy balance and gain some feeling as to how cool, dense material may remain in equilibrium within the much hotter and more tenuous coronal atmosphere. What is the three-dimensional structure of prominences and how does this relate to the magnetic field? Prominence feet represent large variations in the global structure along the prominence and are locations at which material may drain out of the main body of a prominence. There are no satisfactory explanations for these feet, or, indeed, the many other related features, such as the fine structure. Why is the material not uniformly distributed throughout the main body of the prominence but rather clumped into thin vertical threads of diameter, typically 200km? Another bewildering puzzle relates to the mass supply. Continuous slow, downward flows of 1-2 k m s -1 are observed in prominences which implies that the whole structure should drain away within the course of a few days, yet the prominence lifetime is far greater. How is the mass resupplied to the prominence? Is it lifted up by the reconnecting actions of the underlying magnetic field? The eruption of solar prominences is yet another topic about which very little is understood. What causes the magnetic field to lose equilibrium and how is its energy released so as to drive the prominence material into the high reaches of the corona? Priest and Forbes (1990) presented a good two-dimensional model for such an eruption in which no neighbouringequilibriumof their considered magnetic structure is possible when the shear is increased beyond a critical
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value. The problem is yet to be solved in a three-dimensional geometry, however, and so this presents another important theoretical challenge. Quite clearly, there are many puzzles still to be unravelled if we are to develop a fundamental understanding of these structures and the corona as a whole. 2.2
A unique position for the prominence can then be found from the balance of forces. Figure 5 shows the final equilibrium.
Classical Theoretical Models
Traditionally, prominence models have focussed on their transverse structure, concentrating on the supporting mechanisms and the topology of the surrounding magnetic field. With the limited aid of two-dimensionalobservations this has led to a great deal of misinterpretation. This section briefly describes two of the classical theoretical models and points out some of the inevitable ambiguities which have arisen.
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One of the first advances was due to Kippenhahn and Schltiter (1957) (hereafter abbreviated to K-S) who gave a simple model for the support of a prominence in a normalpolarity configuration. The basic topology is shown in Figure 4 where the prominence is represented as a sheet with current I at a height h above the photosphere. The current is directed out of the plane and results from a discontinuity in the vertical field component. If the photospheric footpoints are linetied during the formation of a prominence, the preservation of the footpoint position can be modelled by adding an image current ( - 1 ) at a distance h below the photosphere to the original arcade and prominence sheet. Thus, the prominence mass (m) is supported against gravity both by the line tying (the repulsion p I 2 /(47rh) between I and ( - I ) ) and also by the Lorentz force 1B acting on I in the original background field B at height h. An alternative magnetic topology (inverse-polarity) was proposed by Kuperus and Raadu (1974) (K-R) where the magnetic field passes through the prominence in the opposite direction. They represented the prominence by a line current embedded in the background field of a vertical neutral sheet. Again, to simulate the line-tying during the formation, an image current is required and this provides the upward repulsive force pI2/(4rh) to balance the weight of the filament.
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Fig. 5. The Kuperus-Raadumodel. A linecurrent,representingthe prominence,is supportedin equilibriumin an inverse-polarityconfiguration. It is obvious, even from the two models described, that several different interpretations are possible with 2-D observations. In the K-S model, the field passes directly from the photosphere through the prominence, whereas in K-R models the prominence field is detached. The field in the KR model reverses its sign beneath the prominence and possesses an X-type neutral point. There is also an O-type neutral point present. These are two very contrasting topologies, but which type is correct, if either? Many subsequent models have presented further interpretations, which really fuels the need for 3-D observations to pinpoint the correct structure. To date, most models have assumed invariance in the third, axial direction. This has provided a basic understanding of their two-dimensional properties and has certainly been invaluable in that sense. However, it is clear that prominences are very much three-dimensional structures with substantial variations in the axial direction such as kinks, bends and prominence feet reaching down to the photosphere. Both new models and observations are required to take these important aspects into account. 2.3
Recent Advances
2.3.1 Observations A recent detailed study by Martin et al. (1994) has uncovered several new interesting observational facts about prominences. The main results are as follows: (1) Prominences lie along polarity inversion lines, with fibrils in the filament channel running parallel to the prominence rather than crossing it. Many of the classical models have required that the field crosses through the prominence from one side to the other, such as the K-S and K-R models. (2) If an imaginary observer is standing in the photosphere at the positive side of the prominence and sees that the field is directed to the right, then the field is said to be dextral.
