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Astronomy and Astrophysics in the New Millennium: Panel Reports (2001)

Chapter: 5 Report of the Panel on Solar Astronomy

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Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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5
Report of the Panel on Solar Astronomy

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

SUMMARY

The study of the Sun has revealed fundamental physical puzzles that have resisted understanding for generations of astronomers. The Sun is a typical star, with other stars being at least as complex. Solving mysteries on the Sun—among others, the dynamo process, the intermittency in the surface magnetoconvection, and the heating of the active corona—is important for all of astronomy and astrophysics. The Sun offers unique opportunities for physical insight that go far beyond just resolving astrophysical processes on their intrinsic scales. These opportunities include (1) using the Sun as a plasma physics laboratory, (2) understanding and predicting the impacts of the Sun on Earth’s climate and on “space weather” in the near-Earth environment, and (3) understanding the role of solar evolution in the evolution of life in planetary systems. The successes achieved in solar research since the 1991 survey report1 lead us to expect that many of these mysteries can be resolved by the new projects prioritized in this report. However, it should be kept in mind that at this time, key solar mechanisms are poorly understood even as they are applied in other astrophysical contexts. Or, fascinating new phenomena might be discovered that will give rise to new puzzles to challenge new generations of physicists.

STRATEGY FOR THE DECADE 2001 TO 2010

The progress of the past decade was made possible by investments made in the 1980s that led to revolutionary observational capabilities in space and on the ground, including simultaneous multiwavelength observations of dynamics, precision vector magnetic field measurements, and helioseismology. Breakthroughs in numerical simulations of two-and three-dimensional magnetohydrodynamical (MHD) processes allowed for tailoring solarlike scenarios on the computer. All these advances have led to the formulation of a new strategy—a systems approach—for solar physics in the next decade:

1  

Astronomy and Astrophysics Survey Committee, National Research Council. 1991. The Decade of Discovery in Astronomy and Astrophysics (Washington, D.C.: National Academy Press).

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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  • The domains of the solar interior, photosphere, chromosphere, corona, and heliosphere should be treated as a single system.

  • Diverse data sets should be integrated, as demonstrated in the NASA Solar Data Analysis Center (SDAC).

  • The connection to the operational branches of space weather research should be exploited much as weather research received from the National Weather Service is exploited.

  • International efforts should be integrated as much as possible.

OBSERVATIONAL EFFORTS

CURRENT
  • Observational facilities in operation. The Dunn solar telescope with the Advanced Stokes Polarimeter and its adaptive optics (AO) program, the McMath-Pierce telescope with its infrared program, and the Fourier-transform spectrograph should be operated until the Advanced Solar Telescope (AST) becomes available. The seismology network GONG and the Mauna Loa Solar Observatory should be continued. The various university observatories should be maintained at a level that will ensure a broad educational base. The space-based observatories—Yohkoh, SOHO, Ulysses, and TRACE—should be maintained and given adequate funding for data analysis.

  • Observational facilities under construction. SOLIS (on the ground) and HESSI, Solar-B, STEREO, and Solar Probe (in space) are of utmost importance for expanding some findings of the last decade in critical areas.

FUTURE
  • Primary recommendation, ground-based, medium size: the Advanced Solar Telescope (AST). In view of solar physics’ growing relevance to the climate research and space weather communities, AST should be built and become operational within this decade. Key astrophysical processes will be directly observable with the AST and AO. Half of the $64 million investment would come from international partners.2

  • Secondary recommendation, ground-based, medium size: the

2  

The estimated costs for ground-based initiatives include costs for instrumentation, grants, and operations, as described in the preface.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

Frequency-Agile Solar Radiotelescope (FASR). This state-of-the-art radio observatory would be primarily for solar observations that can be readily scheduled in coordinated campaigns. Cost: $15 million.

  • Primary recommendation, ground-based, small size: the expansion of SOLIS to a three-station network around the globe. This would give nearly continuous coverage in full-disk solar vector magnetic field monitoring and would form the backbone of an assessment of the solar magnetic flux budget over the solar cycle. Cost: $4.8 million.

  • Primary recommendation, space-based, medium size: Solar Dynamics Observatory (SDO). SDO would pursue in particular the newly discovered tomography of subsurface structures through time-distance analysis of running waves at the solar surface and impulsive helioseismology from oscillations of loops in the corona. Cost: $300 million.

THEORY AND DATA MINING

The panel recommends a broadened Solar Magnetism Initiative (SMI) as a comprehensive research framework for theory and data mining for all of the above projects. Understanding in solar physics can be advanced through detailed multidimensional numerical modeling. SMI will provide the coordination between observational activities and numerical experiments in forward modeling, to be done by modelers in solar physics as well as plasma physics and turbulence theory. SMI will be a community-wide research program that has been broadened from its original scope to become a multiagency enterprise. The cost of the program is estimated to be $3 million per year for 5 years, with the option of extension for another 5 years. Since SMI is proposed as a multiagency enterprise, it is not ranked with respect to ground-based or space-based projects but stands on its own.

NEW TECHNOLOGIES

An adaptive optics system for a 4-m-class AST needs to be pursued based on recent dramatic progress in the existing project at the National Solar Observatory (NSO) and international cooperation. The development of lightweight mirrors (like Solar-Lite) to achieve high resolution in the medium and far IR will break the cost curve for future space missions. The development of Stokes polarimeters in the UV will allow for measurements of the magnetic field in the chromosphere.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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POLICY ISSUES

NSO should be enabled to take its lead role in developing the AST through changes in its managerial structure. Broad community participation needs to be ensured. The allocation3 between the National Science Foundation’s (NSF’s) university grants program (about 63 percent) and its funding to centers in the U.S. solar and solar terrestrial community (37 percent) is appropriate and healthy. Increases in overall funding are necessary, however, given the increased need to understand the Sun for space weather forecasting and for the driving of climate. Additional educational outreach activities will also be required.

WHY DO SOLAR PHYSICS RESEARCH?

The complexities of the Sun—its internal structure, rotation, and convection and the resulting cyclic and random generation of its magnetic fields and the magnetoactive, hot, explosive, extended solar atmosphere and solar wind—are fascinating and challenging (see Figure 5.1). Because these solar phenomena occur over physical scales that cannot be simulated in laboratories on Earth, their study tests and expands our understanding of magnetofluid dynamics and plasma physics. Solar physics is key to much of astrophysics and central to the Sun-Earth connection, and it bears on the quest to determine the origin and extent of life in the universe.

KEY TO THE MAGNETODYNAMIC UNIVERSE

Dynamic magnetic fields are widespread throughout the universe; they are an active ingredient of many astronomical objects, from dwarf stars to accretion disks to clusters of galaxies. Our understanding of the origins and effects of these distant astrophysical magnetic fields is rudimentary at best. The Sun has the most intense magnetic field in the solar system. The entire corona and solar wind and diverse explosive events (many producing bursts of high-energy particles, x rays, and gamma rays

3  

From the Task Group on Ground-based Solar Research, National Research Council. 1998. Ground-based Solar Research: An Assessment and Strategy for the Future (Washington, D.C.: National Academy Press). Also known as the Parker report for committee chair Eugene Parker.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

FIGURE 5.1 Superposed images of aspects of solar variability. Top left: composite of an event with a modest flare, the brightest eruptive prominence seen so far by SOHO/EIT, and a 400 km/s CME seen by SOHO/LASCO. Top right: the Sun from RISE/PSPT in CaK. Bottom from left to right: plot of solar luminosity and the sunspot cycle; auroral curtain during a magnetic storm; H.Averkamp painting of skaters during the Little Ice Age, when solar activity was low during the 17th century (Hendrik Averkamp, Winter Scene on a Canal, c. 1615, oil on panel, 18 7/8×37 5/8 in., Toledo Museum of Art, Toledo, Ohio; purchased with funds from the Libbey Endowment, Gift of Edward Drummond Libbey, acc.no. 1951.402). Top left images courtesy of the SOHO/EIT and SOHO/LASCO consortia. SOHO is a project of international cooperation between ESA and NASA. Top right image courtesy of Radiative Inputs of the Sun to Earth/Precision Solar Photometric Telescope Project.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

and other large events blasting magnetized matter out past the planets) are all magnetodynamic effects. The Sun is a unique laboratory that will lead to an understanding of the dynamic behavior of cosmic magnetic fields.

SOLAR-TERRESTRIAL PHYSICS

Life on Earth depends on the Sun’s heat and light. Earth’s climate, the state and extent of the upper atmosphere and magnetosphere, and space weather inside and outside the magnetosphere are determined and driven by the Sun’s luminosity, by its UV and x-ray spectrum, by the solar wind, and by explosive events on the Sun. Solar irradiance variations appear to be correlated with the level of the Sun’s magnetic activity. Extrapolated luminosity changes due to changes in the Sun’s production of magnetic field over decades and centuries are large enough (>0.1 percent) to significantly affect Earth’s temperature, contributing to global warming and “little ice ages.” Changes in the Sun’s magnetic activity change the output of UV and x rays by factors as large as 10 or more. These radiations control Earth’s thermosphere, ionosphere, and protective ozone layer. Coronal mass ejections (CMEs) on the Sun blast out massive magnetic clouds that plow through the solar wind and impact Earth, causing magnetic storms that can disrupt power systems. In near-Earth space and throughout the solar system, high-energy particles from these events often reach levels that can be lethal to spacecraft and astronauts. To better understand and predict global change and space weather, we need to understand and predict the mechanisms and behavior of their driver, the magnetic Sun.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

ORIGIN AND EVOLUTION OF LIFE ON PLANETS

Acting over the age of the solar system, the solar wind may well have played a major role in the evolution of planetary atmospheres. It has been suggested that Mars currently has little atmosphere because it has long had, as now, little magnetic field, allowing its early atmosphere to be blown away by the solar wind. To evaluate the magnitude of such effects, astronomers need to understand the Sun’s production of magnetic field, the mechanisms underlying the acceleration of the solar wind, and their variation over solar cycles and longer times. Similar considerations apply to the Sun’s output of UV and x rays over its history—with this output being controlled by the solar magnetic cycle. Some stars with activity cycles exhibit much greater variability than the present Sun, suggesting that the Sun might have had very active phases in the past. An understanding of the magnetic Sun will form a basis for estimating how stellar magnetism could influence the possibility of life arising on other planets throughout the universe.

