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Working Papers: Astronomy and Astrophysics Panel Reports Solar Astronomy Table of Contents 0. Executive Summary 2 1. An Overview of Modern Solar Physics 5 1.1 General Perspectives 5 1.2 Frontiers and Goals for the 1990s 8 1.2.1 The Solar Interior 8 1.2.2 The Solar Surface 9 1.2.3 The Outer Solar Atmosphere: Corona and Heliosphere 9 1.2.4 The Solar-Stellar Connection 10 2. Ground-Based Solar Physics 11 2.1 Introduction 11 2.2 Status of Major Projects and Facilities 11 2.3 A Prioritized Ground-based Program for the 1990s 12 2.3.1 Prerequisites 12 2.3.2 Prioritization 12 2.3.3 The Major Initiative: Solar Magnetohydrodynamics, and the LEST 12 18.104.22.168 The Large Earth-based Solar Telescope (LEST) 14 22.214.171.124 Infrared Facility Development 15 2.3.4 Moderate Initiatives 15 2.3.5 Interdisciplinary Initiatives 18 2.4 Conclusions and Summary 20 3. Space Observations For Solar Physics 20 3.1 Introduction 20 3.2 Ongoing Programs 20 3.2.1 U.S. Programs 20 3.2.2 Non-U.S. Programs 21 3.3 New missions 21 3.3.1 The Orbiting Solar Laboratory 21 3.3.2 The High Energy Solar Physics (HESP) mission 22 3.3.3 "Quick" opportunities in space 23 3.3.4 Other missions 24 3.4 The Space Exploration Initiative 25
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Working Papers: Astronomy and Astrophysics Panel Reports 3.5 Solar-Terrestrial Physics 26 3.6 Conclusion and Summary 26 4. Technology for Solar Physics in the 1990s 26 4.1 Introduction 26 4.2 Ground-based Solar Physics 26 4.2.1 Adaptive Optics 27 4.2.2 Analysis of Extremely Large Data Sets 27 4.2.3 Instrumentation 27 4.3 Space-based Solar Physics 28 5. Policy and Related Programmatic Recommendations 28 5.1 University Research and Education 28 5.2 Facilitating Solar Research 28 5.3 Integrated Support of Solar Research 29 5.4 Computing 29 5.5 Theory Initiatives 29 5.6 Recommendations by the NAS Study on Solar Physics from the Ground 30 6.0 Figures 31 0. EXECUTIVE SUMMARY The following is an abbreviated overview of our Panel recommendations, including the priority ranking of major missions, brief descriptions of these major projects, and a summary of recommendations regarding small-scale science, theory, and programmatics. 0.1 Strongly-Supported Major Ongoing Projects Global Oscillations Network (GONG); cost: $15M; start: 1987 Solar Heliospheric Observatory (SoHO); cost: $211M; start: 1988 GONG. The Global Oscillations Network Group will produce definitive observations of solar global oscillations (p-modes), based on an Earth-encircling network of automated observing stations. The major aim is to reduce the effects of sidelobes (by dramatically increasing the time during which the Sun is observed), thus increasing the S/N, and making mode measurements much easier (and more reliable). These measurements, in conjunction with the on-going and planned solar neutrino experiments, allow us to peer into the solar interior, and thus allow us to test our understanding of how a star like the Sun is constructed. SoHO. The Solar Heliospheric Observatory contains coronal imagers and spectrometers, and a helioseismic instrument, and will be placed at the L1 Lagrangian point. It is a European spacecraft, but has U.S. participation in the form of instruments, ground support, and subsequent data analysis. This observatory has probably the best "shot" at observing g-modes (which will allow us to probe the deep interior), as well as the lowest-degree p-modes. In addition, the solar imaging instruments on SoHO are designed to look in great detail at a totally different aspect of solar activity than is normally examined, namely the mass outflows. They will also be able to look at the "closed" corona, and here their principal virtue is their spectroscopic capabilities, combined with imaging, which also allows detailed density and temperature diagnostics to be performed. SoHO will be able to measure temperature and density of coronal material out to several solar radii; to measure outflow velocities; and possibly to measure motions with sufficient accuracy that one could detect with waves in the atmosphere. Thus, these instruments will directly address the recalcitrant problem of accounting for the acceleration of the solar wind. 0.2 New Ground-Based Solar Projects High Resolution Optical Imaging (LEST); cost: $15M; start: 1991 Large-aperture Solar IR Telescope; cost: $10M; start: 1996 LEST. Our highest priority ground-based project is a new moderate-to-large-aperture adaptive-optics telescope in the visible region. This entails: a vigorous development program in adaptive optics, based on the existing facilities; the development of a moderate to large-aperture high-resolution telescope, either separately or jointly with other countries. The United States is now a participant in the Large Earth-based Solar Telescope
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Working Papers: Astronomy and Astrophysics Panel Reports (LEST) consortium, which plans to build such a telescope starting in 1991. Contingent upon progress in the adaptive optics area, we propose to contribute up to 1/3 of the total cost of that telescope. The main improvements over existing telescopes will be in resolution (down to 0.1"), intrinsic polarization (less than 1%), and improved detectors. LEST will thus function as a high angular resolution ground-based optical imaging tool for examining flow-magnetic field interactions; for example, its polarimetric capabilities will enable it to measure Stokes parameters with higher accuracy than OSL, though over a much smaller field of view. LARGE-APERTURE SOLAR INFRARED TELESCOPE. The infrared solar spectrum offers several innovative ways to study solar magnetism. The opacity minimum at 1.6 microns gives us the deepest look into the solar photosphere. At 12 microns, emission lines arising from highly excited states of magnesium give us our first good look at magnetic regions that are difficult or impossible to probe in the visible. A further powerful advantage for magnetic field research in the infrared is the quadratic wavelength variation of Zeeman splitting. Our proposal again entails two distinct objectives: Acceleration of existing programs for the development of focal plane instrumentation suited to measure magnetic fields in the infrared; Building a larger telescope than the 1.5m McMath (because of the improvement of seeing that occurs in the infrared, combined with the decreasing resolution of a fixed aperture at increasing wavelength, and the decreasing photon flux per spectrum line doppler width in the infrared), which means one of the following three options: (i) join with our nighttime colleagues in construction of a 10m class telescope capable of doing solar infrared daytime work (with suitable protection); or (ii) construct a new, large-aperture solar infrared telescope, possibly in combination with a large-aperture reflecting coronagraph; or (iii) upgrade the McMath telescope to a 4m aperture. This facility will allow us to probe the 3-D structure of magnetic flux tubes in the solar surface layers, and thereby permit us to understand the processes which lead to these highly fragmented structures in the solar photosphere. 