Chapter 1

Ground-based Solar Research: A Scientific Synopsis

THE SUN AMONG STARS

The pedestrian star we call the Sun is remarkably active. Close scrutiny forces the realization that stars are not the ideal, perfect, celestial orbs expected from standard, spherically symmetric, hydrostatic models. The Sun's atmosphere is highly variable as it responds to a changing magnetic field that is structured on a range of scales. Magnetic fields at the surface of the Sun occur in a curious interactive fibril state that leads to the explosive dissipation of magnetic energy on all scales and intensities—unfortunately, often below the threshold for detection and resolution by the best existing telescopes.

The fibril diameters are estimated to be about 100 km, and the field intensities are 1 to 2 kilogauss within the fibrils (see Figure 1.1). It appears that the dissipation of magnetic energy is the cause of the Sun's supersonic wind, its X-ray emission, coronal mass ejections, and the production of flares and solar energetic particles. Large flares can be accompanied by highly energetic particles that pose hazards ranging from excessive levels of radiation harmful to astronauts to disruption and/or destruction of satellites in space. The associated coronal mass ejections produce interplanetary blast waves that impact Earth's magnetic field, occasionally so violently that they disrupt electrical power grids on Earth. All of these processes wax and wane over varying time scales, including the 11-year solar activity cycle and longer periods of many decades to centuries. It appears that the microstructure, i.e., the fibril structure, of magnetic fields plays an essential role in the physics of these large-scale phenomena, so that direct observational study of the microstructure is essential for discovering how the Sun's activity comes about.

Advances in optical technology now hold the promise of achieving ground-based spatial resolutions of 50 to 100 km (75 km is equivalent to 0.1″), opening up the microscopic world of the Sun's basic active magnetic elements, their associated fluid motions, and their local three-dimensional atmospheric structure. Such observations are necessary to discover the basic nature of the fibril activity and to elucidate the physics of the many diverse phenomena of solar activity (see Appendix D). The overall endeavor encompasses domains far below the Sun's visible surface (using helioseismology and theoretical modeling), and far above the surface (by means of space-based observations).

Moreover, the recent triumphs of solar physics provide a solid foundation for ongoing scientific investigation and interpretation of the Sun 's activity. Helioseismology has established the validity of the classic theory of stellar interiors deduced from the opacities and the elemental abundances inferred from observations of the Sun's surface. In particular, with the inclusion of such subtleties as gravitational settling of the heavier ions, the theoretical models of the solar interior have been fine tuned so that predictions of the sound speed based on the standard model now match to within 1 part in 1000 the sound speed deduced from helioseismology. Quite literally, the observations have checked out the solar interior, leading also to discovery of the wholly unanticipated internal rotation profile (see cover illustration). This sets the stage for the time-distance analysis of helioseismology, or helioseismological tomography, to probe the subsurface



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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE Chapter 1 Ground-based Solar Research: A Scientific Synopsis THE SUN AMONG STARS The pedestrian star we call the Sun is remarkably active. Close scrutiny forces the realization that stars are not the ideal, perfect, celestial orbs expected from standard, spherically symmetric, hydrostatic models. The Sun's atmosphere is highly variable as it responds to a changing magnetic field that is structured on a range of scales. Magnetic fields at the surface of the Sun occur in a curious interactive fibril state that leads to the explosive dissipation of magnetic energy on all scales and intensities—unfortunately, often below the threshold for detection and resolution by the best existing telescopes. The fibril diameters are estimated to be about 100 km, and the field intensities are 1 to 2 kilogauss within the fibrils (see Figure 1.1). It appears that the dissipation of magnetic energy is the cause of the Sun's supersonic wind, its X-ray emission, coronal mass ejections, and the production of flares and solar energetic particles. Large flares can be accompanied by highly energetic particles that pose hazards ranging from excessive levels of radiation harmful to astronauts to disruption and/or destruction of satellites in space. The associated coronal mass ejections produce interplanetary blast waves that impact Earth's magnetic field, occasionally so violently that they disrupt electrical power grids on Earth. All of these processes wax and wane over varying time scales, including the 11-year solar activity cycle and longer periods of many decades to centuries. It appears that the microstructure, i.e., the fibril structure, of magnetic fields plays an essential role in the physics of these large-scale phenomena, so that direct observational study of the microstructure is essential for discovering how the Sun's activity comes about. Advances in optical technology now hold the promise of achieving ground-based spatial resolutions of 50 to 100 km (75 km is equivalent to 0.1″), opening up the microscopic world of the Sun's basic active magnetic elements, their associated fluid motions, and their local three-dimensional atmospheric structure. Such observations are necessary to discover the basic nature of the fibril activity and to elucidate the physics of the many diverse phenomena of solar activity (see Appendix D). The overall endeavor encompasses domains far below the Sun's visible surface (using helioseismology and theoretical modeling), and far above the surface (by means of space-based observations). Moreover, the recent triumphs of solar physics provide a solid foundation for ongoing scientific investigation and interpretation of the Sun 's activity. Helioseismology has established the validity of the classic theory of stellar interiors deduced from the opacities and the elemental abundances inferred from observations of the Sun's surface. In particular, with the inclusion of such subtleties as gravitational settling of the heavier ions, the theoretical models of the solar interior have been fine tuned so that predictions of the sound speed based on the standard model now match to within 1 part in 1000 the sound speed deduced from helioseismology. Quite literally, the observations have checked out the solar interior, leading also to discovery of the wholly unanticipated internal rotation profile (see cover illustration). This sets the stage for the time-distance analysis of helioseismology, or helioseismological tomography, to probe the subsurface

