Executive Summary

Solar physics is a critical part of the nation's natural science program and a research area of fundamental importance to physics and astrophysics. The Sun is the only star that can be resolved in detail in its interior, on its surface, and in its outer atmosphere, thus making it an important and unique laboratory for fundamental physics, astrophysics, fluid mechanics, and magnetohydrodynamics. Further, the radiative and particle outputs of the Sun, and their variation, have a controlling influence on Earth's atmosphere, climate, and near-space environment. The field of stellar astrophysics would not include knowledge about starspots, prominences, differential rotation, flux tubes, flares, coronal mass ejections, tenuous supersonic winds, the nature of hot, dense X-ray coronal loops, or the variation of the total brightness of a star over years and decades without the discoveries of these phenomena on the Sun.

Scientists understand the physics of these diverse phenomena only insofar as they have worked out the physics from studies of the Sun. In the broadest terms, it remains only partially understood why the Sun, or any other star, is obliged by the laws of physics to carry on the many curious phenomena that are collectively known as magnetic activity. In particular, the existence of a million-degree corona surrounding a 6,000-degree surface is not understood except in outline. This is a fundamental problem in the physics of stellar systems, and a solution is required if there is to be any confidence in the interpretation of X-ray and extreme-ultraviolet emission of astrophysical objects. The Sun is the only laboratory where these questions can be studied in some detail.

This report reviews the scientific challenges posed by the active and variable Sun, and it is those challenges that drive the recommendations of the report. The general behavior of the major phenomenological components of solar activity are effectively pursued with the ongoing space program and existing and proposed ground-based telescopes. However, scientific understanding of the basic physics of these phenomena is stymied by an inability to resolve many aspects of the fundamental magnetic energy release processes that are occurring at scales of approximately 75 km or less. Clearer understanding requires observations with better than 0.1 arc-second (″) resolution, but existing telescopes provide at best approximately 0. 3″ to 1.0″ resolution.

THE ROLE OF GROUND-BASED PROGRAMS IN SOLAR RESEARCH

The activity of the Sun varies over years, decades, and centuries, evidently reflecting diverse internal magnetic and convective states. A number of the fundamental scientific challenges noted above require spatial and temporal resolution and long-term synoptic coverage that can only be realistically achieved through a program of ground-based observations. Ground-based solar research programs provide easy accessibility to facilities for the entire solar-physics community and are a means for the hands-on education of the next generation of solar researchers. They can be responsive to rapid intellectual, technical, or solar activity developments, as instrumentation on the ground is comparatively easy to repair, modify, calibrate, and replace. In addition, the costs of a ground-based facility are also typically far lower than those of space-based equivalents.

Observations from space 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



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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE Executive Summary Solar physics is a critical part of the nation's natural science program and a research area of fundamental importance to physics and astrophysics. The Sun is the only star that can be resolved in detail in its interior, on its surface, and in its outer atmosphere, thus making it an important and unique laboratory for fundamental physics, astrophysics, fluid mechanics, and magnetohydrodynamics. Further, the radiative and particle outputs of the Sun, and their variation, have a controlling influence on Earth's atmosphere, climate, and near-space environment. The field of stellar astrophysics would not include knowledge about starspots, prominences, differential rotation, flux tubes, flares, coronal mass ejections, tenuous supersonic winds, the nature of hot, dense X-ray coronal loops, or the variation of the total brightness of a star over years and decades without the discoveries of these phenomena on the Sun. Scientists understand the physics of these diverse phenomena only insofar as they have worked out the physics from studies of the Sun. In the broadest terms, it remains only partially understood why the Sun, or any other star, is obliged by the laws of physics to carry on the many curious phenomena that are collectively known as magnetic activity. In particular, the existence of a million-degree corona surrounding a 6,000-degree surface is not understood except in outline. This is a fundamental problem in the physics of stellar systems, and a solution is required if there is to be any confidence in the interpretation of X-ray and extreme-ultraviolet emission of astrophysical objects. The Sun is the only laboratory where these questions can be studied in some detail. This report reviews the scientific challenges posed by the active and variable Sun, and it is those challenges that drive the recommendations of the report. The general behavior of the major phenomenological components of solar activity are effectively pursued with the ongoing space program and existing and proposed ground-based telescopes. However, scientific understanding of the basic physics of these phenomena is stymied by an inability to resolve many aspects of the fundamental magnetic energy release processes that are occurring at scales of approximately 75 km or less. Clearer understanding requires observations with better than 0.1 arc-second (″) resolution, but existing telescopes provide at best approximately 0. 3″ to 1.0″ resolution. THE ROLE OF GROUND-BASED PROGRAMS IN SOLAR RESEARCH The activity of the Sun varies over years, decades, and centuries, evidently reflecting diverse internal magnetic and convective states. A number of the fundamental scientific challenges noted above require spatial and temporal resolution and long-term synoptic coverage that can only be realistically achieved through a program of ground-based observations. Ground-based solar research programs provide easy accessibility to facilities for the entire solar-physics community and are a means for the hands-on education of the next generation of solar researchers. They can be responsive to rapid intellectual, technical, or solar activity developments, as instrumentation on the ground is comparatively easy to repair, modify, calibrate, and replace. In addition, the costs of a ground-based facility are also typically far lower than those of space-based equivalents. Observations from space 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