0.3 New Space-Based Solar Projects 0. Orbiting Solar Lab (OSL); cost: $500M; start: 1992 1. High-energy Solar Physics mission (HESP); cost: $200M; start: 1995 2. High-resolution Coronal Imager (Context); cost: $250M; start: 1998 OSL. The Orbiting Solar Laboratory represents a marriage of a high angular resolution optical telescope with a high angular resolution UV spectrometer and X-ray imager on a free-flying polar orbiter. The main objective of OSL is to image the solar surface layers with a sufficiently high angular resolution (0.1-0.3") so as to observe directly the interactions between surface motions and the magnetic fields on the solar surface. To date, we have only been able to do this from the ground (through the highly obscuring terrestrial atmosphere) and on one brief Shuttle flight; OSL will allow us to view the interactions we believe give rise to virtually all of solar activity over time scales comparable to the duration of the solar activity cycle. The physical processes we want to understand, and which will be observed with this mission, are: (a) The formation of magnetic flux bundles -we want to understand why solar magnetic fields are so highly spatially intermittent; (b) the evolution of surface magnetic fields -- we want to understand how the distortions of surface magnetic fields by the surface convection can lead to plasma heating, including the creation of a multi-million degree corona enveloping the Sun; and how the process of magnetic field evolution proceeds (we know that solar magnetic fields must decay, but the observed decay rate is many orders of magnitude larger than what classical theory would predict.) HESP. The High Energy Solar Physics mission will carry out high-resolution spectroscopy and imaging of solar hard X-rays and gamma-rays, and will detect neutrons. It will image X-and gamma-ray sources with resolution better than 1", and will resolve spectral features with E/ΔAE ~ 1,000 up to energies of 10 MeV. HESP will study the processes of nonthermal energy release in the active solar atmosphere by observing the X-rays and gamma-rays produced by particles accelerated concomitantly with flare energy release. We know that in solar flares the release of nonthermal energy is mediated by efficient particle acceleration, that energy is transported rapidly by the accelerated particles, and that the acceleration and transport must result from complex interactions of plasma with shocks, turbulence, and rapidly varying electric fields. However, we do not understand the detailed nature of these processes, not can we reliably differentiate among different mechanisms. We must determine the location of the site or sites of particle acceleration observationally,
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Working Papers: Astronomy and Astrophysics Panel Reports we must define the properties of the particles directly from the observations, and we must have a clear picture of the relevant magnetic configurations. Observations with high spatial resolution will allow the localization of the sources of the particles and the tracing of their transport paths. Observations with high spectral resolution will allow the deciphering of the rich information encoded in gamma-ray lines, such as energy spectra, angular distributions, abundances, and energy content of the accelerated particles. Because of the proximity of the Sun, the study of the solar high-energy processes provides one the best techniques for investigating similar processes which are known to play a dominant role at many astrophysical sites. HESP has a definite time-critical need -- it must by flown by the year 1999 to take advantage of the next solar maximum. For this reason, we aim at a modest mission (in the ''intermediate class'', in the context of NASA/Space Physics Division plans). HESP will consist of a single major high-energy instrument capable of carrying out high-resolution imaging and spectroscopy simultaneously. It is by virtue of this simplicity of design that a mission in orbit by 1999 is possible. High-resolution Coronal Imager (Context). The Context mission addresses a major problem in astrophysics: How are hot plasmas in stellar atmospheric structures created? Context takes advantage of the opportunity created by the measurements of Solar Probe, which will fly to within 4 solar radii of the photosphere and conduct in situ measurements there. Through high-resolution imaging and spectroscopy, Context can identify the structures through which Probe has flown, in order to understand the environment of Prohe's particles-and-fields measurements. Such a combination of in situ and remote sensing will be unprecedented in astrophysics. 0.4 Strongly-Supported Major Interdisciplinary Projects Solar Neutrino Experiments; no costs available mm Array; cost: $115M; start: 1991 • Neutrino experiments refer here to the panoply of solar neutrino experiments supported by DOE and NSF, many carried out in collaboration with foreign scientists, including the Homestake mine experiment, the Japan/US collaboration Kamiokande II, the Canadian/US collaboration in the Sudbury experiment, the Gallium experiments, and so forth. The long-standing solar neutrino problem involves a remarkably broad range of physical issues, from stellar structure and opacity calculations to the physics of neutrinos themselves. More recent data have only deepened the mystery of these particles, as the possibility of correlations of the solar neutrino flux with the solar activity cycle has been raised. The discrepancy between what we observe and what can be explained by theory, is so substantial, and is so basic to physics, that the new experiments must be assured of continued funding. The mm array is a facility which solar radio astronomers are enthusiastically looking forward to sharing with their astrophysics colleagues. Its main capability will be to allow imaging of the solar outer atmosphere at radio wavelengths with a resolution comparable to the what can be done at other wavelengths (from the optical to X-rays), and to do so in a wavelength window which has been essentially unexplored. A particular feature of observations at these wavelengths is that one can look "deeper" into the atmosphere than at longer wavelengths. 0.5. OTHER RECOMMENDATIONS AND POLICY CONCERNS The Solar Panel also focussed on a number of issues not tied to the larger projects just discussed: 0.5.1 The role of small-scale projects. The Solar Panel strongly encourages the continued vigorous support of small-scale scientific projects, such as balloon and sub-orbital rocket projects; small PI-class space missions; and strong individual investigator grants programs at both NSF and NASA. 0.5.2 University revitalization. Solar physics is poorly represented at universities, a demographic fact which bodes ill for the future of the discipline. We therefore strongly support efforts to fund graduate students, such as the NASA Graduate Student Research Fellowship program; encourage more flexibility on the funding time period for NSF fellowships; and advocate stronger research ties between the universities and the national research centers. 0.5.3 Funding flexibility. We urge NASA and NSF to allow greater flexibility in the duration of funding periods.