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE FIGURE 1.1 This recent picture of the surface of the Sun in white light was obtained using a CCD camera on the Swedish Vacuum Solar Tower telescope on La Palma, taking advantage of the excellent seeing conditions on the Canary Islands. The fuzzy bright spots of about 200 km diameter (~ 0.25”) represent the positions of the individual unresolved magnetic fibrils in the dark downdrafts between granule convective cells with diameters of about 103 km. SOURCE: Courtesy of Alan Title of the Lockheed Martin Advanced Technology Center.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE fluid motions, already showing both a downdraft and a magnetic field intensity increasing downward beneath a sunspot. Other achievements of solar research include: Elucidation in detail of the remarkable structure of the Sun's X-ray-emitting loops. Scientists can now picture the X-ray coronal structure of other stars; they also have a clearly defined basis for working out the nature of the heat input to the X-ray emission. Direct observation of the expanding corona and solar wind. The corona and solar wind have now been viewed directly and studied in detail with instruments on SOHO. In addition, instruments on the Ulysses spacecraft and on a variety of other spacecraft have enabled studies of the corona and solar wind over the poles of the Sun and far out into the heliosphere. The discovery that the Sun's brightness varies with its level of activity, and verification of this effect in other solar-type stars. Evidence of a close relationship between terrestrial climate and extreme changes in solar activity over the centuries has opened up a whole new field of inquiry, with profound implications for better understanding Earth's environment. 1 The discovery that the magnetic fields of a star like the Sun are in an intensely fibril state at the visible surface. This is a startling revelation, evidently responsible for otherwise inexplicable aspects of solar activity, but still inexplicable in itself. It is the basic dynamical role of the interacting fibrils that drives the need for high spatial resolution in observations of the Sun. The inference from the dynamics of rising azimuthal flux bundles that there are magnetic fields of 0. 5 × 105 to 1 × 105 gauss at the base of the convective zone, presumable in some fibril form. The discovery of the low-level emission of electron neutrinos from the thermonuclear core of the Sun.2 This observation has led to new opportunities to study the physics of leptons. Neutrino observations have been taken up in earnest by the particle-physics community, and the recent confirmation of a neutrino rest mass (based on the diurnal variation of the neutrino intensity in cosmic-ray showers), gives strong support to the hypothesis that the reduced neutrino emission from the Sun is a consequence of neutrino oscillations. Studies of solar neutrinos should go a long way toward elucidating the problem. In summary, studies of the Sun play a fundamental role in basic physics and in the physics of stars. It now appears that modern technology can provide the necessary sensitivity and spatial and temporal resolution for the next epoch in solar research—one that will elucidate in detail the physics of solar activity and variability. 1   See Friis-Christensen, E., and Lassen, K. 1991. Length of the solar cycle: an indicator of solar activity closely associated with climate. Science 254: 698-700. 2   Bahcall, J.N., Calaprice, F., McDonald, A.B., and Totsuka, Y. 1996. Solar neutrino experiments: The next generation. Physics Today 49: 30-36.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE SCIENTIFIC CHALLENGES FOR SOLAR RESEARCH Among the challenging questions that solar physicists seek to address are the following: What are the details of the nuclear processes occurring in the deep interior of the Sun? What are the details and effects of the (generally slow) large-scale circulation patterns within the Sun? How does the Sun generate the ever-changing magnetic field? How does that field interact with radiation and convection below, at, and above the surface? why is the field in a fibril form? what (generally weak) interfibril magnetic fields are present, and how do the fibrils interact with each other? How is nonradiative energy channeled into the outer atmosphere where it is heated to millions of degrees, thereby producing the solar X-ray emission from the magnetic active regions? How is the solar wind heated, structured, and driven? What variations in brightness has the Sun experienced in past centuries and millennia? How are these excursions related to the Sun's magnetic activity? what are the most effective synoptic observations for answering these questions? Which aspects of variations in solar irradiance most affect life on Earth? How do the effects of varying UV emission on upper atmospheric chemistry compare with the effects of general heating and cooling of Earth 's surface owing to changes in total luminosity? How does warming of the oceans' surface water affect the carbon dioxide balance between the ocean and the atmosphere? Answering these questions will require close coordination of studies in solar physics and the atmospheric sciences. In addition, continued precision monitoring of the solar irradiance with spaceborne instruments is required. Ongoing study of solar phenomena suggests that although they occur on vastly different scales, they are, in fact, intimately