Imaging Solar Coronal Magnetic Structures in 3D
Conversely, if the field points to the left, it is said to be sinistral. In addition, there are two structural classes for prominences, corresponding to a directional asymmetry in the angle the legs (also known as appendages or barbs) make with the main spline of the prominence. For some prominences, the legs bear off the spline to the right and for others the legs bear off to the left. These two structural classes of prominence are said to be either right-bearing or left-bearing, respectively. This brings us to the second result which states that there is a near-perfect correlation between the magnetic class and the structural class: dextral prominences have rightbearing structure and sinistral prominences are left-bearing. This is a very significant result that provides a major clue to the relation between the magnetic field and the basic prominence structure. It also emphasises the importance of understanding prominence barbs. Figure 6 shows examples of dextral and sinistral prominences and their associated magnetic structures. The next result is possibly the most intriguing, however, and relates to the discovery of a global pattern in the magnetic field and structural patterns of quiescent prominences. (3) There is a clear hemispherical ordering of high-latitude and quiet-region prominences: dextral prominences dominate in the northern hemisphere while sinistral prominences dominate in the southern hemisphere. Moreover, this global pattern seems to be independent of the solar cycle, i.e. dextral prominences always dominate in the northern hemisphere. This is a previously undiscovered fact about prominences and has definite consequences for the global organisation of the magnetic field on the Sun including possible clues for the nature of the solar dynamo. Finally, there is a fourth fact which has been established by Bothmer and Schwenn (1994), namely (4) Magnetic clouds in interplanetary space have lefthanded twist in the northern hemisphere and right-handed in the southern hemisphere. This appears to be consistent with the magnetic ordering of quiescent prominences described above and so it is possible that some of the helicity of the prominence field is conserved through an eruption and transferred into interplanetary space. These results, along with other recent observations, are dramatically altering our traditional ideas about prominences, particularly the details of their structure and the physical processes responsible for their formation, evolution and eruption. In fact, the main paradigms of classical prominence theory are currently being challenged in an attempt to reconcile their newly discovered properties with advanced physical ideas. 2.3.2
Theory
The main existing paradigms Priest (1996) are that: (a) prominence formation is by radiative instability when the length of a flux tube exceeds a critical value such that the conduction time exceeds the radiation time; (b) the basic structure of the magnetic field around a prominence is that of a highly sheared force-free arcade with
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DEXTRAL
Fig. 6. Examples of dextral and sinistral prominences (Courtesy S. Martin, Big Bear Solar Observatory).