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Working Papers: Astronomy and Astrophysics Panel Reports 0.5.4 Balanced support of experiments and theory. Balanced development of solar physics calls for consistent and realistic support of both theory and experimental efforts, including the support of theoretical work which is not directly tied to observational programs. 0.5.5 Interdisciplinary research. Neither the NSF nor NASA have a regular mechanism in place for handling interdisciplinary research programs. This is a particularly deeply felt problem in solar physics because solar physics research by its very nature tends to cross the discipline boundaries defined by the agencies. We therefore urge that provisions be made to allow such research programs access to funding. 0.5.6 Access to large-scale computing. Despite the vastly improved access to supercomputing afforded by the NSF Supercomputer Centers, it remains difficult to obtain large (> 200 hrs) contiguous allocations of supercomputer time. We therefore urge that exploratory programs initiated by NASA/Ames, NASA/Goddard, NCAR, and others to address this problem be strongly encouraged. 0.6. ENABLING TECHNOLOGIES The following examples are just illustrative of the range of technologies with which solar physics is involved, and in which it plays a leading role. We strongly endorse a continued vigorous program in technological developments. Active control of large structures in space. This will be needed in order to carry out the Pinhole Occulter Facility (P/OF), a major imaging facility at hard X-ray and gamma-ray energies with sub-arcsecond capabilities. Adaptive optics, optimized for relatively low-contrast, extended images. Capabilities for analysis of extremely large data sets (>1 Terabyte). Improved, new-generation detectors for high energies (e.g., wide-band gap semiconductors, gamma-ray channeling detectors, etc.); and at infrared wavelengths (large format CCDs). Further development of normal incidence EUV and X-ray optics for high angular resolution studies at these wavelengths. 1. AN OVERVIEW OF MODERN SOLAR PHYSICS 1.1 General Perspectives Recent 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 models, but it must be remembered that the ideal standard stellar model contains many arbitrary assumptions. The Sun is the only star that has been studied in detail, and the only detailed information we have is from scrutinizing its more or less inscrutable exterior. Its interior possesses internal degrees of freedom that are only gradually being discovered and described, and, once described, are only gradually being understood. Present knowledge of the interior of the Sun and stars is based largely on simplified static models constructed from the theoretical properties of particles and radiation as we presently understand them. Parameters can be adjusted to provide a static solar model whose radius and surface temperature agree with observation, and which represents a starting position for the next phase of the inquiry into stellar physics. This next phase concerns the dynamic and magnetic aspects of a star, a phase which is already well underway, and which is the primary focus of our report. Now the dynamical effects ignored in the static models are already suggested by these models. Thus, for instance, the calculated temperature gradients indicate the existence of the convection zone, extending from the surface down a distance of about 0.3 solar radii. The gas continually overturns and operates as a heat engine whose work output is not subject to the usual thermodynamic limitations that apply to thermal energy. The activity at the surface of the Sun is a direct manifestation of this convective heat engine, which produces such diverse phenomena as sunspots, flares, coronal transients, the X-ray corona, and the solar wind, largely through the magnetic field as an intermediary. It seems not to be generally recognized in the astronomical community and elsewhere that the precise causes of solar activity are not yet reduced to hard science. For instance, it cannot be stated why the Sun, or any other solitary star, is compelled to emit X-rays, nor is it understood why a star like the Sun is subject to a mass loss of 1012 gm/sec. Indeed, it is not altogether clear why the Sun chooses to operate a 22-year magnetic cycle, producing the other aspects
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Working Papers: Astronomy and Astrophysics Panel Reports of the activity largely as a by-product. 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 phenomenological interpretation. Indeed, the present ignorance of 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 surface activity. There are the mysteries posed by the different surface abundances of Li, Be, and B and in more stable elements such as Ca and Fe in some F and G dwarfs. There is the mystery posed by the theoretical evolutionary brightening implying that the Sun was 30 percent fainter 3 or 4 x 109 years ago, while over the same period of time, mean temperatures on the terrestrial equator have not varied more than a few degrees. Turning to more direct problems, observations of the solar neutrino emission have failed to corroborate the conventional theoretical models of the Sun. The failure to do so has stimulated a careful review of the theoretical complexities and uncertainties of the model, leading to significant downward revisions of the predicted neutrino flux. Nonetheless the present discrepancy between the observed and predicted neutrino emission seems to be stuck at a factor of three. Until that dilemma is resolved, we cannot be sure of the rest mass of the neutrino or of the amount of the dark matter in the universe. 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 or of 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 probe of the physical conditions (temperature, density, mean molecular weight, angular velocity and magnetic field) throughout the entire Sun. Complete success depends upon suitably long unbroken runs of data and on the detection and identification of g-modes. The analysis of the data currently available points to peculiar and puzzling effects, including interior velocity fields inconsistent with detailed hydrodynamic simulations and with classical dynamo theory. The frequencies of the p-modes differ by many sigmas from those calculated from the standard models of the solar interior. How drastic will be the necessary revisions to the standard solar model is a matter of conjecture. The present rapid development of seismological probing of other stars is an exciting and important adjunct to the exploration of the interior of the Sun. It should be emphasized that there is far more at stake than the standard model of the solar interior. Our knowledge of the static structure of most stars is founded on the success of the solar model, and it is on the theoretical static structure of stars that our ideas of the age and evolution of the galaxy are based. From the broadest perspective, one of the fundamental tasks of solar physics is to develop independent observational checks on this central bastion of astrophysical knowledge. Now, the remarkably active state of the solar periphery, driven by the convective heat engine, has been studied with increasing angular resolution, spectral resolution and wavelength range for several decades. Knowledge has expanded enormously, without, however, bringing immediate theoretical understanding. Solar activity, like the Earth's weather, is a tempestuous and complicated affair. To obtain some measure of the possible theoretical complexity, note that the Reynolds number NR of the convective heat engine is of the order of 1012 - 10 13, which means that the fluid is active on all scales from one solar radius R down to the fraction 102/NR of R, or cm. Hence, the convection has approximately degrees of freedom, and for complete numerical simulation would require a grid with intervals in each of three dimensions. What is more, the magnetohydrodynamic Reynolds number NM is 1010, whereas the terrestrial laboratory can achieve no more than 102 or 103, so there is no general body of knowledge from which the subtleties of solar magnetic activity can be interpreted. The enormous heat flux in the convective zone producing the superadiabatic temperature gradient and driving the convective heat engine on all scales, and the extreme magnetohydrodynamic effects of solar activity combine to provide a dynamical scenario of exotic character that can be understood only after it is described and studied in detail - it is far too complex for a priori predictions. First, detailed observations are required to describe the situation. Then, numerical modeling and theoretical studies of individual dynamical effects can be brought to bear. That is the nature of the activity of a star: There is no single effect, no single new principle that throws 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 concerted close theoretical and observational study in its own right, progressing past dozens of "milestones" and enjoying dozens of "breakthroughs." The milestones