linked. For example, the collective small-scale behavior of fibrils causes the large-scale effects associated with the cyclic magnetic field. The intense fibril state of the field at the surface is itself a puzzle and is evidently responsible for the localized heating in long coronal connections, the observed small modifications of globally resonant acoustic p-modes,3 and the highly structured heating of the outer atmosphere. As a result of such interrelationships, solar physics studies the system of the Sun through a variety of methods. It necessarily focuses on the smallest observable details, the level at which magnetic dissipation occurs, but it also involves study of centuries-long global changes. These are all aspects of the grand challenge: understanding the magnificent complexity of the star that sustains life on Earth. As observational capabilities have improved and many traditional ideas about solar activity have been overthrown, the mystery and scientific challenges posed by the Sun have only intensified. Based on current knowledge of the Sun, scientists presume 3   Researchers have observed that the amplitude of the p-modes is reduced in regions of the magnetic field.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE that distant stars are equally complex, an interpretation supported by observations of stellar surfaces that are beginning to be resolved in ever greater detail with the use of larger telescopes and the development of new spatial imaging methods, including interferometry and Doppler and modulation imaging.4 Long, quasi-continuous monitoring studies of stars are revealing a variety of patterns in the temporal evolution of stellar activity. Thus, success in the study of the Sun aids comprehension of other stars, whose study in turn refines the questions researchers ask about the Sun and solar activity. What is known about the existence and nature of starspots, stellar dynamos, X-ray coronal loops, coronal holes, fast tenuous stellar winds, magnetic flares, magnetic flux bundles, coronal mass ejections, internal differential rotation, and brightness changes, for example, has been inferred from subtle details in the integrated light and from detailed observations of the Sun and their theoretical interpretation. With increasing knowledge comes the desire for proper study and understanding of the solar atmosphere. Today, comprehension of the diverse stellar phenomena reaches only as far as researchers have been able to work out the physics of solar prototypes. At the same time, models of solar phenomena are put to the test when extrapolations are made to conditions that cannot be found on the Sun. Diverse scientific challenges are posed by the sun (these are outlined in somewhat more detail in Appendix D). To summarize: the Sun ejects matter in the form of the solar wind and coronal mass ejections. Cool magnetized patches (sunspots) form and disperse on the visible surface. Massive prominences hover overhead, and the whole is wrapped in the million-degree coronal cocoon. The UV brightness may vary by tens of percent and the X-ray brightness by more than a factor of ten, depending on the passband or emission lines in which observations are made. The total brightness varies by 1 or 2 parts in 1000 with the decadal variation of magnetic activity, 5 and evidently has varied by substantially more in past centuries of extreme activity and extreme inactivity. The nonuniform rotation of the Sun varies slightly, and the meridional circulation is still being defined by observational studies. Scientists do not yet fully understand why the Sun is compelled by the laws of nature to do these things. THE ROLE OF GROUND-BASED PROGRAMS IN A BALANCED PROGRAM OF SOLAR RESEARCH Solar research has many dimensions, including theory, data analysis, observations made from spacecraft, and ground-based observations. Theoretical investigations and data analysis are carried out mainly in universities, but also in NASA centers, the High Altitude Observatory (HAO), and the National Solar Observatory (NSO). Space-based studies are conducted mainly in universities, NASA centers, and some corporate laboratories. Ground- and space-based observations are complementary. Above the atmosphere, the Sun can be observed at ultraviolet, X-ray, and gamma-ray wavelengths, and direct measurements can be made of the flux of energetic particles that make up the 4   Strassmeier, K.G., and Linsky, J.L., eds. 1996. Stellar Surface Structure, IAU Coll. 176, Kluwer: Dordrecht. 5   Hoyt, D.V., Kyle, H.L., Hickey, J.R., and Maschoff, R.H. 1992. The Nimbus 7 solar total irradiance: A new algorithm for derivation. J. Geophys. Res. 97: 51-64.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE solar wind and solar flares. However, space-based instruments must be defined years to decades in advance, and repair and modification normally are not possible.6 In contrast, ground-based instruments can use state-of-the-art technology and can be configured and optimized for problems of current interest. Ground-based optical, infrared, and radio observatories study the photosphere, chromosphere, and corona of the Sun with a spatial resolution and time span that cannot be duplicated from space platforms. Moreover, the use of adaptive optics combined with novel data capture and data-processing techniques can enable future ground-based telescopes to have much higher spatial resolution than is currently feasible in space-based instruments. In addition, the costs of ground-based