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE solar magnetic fields, and the surface pressure waves (p-modes). Further, ground-based observations provide the critical data required by the designers of space missions. Finally, ongoing nighttime observations of brightness and magnetic activity of distant solar-type stars are demonstrating the varying states of activity the Sun may have achieved in other centuries. These observations, combined with data on Earth's atmosphere, indicate that variations in the Sun's radiative and plasma emissions are capable of influencing the weather and climate at Earth's surface. 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: The National Solar Observatory, the High Altitude Observatory (HAO), and 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. THE CURRENT U.S. GROUND-BASED SOLAR RESEARCH PROGRAM For convenience, the task group divided its discussion of the current U.S. ground-based solar research program into (1) major solar observational facilities; (2) data, theory, and modeling; and (3) people, programs, and institutions—the means by which elements 1 and 2 are integrated to advance scientific understanding. Major Solar Observational Facilities The National Solar Observatory, a multisite facility of the National Optical Astronomy Observatories (NOAO), is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. The current responsibilities of the NSO include the following: Continued operation of the Kitt Peak (NSO/KP), Sacramento Peak (NSO/SP, “Sac Peak”), and Tucson facilities; Operation and upgrade of the multisited telescopes of the Global Oscillations Network Group (GONG) for continuous studies in helioseismology; Fabrication and operation of the SOLIS array for synoptic optical long-term investigation of the Sun; and Archiving and distribution of data, and providing specialist-supported access to NSO observing facilities. The NSO operates the two largest U.S. telescopes for ground-based solar observation—the McMath-Pierce telescope at Kitt Peak (commissioned in 1961) and the Vacuum Tower Telescope (VTT) at Sac Peak (commissioned in 1969). NSO facilities are available to both local staff and visiting scientists worldwide. To maximize scientific productivity, NSO policy