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Working Papers: Astronomy and Astrophysics Panel Reports and breakthroughs already add up to an impressive body of knowledge, but represent only a beginning. A particularly important revelation occurred about two decades ago, with the realization from detailed observational and theoretical considerations that, outside sunspots, the magnetic field at the surface of the Sun, rather than being the expected continuum distribution, is effectively discontinuous, composed of small individual intense and widely separated magnetic flux tubes of 1 - 2 x 103 Gauss. The measured mean field in any region then depends mainly on 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 the 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 Sun's magnetic activity) are: (a) the origin and structure of the individual fibrils and (b) their individual motions. So the pursuit of solar activity becomes solar "microscopy". This is a field in its infancy, although with 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 UV observations from space, although nowhere near the ultimate necessary resolution of 50-100 km, have already established the general occurrence of myriads of tiny explosive events (nanoflares) and high speed jets in the solar corona, providing a clue about the heat input that causes the corona. The individual bursts of energy (1024 - 1027 ergs per event), and indeed the entire supply of energy to the corona, is 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 at the present time, there is no direct measure of any aspect of the fibril motions, nor any direct detection of waves or wrapping in the coronal magnetic fields. Only the effects of the myriad of 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 frequent coronal transients, involving the eruption of matter from the low corona outward into space, often accompanied by flare activity at the surface of the Sun. Now many years 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, the observations show clearly that coronal mass ejections and chromospheric eruptions (i.e., flares) can proceed quite independently. Thus, the coronal mass ejections reflect 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 yet clear. The remarkable X-ray photographs of the Sun, showing clearly the magnetic loop structure of the active corona and the interweaving coronal holes, have gone a long way toward formulating the problem posed by the existence of the active X-ray emission. Also, the high speed streams of solar wind issuing from the coronal holes demonstrate that the corona is active even outside of active regions. The X-ray and EUV studies of the solar corona, together with the discovery by the orbiting Einstein Observatory that essentially all stars emit X-rays, provide a profound scientific challenge as to why ordinary stars are compelled to such extreme suprathermal exploits. The ability to release energy impulsively and 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, such as cosmic rays, strongly suggest that at many sites a significant - and in some cases even a major - fraction of the available energy is converted into high-energy particles. Detailed understanding of the processes which accomplish this is one of the major goals of astrophysics. Solar flares offer an excellent opportunity for achieving this goal. A large solar flare releases as much as 1032 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 non-potential 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 the gamma-ray and hard X-ray continuum emissions which result from electron bremsstrahlung, and by 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 of the relative
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Working Papers: Astronomy and Astrophysics Panel Reports abundances of some isotopes and atomic species among the accelerated particles provides an indirect but detailed view of the physics operating within one of the flare particle-acceleration processes. These high energy emissions are the best tools for studying acceleration processes in astrophysics. Solar flares are one of the very few astrophysical sites where it has been possible to study simultaneously the acceleration of electrons and protons, and only for solar flares can the escaping accelerated particles be directly detected and correlated with the electromagnetic radiations produced by the interaction particles. In addition, lower-energy emissions (soft X-ray, EUV, UV, and radio emissions), which are also observed from flares, reveal much of the detailed properties of the ambient plasma (e.g., temperature, density, and magnetic configuration) before, during, and after the flare. This is the broad view of the mystery of the Sun and the stars. The specifics are complex, but it is essential, if we are to grasp the scope of the problem posed by the Sun, to spell out these complexities in somewhat more detail. The next section, then, details some of the specific problems, measurements, observations, and theoretical studies that are necessary along the way to probe the mystery. 1.2 The Frontiers and Goals For the 1990s By the year 2000 we hope to have a fairly detailed picture of the structure and dynamics of the solar interior, a better understanding of how the Sun generates magnetic fields, considerable measurements of how magnetic fields modulate the smooth outward flow of energy, a better description of the morphology of flaring plasmas, and some predictive capability for the flow of non-radiative energy through the heliosphere to Earth. These and other advances will be achieved by observational improvements in spatial resolution, temporal coverage, and new exploitation of radio, infrared, EUV, and X-ray spectral regions. Up to the present moment, the Sun has become increasingly mysterious the more we have studied it; by 2000 AD, we may reasonably hope to begin closing in on some of the more important mysteries. As in any subfield of astronomy, there are more solar problems ripe for attack, and hence more projects in this report, than can be accomplished in a decade. Uncompleted tasks will form the basis of a program for 2000 and beyond. There is no likelihood that access to space will become rapid, cheap, frequent, or reliable enough to replace the need for a strong ground-based program. Hence, both space-based and ground-based components will be required in solar physics for several decades into the future. In this section we focus on the principal specific goals for solar research which we believe are most important and also realizable in the decade of the '90s or shortly thereafter. This list will then determine the specific initiatives we will recommend in Sections 2 through 5. For convenience, we deal with the three principal components of the Sun - interior, surface layers, and outer layers - in sequence. 1.2.1 The Solar Interior Two powerful and complementary techniques are available for probing the solar interior: neutrino spectroscopy and helioseismology. The flux of high energy neutrinos has been measured since the late 1960's. A discrepancy of about a factor of three between observations and model predictions gives rise to the well-known solar neutrino problem. Significant progress toward solving this problem can be made by a new generation of detectors that will allow the spectrum of solar neutrinos to be measured. An appropriate goal is to measure, by the end of the decade, the flux of solar neutrinos and its possible time variation as a function of energy, from the low energies associated with the neutrinos resulting from the main p-p reactions up through the energies of the 8B neutrinos long-studied by the chlorine experiment. In addition, the fluxes (and their possible time variations) should be understood in terms of the structure of the interior (and its possible time variations), as well as the particle physics of the neutrino. Helioseismology uses the frequencies of millions of normal modes of oscillation of the Sun as probes of the structure and dynamics of the interior. Since 1975, exploration of much of the solar interior has been done using this technique. Better measurements are required to explore both the deep and shallow interior regions as well as to refine the present fairly crude picture of the middle regions. A major goal is to discover why the theory of stellar structure and evolution fails to correctly yield either the structure or the dynamic picture now emerging from helioseismology. Some of the required measurements can be done only from space but others can be done from the-ground more effectively. A newly emerging technique is seismic imaging of local regions on the Sun. This promises to give us the first subsurface views of solar activity, which should revolutionize our understanding of enigmatic features such as sunspots. A realistic goal is, by the end of the decade, to have made accurate measurements of the entire spectrum of p-modes; to have developed a
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Working Papers: Astronomy and Astrophysics Panel Reports new solar model consistent with these data within their errors; to have determined empirically the internal rotation as a function of depth and latitude; to have obtained a quantitative theoretical understanding of the physical processes giving rise to this differential rotation; to have detected the elusive large-scale circulation flows in the deep convection zone; to have mapped the three-dimensional structure of surface active regions and sunspots; and (if they penetrate to the surface with observable amplitude) to have made the initial detection of g-modes generated in the deep interior. This ambitious set of goals is made realistic because of the extraordinary observational and theoretical progress in helioseismology made in the '80s, coupled with realistic expectations for observational and theoretical capabilities to be developed in the '90s. The cycle of solar magnetic activity is believed to arise in the interior by a combination of differential rotation and cyclonic convection acting on magnetic fields. More accurate and higher resolution measurements of surface magnetic fields are required to understand how magnetic flux evolves and is dissipated from the surface as part of the solar cycle. Also needed are accurate measurements of mass flows both on small and large scales to gain a better understanding of the role of magnetoconvection. Such observations, combined with some of the helioseismic results described above, should go a long way toward reaching the goal of an accurate model of the solar dynamo. A realistic goal for the '90s is one which has been sought for decades: finally to obtain a real physical understanding of the origin and nature of magnetic activity in the Sun and stars. 1.2.2 The Solar Surface Observations have shown that the solar surface layers - from the chromosphere to the corona - are permeated, heated and controlled by the magnetic field rooted in the photosphere. The physics of this region is complicated by large density and magnetic variations and violent mass motions. A major observational problem is the difficulty in making accurate physical measurements in the face of spatially blurred observations. Physical quantities deduced from such blurred measurements may apply to an average within the measured volume of the quantity, but because of extreme nonlinearities they may apply to no physically realizable state at all. It is little wonder that important problems such as heating of the upper solar atmosphere or storage of magnetic energy and its violent release in flares have not been solved. The frontier in this research is very much controlled by how small a volume can be measured accurately. While space offers the most certain route to improvement, the development of adaptive optics promises significant benefits using ground-based telescopes. A realistic goal for the '90s is to obtain a clear physical understanding of the interaction of magnetic fields and convective motions at and immediately beneath the surface, and specifically to understand the surprising shredding of the field into spatially intermittent "flux knots," which appears fundamental both to the evolution of surface magnetic fields and to their consequences for atmospheric heating. New measurement techniques will also be needed to characterize accurately the physics of the lower atmosphere. Particularly important will be observations of the magnetic field as a vector varying with height and time, along with corresponding vector mass motion measurements. The technology to make such measurements is under active development at several observatories. Advantages of diverse spectral regions, from the extreme ultraviolet through the infrared to mm wavelengths, are being exploited. Extremely important is the capability to relate physical conditions measured in the surface layers (through visible and infrared data) to conditions in the overlying heated chromosphere (through UV, extreme UV, and mm data); the data must have adequate spatial and temporal resolution to isolate physically near-homogeneous regions and to establish cause-and-effect relations between phenomena at the various levels. A reasonable goal for the end of the decade is to make substantial progress in developing and testing a specific physical description of the magnetohydrodynamic processes giving rise to atmospheric heating in these layers. 