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Solar Research Today: A Science Overview INTRODUCTION In recent years, theory and observation have established that the Sun is a complex dynamical structure whose interior represents an active and mysterious universe of its own. There is no reason to doubt the basic features of stellar structure, but it must be remembered that the ideal standard stellar model contains many arbitrary assumptions. There is evidence from the study of meteorites that the relative atomic abundances may vary throughout the interior of a star. We know from spectroscopy that- composition varies from one star to the next, as do the rotation rates and presumably the primordial magnetic fields. It must be remembered, too, that the Sun is the only star that has been studied in detail and that our only detailed information has come from scrutinizing its more or less inscrutable exterior. The interior possesses internal degrees of freedom that are only gradually being discovered and described, and, once described, are only gradually being understood. The basic reality is that current knowledge of the solar interior is based entirely on theoretical deduction limited largely to simplified, static models constructed from the theoretical properties of particles and radiation as we now understand them. The deductions provide a static solar model whose radius and surface temperature can be adjusted to agree with observation, so that it represents a starting position for the next phase of the inquiry into the physics of a star. This is already well under way. Now the dynamical effects ignored in the static models are already 4

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s suggested by the static models. Thus, for instance, the calculated temper- ature gradients indicate the existence of the convection zone, extending down from the surface for a distance of about 0.3 solar radii. The gas continually overturns and operates as a heat engine. In fact the activity at the surface of the Sun is a direct manifestation of the convective heat engine and involves such diverse phenomena as sunspots, flares, coronal transients, the X-ray corona, and the solar wind. It seems not to be generally recognized in astronomy and elsewhere that the precise causes of the activity are not yet reduced to hard science. Thus, for instance, it cannot be stated why the Sun, or any other solitary star, emits X rays, nor can it be asserted why a star like the Sun is sub- ject to a mass loss of 10~2g/s. Indeed, it is not altogether clear why the Sun operates on a 22-year magnetic cycle, producing the other phenomena related to the activity largely as by-products. This means, then, that we do not understand the origins of stellar X-ray emissions; this branch of X-ray astronomy, with its remarkable powers of penetration into the active component of the universe, is for the present limited largely to phenomeno- logical interpretation. Indeed current ignorance about the Sun reflects the general lack of progress in understanding stellar activity of all kinds. We cannot fully interpret nuances of the surface emissions of the distant stars until we understand the physics of surface activity through close scrutiny of the Sun. However, the problems are deeper than the puzzles of the Sun's surface activity. Mysteries are posed by the different surface abundances of lithium, beryllium, and boron and by the presence of more stable elements such as calcium and iron in some F and G dwarfs. Another puzzle is that theoretical evolutionary brightening predicts that the Sun was 30 percent fainter 3 x 109 or 4 x 109 years ago, whereas over the same period of time, mean temperatures on the terrestrial equator did not vary by more than a few degrees. A more direct problem is that observations of solar neutrino emission have failed to corroborate the conventional theoretical models of the Sun. The failure to achieve such corroboration now being confirmed by the independent Kamiokande II experiment-has stimulated a careful review of the theoretical complexities and uncertainties of the model. Nonethe- less, the present discrepancy between the observed and predicted neutrino emission seems to be stuck at a factor of at least 3. If this dilemma can be resolved, we can limit the rest mass of the neutrino and the dark matter in the universe. Without this step, we cannot be sure of the theoretical evolution of a star on the main sequence. We cannot be sure of the age of globular clusters and the age of the galaxy. We cannot be sure of any theoretical interpretation of anomalous abundances in main sequence stars. Helioseismology shows promise of providing a detailed and quantitative l