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE provides for visiting observers to be assisted by experienced NSO staff. This support is unique among all other solar observatories worldwide and exemplifies the collaborative role of the NSO in the solar physics community. Although the task group's assessment of U.S. observatories focused on NSO facilities, important solar research facilities exist elsewhere. For example, vector magnetograms are recorded by the University of Hawaii's Mees Solar Observatory, California State University Northridge's San Fernando Solar Observatory, NASA's Marshall Space Flight Center, and the New Jersey Institute of Technology's Big Bear Solar Observatory.1 Similarly, the Stanford University Wilcox Solar Observatory specializes in low-resolution magnetograms designed to show the current sheet separating the northern and southern magnetic hemispheres of the Sun and the large-scale surface velocity patterns. Facilities outside the NSO also provide data essential to supporting ongoing NSO programs. For example, data from the Mt. Wilson 60- and 150-foot solar tower telescopes complement data from GONG and other helioseismic experiments. Finally, U.S. solar radio observatories also are outside the NSO set of solar observation facilities. Solar observations in the radio part of the electromagnetic spectrum provide a unique perspective on phenomena in the solar atmosphere. Many excellent solar observing facilities also exist outside the United States, although none operates a wide range of well-documented instruments and also provides resident observers who aid in their operation. Nevertheless, the non-U.S. programs illustrate the worldwide interest in the active Sun and suggest the possibility of a vigorous international collaborative effort should the United States choose to go forward with plans for the demonstration of adaptive optics and development of the Advanced Solar Telescope discussed in this report. The past decade of solar research has shown that spatial resolution of fractions of an arc-second and temporal resolution of a few seconds are required to characterize the interaction of solar magnetic fields and convective flows on the scales at which they actually occur. Further, the need for high sensitivity to magnetic fields necessitates an infrared observational capability out to wavelengths of approximately 15 microns. Operating at such long wavelengths with even minimally acceptable resolution requires a larger-aperture telescope than any that currently exists. No solar optical telescope operating today has the attributes needed to enable studies of the energy release processes that occur at very small scale sizes. In addition, addressing several of the fundamental solar science questions mentioned in this report would require upgrades to the capabilities of existing solar radio observatories. Data, Theory, and Modeling Studies of the Sun and its influence on the interplanetary and Earth environment often involve making correlations between various observed physical parameters. Achieving a readily usable and accessible data archive requires developing an easily searchable catalog of data, ensuring access to data through user-friendly software, and guaranteeing the ability to handle the large quantities of data now available and that are planned in the future. Such data archiving is essential to maintaining and providing ready access to already existing, ongoing, and future data sets. Several centers and institutions in the United States have, or will soon have, online images and other data available through the Internet. However, sifting through the vast 1   On July 1, 1997, the management of Big Bear Solar Observatory was transferred from the California Institute of Technology to the New Jersey Institute of Technology.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE quantities of data for observations of specific solar phenomena is often a formidable task without an intimate knowledge of a particular institution's archive structure or a catalog that goes beyond a simple list of the available data. As a result, several efforts are being advanced to provide a mechanism to identify and retrieve data from a large number of sources. Acknowledging the importance of providing data to the community, the task group encourages the cooperation and participation of observatories and institutions in efforts to archive and provide access to their data. Historically, many of the innovations that have led to new observational facilities have had their origins in small-scale university and institutional research. Although much of the current solar data analysis and theoretical work continues to be done in universities and national institutes with NSF funding, today an increasing amount of solar physics research is conducted at institutes and universities that focus more on space-based projects, with only indirect attention to ground-based studies supported by research and technology contracts from NASA. Thus, for example, the Big Bear Solar Observatory, the Lockheed Martin Solar and Astrophysics Laboratory, NASA's Marshall Space Flight Center, and others reflect the changing pattern away from research traditionally supported by NSF to research oriented toward space-based observations of the Sun for which there is support. New academic research groups at Michigan State University, California State University at Northridge, Montana State University, and the New Jersey Institute of Technology also reflect this trend. In addition to an adequate complement of supporting instruments, the success of the priority facility projects discussed in this report (SOLIS, the upgrade of GONG, and the Advanced Solar Telescope) rests on the availability of adequate funds for data analysis, modeling, and theoretical investigations. Plans for this intellectual infrastucture need to be incorporated at the start of new projects, as has been proposed for the Solar Magnetism Initiative, a multi-institutional proposal to the NSF for an integrated study of solar magnetism and variability. People, Programs, and Institutions The critical elements that enable the capacity of state-of-the-art observing systems and the potential of richly populated data collections to be translated into scientific understanding are people, programs, and institutions. That is, progress in science depends on being able to draw on a critical-mass-size research community (people) who, in turn, are supported by adequate intellectual and physical infrastructure (institutions). Specific programs can serve to integrate the contributions of various kinds of research (e.g., observations, theory, and modeling) and promote the synthesis of new perspectives on critical scientific problems. The task group notes the importance of a balanced NSF approach to facility development and scientific grant support for the optimum long-term handling of solar research. This requires NSF research grants for individual solar scientists in universities, institutes, and observatories, as well as active communication and coordination between the major solar research centers (NSO and HAO), agencies (NSF, NASA, DOD, and NOAA), and the national infrastructure of universities, observatories, and institutes. The task group believes that the magnitude of the grants program for individual researchers should be commensurate with the funding of the centers. At present, there is a strong space-based solar research program that is able to analyze and interpret observational data effectively. Solar space research is conducted in university