1.2.3 The Outer Solar Atmosphere: Corona and Heliosphere Our understanding of the corona and heliosphere was revolutionized by space observations during the last three decades. While much was learned, we still do not have a good understanding of what compels the Sun to produce a hot, X-ray emitting corona. Evidently most other stars also have coronas. Observations have demonstrated that magnetic fields play a controlling role in the morphology and large-scale dynamics of the solar corona. There is also evidence, but not proof, that magnetic processes are responsible for supplying the energy to heat the corona and to produce violent events such as flares and coronal mass ejections. The key to further progress lies in obtaining improved observations on all accessible spatial and temporal scales, but particularly on the smallest size scales. Progress in X-ray and gamma-ray imaging promises to allow
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Working Papers: Astronomy and Astrophysics Panel Reports major advances in our observational understanding of the corona's structure, and of the relation between heated plasma and the sites of energy release. The outer corona merges into a heliosphere that dominates interplanetary space. The heliosphere is Earth's non-radiative connection to the Sun, and events within it are of considerable practical as well as scientific interest. Much has been learned about the structure, dynamics and physics of the heliosphere by combining in situ and indirect measurements with correlative studies of driving phenomena at the solar surface and in the lower atmosphere and corona. The same cannot be said for violent events such as coronal mass ejections that occur frequently during active times of the solar cycle -- better observations from the photosphere through the heliosphere, involving in particular coordinated in situ and remote sensing observations, are required for significant further progress. Appropriate goals for the decade of the 1990s include localizing coronal energy release sites, understanding in detail the mechanism of solar wind acceleration and fixing the height above the surface at which it occurs, and determining how the speed of the resultant wind depends on magnetic geometry. Finally, an extremely important goal is to take advantage of the activity maximum around 1998-2002, using state-of-the-art imaging instruments at X-ray and gamma-ray energies, in order to achieve a major advance in our understanding of the basic mechanisms of solar flares, including very high energy particle acceleration. 1.2.4 The Solar-Stellar Connection It should be emphasized in this overview of solar physics that the solar-stellar connection is an important part of the physics of the Sun and the physics of stars in general. For we may safely assume that most, if not all, rotating and convecting stars would prove as active and mysterious as the Sun if we could observe them as closely. These stars do not fail to exhibit great complexity in those aspects that can be studied. As already noted, it is astonishing to see that some stars support gigantic flares and starspots. Some exhibit mass loss enormously greater than 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. The same 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 for the Sun. The best that can be foreseen is to understand the Sun and then perhaps to infer the solutions for the other stars. It is essential, therefore, 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 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 spindown of the Sun at an early age may have involved profoundly different conditions from those that obtain today. In a similar vein, it appears that the Sun occasionally passes through centuries of suppressed activity (e.g., the Maunder Minimum), and centuries of enhanced activity. The human research program cannot encompass such fundamental long-term shifts in the nature of the activity, so one must turn to the hundreds of solar-type stars to provide a record of the many different moods of the Sun in the span of a human lifetime. Thus, as a direct by-product of obtaining the goals described above, we may anticipate corresponding great advances in our understanding of many long-standing problems of stellar physics. But beyond this "spin-off" result, it is reasonable to adopt the goal of making far more detailed studies of stellar phenomena related to those studied on the Sun, through emerging capabilities of stellar seismology, through stellar observations with new advanced ground-based and space instruments expected to be operational during the decade, and through continued monitoring of time-varying stellar magnetic activity of existing observatories. 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 beyond the listing of specific research goals as we perceive them today. Even though solar physics is sometimes thought of as a mature field, in the coming decade it may be as unpredictable and full of surprises as any astronomical discipline. The observational
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Working Papers: Astronomy and Astrophysics Panel Reports techniques available in this closing decade of the 20th century have opened up entirely new horizons to solar research. It is too soon to guess where the neutrino observations will lead, but whatever the results of the present gallium detectors, the astronomical implications 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 provide surprises, perhaps of a fundamental nature. The present writing is based only 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 its surface. 2. GROUND-BASED SOLAR PHYSICS 2.1 Introduction For nearly 400 years the physics of the Sun has been studied from the ground. While much has been learned about the Sun - and by most astronomical standards it is well understood - the fact is that the Sun confronts astronomers with many unsolved puzzles, both old and new - a point made in some detail in the preceding discussion. Observations from the ground continue to play a major role in confronting these puzzles, many of which have consequences far beyond solar physics. Within the general framework just discussed, we see the 1990s as the era in which ongoing key initiatives - discussed in Section 2.2 below - will be augmented by a major new initiative, which is needed to make really significant progress in our understanding of solar activity. This new "solar magnetohydrodynamics" initiative depends upon a concerted development program in high angular resolution optical observations and precision polarimetry, using existing ground-based telescopes; and aims for the establishment within this decade of a large-aperture ground-based optical facility - the Large Earth-based Solar Telescope - using adaptive optics techniques to image the solar surface in the subarcsecond range. 2.2 Status of Major Projects and Facilities One major ground-based solar project is in progress: the Global Oscillation Network Group (GONG) project. It is filmed at a ten-fold improvement in the accuracy of intermediate spatial-scale helioseismology measurements for studies of most of the solar interior. The project is a community effort led by the National Solar Observatory. A prototype instrument will be completed early in 1991 and the next phase of the project is to build and install six identical instruments at selected sites around the world. Given timely funding, this network should be operational in late 1993. Observations and data reduction will continue for at least three years, to be followed by an intensive analysis effort by the helioseismology community. GONG and the helioseismology instruments to be flown on the SoHO mission in 1995 were designed to be complementary and interdependent: While GONG emphasizes intermediate spatial scale observations with a high duty cycle, the SoHO instruments emphasize large and small spatial scales difficult to observe from the ground. Thus, both projects are essential for the advance of helioseismology, and together can attain the helioseismology goals discussed in Section 1.2.1. The U.S. program of solar physics includes a wide range of ground-based observational facilities operated by national observatories, federal agencies and individual universities. The national observatories with solar observing capabilities (the National Solar Observatory and the National Radio Astronomy Observatory) provide facilities that are publicly available to qualified scientists by peer review. These facilities generally have a scale that is not appropriate for a single university. Federal agencies (NASA, Air Force, Department of Commerce) operate solar facilities in support of various mission goals. These facilities are not generally available to the wider research community. The federally-funded High Altitude Observatory operates solar facilities for its own research programs, but also provides for the use of facilities to the community. Several universities operate solar observing facilities in support of the research of their faculties and students. The scope of these ranges from major, multi-purpose equipment to modest, single purpose instruments. The observational solar programs with two or more faculty members include the California Institute of Technology, California State University at Northridge, Michigan State University, Stanford University, the University of Hawaii, and the University of Maryland. Smaller programs (one faculty member) are found at Penn State University, University of Arizona, University of California at Los Angeles, the University of Chicago, and the University of Southern California.