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7 characterizing the activity of a star. There does not appear to be a single effect or a single new principle that will throw open the gates to a flood of understanding. The behavior of a convective, highly conducting fluid is a whole field of physics in its own right, which requires years of close theoretical and observational study, progressing past dozens of milestones and enjoying dozens of breakthroughs. The milestones and breakthroughs already add up to an impressive body of knowledge but represent only a beginning. A particularly important milestone was reached about 2 decades ago, when detailed observational and theoretical considerations revealed that the magnetic field at the surface of the Sun, rather than being smoothly distributed as expected, is effectively discontinuous. The photospheric magnetic field consists of small, individual, intense and widely separated magnetic flux tubes of 1 x 103 to 2 x 103 Gauss. The mean held over any region is then a measure of the distance between the individual magnetic fibrils, because the individual fibrils or flux tubes are too small (about 200-km diameter), for the most part, to be resolved in a telescope. The crucial information for understanding the large-scale behavior of the magnetic fields on the Sun (which are, it must be remembered, the perpetrators of the peculiar activity of the Sun) are (1) the structure and origin of the individual fibrils and (2) their individual motions (see Figure 2.1~. So the pursuit of solar activity becomes solar "microscopy," a field in its infancy that has great potential through the development of adaptive optics on ground-based telescopes and the development of diffraction-limited telescope systems in space. Indeed, the high-resolution ultraviolet (UV) observations from space, although not yet approaching the ultimate necessary resolution of 50 to 100 km, have already established the general occurrence of myriad tiny explosive events (nanoflares) and high-speed jets in the solar corona, providing a clue as to the heat input that causes the corona. The individual bursts of energy (1024 to 1027 ergs per event), and indeed the entire supply of energy to the corona, are evidently a result of the motions of the individual magnetic fibrils in the photospheric convection. The motions undoubtedly involve both jitter and intermixing of the individual fibrils, producing Alfven waves and a general wrapping, respectively, of the lines of force in the fields in the corona. But currently there is neither a direct measure of any aspect of the fibril motions nor any direct detection of waves or wrapping in the coronal magnetic fields. Only the myriad small explosive nanoflares can be seen. So the causes of the solar and stellar corona, although extensively developed theoretically, are still without a hard observational foundation. Another important milestone was the Skylab discovery of the frequent coronal transients involving the eruption of matter from the low corona outward into space, often accompanied by flare activity at the surface of

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8 :~ ~ :: . :::::::: FIGURE 2.1 Small-scale solar magnetic fields in an active region, September 29, 1988. The line-of-sight component of the photosphenc magnetic field is shown as bright or dark, depending on polarity of the field, with an intensity proportional to field strength. Ticl;s correspond to 2 arcseconds, or about 1500-km spatial resolution. These observations were obtained by Lockheed Palo Alto Research Laboratory, with equipment developed for space flight, at the Swedish Solar Observatory at La Palma, Canary Islands, Spain. (Reproduced by permission of the Lockheed Palo Alto Research Laboratory.) the Sun. Several spacecraft epochs later, we are beginning to realize that these mass ejection events apparently result from large-scale magnetic field eruptions but why they occur is not clear. Further, it is now suspected that these events precede solar flares or eruptive events rather than result from them. Thus they seemingly are the result of a form of solar activity not heretofore recognized. Their relation to the large-scale evolution of the solar magnetic field-and to stellar magnetic changes is not clear at present. The remarkable X-ray photographs of the Sun, showing clearly the magnetic loop structure of the active corona and the interweaving coronal

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9 holes, have gone a long way toward allowing physicists to formulate the problem posed by the existence of the active X-ray emission. The high- speed streams of solar wind issuing from the coronal holes demonstrate the active nature of the corona outside the active corona. Combining the X-ray and extreme ultraviolet (EUV) studies of the corona of the Sun with the discovery by the Einstein X-ray Observatory that essentially all stars emit X rays challenges scientists to understand why an ordinary star has such extreme suprathermal activity. The ability to release energy impulsively and to accelerate particles is a common characteristic of cosmic plasmas at many sites throughout the universe, ranging from magnetospheres to active galaxies. Observations of gamma rays and hard X rays, radiations that' can be unmistakably associated with accelerated particle interactions, as well as the direct detection of accelerated particles, for example the cosmic rays, strongly suggest that at many sites a significant fraction and in some cases even a major fraction- of the available energy is converted into high-energy particles. The detailed understanding of the processes that accomplish this conversion is one of the major goals of astrophysics. Solar flares oDer an excellent opportunity for achieving this goal. A large solar flare releases as much as 1~2 ergs, and a significant fraction of this energy appears in the form of accelerated particles. It is believed that the flare energy comes from the dissipation of the nonpotential components of strong magnetic fields in the solar atmosphere, possibly through magnetic reconnection. Immediate evidence for the presence of accelerated particles (electrons and ions) is provided by gamma-ray and hard X-ray continuum emissions, which result from electron bremsstrahlung, and gamma-ray line and pion decay emissions from nuclear interactions. Nuclear interactions also produce neutrons, which are likewise directly observable at Earth. The accelerated charged particles enter interplanetary space and arrive at Earth somewhat later, delayed by their circuitous paths of escape from the magnetic fields of the flare. The wide variation in the relative abundances of some isotopes and atomic numbers among the accelerated particles provides a direct view of the special aspects of the acceleration process in the flare. These high-energy emissions are one of the best-known tools for study- ing acceleration processes in astrophysics. Solar flares are among the very few astrophysical sites for which it has been possible to study simulta- neously the acceleration of electrons and protons and to directly detect and correlate the escaping accelerated particles with the electromagnetic radiations produced by the interaction particles (Figure 2.2~. In addition, lower-energy emissions (soft X-ray, EUV, UV, and radio emissions), which are also observed from flares, reveal many of the detailed properties of the