the field lines crossing the axis at a small angle and so being highly non-potential; the shear is produced by photospheric motions parallel to the prominence; in a plane normal to the prominence axis it may be of normal or inverse polarity; (c) prominence material is static; (d) eruption occurs when the twist in a prominence is too great. These are being challenged, respectively, as follows: (A) theoretical approaches now suggest that, as the length of a hot coronal loop increases, there is no onset of radiative instability or lack of a hot equilibrium: rather, the hot equilibrium remains in stable existence and it is rather difficult to transfer it to a cool equilibrium. (B) The observations of Martin (1990) and Martin et al. (1994) suggest that some prominences form without evidence of appreciable shear flow; also the fact that there is a strong field component along the filament does not imply the field has to be non-potential, since the distribution of flux sources along the filament channel could produce such a strong component. (C) Prominence material is definitely not static! Many complex motions have been observed in and around prominences. (D) Although twist and twisting motions are observed in some prominences, and there are theoretical grounds for believing paradigm (d), it has yet to be carefully shown from observations. In a recent paper Priest et al. (1996) have suggested a new theory for the formation and structure of a filament channel and prominence. In their paper, they addressed the above paradigms and proposed a new model which accounts for some of the key points: • Their model is dynamic with a continual input and out-
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put of mass and magnetic flux, driven by flux reconnection; emergence of flux builds up a filament channel with flux parallel to the polarity inversion line, and then flux reconnection forms and maintains a prominence in the filament channel. In this way, cool mass may be added to a prominence by a second mechanism in addition to the radiative condensation referred to in paradigm (a), namely the lifting up of photospheric and chromospheric mass in response to flux reconnections beneath the prominence. This mechanism would then account for both prominence formation and the continued supply of material to the prominence. n They consider the prominence field to be that of a threedimensional magnetic flux tube which may be untwisted or twisted, but the twist is not an essential ingredient. It is not a sheared force-free arcade field and the field lines are basically parallel to the polarity inversion line as opposed to crossing it. It may happen that the field becomes twisted as part of the eruptive process, resulting in the type of event shown in Figure 3. • They propose several local models for prominence feet. This is the first serious attempt to understand feet and it provides a preliminary explanation as to why dextral prominences have right-bearing magnetic structures and viceversa. • They suggest a process by which effects of sub-surface velocity shear (due to differential rotation), flux emergence and flux reconnection combine to produce the global hemispheric patterns outlined in observation (3). The main features of this are shown in Figure 7. The hemispherical ordering of prominence chirality is possibly the most important of the recent findings and so any reasonable future prominence model should take this into account. These are the main points in their model which appears to be very promising in the light of the latest observations. However, many questions remain and a detailed programme of prominence observations is required to confirm and supplement these theoretical ideas. From their studies and the recent observations of Martin et al. (1994), it is quite clear that a good knowledge of the three-dimensional structure of prominences and, in particular, the nature of the feet is an essential step towards understanding their basic properties and overall behaviour.
3
Wider Relevance of Solar Prominences
The study of prominences is an important branch of solar physics as it contributes significantly to the overall understanding of the Sun and its atmosphere. One only needs to be presented with the illuminating fact that there is more mass contained in these bodies than in the remainder of the entire corona to be convinced of their importance. The study of prominences provides essential clues to the nature of the coronal magnetic field. Measurements of the magnetic field in prominences is possible by means of Zeeman and Hanle methods, thus giving direct information about the field in the corona. The stunning X-ray videos from
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Fig. 7. The main features of the new model of Priest et ai. (1996). Subsurfacedifferentialrotation acts on the field in the preparation phase to produce the observedfielddirectionin dextral (a) and sinistral (e) prominences. Emergence of flux builds up the field in the filamentchannel (b) and recon-
nection and flux cancellationacts to raise cool material up into the corona, buildingup the prominencematerial (c). The field produced has the correct sense of twist before eruption (f) to match with observationsof magnetic clouds in interplanetaryspace.
