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A Science Strategy for Space Physics: Chapter 1 A Science Strategy for Space Physics 1 Mechanisms of Solar Variability SCIENTIFIC BACKGROUND The Sun is observed to be a variable star on nearly every time scale from milliseconds to centuries and, according to theory, over much longer times as well. The quest to understand and forecast solar variability is both basic science and strategic science. It is basic science because the Sun is the touchstone for theories of stellar structure and evolution, magnetic activity, and astrophysical dynamos. It is strategic science because solar variability demonstrably affects our everyday lives. As the nearest star, the Sun provides a wide range of opportunities to develop and test theories in such diverse areas as neutrino physics and particle acceleration. During the declining phase of the last solar activity cycle (1980 to 1986), REPORT MENU the ACRIM experiment on the Solar Maximum Mission (SMM) indicated that the NOTICE Sun's total radiative output diminished by 0.1%, while the ERB/Nimbus-7 data MEMBERSHIP indicated that the output diminished by nearly 0.2%. The percentage reduction at SUMMARY PART I ultraviolet wavelengths was 10 to 1000 times larger. The NRC study Solar PART II Influences on Global Change (the Lean report)1 concluded that such changes in CHAPTER 1 the Sun's radiative output directly modify the global surface temperature, the CHAPTER 2 ozone layer, and the structure of the middle atmosphere. Those solar-induced CHAPTER 3 effects must be included when researchers weigh observational evidence of CHAPTER 4 anthropogenic influences such as greenhouse gases. CHAPTER 5 PART III Researchers have not directly observed solar irradiance variations on APPENDIX longer time scales. However, space physicists do know that the Sun has shown greater swings of activity in the past than it has recently. There were extraordinarily few sunspots during the period from 1645 to 1715 (the so-called Maunder Minimum). If the Sun's radiative output followed sunspot number then as it does now, the radiative output was about 0.25% lower during that period, which coincided with the Little Ice Age in Europe and North America. Indirect indicators of solar activity such as 14C in tree rings and 10Be in ice cores strongly suggest that periods similar to the Maunder Minimum have occurred repeatedly over the last few millennia. Solar activity has also undergone extended periods of file:///C|/SSB_old_web/strach1.html (1 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 enhancement that are associated with climatic warming. During the so-called Grand Maximum of 1000 to 1200 AD, agricultural conditions in Northern Europe were good but the American Southwest suffered extended droughts. How typical is this behavior? Other stars of mass, age, and spectral type similar to those of the Sun have shown 0.5% brightness changes in as little as 5 years. The theory of stellar structure and evolution as applied to the Sun and other stars stands as one of the major achievements of 20th-century astrophysics. Yet, until recently, the connection of this theory with observation has been made only through gross properties such as stellar mass, radius, and luminosity, with no direct information from the interior. Over the last two decades, two new types of solar observations-neutrino-flux measurements and helioseismology-have changed this picture profoundly and challenged "standard" models of the Sun's interior and its magnetic dynamo. These challenges must be taken into account when space physicists study solar variability because all forms of solar activity are thought to derive ultimately from the interaction of magnetic fields with differential rotation and turbulent convection in the solar interior and atmosphere. When refined and extended, the methods of helioseismology and neutrino-flux measurement will be powerful tools for solving the problems they have uncovered. The generation of energy in the core of the Sun produces neutrinos by several different nuclear reactions. For nearly two decades, high-energy neutrinos from the formation and decay of 8B have been observed through their capture by the 37Cl isotope in the Homestake gold mine in South Dakota. The observed capture rate in that experiment is roughly three times smaller than predicted. Neutrinos from the more fundamental proton-proton reaction that begins the fusion process have lower energy and are more difficult to detect. The newer GALLEX experiment in Italy and the SAGE experiment in Russia both use the 71Ga nucleus to detect low-energy neutrinos, including those from the proton- proton reaction.2 The combined results from all experiments cannot be reconciled with standard solar models and strongly indicate that the neutrinos are altered in their travel from the solar core to Earth. The reconciliation may require fundamental changes in our ideas of solar structure or neutrino