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10 MICROWAVE RADIO HARD X RAYS MAGNETIC FIELD DISSIPATION t ~SOFT X RAYS PARTICLE ACCELERATION ~ ~ ~` ENERGY RELEASE _ . HEATING NUCLEAR By RAYS / NEUTRONS - \ /~°e ~ ' Jim ~ ~ /~W , rho I ~;,rmcnc 104 km I / / , ~ 0 0 ~ 0 off KHOT PLASMA ~ ~ ~(107 TO 1o8 K); em MIRRORING ~\;ASERING? \ ~% \ \p ~ e~\ \t 6~1 TRANSITION ZONE it\ /1 ~ o 1 ,\ CHROMOSPHERE / ~ 1 l l ^. '~_~t-~1 1~= ,~ 1 CORONA / SOFT X RAYS 4==_~: PARTICLE l BEAMS /l g~tVArUnA I IUN5;; ~ HARD X RAYS ,, UV CONTINUUM FIGURE 2.2 A possible solar flare scenario. As a result of the deposition of magnetic energy- most probably by magnetic reconnection bulk plasma heating occurs and electrons and protons are impulsively accelerated. The high-energy emissions or signatures of these particles (e.g., nuclear deexcitation gamma rays from the protons and hard X rays from the electrons) peak simultaneously to within a few seconds, indicating that the protons and electrons are accelerated concurrently, possibly by a single mechanism such as shock or stochastic acceleration. (Reprinted from NASA's Solar Maximum Mission: A Look at a New Sun (1987), edited by J.B. Gurman, Goddard Space Flight Center, Greenbelt, Maryland.)

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11 ambient plasma (e.g., temperature, density, and magnetic configuration) before, during, and after the flare. This is the broad view of our understanding of the Sun and the stars. The specifics will provide many more problems, and it is essential, if we are to grasp the scope of the tasks before us, to spell out the problems in somewhat more detail. The next section, then, following the general principles described above, suggests some of the high-priority problems, measurements, observations, and theoretical studies that are necessary along the way to probe for greater understanding. RESEARCH NEEDS Detailed study of the Sun has established that the most common of stars is a complex dynamical system. Even the quietest regions on the surface of the Sun prove to be riotously active when scrutinized at sufficient magnification at the appropriate wavelengths. We are able to see the gross features of the activity at the surface, although much of the physics goes on at the small scales below the limit of telescopic resolution. There is no reason to think that the interior is any less active because we cannot see it. Indeed, studies of neutrino emission and helioseismology probe only the gross features of the interior, and they have already revealed mysteries of the most fundamental kind. The effort to understand the physics of the Sun is motivated by recog- nition of the central astrophysical role of stellar mass, energy, and nucle- osynthesis; by a general interest in physics; and particularly by the simple fact that the Sun is the basis for life on Earth. We are all subject to the vagaries of the Sun's highly variable emission of UV, X rays, radio waves, gamma rays, and fast particles; its short-term variations in luminosity; and in the long run, the evolution of solar luminosity and the temperature balance of our planet. The short period of time in which the Sun has been adequately monitored is insufficient to determine the full scope of the variability. For instance, terrestrial atmospheric i4C production (by solar-modulated cosmic rays), as well as historical records, establish that the Sun operates for decades at a time in a state of suppressed activity (e.g., the Sporer Minimum of the fifteenth century and the Maunder Minimum of the seventeenth century) and at other times in a hyperactive state (e.g., during the twelfth century), in addition to the "normal" moderate level of activity that we are currently experiencing. It is worth noting that, as far as the records go, the centuries of reduced solar activity coincide with the centuries of cold climate in the northern temperate zone, whereas the centuries of enhanced activity coincide with a particularly mild climate. No scientific connection has been proven or disproven. Observations indicate short-term variations In solar luminosity