Yohkoh highlight the large-scale coronal loops and arcades that overlie prominences and provide details of their orientation. The close association of prominences with overlying helmet streamers is evident from eclipse photographs which highlight the global structure of the field and often reveal a prominence lying at the base of the configuration. Also, the chromospheric fibril patterns in H a give more information about the field structure at lower heights. The hemispheric patterns observed by Martin et al. (1994) now provide even more incentive for studying prominences since this may lead to a better understanding of the global organisation of the field on the Sun. This in turn has obvious benefits as it leads to a better understanding of the origins of the field and details of the solar dynamo. The Yohkoh videos also capture many associated dynamic events and emphasise the dramatic effects a prominence eruption can have on the coronal field. During such an event the overlying structures can be broken entirely open, resulting in a large-scale reorganisation of the field. Following the eruption the field immediately reconnects back to a new closed configuration and unexpected shear angles often appear in the reformed arcades which have been strongly heated by the vast release of magnetic energy. This process often
Imaging Solar Coronal Magnetic Structures in 3D leads to a two-ribbon flare (see below). The prominence is often seen to reform again over the course of a few days after the eruption. Dynamical prominence eruptions are often associated with coronal mass ejections (or CME's). In this event, a huge body of overlying material is driven outwards and accelerated to tremendous velocities, often higher than 1000 km s -1 . The initiation of a CME takes place before the onset of the prominence eruption and the whole process occurs over a period of a few hours. Coronal mass ejections are hugely important phenomena as they represent a substantial proportion of the mass lost from the Sun. Prominences are intimately associated also with tworibbon solar flares which are observed when a prominence loses equilibrium. An enormous amount of heating occurs and temperatures can soar to 10rK - 10SK, again due to the release of stored magnetic energy. In this highly complex burst of activity two distinct, bright Ha ribbons (heated photospheric material) on either side of the polarity inversion line are observed. These ribbons correspond to footpoints of the arcade field and are simultaneously heated by reconnection processes at the top of the arcade. As the field closes back down the hot loops appear to move outwards and the two ribbons correspondingly move apart. The hot loops can reach an altitude of 100,000 km. This is another very significant type of coronal event as it provides a large contribution of heat and fast particles to the corona. Solar prominences are clearly of great interest in their own right and there are many solar phenomena with which they are closely associated, but they are also of great importance in a wider, astrophysical context. They provide a natural laboratory for studying fundamental magnetohydrodynamical processes, particularly the subtle and nonlineareffects of gravitational, magnetic and pressure forces on the equilibrium and stability of a plasma. They also provide an ideal opportunity for studying the thermal properties of a plasma in a magnetic field to understand how the field can allow such material to exist in neighbouring cool and hot states.
4
Stereographic Observations of Prominences
It is clear from current observations that solar prominences and their associated magnetic fields are fully threedimensional in nature: Ha images reveal their highly complex forms and structures; NIXT and Yohkoh X-ray data show their close association with overlying coronal arcades; white light eclipse and coronagraph observations inform us of the surrounding coronal cavities and vast overlying helmet streamers; Helium 10830 images highlight the underlyingfilament channels which change spatially in width and kink; and photospheric magnetograms allow us to see their link with the global polarity inversion lines that weave their way across the Sun. Coronal instruments on SOHO are currently engaged in observing programmes to try and determine their local thermodynamic properties, but without the benefit of a stereoscopic view. At present it is not possible to obtain a three-dimensional
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or stereographic view of prominences. It is currently possible to observe a prominence against the disc and observe it again when it has appeared at the limb of the Sun, some 7 days later. Typical quiescent prominence lifetimes permit this, and a rough estimate of their global structure and magnetic fields may be achieved with this method. However, there are problems: the technique of using solar rotation does not work well for east-west aligned prominences located more than 30 degrees north or south of the equator because the barbs on the poleward side are never visible; also, many of the important features of prominences are changing on much shorter timescales. For example, the feet can appear and disappear over the course of two or three days, and the internal fine structure is continuouslyevolving over tens of minutes or an hour. Plasma flows of 1-2 k m s - 1 are persistently acting to change the basic structure, and important magnetic fluctuations are occurring all the time: flux emergence, cancellation and reconnection events are particularly important as they provide essential clues to the overall stability and masssupply to the prominence. A precise interpretation of such variations is only possible with stereographic observations, the availability of which would allow vital important physical questions to be addressed: • What is the true three-dimensional structure of prominences and what is the structure of the associated magnetic field? • Where are the feet located: how do they relate to the photospheric magnetic field and what are the corresponding plasma flows? • What is the overlying structure and how does this vary along the axis of the prominence? • What are the dominant flows in prominences: in limb videos vertical and horizontal motions are present but what would a simultaneous disc video or dopplergram reveal? • Is there a correspondence between cancelling or emerging photospheric flux and mass supply to the prominence? • Is the global equilibrium due to magnetic support, continuous plasma injection or a combination of these effects? This is merely a sample of the many important questions which could be answered with stereographic observations in order to provide a better understanding of prominences and the surrounding medium. Indeed, many other coronal events and structures could be studied with such technology, resulting in a vastly improved knowledge of the complex physical mechanisms occurring in the Sun's rich atmosphere.