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Working Papers: Astronomy and Astrophysics Panel Reports hemisphere of the Sun carries unique information about global processes, especially on the time scales (crucial for active-region development) comparable to the rotation period. For these reasons, an interplanetary network of solar observing instruments would be an extremely desirable goal for solar physics in space. Finally, there is a sociological point to be made: Permanent inhabitants of the Moon, at some future time, will rather naturally have a strong interest in the Sun and its activity because of its practical significance to their survival. Solar observing facilities should have priority, and this priority should include both the early and late phases of development of lunar habitation. 3.5 Solar-Terrestrial Physics By the time of the next solar maximum, environmental issues will likely be much more the focus of the space program than they are today. Already, better understanding of our environment is the premier achievement and most challenging goal of the space program. The EOS mission is a recognition of this. EOS will include solar irradiance monitors, but solar physicists will need to complement EOS with a research satellite dedicated to the predictive foundations of solar magnetic activity and solar-forced geomagnetic activity. While we do not make extravagant promises, there is little question that a vector magnetograph, a full-disk-viewing X-ray telescope, a chromospheric filtergraph, a coronagraph and a complement of high energy burst detectors in Sun-synchronous orbit or at L1 would make strong contributions to our understanding of disturbances of the near-Earth space environment. 3.6 Conclusions and Summary To meet the challenge of the next decade in space, we recommend the continued development of OSL and the other approved missions. Beyond these, our top priority for a new space project is a small-to-moderate mission capable of studying solar activity through the maximum of the forthcoming solar cycle (ca. 2000 A.D.); this High-Energy Solar Physics (HESP) mission will emphasize high-energy observations with an instrument of unprecedented spectral and spatial resolution, and sensitivity. We also strongly recommend the support and encouragement of small observational programs (sub-orbital, international missions, small Explorers, and individual experiments on various spacecraft), as well as ongoing space programs such as Solar-A and SoHO. Finally, we recommend expeditious development of the remaining missions listed in Section 3.3.4, each of which represents unique, first-class science. 4. TECHNOLOGY FOR SOLAR PHYSICS IN THE 1990S 4.1 Introduction This section contains brief descriptions of technology development efforts that must be conducted during this decade to enable advanced research projects during the decade following AD 2000. Several of these efforts are now underway and should be continued. Some of the technology developments are common with other fields of astrophysics but others are unique to solar research. For those efforts that are well defined and already initiated, the implementation strategy is simple: finish the efforts as soon as possible and promptly convert the results of successful efforts into useful research tools. For less well defined activities and ones that this panel cannot foresee, the strategy for implementation is to let the peer review process assign priorities in the normal way. We note that although both NASA and the NSF have a mechanism for funding advanced technology initiatives, the scale of these two efforts is substantially different. Partly because of this difference, we list the specific efforts directed toward space and ground-based research separately, but note that there is often much commonality. 4.2 Ground-Based Solar Physics An observational science such as solar physics depends on improvements in technical capabilities for acquiring and reducing observational data. Two vital initiatives are required to insure a strong ground-based program both for the decade of the 1990s and beyond. The highest priority is a continuation of development of adaptive optical systems to allow existing and future telescopes to achieve high spatial resolution. The second priority is development of advanced data processing for handling the large data sets typical of solar observations.
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Working Papers: Astronomy and Astrophysics Panel Reports 4.2.1 Adaptive Optics High spatial resolution measurements are needed for extended time intervals to investigate the detailed structures and dynamics associated with energy transport and storage, heating, and activity in the solar atmosphere. Image blurring and distortion make it difficult to obtain the needed data. Post-facto image reconstruction techniques in principle can be made to work for solar filtergrams, but real-time correction of the seeing is necessary for spectroscopic studies. A prototype solar adaptive optics system built by Lockheed demonstrated that a segmented active mirror could be made to function on the Sun, using high contrast features such as sunspots as targets. Current efforts are toward: (a) development of a more robust system that is easy to keep phased and that can track any arbitrary region on the Sun, locking on the low contrast granulation; and (b) interfacing such a system to an existing solar telescope in a user-friendly manner. Development of and experience with an operating adaptive optics system on an existing solar telescope is critical for the success of the Large Earth-based Solar Telescope (LEST) project. 4.2.2 Analysis of Extremely Large Data Sets Progress in observational solar physics depends increasingly on analysis of large datasets of high dimensionality. Examples include helioseismology image time series (3-D), high speed flare spectroscopic imaging time series (4-D), and Stokes polarimetric multiband imaging time series (5-D). Data volumes range from a few gigabytes to a few terabytes. Current capability is only marginally adequate for the near term, and will be a major constraint later in the decade if not improved. Several requirements appear in handling large datasets. A need to monitor instrument performance and verify data quality requires quick access to small samples of the data spread throughout the set. Interactive processing of a subset is required to identify the systematic errors and biases, and the nature of gross defects. Finally, the need to reduce whole datasets to scientifically meaningful results depends on a pipeline analysis keeping pace with the data collection. Raw computing power coupled with high data flow is necessary to maintain this currency. The development of suitable hardware will be driven by commercial needs far more than astronomical needs. Exceptions may be massively parallel processors or single language processors, which are still most common in research environments. It is critical to exploit useful developments, and to make available the most modern hardware practical for the needs of solar research. Solar physics has software needs that are seldom met by software developed for nighttime astrophysics. There is a need to develop specific solar research software and make it available to the solar community. 4.2.3 Instrumentation A number of technological developments are important for the highest-priority ground-based solar research. Improvements in infrared narrow-band filters, polarizers, and photodetector arrays will contribute to continuing progress in measurement of magnetic and electric fields in the solar photosphere and chromosphere, as well as improved thermodynamic diagnostic capability in those same parts of the solar atmosphere. Optical photodetector arrays with shorter readout times than are presently available will produce an immediate benefit in any measurements such as the above, including development of adaptive optics. In some polarimetric applications, kilohertz rates would be ideal for processing of detected images in order to eliminate the undesirable effects of seeing. This need could be met by detectors with several (8) storage areas on the chip itself, along with efficient and rapid avenues to transfer the charge among the storage areas and the light-sensitive portion of the chip. Such devices would have to be developed in collaboration with the semiconductor industry. Superpolished mirrors are the key to the extension of coronagraph capability to the UV and IR regions of the spectrum, as well as to improved sensitivity and spatial resolution. Improvements in tunable birefringent filters using liquid crystals as electro-optic elements will bring direct benefit to all polarimetric observations, which encompasses virtually all of the above. Ongoing development of Fabry-Perot filter devices for very narrow-band imaging of the Sun should also be encouraged. Finally, for many applications, a device is urgently needed which allows spectral coverage simultaneously with imaging. This need would be met by an imaging Fourier transform spectrometer operating within a restricted bandwidth of the spectrum. This development will require a large advance in the speed with which the information from area detectors can be stored and processed.