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12 --r r ~ of as much as 1 part in 200, apparently in association with the short- term, daily fluctuations of solar activity. Perhaps more important is the observation, supported by the accumulating data from the Active Cavity Radiometer (ACRIM) onboard the Solar Manamum Mission satellite, that the luminosity of the Sun has varied by about 1 part in 1000 in step with the general 11-year activity cycle. Now it may safely be assumed that the variability and activity of the Sun are typical of other stars, whose distance obscures their idiosyncrasies. Only in the more extreme cases are activity and variability obvious in other stars. The study of stellar activity was pioneered by O. C. Wilson, who traced the subtle variations of chromospheric line profiles of many stars over a period of years to reveal activity cycles similar in character to that so conspicuous in the Sun. This fundamental work has since been taken up and extended by a number of observers, so that there is today a substantial and rapidly expanding body of knowledge on precise stellar rotation rates, pulsations, magnetic cycles, and atmospheric variations in many different classes of stars. The work has led to the identification of patches of activity, gigantic flares, and cool patches (starspots) on the surfaces of many other stars. It provides a glimpse of the broad scope of stellar activity under a variety of circumstances. It is particularly puzzling, for instance, that some of the faint M dwarfs produce flares that have 1000 times more energy (but about the same duration) than the Sun's flares have and that some produce starspots 1000 times larger than the largest spots on the Sun, so that the starspot may cover half the visible disk of the star. Once we can understand the cause of a sunspot, perhaps through seismological probing of its subsurface structure, it may be possible to appreciate the implications of these extreme phenomena in other stars. But that can be achieved only after the observational work on the Sun has progressed from exploration and preliminary description to hard science, which will require the facts eventually gained from low-energy neutrino observations, comprehensive solar seismology, and high-angular-resolution radio, infrared, visible, UV, and X-ray observations. It is clear from the dilemmas presented by neutrino and helioseismo- logical probing of the solar interior that some new ideas are needed on stellar interiors. The formation of a star may invoke atomic abundances, rotation rates, and magnetic fields in different ways than those currently imagined. There may be more mixing of the interior than we realize, sug- gesting quite a different evolutionary track and a greatly different age for the Sun and other stars. It must be remembered that the current assess- ment of the age of the Sun is based only on geological evidence and on the assumption that the Earth was formed at the same time as the Sun. This is an entirely plausible assumption but by no means the only theoretical possibility.

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13 We will certainly have to understand better than we do at present the large-scale circulation and convection in the Sun, and the associated magnetic effects. Neither the observational nor the theoretical picture is clear on meridional circulation, giant cells, and the radial and latitudinal variation of the angular velocity. The solar lithium, beryllium, and boron abundances suggest some limitations on the circulation, but we are mindful of the strikingly different abundances of these elements in certain other solar-trpe stars. Only when these questions are firmly and satisfactorily answered can we begin to attack the question of the loss of angular momentum from a star like the Sun, which is, of course, intimately tied up with magnetic fields and mass loss. And only then can we confidently pursue the theory of the various rotation rates of other stars. The accumulating information on the precise surface rotation rates of other stars, showing individual variations within a given class and age, provides an invaluable guide to the development of the theory. Solar and stellar seismology are essential for developing anything approaching a hard theory. It will be exciting to see how much progress can be made with ground-based seismology and then eventually with space-based instruments. Surface granulation on the Sun lies at the edge of the resolution of current ground-based telescopes. But adaptive optics with large difiraction- limited ground-based and orbiting telescopes should permit the study of the granular structure and its peculiar mode of fonnation and dispersal, currently revealed only grossly by a few of the best observations now being made. There may be a close link between the dynamics of the granule and the formation of the intense magnetic fibrils. The internal structure of the individual fibrils must be determined from direct observation before we can be sure of their origin. The Fourier spectrum of their individual motions must be determined from observation if we are to assess their role in creating the active X-ray corona and their role in heating the coronal holes. As noted earlier, neither the X-ray emission nor the mass loss from the Sun can be understood until the precise form of the energy input from the fibril motions has been determined. In this connection it is essential to explore further the intense small-scale bursts of energy and the low-frequengy radio microbursts throughout the transition region and corona, as well as the larger microflares and flares. The coronal transients are a product of stressed magnetic fields on both small and large scales, the proportion of small- and large-scale stresses determining the degree of flaring associated with the transient. These phenomena all occur in stressed magnetic herds in both quiet and active regions, and their character varies with the phase of the magnetic cycle, which we know is itself highly variable over periods of decades and centunes. The physics of the Sun does not end with the corona, of course, because the outer corona continually accelerates outward into space, gaining