physics. Helioseismic measurements have constrained solar models by precisely locating the boundary between the Sun's convection zone and its radiative interior. Figure 3 shows a schematic representation of the structure of the Sun together with a recent example of helioseismic measurements that quantitatively probe its interior. For 30 years, phenomenological and kinematic models of the solar dynamo have assumed a pattern of differential rotation in the convection zone that is now ruled out by helioseismology. As a result, dynamo theory is in ferment; current theoretical activity centers on the possibility that the seat of the dynamo is just below the base of the convection zone. Longer-term and more accurate seismic measurements will allow researchers to investigate many more aspects of solar structure and variability. For example, as the precision of frequency measurements improves and if the low-frequency gravity modes are detected, it will become possible to deduce accurately the near-center temperatures and densities from helioseismology and to compute neutrino fluxes file:///C|/SSB_old_web/strach1.html (2 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 without relying on the theory of stellar evolution. FIGURE 3 Left: The structure of the Sun. Right: The acoustic power of global solar oscillations as a function of temporal frequency and spherical harmonic degree. Discrete concentrations of power in this diagram correspond to trapped gravity (g) and pressure (p) waves that sample limited regions of the solar interior, as illustrated on the left. (Photographs courtesy of Newton Books and National Solar Observatory GONG project.) Researchers have made substantial progress in understanding small- scale convection (granulation) in the outer solar envelope. Benchmark observations were obtained by the SOUP experiment on Spacelab-2, now augmented by observations from ground-based observatories. Many of the observed characteristics of the granulation and the flows associated with convection are impressively reproduced by recent three-dimensional magnetohydrodynamic simulations that include radiative transfer. By contrast, the large-scale aspects of convection and flows in the outer solar envelope are poorly understood. Many numerical models of convection exhibit patterns that resemble giant banana-shaped cells; observations of the surface velocity have thus far shown no trace of such structure but instead show flows referred to as torsional oscillations: latitude-dependent variations in the rotation rate that track the zones of activity. The present observations suffer from a lack of temporal continuity that limits our ability to detect low-amplitude, large-scale structures. The observations necessary to study solar velocity oscillations will also provide an ideal data set for the study of large-scale convection. As was the case for helioseismology, these studies will benefit from extended-duration data sequences that match the Sun's 11-year activity cycle. Researchers do not know how strong the magnetic fields are in the solar interior, nor to what extent they are filamentary, as they are at the surface. Helioseismology should eventually place useful limits on the strength and uniformity of the internal field, but the present upper limits are too high to discriminate among dynamo models. If strong fields near the base of the Sun's convection zone are the source of surface phenomena such as sunspots and plage regions, then understanding the mechanisms that connect the deep and surface fields is necessary. Helioseismology measurements carried out through a solar cycle would permit the study of subsurface magnetic fields through their effect on the propagation velocities of different oscillation modes. Local magnetic fields on the solar surface are associated with changes in file:///C|/SSB_old_web/strach1.html (3 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 the solar oscillation spectrum that may be caused by subsurface fields. Strong magnetic fields like those in sunspots absorb more acoustic energy than they emit. Some of the effects are apparent even during the days before a new spot group appears and may be general predictors of activity over time scales of a week to a month. Although sunspots are the most dramatic manifestation of magnetic fields on the solar surface (Figure 4), even regions where the average field is weak are inhomogeneously dotted with tiny magnetic flux tubes in which the intrinsic field is almost as strong as in sunspots. These flux tubes interact with plasma motions and with each other to constitute the fundamental building blocks of solar activity. They have a characteristic diameter of only about 100 km (subtending barely 0.1 arc sec at the Earth) and have never been directly observed-although, very recently, the upper end of their size spectrum (~200 km) may have been glimpsed through the technique of speckle polarimetry. In order to validate and extend our understanding of solar magnetism, it is essential to resolve and study magnetic flux tubes. FIGURE 4 Left: The intricate structure of a sunspot in the photosphere. An ever-changing pattern of convection surrounds the spot. Right: A solar active region seen higher in the atmosphere (the chromospheric Ha). Structure of comparable complexity persists even into the solar corona (see Figure 6). (Courtesy of National Solar Observatory and Big Bear Solar Observatory.) Study of the largest individual flux tubes requires observations that achieve a consistent resolution of 150 km for hours at a time, over angular fields many arc seconds on a side-conditions unachievable from the ground, but possible with future space missions. However, both indirect observations and magnetohydrodynamic models show that the physical properties of flux tubes (including their contribution to the solar irradiance) change significantly with size; it is not enough to study only the largest structures. Therefore, the long-term goal remains the one stated by the Solar Astronomy Panel of the NRC's Astronomy and Astrophysics Survey Committee3 and by the NASA Mechanisms of Solar Variability program:4 to achieve 75- to 100-km (~0.1-arc-sec) resolution both from the ground and space. file:///C|/SSB_old_web/strach1.html (4 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 The impulsive brightening in visible light that originally defined a solar flare is now recognized as only one component of a complex magnetic reorganization that liberates energy in the form of electromagnetic radiation (from radio waves to gamma rays), energetic particles, and bulk motions. Among the most important types of events in terms of their effect on the magnetosphere are the large-scale coronal mass ejections (CMEs) that have little or no visible-light signature. The shocks preceding the fastest CMEs are capable of producing large fluxes of energetic particles. Solar flares are often associated with the CMEs and are capable of generating impulsive bursts of energetic particles as well as hard x-rays and gamma rays. Neither the trigger mechanisms for CMEs and solar flares nor the particle acceleration mechanisms are known beyond a rudimentary level. The range in time scale for flares and CMEs (from days during the buildup to milliseconds at the height of outbursts), as well as the range in spatial scale (from 100,000 km for the size of an active region to 100 km for the size of the flux tubes), places competing requirements on observation and analysis systems. In order to achieve a coherent understanding of flares and CMEs, space physicists must study both their buildup, by continuously measuring vector magnetic fields in active regions and near large-scale magnetic boundaries, and their release, by measuring their energetic emissions with high angular, temporal, and spectral resolution. In the last 15 years, several venerable but hitherto unproved notions about flares and mass ejections have been placed on a firm footing by space missions such as P78-1, SMM, and Yohkoh, and by closely coupled ground- based observations (often coordinated through campaigns such as Max '91 and CoMStOC). The importance of twisted and sheared magnetic fields (with associated electric current systems) has been confirmed, although much remains to be learned about their evolution and dynamics. The ablation of chromospheric material in response to downward-directed particle beams has been detected and modeled. The topological reorganization of magnetic fields after a flare- compelling evidence of reconnection-has been clearly observed and studied. Even apart from flares, large regions of the corona are seen to restructure on time scales as short as an hour. In summary, the goals of the study of solar structure and variability are to understand the basic processes governing the nuclear and rotational evolution of the Sun and, by inference, other main-sequence stars, to understand the physics of neutrino production and propagation, to understand the solar cycle and concomitant variations of the solar radiative output, and to understand solar activity well enough to produce useful forecasts of solar flares, coronal mass ejections, solar energetic particle events, and other forms of impulsive activity. Specific research questions that should be addressed to achieve those goals are as follows: What are the mechanisms responsible for variations in the spectral and total irradiance of the Sun and solar-type stars? file:///C|/SSB_old_web/strach1.html (5 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 What are the large-scale velocity fields inside the Sun as functions of depth, latitude, and phase of the solar cycle? What is the structure of the Sun's internal magnetic field? How are the internal velocity and magnetic fields related to the surface fields? How does convection operate? What is the role of magnetic buoyancy in the convection zone? What is the structure of the transition between the radiative core and the convection zone? How is magnetic energy stored and released in the solar atmosphere? How are active regions born? How do they evolve and die? What is the spectral energy distribution of neutrinos from the solar interior and how can it be explained through solar models? What magnetic configurations and evolutionary paths lead to flares and coronal mass ejections? What is the physics of solar transients that cause impulsive emissions of radiation, plasma, and high-energy particles? CURRENT PROGRAM The current program seeks to improve our knowledge of the solar interior and solar variability principally through monitoring of the solar irradiance and measurements of solar oscillations. Solar variability is monitored through several experiments on the Upper Atmosphere Research Satellite (UARS), and major experiments are being developed with substantial international collaboration to study solar oscillations. Additionally, the ultimate source of solar energy, the core nuclear reactions, are being studied by international collaborations through the measurement of the neutrino fluxes. The UARS carries a total solar irradiance monitor (ACRIM) and two ultraviolet spectral irradiance monitors. The Solar and Heliospheric Observatory (SOHO) will measure variations in the total solar irradiance as well as manifestations of solar activity. As the Lean report5 emphasized, however, there file:///C|/SSB_old_web/strach1.html (6 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 is a real danger that the crucial cross-calibration of the irradiance measurements will be lost if components of the Earth Observing System do not overlap UARS or SOHO (Figure 5). A key element of the strategy to understand and forecast- rather than just monitor-irradiance variations is to determine precisely, throughout a solar cycle, to what degree the variations are caused by changes in localized surface and subsurface properties and, therefore, how much must be attributed to global changes in the Sun's output. The ground-based RISE program is designed to fill this need, but it is just getting under way and does not have the assurance of long-term funding. FIGURE 5 Measurements of total solar irradiance (TSI) monitored between 1978 and early 1994 by various spacecraft. The difficulty of obtaining absolute calibrations between the different instruments emphasizes the importance of obtaining overlapping measurements. The currently scheduled future measurements and gaps are shown for the period from 1994 to 2008 by horizontal lines. (Courtesy of Richard Willson, Jet Propulsion Laboratory.) One of the reasons for the continued operation of the Homestake neutrino experiment is the detection of a potentially significant correlation of neutrino flux with solar activity. The newer experiments that detect lower-energy neutrinos from the proton-proton nuclear reaction allow several possible theoretical interpretations within the present statistical uncertainty. Consequently, continuation of all the experiments will produce an important narrowing of the experimental uncertainty and permit fewer interpretations of the combined results. The current program in helioseismology comprises a variety of small- scale experiments with different ranges of spatial resolution, sensitivity, temporal continuity, and duration of operation. There are also two large programs that will reach fruition in 1995: the Global Oscillation Network Group (GONG) project and a suite of three instruments on SOHO. GONG consists of six observatories regularly spaced around the globe. Because the Sun never sets on it, the network will be able to distinguish oscillations that differ in frequency by only 1 part in 10 billion. GONG will measure nonradial oscillation modes that are characterized by spherical harmonics of moderate degree and will provide the best available temporal continuity for those file:///C|/SSB_old_web/strach1.html (7 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 modes. The ground-based nature of the GONG network permits servicing and upgrading of system components but also introduces some uncertainties due to diurnal trends and the variability of the terrestrial atmosphere at each site. The SOHO instruments will provide high spatial resolution and the most complete determination to date of solar interior structure, especially of convection in the solar envelope, together with stable and sensitive measurements of solar velocities. SOHO also offers the best chance of detecting the elusive low- frequency gravity modes. The improved precision and temporal continuity of velocity measurements to be achieved by GONG and SOHO may permit the detection of large-scale convective flows. This is a secondary objective for both projects, however, and there is a risk that those studies may be omitted for lack of adequate support. Temporal changes seen in the Sun's acoustic spectrum indicate the desirability of extending observations to include a major fraction of a solar cycle. These changes have been detected in limited ranges of global structure but have not been measured systematically. Both GONG and SOHO are planned for 2 to 3 years of operation. Depending on the efficiency of launch and transfer orbit insertion, SOHO can be extended for an additional 1 to 4 years before exhausting spacecraft consumables, but GONG could operate for a full solar cycle if funding were available. Although researchers anticipate considerable advancement from the generation of helioseismological experiments soon coming on line, this is a new field of investigation and the experiments have known limitations. None of the experiments uses multiple spectral lines that could permit the study of the vertical structure of the velocities. High-frequency oscillations may play a role in energy transport, but no current experiment has adequate temporal and spatial resolution to study such oscillations. Atomic reference systems have been eliminated from the imaging experiments on both SOHO and GONG, and so their velocity measurements rely on the mechanical stability of their optical systems to provide the velocity zero points. Because only one hemisphere of the solar surface can be measured at a time, the resolution in wave number is fundamentally limited. Studies of small-scale convection and surface flows require subarc- second imaging and extended time series (an hour or more). Ideally, such observations are carried out above the Earth's atmosphere. However, apart from a prospective contribution by the Flare Genesis balloon experiment,6 the current program relies on ground-based observatories, which strive for the necessary resolution through a combination of intrinsic image quality (a good site), frame selection, digital postprocessing, and sophisticated techniques, such as speckle imaging and adaptive optics, that show great potential but have not been supported at a level that enables them to make rapid progress. U.S. participation in the Japanese Yohkoh mission and the European SOHO mission represents two major components of the current program for the study of solar variability and activity. Yohkoh carries a cluster of telescopes that measure x-ray emission from flares and the corona. Soft x-ray images are often file:///C|/SSB_old_web/strach1.html (8 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 made simultaneously with ground-based maps of the magnetic fields and visible- light structures on the Sun. Particularly rapid progress is being made in this way in mapping the overall structure of flares and the effects of flares on the corona. The soft x-ray spectrometer reveals the temperature, density, and flow velocity of hot, dense plasmas. Yohkoh can also probe particle acceleration sites with a hard x-ray telescope (7-arc-sec resolution) and a wide-band x-ray/gamma-ray spectrometer (full Sun). SOHO will also measure atmospheric structure and activity with a diverse complement of instruments. Two rocket programs have successfully demonstrated the use of normal- incidence optics to obtain narrow-band soft x-ray images with high angular resolution. Such instruments will be powerful tools for the study of activity if they can be given a long-duration platform. An imager to be flown on a future NOAA/GOES satellite will provide soft x-ray images on a continuous, high-time- resolution, long-term basis, but at much lower angular resolution. The HIREGS balloon experiment has observed hard x-ray "microflares," nonthermal events up to 100 times weaker in flux than previously detected, but still rising in numbers to the limit of instrumental sensitivity. The likelihood that there is a continuous size spectrum of flares, from the rarest spectacular events to ubiquitous microflares (and perhaps even smaller "nanoflares"), has given rise to new theoretical ideas about flares and their connection with coronal heating. In the gamma-ray regime, the Compton Gamma Ray Observatory (CGRO) provides capability to study events with low spectral resolution and no spatial information. This observatory has detected solar gamma rays with energies up to 1 GeV and has directly measured the energy spectrum of solar neutrons. The CGRO found the high-energy bremsstrahlung emission to be highly collimated along the direction of electron motion. This observation is interpreted to mean that the electrons are trapped in a closed magnetic structure. Ground-based observations-free of the mass and size constraints of space experiments, accessing a wide spectral range, and able to incorporate the latest technology-remain a cornerstone of solar physics. In addition to special- purpose applications such as magnetographs and helioseismology instruments, general-purpose telescopes (including spectral isolators and detectors) have been the testbeds for new ideas and techniques. For example, frequency-agile radio imaging has provided new information about the magnetic configuration of active regions