facilities are far lower than those of space-based equivalents. Space-based observations have opened up a new world unknown to and inaccessible from the ground, but ground-based observations are credited with the discovery of the Sun's cyclic magnetic activity, the million-degree temperature of the corona, the fine scale, fibril state of the solar magnetic fields, and the surface p-modes.7 Ground-based nighttime observations of distant solar-type stars8 are also a key element in research aimed at assessing whether ornot variations in the Sun's radiative and plasma emissions are capable of influencing the weather and climate at Earth's surface. The ground-based observations suggest the variations in activity and brightness that the Sun may undergo in the next millennium, while 14C and Be cosmogenic isotope records show the extreme variations in solar activity in earlier millennia.9, 10 Long-term, ground-based, precision observations of solar conditions of the kind now being made by the Global Oscillations Network Group (GONG) and anticipated in the future for Synoptic Optical Long-term Investigations of the Sun (SOLIS) are an essential complement to the observations of space missions such as Yohkoh and TRACE. As these missions provide more information about the Sun's X-ray emission and corona, there is a corresponding need to learn more about the vigorous small-scale dynamics of the photospheric and chromospheric magnetic fields. Ground-based observations also complement those now being made by SOHO and other ISTP11 spacecraft and will 6   The vulnerability of space systems was illustrated by the loss of communication with the NASA/European Space Agency (ESA) Solar and Heliospheric Observatory (SOHO) spacecraft on June 24, 1998, during maintenance operations. Contact with SOHO was later regained, the spacecraft was returned to Sun pointing, and initial testing of SOHO instruments was successful. 7   Convection acts rather like a random clapper, causing the Sun to ring like a bell. The resulting oscillations are pressure waves or p-modes. In contrast to the pulsations found in most traditional variable stars, they are essentially nonradial oscillations. Information about GONG helioseismology can be found on the World Wide Web at <http://helios.tuc.noao.edu/helioseismology.html>. 8   Baliunas, S.L., et al. 1995. Chromospheric variations in main-sequence stars. II. Astrophys. J. 438: 269-287. 9   Beer, J., et al. 1990. Use of 10 Be in polar ice to trace the 11-year cycle of solar activity. Nature 347:164-166. 10   The historical record of climate in Earth's northern temperate zone gives a rough idea of the changing intensity of sunlight through the ups and downs of solar activity; for example, the warm 12th century is associated with the Medieval Maximum in solar activity, and the Little Ice Age is associated with the Sp örer Minimum of the 15th century and the Maunder Minimum of the 17th century. 11   The International Solar-Terrestrial Physics (ISTP) program is a collaborative effort by NASA, the European Space Agency (ESA), and the Institute of Space and Astronautical Science (ISAS) of Japan. The ISTP Science Initiative consists of a set of solar-terrestrial missions to be carried out during the 1990s and into the next century. Information on ISTP is available on the World Wide Web at <http://www-istp.gsfc.nasa.gov/istp/>.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE provide the context for interpreting the intensive observations of Solar B, HESSI, and other future space missions. Advances in theory and in data analysis likewise lead to new questions that can be resolved only by more detailed ground- and space-based observations. The synergy between theory and space- and ground-based observations is evident in what are generally considered to be the three major challenges of present-day solar physics: coronal heating, the solar activity cycle, and the neutrino deficit. Ground-based observatories provide easy accessibility to state-of-the-art tools for the entire solar-physics community. They also allow modification of specialized detectors and data-capturing methods; offer long-term continuity; can be repaired, calibrated, modified, or exchanged, thus allowing for flexibility and responsiveness to intellectual, technical, and solar developments; and are important in the hands-on education of the next generation of researchers. To enable fuller understanding the Sun on all spatial and temporal scales, as well as to complement and enhance space-based solar research, the task group believes that the primary tasks of ground-based telescopic research should be the following: Obtaining a long-term synoptic record of solar activity: This effort should be supplemented by continued monitoring of the activity and luminosity of other solar-type stars to provide a statistical sample of the states through which the Sun may pass in the next 1000 years. The task group notes that the National Solar Observatory, the High Altitude Observatory, the independent observatories—including, for example, Mt. Wilson, Stanford-Wilcox, Big Bear, San Fernando, and Marshall Space Flight Center— have an important role in this effort. Studying the solar interior and the generation of magnetic fields by mapping subsurface flows and interior magnetic fields through long-term helioseismological observations; and Observing the interaction of convection, magnetic fields, and radiative transfer by imaging with high spatial, temporal, and spectral resolution.