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~4 speed with increasing distance to form the solar wind and the heliosphere, extending out a distance on the order of 100 AU into interstellar space. Flaring adds a fast-particle population to the heliosphere and produces transient bursts of hot gas blast waves to the wind. These blast waves, together with the strong shock interactions between the fast and slow streams of wind, make the heliosphere an active structure whose properties vary markedly with radial distance from the Sun. We are only beginning to get an idea of the detailed structure of the inner and middle heliosphere as the Voyager and Pioneer spacecraft journey past the outer planets. The interaction of the wind with the planetary magnetospheres, creating a local environment that is unique to each planet, is another interesting and important subject that is in a state of rapid development. It should be emphasized in this overview of solar physics that the solar-stellar connection is an integral part of the physics of the Sun and the physics of stars in general. Other stars exhibit great complexity in those aspects that can be studied. Thus we may safely assume that most, if not all, stars would prove as active and complex as the Sun if we could observe them as closely. It is astonishing to see that some stars support gigantic flares and starspots. Some exhibit mass loss enormously greater than that of the Sun. Essentially all of them exhibit X-ray coronae, from which we may infer that their coronal gas expands along the more extended lines of force, carrying the field into space to form a stellar wind much like the solar wind. The general existence of X-ray coronae implies the same nanoflares and microflares and the same coronal transients as can now be observed on the Sun, although there is no foreseeable means for observing them individually on the distant stars. Similar complex magnetohydrodynamic and plasma processes must occur. The same puzzles concerning their internal structure, their internal rotation, and their dynamo confront us, except that it is not possible to come so directly to grips with these puzzles as it is with those posed by the Sun. The best that can be foreseen is to understand the Sun and then to infer the characteristics for the other stars. It is essential, nevertheless, to study the oscillations and seismology of the other stars, to monitor their activity cycles over long terms, and to make precise measurements of their rotation rates. Only in this way can we discover their individual quirks as well as determine the "average" behavior of each class of star. The deviation of the individual from the average provides insight into the variable conditions under which stars are formed, which then helps us to understand the idiosyncrasies of the Sun. Other stars of different ages may provide an idea of the Sun in its youth, to be compared with the geological record for clues to the effects on the planetary environment. The spinning down of the Sun at an early age may have involved conditions profoundly different from those that obtain today. Finally, it should be noted that up to this point this discussion has

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15 focused on single stars, whereas many stars are binary. It is well known that the tidal effects of close binary stars have drastic effects on the behavior of the individual component stars. Perhaps one day we shall understand the internal dynamics of the Sun well enough to deduce what subtle effects may be expected from the tidal ejects of distant, or even close, companion stars. In concluding this general appraisal of current problems in the physics of a star like the Sun, it is appropriate to make some general comments on the future. It is too soon to guess where the neutrino observations will lead, but whatever the results obtained from the present gallium detectors, the implications for astronomy will be profound. Helioseismology may be expected to play an essential role in removing the ambiguities of anomalous neutrino fluxes, unless, of course, the discrepancy is entirely a matter of neutrino oscillations between three or more states, which would have deep cosmological implications. What is more, we can be sure that the investigation of the solar surface and the solar interior on so broad a front will pronde surprises, perhaps of a fundamental nature. The present writing, and the list of opportunities and initiatives that follows, is based on contemporary knowledge and cannot anticipate what lies ahead when we probe into the unknown realm of the solar interior and the small-scale phenomena at the solar surface." 1 Ihe reader is referred to the recent comprehensive reviews of contemporary knowledge of the Sun to be found in the three-volume work The Physics of the Sun (~1986), D. Reidel Publishing Co., Dordrecht, The Netherlands, edited lay P. A. Sturrock, T. E. Holzer, D. M. Mihalas, and R. K. Ulrich; and Volume 100, Solar Physics (1986), D. Reidel Publishing Co., Dordrecht, The Netherlands, edited lay C. de Jager and Z Svestka. Indeed the tables of contents of these works provide a useful list of major topics in solar and stellar physics in greater detail than has been possible in the present writing. Further relevant information can be found in the reports listed in Appendix C.