THE MOST SIGNIFICANT ADVANCES IN THE LAST DECADE

GOALS ACHIEVED

Many of the goals for solar physics laid out in the 1991 survey report have been met or surpassed, as can be seen below.

THE SOLAR INTERIOR
  • Thin flux-tube calculations have been successful in reproducing the synoptic properties of flux emergence over the solar cycle, thereby placing stringent bounds on the magnetic field strength at the base of the convection zone (several 104 gauss).

  • The radiative core of the Sun rotates as a solid body, while the observed surface differential rotation persists to the base of the convection zone.

  • A thin boundary layer, the tachocline (thickness less than 0.05 solar radii), exists at the radiative core convection zone interface and is a propitious site for the solar dynamo. The rotation amplitude between 0.68 solar radii (just below the convection zone) and 0.72 solar radii (just above it) varies locally up to 25 percent over a period of 1.3 years.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×
  • Solar model structures validated by helioseismology rule out an astrophysical solution to the solar neutrino problem and underscore the necessity of elemental diffusion and settling in the Sun’s radiative core.

  • Local helioseismology (Hankel decomposition, time-distance analysis, and acoustic holography) images to a depth of ~20,000 km subsurface signatures of extant and emerging active regions and has detected steady poleward, near-surface meridional flows of 10 to 30 m/s.

THE SOLAR SURFACE
  • Magnetic fields emerge in strong-field concentrations with significant electric currents and helicity. Through convective collapse and the buoyancy of the magnetized plasma, the fields rapidly orient themselves perpendicularly to the solar surface and are enhanced to superequi-partition levels of approximately 1500 G.

  • The rate of appearance (and disappearance) of the surface magnetic flux, particularly in the form of small-spatial-scale ephemeral regions, is such that the average observed unsigned flux in the quiet Sun would be doubled in approximately 40 h.

  • The fact that the average unsigned flux varies by a factor of only 3 to 5 over the entire solar cycle implies that these emerging fluxes must be rapidly “recycled” under the action of a local surface magnetic dynamo. Model calculations indicate these local dynamo processes provide sufficient magnetic energy for the heating of the outer solar atmosphere.

  • A significant fraction of a sunspot’s magnetic flux is contained in the penumbra, which has a deep fluted structure. Radial spokes of nearly horizontal magnetic field alternate with spokes in which the field is inclined some 40 deg.

  • Evidence indicates that the emergence of active regions leads to excess facular emission that exceeds the deficit of the sunspot umbrae and penumbrae. Numerical simulations are beginning to address the question of how deep in the convection zone these irradiance variations first arise. The total change in solar irradiance over a solar cycle, however, remains unexplained.

THE OUTER SOLAR ATMOSPHERE AND HELIOSPHERE
  • Both theory and observation now show that the chromosphere and the transition region cannot be regarded as nested physical atmospheric layers with a distinct identity. Rather, their characteristic spectral

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

signatures arise from radiatively weighted averages of nonlinear magnetohydrodynamic processes that are highly variable in both space and time and that force the tenuous plasma to be far from radiative equilibrium.

  • There is a nascent appreciation that the signatures of wave propagation, magnetic reconnection, and nanoflares are imprinted on the line profiles of UV and EUV emission features, allowing the relative contribution of these processes to the heating of the solar atmosphere to be determined from data with sufficient resolution in wavelength, space, and time.

  • The reconnection of post-CME loops has been detected through their continuous glow in soft x-ray emission. The synthesis of radio, x-ray, and white-light coronal images has begun to reveal the intricate manner in which the corona ejects magnetized material (carrying magnetic helicity) in the guise of CMEs while liberating magnetic free energy through flares (magnetic reconnection events) possessing a continuous spectrum of sizes.

  • The diffuse x-ray irradiance of the corona shows a pronounced variation with the solar cycle. This variation exceeds the variation expected from the number and size of active regions.

  • The first-ionization-potential (FIP) effect is absent in high-speed solar wind streams, implying that the structure and dynamics of the upper chromosphere are fundamentally different in coronal hole regions and the rest of the Sun.

  • Heating in the coronal acceleration region of the high-speed solar wind leads to large ion-temperature anisotropies and very large perpendicular ion temperatures, implying that ion cyclotron heating is a major source of the energy required to drive these streams.

THE SOLAR-STELLAR CONNECTION

This report cannot be a comprehensive review of all of stellar physics. Hence, the panel concentrates on two examples where the synergy between solar and stellar work is particularly beneficial and includes the sharing of instrumentation. During the last decade, studies of stellar magnetic activity have made significant progress in some respects, but in others they have been handicapped by the lack of adequate tools for some of the observations. Ground-based spectrographic and photometric studies of bright-field stars, including the Mt. Wilson HK (hydrogen and calcium line) photometry program, the High Altitude Observatory

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

(HAO)/Lowell Solar-Stellar Spectrophotometer, and the Tennessee State University network of photometric telescopes, have delineated the broad features of stellar magnetic activity. The broad dependencies of stellar activity on stellar temperature and rotation rate are starting to be understood, as are the connections between these properties, a star’s age, and the photospheric abundances of lithium and beryllium. The Sun displays an unusually small ratio of photometric to chromospheric variability; this fact is central to the understanding of sunspot and facular contributions to time variations in the solar flux, but it also has the consequence that accurate solar analogues are difficult to find. In order to gain samples of stars that are larger, more homogeneous, and better defined with respect to their mass, age, and composition, access to larger telescopes to observe fainter stars is required.

Astroseismology of Sun-like stars would make critical contributions to solar-stellar problems, better defining the fundamental parameters of the stars under study and helping to reveal their internal processes. So far there are no methods to reliably measure the tiny oscillating signals produced by stars similar to the Sun. Progress has been made in radial velocity observations of a few of the brighter stars. Further efforts in calibration methods, combined with suitable high-resolution echelle spectrographs, can be expected to bring a hundred or so nearby solar-type stars within the reach of this technique. A more far-reaching avenue for the application of seismic methods is photometry from space. Unhindered by atmospheric absorption and scintillation, a modest-size telescope would be able to analyze stellar pulsations in many Sun-like stars in the nearer open clusters. The first steps toward such precise photometry missions are now being taken, involving observations of a few bright-field stars; recent results have come from the star tracker on the WIRE spacecraft, while small photometry missions have been selected for flight by France, Canada, and Denmark.

A SYSTEMS APPROACH TO SOLAR PHYSICS—TOWARD A DECADE OF UNDERSTANDING

The scope of solar research and the methods for performing it are expanding in several respects: (1) the Sun is being treated as one physical system, (2) solar research is being systematized, (3) diverse datasets are being integrated into a framework, (4) connections are being forged

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

to operational forecasting of space weather, and (5) international cooperation is being sought.

The Sun’s variable output—radiation, particles, and fields—is controlled by the structure and evolution of its magnetic field. The solar magnetic field is present throughout the solar convection zone and the tachocline immediately below it and the photosphere, chromosphere, and corona above it. Thus, all of these domains are magnetically linked. Historically, the dynamics and magnetohydrodynamics of these domains of the Sun were studied and modeled in distinct, relatively unconnected efforts. Much progress has been made in understanding the physical processes at work in each domain. The time is right to try to understand the whole of solar magnetism, by looking at the Sun as a single system from the convection zone out through the corona. This perspective is supported by a combination of well-established facts about the Sun. Hale’s polarity law of sunspots—which says that leader and follower spots have opposite polarities in the north and south hemispheres—is augmented by the observation that the Sun’s magnetic field patterns show a predominance of left-handed twist in the north and right-handed twist in the south. This observation points to a global organization of the field despite its structured and filamentary characteristics. Helioseismology has revealed that the tachocline at the base of the solar convection zone is the likely location for dynamo action. The tachocline may be unstable to global MHD disturbances that in turn act as templates for solar activity seen at the photosphere. Mechanisms for the injection of magnetic flux into the bottom of the convection zone have been identified and modeled, and the rise of flux through the convection zone due to magnetic buoyancy has been demonstrated theoretically, explaining several features of sunspot groups. Measurements of vector magnetic fields at the surface have allowed detailed study of the physical interaction between the magnetic field and the thermally radiating plasma (see Figure 5.2). MHD models have captured the process of convective collapse, by which magnetic flux tubes form. Understanding chromospheric heating through radiation-hydrodynamics has also advanced. The evolution of coronal structures throughout all solar cycle phases has been well described, including the sudden changes due to CMEs. Some of these processes have also been modeled successfully. White-light, UV, and x-ray observations of evolving coronal structures are being integrated into a unified picture.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

FIGURE 5.2 Vector magnetic-field measurements and visible-light and x-ray observations of a small active region composed of a quiescent unipolar sunspot and a smaller delta spot, which evolved significantly over a few days. This evolution is associated with a coronal brightening observed in soft x ray over the delta spot. In contrast, the larger unipolar spot shows no detectable x-ray signature in the corona above it. Courtesy of B.Lites, High Altitude Observatory, University Corporation for Atmospheric Research. Reprinted by permission from Reviews of Geophysics 38(2000):1–36; copyright by the American Geophysical Union.