5
Summary
Solar prominences are of great importance, both in their own right and also in a much wider context. Many of the corona' s intriguing features and dynamical events are closely associated with prominences and so a fundamental knowledge of their basic properties is essential if we are to understand the principles behind other key phenomena. In particular, highly complex events, such as solar flares and coronal mass ejections, are closely linked with prominence eruptions; the con-
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cept o f cool, dense plasma in magnetohydrostatic and thermal equilibrium is o f direct relevance; and the very nature o f the coronal magnetic field and it's origins may be better understood through p r o m i n e n c e observations and models. Over the last few years, new observations have caused the main paradigms o f classical prominence theory to be challenged. It is clear that prominences are fully threedimensional in nature and the role o f prominence barbs in determining many o f their key features is becoming evermore apparent. The recently discovered hemispherical patterns are stimulating many new questions about their formation and structure and also the global organisation o f the magnetic field on the Sun. At present, crude techniques are available to estimate the basic global structure o f prominences but these are far from ideal and we are always left to guess the complete three-dimensional structure. If we could observe prominences from different vantage points (stereoscopically), we could much more readily establish the geometry o f any given prominence. Such technology would provide an essential step towards a fundamental understanding o f these fascinating structures and, indeed, o f the corona as a whole. Acknowledgements. I would like to acknowledge financial support from the UK Particle Physics and Astronomy Research Council. I have benefitted by working with members of the PROM research team funded by NASA grant NAGW-4300 and would like to acknowledge fruitful discussions with
Sara Martin, Jack Zirker and Eric Priest and thank them for their helpful comments.
References Babcock, H. and Babcock, H., Astrophys. J., 121,349, 1955. Bothmer, V. and Schwenn, R., Space Sci. Rev., 70. 215, 1994. Engvold, O., in Dynamics and Structure of Quiescent Solar Prominences, edited by E. R. Priest, Kluwer Academic Publishers, Dordrecht, Holland, 1989. Hundhausen, A. J., Sawyer, C. R., House, L., Illing, R. M. E., and Wagner, W. J., J. Geophys. Res., 89, 2639, 1984. Kim, I. S., in Dynamics of Quiescent Prominences, Proc. IAU Conf., edited by V. Ruzdjak and E. Tandberg-Hansen,vol. 117, p. 49, 1990. Kippenhabn, R. and SchlUter,A., Z. Astrophys., 43, 36, 1957. Kuperus, M. and Raadu, M., Astron. Astrophys., 31, 189, 1974. Leroy, J. L., Bommier, V., and Sabal-Br~chot,S., Solar Phys., 83, 135, 1983. Martin, S. E, in Dynamics of Quiescent Prominences, Proc. IAU Conf., edited by V. Ruzdjak and E. Tandberg-Hanssen, vol. 117, p. 1, 1990. Martin, S. E, Bilimoria, R., and Tracadas, P. W., in Solar Surface Magnetism, edited by R. J. Rutten and C. J. Schrijver, p. 303, SpringerVerlang, New York, 1994. Priest, E. R., Dynamics and Structure of Quiescent Solar Prominences, Kluwer, Holland, 1989. Priest, E. R., in Solar Drivers of Interplanetary Disturbances, edited by K. Balasabramariar, S. Keil, and R. Smartt, Astron. Soc. Pacific, 1996. Priest, E. R. and Forbes, T. G., Solar Phys., 126, 319, 1990. Priest, E. R., VanBallegooijen, A. A., and MacKay, D. H., Astrophys. J., 460, 530, 1996. Tandberg-Hanssen,E., Solar Prominences, D. Reidel, Dordrecht, 1974. Tandberg-Hanssen,E. and Anzer, U., Solar Phys., 15, 158, 1970.