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE space science groups, NASA field centers, Department of Defense research laboratories, and some corporate research facilities. However, historically strong, university-based research groups that carry on ground-based solar research (for example, those at the California Institute of Technology, Stanford University, the University of Maryland, and the University of Colorado) are losing or have lost tenured solar faculty. Institutions such as the New Jersey Institute of Technology and Montana State University have stepped in with new faculty hires, but the task group remains concerned about the trend away from traditional ground-based solar research and the likely effect on training for the next generation of graduate students. The task group is also concerned about the current state of university-based instrumentation programs, which are widely seen as essential to future instrument development, as well as the reduced access of new researchers to hands-on observing experience. Existing instrumentation and training programs are few in number and rely on precarious grant-based funding. A STRATEGY TO STRENGTHEN GROUND-BASED SOLAR RESEARCH The task group believes that although there is great strength in the current ground-based solar research program, it is nevertheless fragile. Specifically, there is an urgent need to develop a coherent strategy for ground-based research that will address the following issues: Aging national facilities; Limited capabilities to pursue the most important scientific problems; Concerns about the health of the research community, especially in academia; and The need for a workable plan to effectively integrate the diverse pieces of ground-based solar research into a synergistic whole. The task group concluded that such a strategy can be built around the three elements mentioned above: (1) major observing facilities; (2) data, theory, and modeling; and (3) people, programs, and institutions. Within this strategy, the task group believes that the highest priority should be accorded to major observing facilities. This is so for several reasons. First, new observing facilities are required to address the major scientific questions in solar research. Second, new facilities are needed to replace certain of the aging facilities now in operation. Third, but especially importantly, major new facilities will constitute the most effective way to attract and engage the next generation of outstanding researchers who will bring vigor and momentum to ground-based solar research in the United States. Recommendations Regarding Facilities The following four recommendations are presented in priority order. Recommendation 1: Complete fabrication of the SOLIS facility over the next 3 years, operate it at an appropriate site, and provide funding for U.S. scientists for data analyses. Recommendation 2: Upgrade the GONG system by installing appropriate 1024 × 1024 CCD sensors, and operate GONG over a whole solar cycle with funding for data analysis in the United States.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE Recommendation 3: Develop, construct, and operate a 3- to 4-meter Advanced Solar Telescope (AST). The AST might also be called the “Solar Microscope” because it would, for the first time, peer into the mysterious world of the active magnetic microstructure. Work toward the AST by: Strengthening the NSO adaptive optics program immediately, including an augmentation in funding of approximately $1.5 million for the next fiscal year; Demonstrating the required adaptive optics (0.1″ or better resolution in visible light at 0.5 microns) on a telescope with an aperture of approximately 1.0 to 1.5 meters; Beginning preliminary design of the 3- to 4-m AST so as to be ready for the final design when the adaptive optics has been convincingly demonstrated; and Carrying on site testing to determine an accessible site with the best available seeing as quickly as possible so as to define the task for the adaptive optics and be ready for construction of the AST. Recommendation 4: Begin exploratory development of a high-resolution, frequency-agile solar radio telescope (FASR), using existing radio observatories to demonstrate its scientific potential. The FASR would provide unique diagnostics of solar flare plasmas, detect and locate the myriads of microflares, and provide maps of magnetic fields over surfaces of constant density within active regions. Recommendations Regarding Data, People, Programs, and Institutions The four recommendations above relate to priority actions for major observing facilities. The next set of recommendations focuses on addressing the issues that emerge in the two other major elements of the U.S. ground-based solar research program, which are (1) data, theory, and modeling and (2) people, programs, and institutions. Unlike the recommendations on facilities, they are not presented in priority order. Recommendation 5: Facilitate efficient and timely development and utilization of the AST through enhancements to the management and organization of the NSO. Recommendation 5a: Consolidate the NSO science, engineering, and operations site when the AST is operational. Recommendation 5b: Establish an independent management council of the NOAO management organization to represent solar research, thereby recognizing the unique requirements for a program in ground-based studies of the Sun and placing such a program on an equal footing with the other major initiatives of NOAO. Recommendation 5c: Establish an advisory committee to the NSO director that would include leading solar physicists from NSO, HAO, universities, NASA, DOD, NOAA, and U.S. and international research partners. Recommendation 5d: Foster communication within the solar physics community by considering creation of an NSO national fellowship program, perhaps structured along

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE the lines of the visiting scientist program that has been in place for many years at HAO. Recommendation 6: Establish the essential national infrastructure for the effective operation and scientific exploitation of the U.S. solar observing facilities. Recommendation 7: Develop a collaborative NSF and NASA distributed data archive with access through the World Wide Web.