THE CONCEPT BEHIND THE SOLAR MAGNETISM INITIATIVE

An understanding of the Sun’s entire magnetic field requires an integrated program of observations and the incorporation of diverse datasets into a common database for community use. This effort has to be closely coupled to theoretical modeling that uses existing models and develops new ones. Since the domains of the Sun are physically linked by the solar magnetic field, an understanding of the Sun and its variability will require that all domains be considered together in a consistent

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

physical model. The solar physics community has defined an initiative to do this: the solar magnetic initiative (SMI), which is described in the section on theory and data mining.

GLOBAL SOLAR DATABASES

Starting with the Japanese Yohkoh mission, launched in 1991, solar physicists worldwide adopted the concepts of distributed software development and integrated ground- and space-based data access and analysis. Concurrent software development at the Solar Data Analysis Center (SDAC) at NASA led to the evolution of the solar software tree. Separate master sites exist for the Yohkoh, SOHO, and TRACE branches. Via a Web interface, users can configure an installation package, download, and install it. The archive consists of data from the 12 SOHO experiments, synoptic data from 14 ground-based instruments (optical and radio), and synoptic data from Yohkoh and TRACE.

OPERATIONAL FORECASTING

Variations in the space environment near Earth that adversely affect mankind and technological systems are driven by variations in solar output. The National Oceanic and Atmospheric Administration’s (NOAA) Space Environment Center, the nation’s provider of space weather services, uses a variety of means to predict solar activity on timescales as short as a day to as long as the solar cycle. If the global generation and eruption of solar magnetic flux were better understood and modeled, as suggested in the recommendations of this study, earlier watches, accurate warnings, and long-term activity profiles could be issued. Basic solar research will benefit from the feedback from operational forecasting, much as basic weather research did.

INTERNATIONAL COOPERATION

European Space Agency (ESA)-NASA cooperation on the SOHO spacecraft and Japanese-U.S.-U.K. cooperation on the Yohkoh spacecraft have surpassed even the most optimistic expectations in the richness of their scientific returns. Similarly, ground-based networks around the world allowing for near-continuous observations have had great success.

Two telescopes are under construction on the Canary Islands: the Swedish 1-m telescope and the German 1.5-m GREGOR telescope.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

Adaptive optics (AO) has been demonstrated successfully on the Dunn solar telescope at Sacramento Peak and on the 50-cm Swedish telescope at La Palma (Figure 5.3). A realistic roadmap for using AO in a 4-m-class AST would involve using the new telescopes as stepping-stones in a collaborative effort.

The panel met with S.Solanki from the Eidgenössische Technische Hochschule in Zürich, now the director of the Max-Planck-Institut in Lindau; with T.Kosugi from the Japanese Space Research Agency (ISAS), head of the Solar-B project; and with O.von der Lühe, director of the Kiepenheuer-Institut für Sonnenphysik in Freiburg, to assess prospects for international cooperation. From the presentations of these

FIGURE 5.3 Snapshots of observations of solar granulation at the Dunn solar telescope without and with adaptive optics. Courtesy of T.Rimmele, National Solar Observatory.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

experts and the discussions with them and other international contacts, there is a good basis for international collaboration beyond the projects already mentioned. In particular, there is strong interest in substantial participation in the AST.

EXISTING PROGRAMS

GROUND-BASED OBSERVATIONAL EFFORTS

NATIONAL SOLAR OBSERVATORY (NSO)
  • Evans Facility—40-cm coronagraph at Sacramento Peak;

  • Dunn solar telescope—0.76-m vacuum telescope at Sacramento Peak with a prototype AO and the High-Altitude Observatory (HAO)/ NSO Advanced Stokes Polarimeter;

  • Kitt Peak vacuum telescope—synoptic instrument for full-disk solar magnetograms and He I 1083-nm spectroheliograms;

  • Kitt Peak McMath-Pierce telescope—1.5-m open telescope for IR and optical observations;

  • Synoptic Optical Long-term Investigations of the Sun (SOLIS) — currently under construction. The three SOLIS instruments are (1) a full-disk vector spectromagnetograph, (2) a full-disk imager for high-fidelity spectral images of the solar disk, and (3) a solar spectrometer for measurements of line profiles of the Sun as a star. SOLIS is expected to become operational in 2001. Two additional vector spectromagnetographs should be built and installed at much different longitudes to obtain nearly continuous time coverage (see below at “Extension of SOLIS to a Network”); and

  • Global Oscillation Network Group (GONG) —worldwide network of six seismology instruments. GONG has recently been upgraded to higher spatial resolution. It will continue to operate over a full solar cycle. There will be increased costs ($750,000 per year, beginning in FY2001) associated with operation and data analysis of the enhanced GONG network.

HIGH ALTITUDE OBSERVATORY (HAO)
  • The Advanced Coronal Observing System—synoptic instrument set to study coronal dynamics;

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×
  • The Low-degree/Experiment for Coordinated Helioseismic Observations (LOWL/ECHO) Oscillations Experiment—operated on Hawaii and on Tenerife (jointly with the Institute de Astrofísica de Canarias); and

  • Precision Solar Photometric Telescope (PSPT) —one of a network of three such telescopes (all built by NSO) to measure solar radiative variability as part of the NSF RISE program.

UNIVERSITY OBSERVATORIES

University-based solar observatories in the United States are critical for the training of the next generation of solar experimental scientists:

  • Big Bear Solar Observatory—65-cm telescope operated by the New Jersey Institute of Technology (NJIT);

  • Mees Solar Observatory—operated by the University of Hawaii on Haleakala;

  • San Fernando Observatory—61-cm vacuum telescope operated by California State University at Northridge to study solar irradiance variability;

  • Mt. Wilson—the 60-ft tower telescope operated by the University of Southern California as part of a worldwide helioseismology network. The 150-ft tower telescope, operated by the University of California at Los Angeles, investigates long-term changes of solar magnetic activity and large-scale flow systems;

  • Wilcox Solar Observatory—operated by Stanford University. The observatory began daily observations of the Sun’s global magnetic field in May 1975; and

  • Owens Valley Radio Observatory (OVRO) —array for imaging and spectroscopy operated by NJIT.

Additional facilities are run by NASA in support of space missions and by the Air Force for monitoring space weather events (SOON and ISOON).

INTERNATIONAL GROUND-BASED OBSERVATORIES

Other countries have made significant investments and have seen significant successes with new instruments. These efforts complement U.S. facilities globally, providing good coverage for monitoring dynamical events on the Sun and sometimes coordinating their measurements with

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

space-based observatories. Large facilities around the globe include the following:

  • Swedish vacuum solar telescope on La Palma, Spain,

  • Dutch open telescope on La Palma, Spain,

  • German vacuum tower telescope on Tenerife, Spain,

  • German vacuum Gregory telescope on Tenerife, Spain,

  • Franco-Italian THEMIS telescope on Tenerife, Spain,

  • Solar telescope of the Indian Institute of Astrophysics,

  • Huairou solar station in China,

  • Hida observatory in Japan, and

  • Nobeyama 17- and 34-GHz radio telescope in Japan.

SPACE-BASED OBSERVATIONAL EFFORTS

The last decade saw unprecedented successes in solar observations from space-based observatories. It is imperative that now-active missions be sustained as long as possible to cover the solar cycle. The guest investigator programs under which the United States participated in the Japanese-U.S. Yohkoh mission, the ESA-NASA-SOHO mission (see Figure 5.4), and the TRACE mission (see Figure 5.5) have led to vigorous data analysis efforts at universities and other U.S. research institutions. Sustaining and expanding a support program for data analysis is essential to fully exploit these missions. Four additional missions (HESSI, STEREO, Solar-B, and Solar Probe) are in preparation. The panel considers these missions as approved and/or on their way and therefore does not rank them in this report; however, it draws attention to their significant scientific value. The scientific return of all these missions would be increased significantly if the participation of guest investigators could be increased.

MISSIONS IN FLIGHT
Yohkoh

Yohkoh is a Japanese mission in cooperation with the United States and United Kingdom to observe high-energy radiation in the solar atmosphere. Launched in the autumn of 1991, Yohkoh has provided continuous coverage of solar coronal activity throughout nearly a complete solar cycle. As of the time of this writing, the data have re-

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

FIGURE 5.4 Solar rotation and polar flows of the Sun as deduced from measurements by the SOHO/MDI instrument. The left side of the image represents the relative rotation speed of various areas on the Sun. Red-yellow-orange is faster and blue is slower than average. The light-orange bands extend down approximately 20,000 km into the Sun. The cutaway reveals rotation speed inside the Sun. The large red band is a massive fast flow beneath the solar equator. A much more subtle stream can be seen in the cutaway at the poles as the light-blue areas embedded in the slower moving dark-blue regions. The blue lines in the cutaway represent the surface flow (10 to 20 m/s) from the equator to the poles, which extends to a depth of at least 26,000 km. The return flow at the bottom of the convection zone is from a simple model and has not been observed yet. Courtesy of the SOHO/MDI consortium.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

FIGURE 5.5 Sample picture of coronal loops from TRACE observed in Fe IX/X at 171 Å. Courtesy of NASA and the Stanford-Lockheed Institute for Space Research.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

suited in approximately 500 refereed papers, 38 master’s theses, and 34 doctoral theses. Yohkoh is expected to continue operations until 2002– 2003.

SOHO

The Solar and Heliospheric Observatory (SOHO), launched in December 1995, is an international cooperation between ESA and NASA. Located at the L1, SOHO has enough fuel to fully cover a solar cycle. The SOHO mission has three principal goals—to gain an understanding of the mechanisms responsible for the heating of the Sun’s outer atmosphere; to determine where the solar wind originates and how it is accelerated; and to measure the properties of the solar interior and flows into it. By flying at L1, which is ≈ 1 percent of the distance to the Sun on the Sun-Earth line, SOHO is ideally situated to continuously monitor the Sun, the heliosphere, and the solar wind particles streaming toward Earth. The SOHO instruments function together as a coordinated system.

The helioseismology instruments on SOHO measure the surface magnetic fields, the surface flows, and flows and plumes below the surface. The coronal instruments provide both high-spatial-resolution maps of the locations of the energy releases and spectral diagnostics to determine the mechanisms of the release processes. The all-sky Lymanalpha imager shows the extent of the wind in the heliosphere, and the particle and field detectors measure the energy and constitution of the particles accelerated toward Earth.

Missions have often been planned as coordinated systems. In this case particularly, a unique combination of an excellent instrument selection; pre-mission coordination of data formats, analysis software, and instrument operational methods; an agreement to share a basic data set in near real time; a joint central experimental operations center; topical workshops; a regular schedule of joint planning sessions; and daily coordination of these plans have made SOHO function as a science system rather than a collection of instruments.

TRACE

The Transition Region and Coronal Explorer (TRACE) telescope carries a Cassegrain design with a 30-cm aperture and a field of view of 8.5 arcmin×8.5 arcmin. The TRACE instrument employs heritage from

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

SOHO MDI flight spares, making it much more capable than one would expect from a NASA Small Explorer (SMEX) mission. TRACE is in a polar orbit, allowing for continuous uninterrupted solar observing for approximately 9 months each year. TRACE data and analysis software are freely available to the whole community. The TRACE spacecraft continues to operate nominally; the estimated orbital lifetime is about 5 years.

MISSIONS UNDER CONSTRUCTION
HESSI

The High Energy Solar Spectroscopic Imager (HESSI) is slated for launch in March 2001. The combination of arcsec imaging and spectroscopy will allow HESSI to study impulsive energy release, particle acceleration, and particle and energy transport in solar bursts. Bursts with sufficient count-rate can be imaged in as short as 10 ms, although the best imaging will be obtained in 2 s, set by the spacecraft rotation rate. HESSI will locate the energy release site of flares, trace the propagation of particles from the release site, locate and determine the role of secondary particle acceleration, determine the composition of accelerated ions, and examine the role of long-term storage and acceleration of ions in flares.

HESSI mission plans include direct support of key ground-based observations, needed to place the high-energy processes into the overall context with other processes not observable by HESSI. The HESSI data are to be completely open for unrestricted use, and both data and analysis software will be disseminated through the HESSI European Data Center.

Solar-B

Solar-B is a Japan-U.S.-U.K. mission led by ISAS. The cost to NASA is about $65 million, a quarter to a third of the total mission cost. NASA’s contribution includes the focal plane package, including the vector-spectromagnetograph for Solar-B’s 50-cm optical telescope as well as parts of the x-ray telescope and the EUV imaging spectrometer. Solar-B is planned to launch in 2004 and to operate for at least 3 years.

Solar-B has a coordinated set of optical, EUV, and x-ray instruments. Together, these instruments allow the interaction between the Sun’s magnetic field and its heated outer atmosphere to be investigated as a

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

system, by observing the direct response of the chromosphere and corona to changes in the photospheric magnetic field. Solar-B will, with the perfect seeing of space, provide quantitative measurement of the full vector magnetic field in the photosphere on small enough scales (down to 0.2 arcsec) to isolate elemental flux tubes with a sensitivity to the transverse component of the field vector of about 100 G.

STEREO

The Solar-Terrestrial Relations Observatory (STEREO) is a Solar-Terrestrial Probe mission. STEREO will consist of two SMEX-size spacecraft carrying identical scientific payloads. The spacecraft will be launched into orbits near 1 AU that will allow increasing angular separation between the two spacecraft, with one trailing Earth and one leading.

The STEREO payloads will include both in situ and radio instruments for probing the interplanetary medium and a set of solar imaging instruments to characterize the structure and development of CMEs in the plane of the ecliptic. In addition, when the separation of the two spacecraft is still relatively small, the stereoscopic view from the two sets of imagers will allow new insight into the three-dimensional structure of active region loops. The scientific return from STEREO would be significantly enhanced by the presence of similar imaging and in situ instrumentation along the Sun-Earth line.

Solar Probe

The Solar Probe, evaluated in A Science Strategy for Space Physics,4 will fly from pole to pole through the solar atmosphere, as close as 3 solar radii above the surface at perihelion, and will perform the first close-up exploration of the Sun. Scheduled for launch in February 2007, Solar Probe will travel along a polar trajectory to the Sun, where it will arrive in 2010. The second flyby will be near the solar minimum in 2015. Embedded in the acceleration region of the solar wind, Solar Probe will address the basic questions surrounding the origin of the fast and slow solar wind. By flying over the poles, it will sample the slow and fast solar

4  

Committee on Solar and Space Physics, National Research Council. 1995. A Science Strategy for Space Physics (Washington, D.C.: National Academy Press).

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

wind directly where their acceleration takes place, and it will image the solar atmosphere with better spatial resolution than is currently possible. It will also determine for the first time the solar surface properties of the polar regions of the Sun. The payload will consist of miniaturized imaging and in situ instruments. Together they will provide the first three-dimensional view of the corona, high-resolution spatial and temporal observations of the magnetic fields, helioseismic measurements of the solar polar regions, and local sampling of plasmas and fields at all latitudes.

NEW INITIATIVES

Figure 5.6 illustrates the spatial resolution and wavelength coverage of major instruments on the ground and in space. The new ones will be ranked separately below.

FIGURE 5.6 Schematic overview of the coverage in wavelength and spatial resolution provided by some of the observational facilities discussed in this report. Note that some of the overlap is necessary because of the different time resolution of the various instruments.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

FIGURE 5.7 Schematic overview of a global approach to understanding the Sun and the coverage of the various physical regimes by existing and future observational facilities.

FROM THE GROUND

The priority of the projects below, as ranked by the panel, is in agreement with the conclusions formulated in the Parker report. The third project, a small one, was part of the original proposal for SMI (see Figure 5.7). It has since been separated out as a stand-alone astronomical project. Several groups have brought other plans for new instrumentation to the attention of the committee (without entering them formally into the ranking competition). These projects are summarized under “Other Projects—Not Ranked.”

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×
THE ADVANCED SOLAR TELESCOPE

The Advanced Solar Telescope (AST) will enable the observation of plasma processes with unprecedented resolution in space and time and will provide a unique opportunity to probe cosmic magnetic fields and test theories of their generation, structure, and dynamics. Progress in these issues requires a solar telescope with (1) an angular resolution of better than 0.1 arcsec to resolve the pressure scale height and the photon mean free path, (2) a high photon flux at high spatial resolution for precise magnetic and velocity field measurements, and (3) access to a broad set of diagnostics for a wavelength range from 0.3 to 35 µm. No current or planned ground-based or space-based solar telescope meets these requirements. Recent major advances in technology, such as the successful development of solar adaptive optics; the construction of the Dutch open-air solar telescope on La Palma, which produces superb images without a vacuum sustained in the light path of the telescope; and the development of large-format infrared detectors are making it possible to realize the AST within the decade. The AST will replace NSO facilities at Sacramento Peak and Kitt Peak.

Solar physics has advanced to a point where existing solar telescopes are no longer sufficient to conduct critical observational tests of models for the underlying physical mechanisms. For this reason, two European telescopes on the Canary Islands are being replaced by telescopes with larger apertures. The AST is proposed as a joint project of the United States and its international partners, to be centered at NSO. It will complement other solar facilities in space and on the ground. A major design driver for the AST is the capability to perform precise and accurate measurements of solar magnetic fields over a large wavelength range. The AST will be sited at a location that offers superb seeing and clear weather for sustained periods of time.

The Origin of the Sun’s Magnetic Cycle

The 11-year sunspot cycle and the 22-year magnetic cycle are still a mystery. Dynamo models assuming a large-scale dynamo at the base of the convection zone were promoted for many years, but it was finally realized that they are not self-consistent. Recent observations and modeling indicate that there may be two dynamos: a high-field-strength dynamo at the base of the convection zne and a weak-field, turbulent dynamo near the surface. Indeed, hints of a weak magnetic field compo-

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

nent that covers the entire Sun have been discovered in several recent observations. This global phenomenon may be of crucial importance for the magnetic cycle and its variability. The AST is the ideal tool for quantitative measurements of these weak, turbulent fields.

The Solar-Stellar Connection

The observation of the solar cycle at high precision with modern instrumentation is only a few decades old. Thus, our knowledge of the full range of solar variability that may occur is extremely limited. The observation of solar-type stars can be productively exploited to overcome the temporal confines of the solar database, thereby revealing the potential range and nature of solar variability over timescales that are now inaccessible to the solar database of only a few decades. In particular, a program of high-precision spectroscopic and photometric observations of the numerous solar-type stars in the solar-age and solar-metallicity cluster M67 can reveal all the potential modes of variability in both magnetic activity and radiative output in Sun-like stars and, by implication, in the Sun itself. The study of stars similar to our Sun is a practical approach to understanding and forecasting the long-term behavior of the Sun. The AST will be used at night as a dedicated 4-m-class facility to observe the faint solar-type stars in distant clusters such as M67 at high precision. Studying solar-type stars with ages, chemical compositions, or rotation rates different from those of the Sun will tell us how these stellar parameters relate to the physical principles responsible for them.

Stellar Chromospheres

Measurements of CO around 4.7 µm show extremely cool clouds that appear to fill much of the volume in the low chromosphere. Only a small fraction of the volume is filled with hot gas, as expected from static models that exhibit a homogeneous temperature rise in the chromosphere. The observed spectra appear to be explained only by dynamic models of the solar atmosphere. Numerical simulations indicate that the temperature structures occur on spatial scales that cannot be resolved with current solar telescopes. A test of these models therefore requires a large telescope that provides access to the thermal infrared. Stellar magnetic fields can be measured most accurately in the infrared. Thus

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

such observations should be a major component of the nighttime program for the AST.

The Sources of Global p-Mode Oscillations

A process that occurs on very small scales (<100 km) causes global solar oscillations: narrow, supersonic downdrafts in intergranular lanes continuously produce “acoustic noise” that powers the oscillations. Numerical simulations provide detailed predictions on how convective energy is converted into acoustic energy that powers the p-modes. Acoustic events may also contribute to the heating of the lower chromosphere through the formation of acoustic shock waves. While the resolution of current facilities is sufficient to verify the existence of acoustic events, it is insufficient to determine in any detail the underlying physical mechanisms.

Structure of Sunspots

Strong photospheric magnetic fields are concentrated in flux rope units in which local fields are strong enough to control the local environment but whose collective behavior is controlled by the photospheric convection patterns. In sunspots the total magnetic field is large enough to dominate the hydrodynamic behavior of the solar atmosphere. To test numerical simulations of sun- and star-spots in general, 0.05- to 0.1-arcsec-resolution vector polarimetry with low-scattering optics is required.

Confronting Models with Observations

Figure 5.8 shows magnetic fields in dynamic pressure equipartition (~400 G) with convective motions from numerical simulations. Magnetic field elements are small scale (<70 km), of mixed polarity, intermittent, and mostly concentrated in the narrow intergranular lanes where strong downflows are present. To understand the importance of these fields in the dynamo process astronomers need to observe how and at what rate these weak fields form and to determine the spectrum of field strength. Polarimetric measurements show that the “quiet” Sun appears to be covered with weak magnetic fields. IR observations of sufficient spatial and temporal resolution would allow observing the formation of kilogauss flux tubes, the building blocks of solar and stellar activity, by

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

FIGURE 5.8 Snapshot of the appearance of temperature and magnetic field from a three-dimensional MHD simulation. Color plot of temperature fluctuations (top; bright indicates hot material) and gray-scale plots of the vertical component of the magnetic field (middle panel). The middle panel corresponds to a layer near the upper boundary; the bottom panel corresponds to a deeper layer Courtesy of F.Cattaneo, University of Chicago.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

convective collapse. No direct observational evidence for this process exists to date.

AST Baseline Parameters and Their Science Drivers

Basic design guidelines for the AST and their link to science requirements are given below. Conceptual design efforts are preliminary at this time.

  • Spatial resolution. A 4-m diffraction limit using AO is needed to (1) resolve the photon mean free path and the pressure scale height in the photosphere and (2) probe the IR signature of cool clouds in the chromosphere and test models of their radiative cooling.

  • Sensitivity. A 4-m photon collection aperture is needed to (1) study the ubiquitous weak magnetic field and test models of a turbulent dynamo in the upper convection zone and (2) measure waves in magnetic flux tubes and test models of chromospheric and coronal heating.

  • Field of view. A field of view of 5 arcmin is needed to (1) test models of the eruptions of flux that form active regions from the strong-field dynamo in the lower boundary layer of the convection zone, (2) test models of large-scale coherent processes that lead to flares and CMEs, and (3) observe large-scale oscillations in prominence and compare with models.

  • Wavelength range. A wavelength range of 0.3 to 35 µm is needed to (1) provide access to a broad range of diagnostics from the photosphere to the corona and (2) observe the widest variety of diagnostic spectral lines to constrain atmospheric properties. The Mg lines at 10 mm allow for magnetic field measurements with large Zeeman splitting, and they form in local thermodynamic equilibrium as opposed to the UV and visible lines formed in the same region, which do not. The range from 20 to 30 µm is unknown territory. The telescope technology to go beyond 20 µm does not change.

  • Polarization accuracy. A polarization accuracy of 10−4 intensity is needed to (1) precisely measure the vector magnetic field and test models of wave generation in magnetic flux tubes by the surrounding granulation and (2) use the Hanle effect to test models of extremely weak magnetic fields in the photosphere, chromosphere, and prominences.

  • Scattered light. Large sunspots with field strengths in excess of 3 kG often have residual intensities of less than 10 percent of the mean

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

TABLE 5.1 AST Investment Strategy (thousand dollars)

 

Year

 

Cost Increment

1

2

3

4

5

6

7

8

9

Total

Adaptive optics

1,500

1,500

1,500

500

 

5,000

Site tests

550

150

150

150

1,000

Conceptual design

500

1,100

1,500

 

3,100

IR technology

100

300

300

300

1,000

Focal-plane instrument packages

 

200

500

1,000

3,000

3,000

2,000

2,000

11,700

AST construction

 

14,000

13,000

12,000

3,000

 

42,000

Total

2,650

3,050

3,650

1,450

15,000

16,000

15,000

5,000

2,000

63,800

granular intensities, even in the red. To isolate effects intrinsic to the umbra, the umbral signal should be greater by a factor of 10 than the signal from the stray and scattered light. The scattered light from the instrumentation must then be on the order of 1 percent or less.

  • Location. The best possible site in terms of seeing and sunshine hours is needed to maximize telescope performance and minimize the cost of AO.

  • Costs. Table 5.1 gives the incremental costs of designing and building the AST. About one quarter of the cost for the construction will go into the focal plane instrumentation packages (FPIP). Costs include a 15 percent contingency. Both Germany and Japan have indicated interest in the project, and plans are being made to share the cost of developing of the AO system, which is a critical technology for the AST. It is planned to seek sufficient international and domestic partnerships to bring the cost to the United States to approximately one half of the total cost, or about $32 million.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×
Development Plan

The AST will represent the first major U.S. investment in ground-based solar physics in over 40 years. As such, it must involve a substantial fraction of the solar community. Much of the instrumentation development, site testing, and design will be done by universities and international partners. Once completed, the international operating organization must also run the AST facility and ensure continued instrument development. Figure 5.9 gives a time line for the various ingredients and phases of the AST development. Since the AST should involve a wide spectrum of the solar community and since it will require international partners, an AST board consisting of representatives of the national

FIGURE 5.9 Roadmap for the development of the Advanced Solar Telescope. Courtesy of S.Keil, National Solar Observatory.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

centers, the university community, other interested government agencies, and non-U.S. partners should be formed. This board will provide overall project guidance and oversee project execution through advisory subcommittees. It will also oversee the financing of the project. Funds will be provided by the NSF, by other interested agencies, and by non-U.S. partners. The board will organize the distribution of these funds to the various AST functions, AST design, site testing, construction, instrument development, and operation. These functions will be distributed among the partners, with the NSO taking overall responsibility for developing the telescope.

THE FREQUENCY-AGILE SOLAR RADIOTELESCOPE

Solar radio instrumentation worldwide lags far behind instrumentation for other wavelength regimes. The Frequency-Agile Solar Radiotelescope (FASR) would represent a major advance over existing facilities, with capabilities specifically for observing the Sun. It is designed to produce high-quality images of solar processes over a core frequency range of 0.3 to 30 GHz, with sufficient angular, spectral, and temporal resolution to fully exploit radio emission as a diagnostic of the wide variety of physical processes that occur on the Sun. It improves on the 84-antenna Nobeyama Radioheliograph by working at a multitude of frequencies rather than just two, and its spatial resolution is better by a factor of 10.

The science goals of the FASR include the following: (1) transient energetic phenomena: energy release, plasma heating and electron acceleration, electron transport, and formation and destabilization of large-scale structures, (2) the nature and evolution of coronal magnetic fields: precise measurement of coronal magnetic fields, temporal evolution of coronal magnetic fields, the role of coronal currents, and the storage and release of magnetic energy, and (3) the solar atmosphere: coronal heating, structure of the quiet solar atmosphere, origin of the solar wind, and the formation and structure of filaments.

The approach to attaining these goals is through the measurement of broadband, spatially resolved microwave, and decimeter-wave spectra. Such spectra are the key to unraveling the multiparameter dependence inherent in radio emission, which would make possible the precise measurement of physical quantities such as magnetic field strength and direction, temperature, electron density, and the shape of the electron energy distribution. The FASR specifications, listed in Table 5.2, give the

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

TABLE 5.2 Specifications for the Frequency-Agile Solar Radio Telescope (FASR)

Parameter

Specification

Frequency range

0.3–30 GHz

Frequency resolution

 

 

~3%, 3–30 GHz

~1%, 0.3–3 GHz

Time resolution

 

 

<1 s, 3–30 GHz

<0.1 s, 0.3–3 GHz

Antenna size

2–5 m

Number of antennas

~100

Number of baselines

~5000

Polarization

Dual

Number of IF pairs

4–8

Angular resolution

20″ (1 GHz) to 0.5″ (30 GHz)

Field of view

Full Sun

requisite spatial and spectral resolution, image quality, and temporal resolution to address the following topics:

  • Spatial, temporal, and spectral characteristics of the site of energy release;

  • Measurement of magnetic field strength at coronal heights in flares and active regions;

  • Determination of the electron energy distribution;

  • Detection of CMEs, both off the limb and on the solar disk;

  • Elucidation of causes of coronal heating (nanoflares or destabilization of coronal currents); and

  • Synoptic measurement of both thermal and nonthermal activity.

The FASR design can be entirely based on existing technology, although innovative concepts should be explored to cut costs and extend capabilities. Many aspects of the system (antenna size, broadband frequency coverage, signal transmission, and perhaps even digital signal processing and correlation) overlap with the One Hectare Telescope (1HT), now under way at the University of California at Berkeley and the Search for Extraterrestrial Intelligence (SETI) Institute.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×
EXTENSION OF SOLIS TO A NETWORK

A single SOLIS facility is severely limited by the day-night cycle because many important phenomena connected with magnetic fields occur on timescales of a day. To study and understand the magnetic origin of solar variability, two additional vector spectromagnetographs should be built and installed at widely separated longitudes to obtain nearly continuous time coverage. Precision spectropolarimetry with a SOLIS network affords the possibility of monitoring the evolution of solar surface vector magnetic fields as toroidal flux rope systems, created by dynamo action at the base of the solar convection zone, rise through the solar surface. The full history of the birth, evolution, and decay of solar magnetic flux during the disk passage of active regions (10 to 14 days) is now recognized to be essential to an understanding of the physical processes underpinning flux emergence and activity. Fortunately, most active regions pass through their critical early emergence phase within a few days to a week, so that one can be reasonably optimistic that the passage of emerging regions across the visible solar hemisphere will illuminate much of the crucial physics. A SOLIS network will provide continuous coverage of such an event. Operation of such a network over the solar cycle will provide critical insight into the magnetic origins of solar activity and variability. Expansion of SOLIS from one station to a three-station network will cost about $4.8 million, with an additional $200,000 needed annually for operation of the two additional stations and the associated increase in data volume.

OTHER PROJECTS—NOT RANKED
Large-Aperture, Ground-Based Coronagraph

There is evidence from a recent eclipse expedition that a Si I line is present prominently in solar coronal structures. This spectral line could be the best tool for direct measurements of coronal magnetic fields. To gather enough photons to quantitatively exploit this line and other infrared lines in the solar corona, much bigger coronagraphs than the existing ones are needed. A group at the University of Hawaii is pursuing this with a proof-of-concept telescope of about 50-cm aperture. A coronagraph of several meters aperture is targeted by this group in the context of replacing the NASA infrared facility on Mauna Kea. Such a telescope with an aperture of 6.5 m would have tremendous value for

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

nighttime observations in planetary astrophysics as well as for observations of faint objects around bright sources.

LOFAR

A project for a long-wavelength array for general astronomical use is under way. It is a merger of two previous concepts—the Naval Research Laboratory’s (NRL’s) Long-Wavelength Array and the Dutch Low-Frequency Array (LOFAR). This array would have significant impact on space weather research. It will operate in the frequency range below 150 MHz and will obtain high-resolution (1- to 10-arcsec) images with excellent sensitivity in the region of the corona approximately 0.5 to 2 solar radii above the surface. In this region, the imaging of large-scale thermal and nonthermal coronal structure will be possible, including likely signatures of CMEs, shock waves (type II bursts), noise storms (type IV), and electron beams (type III), all of which directly affect the space weather environment. The solar science to be addressed by the array needs to be more fully investigated, and full consideration for solar science is urged in the design of the instrument, to ensure fast imaging over the entire spectrum.

SRBL

The Solar Radio Burst Locator (SRBL) is a project in the prototype stage and slated to become an operational four- or five-station system operated by the U.S. Air Force. In addition to the operational use of the system for locating bursts, it will provide broadband spectral coverage (in the 2- to 18-GHz range) of solar bursts. The panel urges that provision be made for proper calibration and wide dissemination of the spectral data to the scientific community.

Ballooning

Recently, a group led by Johns Hopkins University flew a balloon experiment in Antarctica with the goal of long-duration observations of solar processes. A second flight has just been completed. NASA developed the concept of a long-duration balloon facility offering the option of solar observations largely undisturbed from seeing in the Earth’s atmosphere and with fairly long observation cycles.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

IN SPACE

The Solar Dynamics Observatory is the recommended mission for the next decade. All the other missions are either part of the Solar Terrestrial Probe line (category “small” for NASA) or will be evaluated by the Space Studies Board of the NRC.

SOLAR DYNAMICS OBSERVATORY

The Solar Dynamics Observatory (SDO) is a mission to explore the Sun from the subsurface layers of the convection zone to the outer atmosphere. Breakthroughs in techniques for analysis of solar oscillations using MDI (see Figure 5.10) data have opened a new frontier. Acoustic imaging reveals that active regions generate a complex pattern of wave absorption. However, existing instruments cannot provide the combination of temporal and spatial resolution and coverage necessary to exploit these techniques to study the eruption and evolution of active-region magnetic structures. SDO will explore the complete life cycle of solar active regions using 1-arcsec-resolution, full-disk Doppler velocity and vector-magnetic-field observations.

Because of its location at L1, SOHO was limited to a data rate of 40 kbps for most observations (see Table 5.3). For 8 hours a day, the MDI data rate was increased to 640 kbps. Also, for a 2-month period every year, the MDI rate was 640 kbps. The SOHO imagers used a total of five 10242 charge-coupled devices (CCD) detectors. SDO operates 24 hours a day in a geostationary orbit at a data rate of about 160 Mbps, which is greater by a factor of 250 than the 8-h data rate of SOHO and greater by a factor of 4000 than the normal operational mode (see Table 5.4). The SDO uses twelve 40962 CCD detectors, which allow simultaneous observation in all the EUV channels to separate temporal changes from evolutionary ones (SOHO requires sequential operations). In its normal mode, SDO can make 40962 images 250 times as fast as SOHO makes 10242 images.

SOHO/EIT and TRACE observations have demonstrated a requirement for high temporal and spatial resolution over the full disk to understand the interconnected dynamic structures of the transition region and the low corona and to conduct coronal helioseismology. EUV spectroscopic imaging with 1-arcsec resolution will determine the atmospheric magnetic connectivity of active regions. SDO should be launched early in the rising phase of solar cycle 24 to observe relatively isolated active

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

FIGURE 5.10 Example of tomography of a sunspot’s subsurface structure in sound speed as inferred from a time-distance analysis of running waves. Courtesy of the SOHO/MDI consortium.

regions and follow the rapid increase in complexity as the solar activity maximum is approached.

SDO carries the Atmospheric Imaging Assembly, an array of telescopes that image the Sun in the temperature range 4000 to 9,000,000 K; an advanced Doppler and vector magnetogram instrument package (Helioseismic and Magnetogram Imager, or HMI) that images subsurface structures, detects spots on the opposite side of the Sun, and produces vector magnetograms; and a pair of coronagraphs (the Coronal Imaging Assembly, or CIA) that make precise measurements of the Sun’s white-light corona from 1.05 to 18 solar radii. In addition, SDO carries an array

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

of precision radiometers that measure the solar irradiance from the far UV to the IR.

Measurement Strategy

All imaging instruments will observe the full Sun continuously, and there will be one basic observing mode to simplify operations and data analysis. Continuous full-disk observations will provide the ability to follow the location of every active region as it transits the solar disk, whenever it emerges. Measurements that simultaneously achieve high spatial and temporal resolution and wide spatial and temporal range are the critical elements of SDO mission strategy.

Science Objectives

SDO should help to answer the following questions:

  • Why are there sunspots and solar active regions?

  • How do magnetic regions emerge, evolve, and decay?

  • How do the active-region fields interact with the small-scale fields?

  • Do local dynamo processes occur?

  • How does the large amount of magnetic energy created at small scales dissipate?

  • How do small- and large-scale coronal magnetic field reorganizations occur?

  • What are the surface and subsurface magnetic configurations that lead to CMEs and flares?

  • How important is the inverse cascade of small-scale flux emergence to large-scale flux expulsion?

  • To what extent are CMEs and flares predictable?

  • Does activity affect solar convection and irradiance?

  • How are the dynamics of the interior and the quiet and active solar corona linked?

SOHO/MDI has shown that we can make detailed maps of magnetic fields before they reach the surface by the technique of acoustic imaging. Then the surface magnetic fields and flow systems can be tracked using polarized spectral images. TRACE has shown that it is necessary to take data with a cadence of less than 10 s to follow the small- and large-scale

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

TABLE 5.3 SOHO Imagers

Instrument

Measurement Type

Field of View (arcmin)

Spatial Resolution (arcsec)

MDI

Dopplergram

34×34

30

(Full Sun)

Dopplergram

34×34

4

(HiRes)

Dopplergram

10.5×10.5

1.2

(Full Sun)

Line-of-sight magnetogram

34×34

4

(HiRes)

Line-of-sight magnetogram

10.5×10.5

1.2

EIT

Intensity

40×40

5

 

Intensity

40×40

5

Intensity

40×40

5

Intensity

40×40

5

LASCO

 

C1

Intensity

1.1–3 solar radii

11

C2

Intensity

1.5–6 solar radii

25

C3

Intensity

3.7–30 solar radii

110

NOTE: MDI is Michelson Doppler Imager, EIT is Extreme Imaging Telescope, LASCO is Large-Angle and Spectrometric Coronagraph Experiment, and C1, C2, and C3 are detectors on LASCO.

reconnections of magnetic fields in the corona. SOHO/EIT has shown that waves can propagate at least over a visible hemisphere.

Analysis of the high-cadence TRACE data has revealed waves in the corona excited by flares, CMEs, filament activation, and magnetic reconnection. The new science of coronal helioseismology has already revealed preliminary estimates of the magnetic field in the corona and has started a fundamental reevaluation of the basic magnetohydrodynamic wave damping processes. Like earthquakes, coronal explosions are episodic, and the measuring system must be in place before the event

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

Spectral Resolution

Temporal Cadence

Data Rate (kbps)

Time Coverage (h/d)

Detector

94 mÅ at 6768 Å

60s

5

24

One 10242

94 mÅ at 6768 Å

60s

160

8(24)

 

 

60s

 

90min

90min

171 Å

23.5 min

1

23.5

One 10242

195 Å

4.7 min

5.6

0.5

 

284 Å

 

304 Å

 

Three

10242

Broadband

25 min

 

Broadband

 

Broadband

in order to study it. Solar researchers expect that coronal helioseismology will teach them as much about the corona as traditional helioseismology has taught about the solar interior.

Technology Requirements

SDO will use large-format, low-power, fast-readout CCD detectors, and it will rely on fast spacecraft data compression hardware with large, low-power, fast, smart memories. The spacecraft will use artificial

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

TABLE 5.4 Solar Dynamics Observatory Imagers

Instrument

Measurement Type

Field of View (arcmin)

Spatial Resolution (arcsec)

HMI

Dopplergram

34×34

1

 

Vector magnetogram

34×34

1

AIA-EUV

Intensity

36×36

1.1

 

Intensity

36×36

1.1

Intensity

36×36

1.1

Intensity

36×36

1.1

Intensity

36×36

1.1

Intensity

36×36

1.1

Intensity

36×36

1.1

AIA-UV

Intensity

36×36

1.1

 

Intensity

36×36

1.1

Intensity

36×36

1.1

Intensity

36×36

1.1

Intensity

36×36

1.1

CIA-Inner

Intensity, polarization

33.6–72

2.2

CIA-Outer

Intensity, polarization

2–18 solar radii

17

NOTE: HMI is the Helioseismic and Magnetogram Imager, AIA is the Atmospheric Imaging Assembly, and CIA is the Coronal Imaging Assembly.

intelligence for self-operation and monitoring along these lines. Technology developments can increase the scientific return of the mission and lower its cost.

Education and Public Outreach

The SDO data set will be available in real time at the science data center for distribution over the Internet. The data will be of great use to scientists worldwide. In addition, national and international agencies concerned with space weather forecasting will have an invaluable asset—continuous, real-time solar data from the interior to the outer atmosphere. By reformatting the data and using the spacecraft as a repeater, as was done with GOES satellites, a video signal showing the

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

Spectral Band

Temporal Cadence

Data Rate (Mbps)

Time Coverage (h/d)

Detector

94 mÅ (6768)

45s

30

24

One 40962

 

5min

30

24

One 40962

171 Å

10s

10

24

One 40962

195 Å

10s

10

24

One 40962

284 Å

10s

10

24

One 40962

304 Å

10s

10

24

One 40962

211 Å

10s

10

24

One 40962

133 Å

10s

10

24

One 40962

304 Å

10s

10

24

One 40962

1900 Å

10s

10

24

One 40962

1700 Å

 

1600 Å

1550 Å

1216 Å

Broadband

30s

3

 

One 40962

Broadband

30s

3

One 40962

Sun in action can be made available to the entire world for use in schools, by forecasters, by scientists, by the general public, or by the media. It would not be surprising if every major science museum in the world had a high-definition TV showing “Live from the Sun.”

Costs

The costs for SDO are well understood. The instruments are straightforward enhancements of those on SOHO, TRACE, and Yohkoh, and accurate cost estimates exist. A geosynchronous orbit requires a Delta-class launch vehicle. By using a single ground station at the science data center, operating costs are minimized. The preliminary estimate through launch and the first 2 years of operation is $300 million.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×
SOLAR TERRESTRIAL PROBE MISSIONS
Particle Acceleration Solar Orbiter (PASO)

PASO will address the fundamental question of how the Sun accelerates particles to high energies in solar flares and CMEs. It is designed to use solar sailing to achieve a near-synchronous orbit at 0.16 to 0.2 AU, that is, an orbital period about equal to the solar rotation period. This would allow continuous observation of particle acceleration from active regions and CME-related solar features from their birth through their rise to a maximum and decay. PASO would provide hard x-ray/gamma-ray imaging of flares with 25 to 36 times the sensitivity and 5 or 6 times the linear spatial resolution of observations from 1 AU. Neutrons of energies below tens of MeV and down to ~1 MeV, which can provide direct evidence for acceleration of low-energy ions, would only be detectable by getting this close, since they decay in flight (e-folding decay time of ~1000 s). Getting this close is also the only way to obtain measurements of the energetic particles freshly accelerated by CME shocks before they have been significantly modified by scattering and energy changes. Finally, PASO will provide the first systematic exploration of the inner (<0.16 to 0.3 AU) heliosphere.

Reconnection and Microscale Probe (RAM)

The RAM is designed to investigate the structure and dynamics of the magnetized coronal plasma with continuous broadband solar observations from the L1 orbit. It aims to understand the microscale instabilities that lead to reconnection and, ultimately, to flares and CMEs. The probe will be equipped with an ultrahigh-resolution telescope imaging the Sun at 195 Å with a resolution of 0.02 arcsec. It will perform high-resolution spectroscopy (0.2 arcsec from 0.3 to 10 keV) and high-resolution EUV spectroscopy at 170 to 220 Å. The probe is projected to be a Solar-Terrestrial Probe (STP) mission. It is a follow-up mission on TRACE, SOHO-EIT, and Yohkoh, going for the specific problem of reconnection. It will have to be equipped with large-format cryogenic imaging detectors, which are under development.

THEORY AND DATA MINING: THE SOLAR MAGNETISM INITIATIVE

The SMI concept of a comprehensive research program involving

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

the various facilities and the solar community was introduced earlier. SMI would include a series of focus programs on particular aspects of the problem; these would be carried out by groups of scientists collaborating in extended workshops at a single location supported by appropriate computing, observing, and data analysis resources. Candidate topics include the solar dynamo and interior dynamics; magnetic flux transport through the convection zone; an observational description of emerging magnetic flux; the history of magnetic flux; the solar magnetic cycle at the solar surface; and the physics of coronal mass ejections, their causes, and heliospheric effects. It is expected that each focus program would take about 1 year, with intensive workshops every 6 months at which progress would be compared and coordinated. At the end of each focus program, results would be presented to the broader community in a workshop or at a scientific meeting.

The SMI would be overseen by a steering committee that could be modeled after the CEDAR steering committee. In addition to defining the focus programs described above, it would advise on scientific priorities throughout the life of the program, coordinate observing campaigns, and keep the community informed and involved though newsletters and presentations at scientific meetings and workshops.

Two- and three-dimensional simulations of MHD processes mimicking some of the processes on the Sun have been undertaken with great success by many groups in the United States and elsewhere. There are major numerical efforts at the following universities: University of Colorado at Boulder, Harvard University, Michigan State University, Stanford University (Lockheed), University of California at Berkeley, University of Alabama at Huntsville, University of Chicago, University of Rochester, and Yale University, as well as at the Bartol Institute, NASA Goddard Space Flight Center, National Center for Atmospheric Research/High Altitude Observatory, Naval Research Laboratory, and the San Diego Supercomputer Center/Science Applications International Corporation.

To take advantage of the rapidly increasing quantities of solar observational data (from, for example, the SOLIS instruments) and to expand the effort in numerical modeling, SMI will need about $2 million per year in funding for an expanded university grants program. Grants would be awarded following standard NSF procedures for directed programs, such as CEDAR and GEM.

To entrain new scientists with recent Ph.D.s into SMI, a program of at least two SMI postdoctoral fellowships should be established, to be

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

TABLE 5.5 Estimated Annual Investments for the Solar Magnetism Initiative

Initiative

Annual Investment ($)

University grants program and SMI postdocs

2,200,000

Focus programs, workshops, and coordination

90,000

Centralized scientific support and community service

500,000

hosted by any institution involved in SMI. The appointees would be chosen in a community-wide competition according to a process to be determined by NSF.

Some centralized support would also be needed to develop the community Stokes inversion program, which is essential for utilizing the new and greatly expanded vector magnetograph data expected from SOLIS, and to support observing campaigns, focused programs, and the SMI database, which provides both observational and modeling data (Table 5.5). There would also be some one-time costs for hardware for the SMI database, proposed to reside at the National Center for Atmospheric Research (NCAR)/HAO. It is expected that supercomputing requirements for SMI, particularly for numerical modeling and Stokes inversions of data, while not small, can be accommodated within current plans to upgrade the NCAR Scientific Computing Division’s supercomputing capability and by some of the NASA centers.

TECHNOLOGIES FOR THE FUTURE

ADAPTIVE OPTICS

The AST project will develop a visible adaptive optics (AO) system. A multiconjugate adaptive optics (MCAO) system based on this technology will achieve diffraction-limited resolution over fields of view (FOVs)

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

significantly larger than the isoplanatic patch size of, typically, a few arcseconds. The Sun is an ideal object for the development and application of MCAO since the multiple wavefront measurements as a function of the FOV required for MCAO can be performed using solar structure as the wavefront sensing target. The complexity involved in having to use multiple laser guide stars can be avoided. MCAO should therefore be developed for the Sun first. The AO development is proposed as a collaborative effort of NSO and NJIT/Big Bear Solar Observatory, the Center for Adaptive Optics, and international partners.

SOLAR-LITE

Solar-B will have 150-km resolution, 10-G sensitivity to the line-of-sight field component and 100-G sensitivity to the transverse component. Solar Probe will provide the first glimpse of the line-of-sight magnetic field with 10-G sensitivity at 50- to 25-km resolution. Solar-B will have the resolution to isolate separate elementary flux tubes (150 km in diameter); Solar Probe will have the resolution to look within a tube but will not measure the transverse component. Measurement of the transverse component is essential for determining the three-dimensional configuration of the field. Now is the time to begin defining and developing the science and technology for the next-generation, high-resolution solar mission. The development of a lightweight mirror larger than the 50-cm mirror of Solar-B, as begun in the Solar-Lite technology studies, will lead to less expensive instruments than those envisioned for OSL/SOT, which was planned for in the 1991 survey report but never built, for financial reasons.