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Working Papers: Astronomy and Astrophysics Panel Reports 4.3 Space-Based Solar Physics 4.3.1 New Technology The development of new technology for space observations, traditionally the domain of small space programs, has dwindled alarmingly with the lack of emphasis given by NASA to the suborbital programs and university groups. The recent successes of normal-incidence X-ray optics are a classic example of the kind of rewards that can be reaped by small programs. Urgently needed technology areas include advanced detectors for all spectral ranges, mirror technology, filters, lightweight large structures and data systems. Many of these developments benefit more than one field of research, and should be energetically pursued in NASA's Office of Applications and Space Technology, in conjunction with OSSA. 4.3.2 Large Structures in Space Most disciplines of astrophysics, including solar physics, envision large future instruments in space. These include telescopes of large aperture and/or focal length, interferometers for a variety of wavelengths, and large-scale occulters such as the Pinhole/Occulter Facility. Such structures can be made extremely lightweight and robust by using active servo control of the structure itself. This technology exists where needed for structures on the Earth's surface, for example in active-optics telescopes, but has never been utilized in space. NASA may also need this technology for its large engineering structures (for example, Space Station Freedom itself). We therefore encourage the orderly development of "Controls-Structures Interactions" techniques. 4.3.3 High-Energy Instrumentation A variety of new high energy (> 10 keV) radiation detection techniques which offer the potential advantages such as high spatial resolution, low background, polarization measurements, large detection area, and high spectal resolution at room temperature should be developed. These may include high pressure gas scintillation detectors, wide band-gap semiconductors such as HgI2 or CdTe, position-sensitive germanium detectors, and crystal diffraction and channeling techniques. 5. POLICY AND RELATED PROGRAMMATIC RECOMMENDATIONS 5.1. University Research and Education The discipline of solar physics is currently poorly represented in universities, and is consequently hampered in the development of a strong theoretical and experimental personnel base for future research. There are several initiatives that would specifically address this problem. First, the NASA Graduate Student Research Fellowship (GSRA) program, which funds graduate students semi-independently of a primary advisor, should be greatly expanded. This very successful program has commendably increased its student enrollment beyond allocated levels by ad hoc additional funding supplied by various OSSA Divisions, but we urge that NASA instead consider simply increasing the scale of this program. Second, the NSF graduate fellowships should be awarded over a flexible time period that would allow support to be shifted into the latter part of a student's graduate education, when he or she is most likely to be doing research (NSF Fellowships are currently awarded for the first three years of study). Third, stronger connections between universities and national research centers should be encouraged. This could be done by establishing additional cooperative research programs, expanding visitor programs (at both the postdoctoral and faculty level), and increasing the size of student visitor programs. The latter could be done through the NASA GSRA program, which at present funds students only at universities or NASA centers. 5.2. Facilitating Solar Research Two steps can be taken by NASA and NSF to facilitate solar research through minor restructuring of existing funding programs. NASA and NSF contracts/grants should have flexible funding periods, ranging from one year to five years, with the length of the funding period being determined by the quality and the requirements of the proposal.
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Working Papers: Astronomy and Astrophysics Panel Reports NSF grants should allow for release time for teaching faculty, which could, for example, provide one half year of salary every three and one half years to support dedicated research time either at the home university or at another institution. 5.3. Integrated Support of Solar Research It is important that both NASA and NSF take an integrated approach to the support of solar physics, particularly with regard to theoretical and observational research and to solar and related astrophysical research. Whenever possible, a balanced support of theoretical and observational research should be provided; in addition, it is wise to provide a balance between theorists and observers tied to large research groups and/or projects, and theorists and observers who operate independently of such research groups. At the same time, we strongly urge NASA to improve its past and present record of funding only minimal amounts of science in conjunction with instrument proposals; past funding practices have led to the unfortunate situation that theoreticians are recruited as co-investigators on instrument proposals, contributing to the credibility of the proposal during peer review, but then are not funded to provide substantive scientific input prior to launch. NASA Space Physics and Astrophysics should develop a mechanism for supporting interdisciplinary solar/astrophysical research. Other agencies should consider establishing similar arrangements, as appropriate. 5.4. Computing Supercomputers: Although we applaud the recent initiatives in supercomputing by the NSF and, more recently, by NASA, we have some concern that this new capability is not optimally used. In particular, supercomputers are a singular resource for very large simulations (viz., calculations which consume of the order of 200 or more Cray X/MP CPU hours), which cannot be replaced by machines of the minisupercomputer class. However, national resources devoted to supercomputing in fact generally do not allow realistic access to such use of supercomputers. We urge the NSF and NASA to encourage the provision of additional resources to such very large scale computing, as has in fact been done by the NASA Ames supercomputing facility and, in isolated instances, by some of the NSF Supercomputing Centers. Workstations: Workstations are becoming essential tools for both experimentalists and theorists in the analysis of their "data". For this reason, we recommend that contracts and grants should continue to be able to provide for both the purchase and maintenance of such workstations as a matter of standard practice. Networks: Efforts to provide high speed digital networking capability to large numbers of solar researchers should be enhanced. Frequently used data sets should be made readily available via these networks. 5.5. Theory Initiatives The NASA Solar-Terrestrial Theory Program (STTP), and its successor, the Space Physics Theory Program (SPTP), have made a commendable start in the support of theoretical work which is not directly mission-oriented. As a number of previous National Academy reports have noted, provision of such support—in addition to more mission-related theoretical studies—is essential for the health of the space physics disciplines. These reports also noted that theory needs to be supported on two distinct scales: first, at the individual investigator level; second, at the level of group efforts with significant "critical mass". The STTP program indeed was created initially with the specific intention of responding to this second need. However: The current typical grant size of the NASA SPTP is well below what is desirable for support of "critical mass" theory groups at universities; this desired mean support level has been discussed by NASA, and projected at the roughly $300K level, but has never been implemented. We recommend augmentation of the current program in order to increase the mean grant size to the previously-discussed support levels. Much of current NASA theory grants to individuals is funded through the SR&T budget, in which there is substantial pressure to focus funding on directly mission-related work. This means that theoretical studies in solar physics, which are carried out by individuals but are not directly mission-related, are strongly discouraged, contrary to the recommendations of previous National Academy studies. We recommend that NASA modify the "ground-rules" so that such grants can be funded, based on the quality of the peer reviews.