before and after flares. There has been an explosive growth in infrared observations, enabled by the development of infrared array detectors, with applications ranging from the deepest photosphere to the corona. Ground-based observations of the solar magnetic field fall into two general categories. Full-disk synoptic measurements emphasize high sensitivity to the line-of-sight field, stable calibration, and continuity over many years. Together with the international sunspot number and the 10.7-cm radio flux, these are among the most widely used data in solar and solar-terrestrial physics. A second category of magnetographs is intended for the detailed study of active regions and magnetic flux tubes. Such instruments emphasize measurements of file:///C|/SSB_old_web/strach1.html (9 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 the full vector field and electric currents, subarc-second angular resolution, good temporal resolution, and precise measurements of the intrinsic field strength using infrared spectral lines. These instruments, most of which are less than 5 years old, represent a major technical and scientific advance over what prevailed a decade ago. However, two major preconditions for understanding surface magnetic fields are still unmet: the ability to study flux concentrations at their intrinsic size scale (corresponding to 0.1 to 0.2 arc sec), and continuous measurements at intermediate resolution (~1 arc sec) over the full lifetime of an active region. FUTURE DIRECTIONS The natural time scale of the solar cycle dictates that some types of observation must be maintained for decades or longer. This requirement is difficult to meet when budgets and political priorities change on a shorter time scale. Nevertheless, a high scientific priority must be accorded the continuation of modest long-term studies of cycle-dependent phenomena such as magnetic flux, sunspots and plages, coronal brightness, and large-scale velocity patterns. It is also vital to assure the continuity of existing spaceborne irradiance monitoring (total and spectral). The whole-Sun fluxes recorded by the irradiance instruments should be supported by spatially resolved, multisite photometric measurements from the ground or space. Progress in the analysis of solar interior structure using the methods of helioseismology requires improved precision in the oscillation frequencies and a larger range of detected modes. The details of the most effective strategy to extend helioseismology studies should be determined after GONG and SOHO have been in operation for about a year. However, there is already a compelling case for maintaining GONG through a full solar cycle, with adequate support for the analysis of large-scale flows. A major advance in the study of energetic events is possible by combining high detector sensitivity with high spatial and spectral resolution. Germanium detectors behind rotating linear masks can provide time resolution as short as 0.01 second with spatial resolution in the arc-second range and enough spectral resolution to reveal dynamics through Doppler distortion of spectral line profiles. This combination yields powerful diagnostics for following the development of the impulsive phase of hard x-ray and gamma-ray flares and for studying energetic particle propagation. Other frontiers in research on flares and active regions lie in the exploration of poorly known regions of the flare electromagnetic spectrum, such as the infrared and submillimeter domains; in multifrequency radio imaging; in continuous observations of vector magnetic fields and velocity fields in active regions through their entire life cycle; and in acquiring a three-dimensional view of coronal structures and mass ejections. file:///C|/SSB_old_web/strach1.html (10 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 The complex and small-scale character of the Sun's surface magnetoconvection demands high angular resolution (0.1 to 0.2 arc sec), good temporal resolution (~10 s), accurate polarimetry (<0.1%), and the ability to simultaneously measure the magnetic field strength in sunspots and weak-field regions as well as diagnostics of the thermodynamic state of the magnetic and nonmagnetic gas. Current instruments achieve (or, in the case of angular resolution, approach) only subsets of these requirements. Fully capable instruments should be developed and fed by telescopes able to supply the requisite photon flux (aperture >1 m). A global network, able to provide 1-arc-sec resolution and 24-hour coverage over a week or more, is needed to characterize the magnetic evolution of active regions over their lifetimes. Adaptive optics and image-reconstruction techniques should be supported to advance the goal of achieving 0.1-arc-sec angular resolution over a restricted field of view. However, for precision photometry at this resolution over an active-region field of view, balloon or spacecraft systems are required. Viewing the Sun from more than one solar longitude has several unique and practical advantages. For example, an observing station over the solar east limb would increase the chance of obtaining an extended set of observations beginning with a change in the oscillation spectrum and ending with the appearance of an active region. This is perhaps the best hope for developing the ability to predict the emergence of activity. Other motivations for a global-view array of spacecraft include early detection and warning of solar activity, tomographic reconstructions of the three-dimensional structure of the outer atmosphere, and understanding of irradiance variations. Achieving all of these objectives would require several types of instruments on each spacecraft; simpler missions could address subsets of these goals. The measurement of neutrino fluxes with different detection systems will provide the most direct observations of the solar core. The gallium experiments, such as SAGE and GALLEX, are especially critical until their statistical uncertainties are reduced to <10%. Although non-U.S. groups have primary responsibilities for those experiments, the support of U.S. investigators should be given high priority as a good return on investment. The Homestake mine experiment should be continued through the current solar cycle to assess the reality of the reported solar-cycle modulation. Asteroseismology poses a technical challenge that has great promise for validating and extending our theories of stellar structure and evolution. Current synoptic programs to monitor activity and magnetic fields on late-type stars should be maintained. However, the sample of stars that can be studied with existing facilities is very limited, particularly for the latest types; access to a 4-m- class telescope would expand the sample by an order of magnitude. The Solar Astronomy Panel of the NRC's Astronomy and Astrophysics Survey Committee7 emphasized the need for a major effort to develop observational capabilities in the infrared. For solar physics, infrared wavelengths allow unique diagnostic capabilities for the measurement of magnetic field strength and temperature. The realization of the potential for new discoveries in file:///C|/SSB_old_web/strach1.html (11 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 the infrared will depend on the continued development of infrared technology (such as large-array detectors and tunable narrow-band filters) and, ultimately, a large infrared-capable telescope to provide subarc-second angular resolution in the range from 3 to 10 microns. It is possible that new insights into two of the more important issues in solar physics-the solar dynamo and magnetic reconnection in solar flares-may emerge from a combination of new laboratory experiments and theoretical research. The potential for the application of laboratory plasma physics to space physics problems has been summarized in reports by the NRC's Plasma Science Committee8 and Panel on Opportunities in Plasma Science and Technology.9 NOTES 1. Board on Global Change, National Research Council, Solar Influences on Global Change, National Academy Press, Washington, D.C., 1994. 2. Board on Physics and Astronomy, National Research Council, Neutrino Astrophysics: A Research Briefing, National Academy Press, Washington, D.C., 1995. 3. Board on Physics and Astronomy, National Research Council, Working Papers: Astronomy and Astrophysics Panel Reports, National Academy Press, Washington, D.C., 1991. 4. The Mechanisms of Solar Variability Program, report of a workshop presented by the Space Physics Subcommittee of NASA's Office of Space Science and Applications, Washington, D.C., January 7-9, 1992. 5. Board on Global Change, National Research Council, Solar Influences on Global Change, National Academy Press, Washington, D.C., 1994. 6. Rust, D., D. Lohr, G. Murphy, and K. Strohbehn, "The Flare Genesis Experiment: Studying the Sun from the Stratosphere," presented at the AIAA 32nd Aerospace Sciences Meeting and Exhibit, January 10-13, 1994, Reno, Nev. 7. Board on Physics and Astronomy, National Research Council, Working Papers: Astronomy and Astrophysics Panel Reports, National Academy Press, Washington, D.C., 1991. 8. Board on Physics and Astronomy, National Research Council, Research Briefing on Contemporary Problems in Plasma Science, National Academy Press, Washington, D.C., 1991. 9. Board on Physics and Astronomy, National Research Council, Plasma file:///C|/SSB_old_web/strach1.html (12 of 13) [6/18/2004 2:18:51 PM]

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A Science Strategy for Space Physics: Chapter 1 Science: From Fundamental Research to Technology Applications, National Academy Press, Washington, D.C., 1995. Last update 2/17/00 at 4:32 pm Site managed by Anne Simmons, Space Studies Board The National Academies Current Projects Publications Directories Search Site Map Feedback file:///C|/SSB_old_web/strach1.html (13 of 13) [6/18/2004 2:18:51 PM]