HIGH-RESOLUTION VECTOR MAGNETOMETRY OF UV LINES

Measurements are needed of the three-dimensional vector of the magnetic field in lines formed above the photosphere, in the field-dominated, force-free domain of the solar atmosphere. This requirement is motivating the development of new filters and polarimeters for vector magnetography of UV lines formed in the chromosphere and low transition region.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

CONNECTION TO LABORATORY ASTROPHYSICS

ATOMIC/MOLECULAR/NUCLEAR PHYSICS

  • Identification of EUV line spectra. Of more than 800 observed lines, SOHO has revealed that 300 or so between 500 and 1600 Å are unidentified.

  • Accurate laboratory wavelengths. Laboratory measurements for Mg X and Ne VII lines found in the corona/transition region are of insufficient accuracy or missing altogether. The NIST spectrograph should follow up with measurements accurate to 1 part in 200,000.

  • Photoionization resonances. The OPACITY project data for He I show resonances close to the Fe IX/X and Fe XII lines emitting in the TRACE bandpasses, with energies uncertain to 1 eV. High-resolution measurements of the photoionization resonance structure of neutral or singly (doubly) charged ions of abundant elements are needed.

  • Collision cross-sections for particle impact. A new area of research recently opened up using the Hanle effect in Stokes spectra obtained inside the solar limb. Many details of the atomic physics involved in the collisional depolarization need to be measured to much greater accuracy than can presently be achieved in the laboratory.

  • Landé g factors. The Landé g factors of absorption lines of complex atoms (e.g., Fe I and Fe II) depend on the configuration mixing. As the infrared becomes accessible to high-resolution Stokes measurements, precise g factors for Fe I and Si I are needed.

  • Neutrinos. The LOWL instrument provided proof that the neutrino deficit as it is measured for the solar neutrino flux on Earth is not due to a deviation of the solar structure from the standard model. The key lies in the regime of particle physics. A continuation of measurements of neutrinos is, however, essential to determine finite mass and possible magnetic moment of the neutrinos.

PLASMA PHYSICS

Basic plasma physics and magnetohydrodynamic processes, which are thought to be central to solar physics, can be studied in the laboratory. The Magnetic Reconnection Experiment (MRX) device at Princeton has been used for a series of magnetic reconnection experiments. One of the main issues studied by MRX is the relationship between the

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

reconnection region, which is extremely small, and the global magnetohydrodynamic equilibrium. The results have been successfully interpreted as magnetic reconnection at the Sweet-Parker rate. The basic device is a modified plasma fusion reactor design of a type known as Spheromak. There have been other contributions from laboratory plasma physics in the areas of anomalous thermal conductivity and anomalous resistivity, relaxation of plasma to a force-free state, and dynamo activity. Such experiments have an important role in solar physics, providing a basis for theory and interpretation of observation.

POLICY AND EDUCATIONAL ASPECTS

The panel examined issues surrounding the standing of solar physics in the U.S. university community and the overall balance between the NSF grants program and the two NSF solar physics centers—NSO and HAO. It also considered how a major development project like the AST should be optimally organized within the United States as well as with international partners.

THE UNIVERSITY-BASED SOLAR PHYSICS COMMUNITY IN THE UNITED STATES

Driven by the successes of solar space missions and by helioseismology, there has been a rejuvenation of solar physics at U.S. universities, with two new departments having been established. There is a vigorous university research community in solar physics built mostly on research faculty positions rather than on regular faculty. However, several traditional chairs for solar physics at some major universities were not refilled as they became vacant. This development continues to be of concern in view of the growing importance to society of understanding the Sun in the context of space weather and climate change. It also contrasts starkly with the scientific opportunities and the apparent strong interest of the many excellent young researchers who work in the field supported by soft money. The reasons for this development lie partly in the shift in emphasis from solar (and stellar) physics in astronomy to solar physics in the geophysical context. Neither the funding agencies nor the universities have yet been able to address the challenge posed by the changing astrogeophysical framework for solar physics. The panel urges them to do so.

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

FUNDING ASPECTS

Appendix G of the Parker committee report documents the balance of funding between NSF grants to universities and funds spent on NSF-supported centers (NSO and HAO). In solar physics research, about 63 percent of NSF funding goes to grants and 37 percent to centers. This is very similar to the balance between NCAR (the largest NSF center) and grants in atmospheric science. The panel regards the balance of funds in solar support by NSF as healthy. The overall demographics in the solar community should be adequate to support and fully exploit the missions and programs planned for the next decade, except that more faculty positions are needed. If NASA provides funding for strong guest investigator programs and NSF provides funds sufficient to run and exploit new ground-based observing capabilities, there will be new incentives for universities to hire faculty.

THE NATIONAL SOLAR OBSERVATORY

The panel endorses the recommendation of the Astronomy and Astrophysics Survey Committee’s Panel on Education and Public Policy that the NSO should be separated from the nighttime parts of the National Optical Astronomy Observatories as soon as is reasonable. This would allow the best possible posture for NSO and the solar community to advocate and develop the AST, which should be (and is becoming) the primary future focus for NSO. NSO should then establish structures that ensure broad community participation in preparing and building the AST. The panel also recommends that postfocus instrumentation for the AST be developed in collaboration with the community, with instrument packages outsourced but developed under overall guidance from a central authority for AST. In addition, the panel sees many advantages to having international partners in the AST project, provided adequate control remains with the United States.

EDUCATION

For the broader educational outreach aspects the panel refers the reader to Chapters 4 and 5 of the survey committee report. Solar physics can contribute considerably to the educational effort in astronomy. In particular, the highly dynamical nature of solar processes—like CMEs, which can be observed with high time and spatial resolution—make solar

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

observations an attractive means of portraying astrophysical concepts to the public. There is a need to educate the growing community of space weather forecasters in solar physics as well as decision makers in government and in industry.

ACRONYMS AND ABBREVIATIONS

1HT

—One Hectare Telescope

ACOS

—Advanced Coronal Observing System

AIA

—Atmospheric Imaging Assembly

AO

—adaptive optics

ASP

—Advanced Stokes Polarimeter

AST

—Advanced Solar Telescope

AU

—astronomical unit

CCD

—charge-coupled device

CDR

—concept and design review

CEDAR

—Couplings, Energetics and Dynamics of Atmospheric Region, a part of the NSF solar influences program

CIA

—Coronal Imaging Assembly (on SDO)

CME

—coronal mass ejection

EIT

—Extreme Imaging Telescope (part of SOHO)

ESA

—European Space Agency

EUV

—extreme ultraviolet

FASR

—Frequency-Agile Solar Radiotelescope

FIP

—first-ionization potential

FOV

—field of view

FPIP

—focal-plane instrumentation packages

GEM

—Geospace Environment Modeling, a part of the NSF solar influences program

GOES

—Geostationary Operational Environmental Satellite, a series of meteorology satellites

GONG

—Global Oscillations Network Group

HAO

—High Altitude Observatory

HESSI

—High Energy Solar Spectroscopic Imager

HMI

—Helioseismic and Magnetogram Imager (on SDO)

IF

—intermediate frequency produced when a radio receiver mixes the input signal with a local oscillator

IR

—infrared

ISAS

—Institute of Space and Astronautical Sciences (Japan)

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

ISOON

—Improved Solar Observing Optical Network

KPVT

—Kitt Peak Vacuum Telescope

LASCO

—Large Angle and Spectrometric Coronagraph Experiment

LOFAR

—Low-Frequency Array

LOWL/ECHO

—Low-degree/Experiment for Coordinated Helioseismic Observations

MCAO

—multiconjugate adaptive optics

MDI

—Michelson Doppler Imager, an instrument on SOHO

MHD

—magnetohydrodynamics

MPT

—McMath-Pierce Telescope (on Kitt Peak)

MRX

—Magnetic Reconnection Experiment (at Princeton University)

NASA

—National Aeronautics and Space Administration

NCAR

—National Center for Atmospheric Research

NJIT

—New Jersey Institute of Technology

NOAA

—National Oceanic and Atmospheric Administration

NRL

—Naval Research Laboratory

NSF

—National Science Foundation

NSO

—National Solar Observatory

OPACITY

—A solar physics project conducted by the Institute of Physics, Bristol

OSL

—Orbiting Solar Laboratory (never-built NASA project)

OVRO

—Owens Valley Radio Observatory

PASO

—Particle Acceleration Solar Orbiter

PSPT

—Precision Solar Photometric Telescope

RAM

—Reconnection and Microscale Probe

RISE (SunRISE)

—Radiative Inputs of the Sun to Earth, a part of NSF solar influences program

SAIC

—Science Applications International Corporation

SDAC

—Solar Data Analysis Center

SDO

—Solar Dynamics Observer

SETI

—search for extraterrestrial intelligence

SMEX

—Small Explorer (NASA)

SMI

—Solar Magnetism Initiative

SOHO

—Solar and Heliospheric Observatory

Solar-B

—NASA mission to measure magnetic field and luminosity of the Sun

SOLIS

—Synoptic Optical Long-term Investigation of the Sun

SOON

—Solar Observing Optical Network

SOT

—Solar Orbiting Telescope (never-built NASA project)

SRBL

—Solar Radio Burst Locator

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

STEREO

—Solar-Terrestrial Relations Observatory

STP

—Solar-Terrestrial Probe

THEMIS

—Heliographic Telescope for the Study of Magnetism and Instabilities on the Sun (French/Italian project)

TRACE

—Transition Region and Coronal Explorer

UV

—ultraviolet

WIRE

—Wide Field Infrared Explorer

Suggested Citation:"5 Report of the Panel on Solar Astronomy." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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In preparing the report,

Astronomy and Astrophysics in the New Millenium

, the AASC made use of a series of panel reports that address various aspects of ground- and space-based astronomy and astrophysics. These reports provide in-depth technical detail.

Astronomy and Astrophysics in the New Millenium: An Overview summarizes the science goals and recommended initiatives in a short, richly illustrated, non-technical booklet.

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