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Working Papers: Astronomy and Astrophysics Panel Reports Maintaining an appropriate balance between experimental programs, data analysis, and theory is a major challenge for both the NSF and NASA. We recommend that both agencies establish rough guidelines for balancing these programs, based on input from the appropriate advisory panels. 5.6 Recommendations by the National Academy of Sciences Study on Solar Physics from the Ground Finally, we note that the National Academy of Sciences has recently published the report entitled "The Field of Solar Physics: Review and Recommendations for Ground-Based Solar Research" (National Academy Press, 1989), which contains the following recommendations which we endorse and incorporate into this report: " Develop a coherent, well-defined infrastructure for solar physics within NSF, with that agency properly assuming the lead role in support of basic research in ground-based solar physics. Thus the committee recommends that the internal structure for funding of solar research within NSF be changed so that support for both grants and centers is administered by a single entity within NSF whose primary responsibility is solar physics. Such a reorganization will permit the development of appropriate advocacy within NSF, the definition of an overall coherent approach to the subject, a unified vision of the field's national facilities and university grants program—its scope and its development—and the implementation of new efforts. The directorate in which to place the recommended section could be either the Geosciences Directorate (the residence of support for solar-terrestrial sciences and the High Altitude Observatory) or the Mathematical and Physical Sciences Directorate (the residence of support for astronomy and the National Solar Observatory). Placement of the recommended section is a matter for NSF decision. The committee believes that such a section will benefit the nation's solar physics efforts. Support and encourage university programs in experimental and observational solar physics and take steps to strengthen the partnership between, on the one hand, federally supported research centers and, on the other hand, universities. In particular, the committee recommends that specific programs to enhance education and training of students in solar instrumentation and observational techniques be established in the university community and that those universities willing to commit themselves to such programs receive support for the extended periods required to carry out such efforts. In addition, the committee recommends that more effective partnerships be forged between federally funded centers and universities—partnerships involving the exchange of faculty and technical staff, hardware and software, and workshops and short courses. Protect newly funded initiatives in solar physics by ensuring their continued support until they are completed. Unless funding for such initiatives can be assured within the limits imposed by general federal budget restrictions, avoid pursuing additional new initiatives. The committee further recommends that NSF refrain from commingling funds targeted for new initiatives with base-level support funds in response to budget-cutting pressures." The second and third points apply equally well to the solar programs within NASA, and we hence also recommend them to NASA.
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Working Papers: Astronomy and Astrophysics Panel Reports FIGURE 1. Observed gamma-ray spectrum of a solar flare. The comparison of the calculated spectrum (solid curve) with the data allows the determination of abundances of both the ambient gas in the gamma ray production region (thought to be the chromosphere) and the accelerated particles (thought to be accelerated in the corona). The aim of HESP is to improve our understanding of these gamma-ray spectra.
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Working Papers: Astronomy and Astrophysics Panel Reports FIGURE 2. The solar X-ray corona photographed on September 11, 1989 from a rocket carrying a normal-incidence X-ray telescope. Normal-incidence optics represents a great advance over previous methods of solar X-ray imaging and will be the basis for high-resolution X-ray imaging on the Orbiting Solar Laboratory. Image courtesy of IBM Research and the Smithsonian Astrophysical Observatory.
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Working Papers: Astronomy and Astrophysics Panel Reports FIGURE 3. The solar chromosphere, imaged in H-alpha light at the Sacramento Peak Observatory. Spicules are seen near the limb as dark thread-like features (bottom), and are seen in emission above the limb (top). Spicules may be very important to our understanding of the energy balance of the solar atmosphere. Unfortunately, they are only poorly resolved in existing ground-based telescopes, but they could be thoroughly investigated using the high-resolution capabilities of LEST on the ground, or OSL in space.
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Working Papers: Astronomy and Astrophysics Panel Reports FIGURE 4. A composite magnetogram image, with the bottom half showing the magnetic pattern at maximum of the 11-year solar magnetic activity cycle and the top half the pattern at solar minimum. The dramatic change holds important clues to the nature of the magnetic cycle. It is important that new high-resolution observations, both from OSL and LEST, follow the changing magnetic patterns, and their consequences in the overlying atmosphere, throughout a large fraction of a solar cycle.
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Working Papers: Astronomy and Astrophysics Panel Reports FIGURE 5. A computer representation of the two-dimensional spectrum of some of the 10 million solar oscillation modes. This covers the range of degrees from 0 to 150 (from left to right) and frequency from 2 to 5 mHz (bottom to top). These observations, made in a single day, do not have sufficient frequency resolution for accurate characterization of solar internal structure or rotation; needed are continuous observations for extended periods, such as are planned with the GONG experiment as well as helioseismology instruments on the SOHO mission.
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Working Papers: Astronomy and Astrophysics Panel Reports PLANETARY ASTRONOMY PANEL DAVID MORRISON, NASA Ames Research Center, Chair DONALD HUNTEN, University of Arizona, Vice-Chair MICHAEL F. A'HEARN, University of Maryland MICHAEL J.S. BELTON, National Optical Astronomy Observatories DAVID BLACK, Lunar and Planetary Institute ROBERT A. BROWN, Space Telescope Science Institute ROBERT HAMILTON BROWN, Jet Propulsion Laboratory ANITA L. COCHRAN, University of Texas, Austin DALE P. CRUIKSHANK, NASA Ames Research Center IMKE DE PATER, University of California, Berkeley JAMES L. ELLIOT, Massachusetts Institute of Technology LARRY ESPOSITO, University of Colorado, Boulder WILLIAM B. HUBBARD, University of Arizona DENNIS L. MATSON, Jet Propulsion Laboratory ROBERT L. MILLIS, Lowell Observatory H. WARREN MOOS, Johns Hopkins University MICHAEL J. MUMMA, NASA Goddard Space Flight Center STEVEN J. OSTRO, Jet Propulsion Laboratory CARL B. PILCHER, NASA Headquarters CHRISTOPHER T. RUSSELL, University of California, Los Angeles F. PETER SCHLOERB, University of Massachusetts, Amherst ALAN T. TOKUNAGA, University of Hawaii JOSEPH VEVERKA, Cornell University SUSAN WYCKOFF, Arizona State University
Representative terms from entire chapter: