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69 5. Air-Shower Observations Two new installations, the Fly's Eye and the Homestake underground detector, have recently become operational. They offer the promise of significant advances in knowl- edge of the composition and energy spectra of ultra- high-energy cosmic rays. These measurements should be continued. Additional surface arrays for measuring elec- trons, muons, and hadrons may help in determining the elemental composition of the primary particles, which is a problem of fundamental importance. VI I I . HIGHENERGY SOLAR ASTRONOMY A. Introduction Among all investigations in astrophysics the study of the evolving and transient properties of the Sun has the most direct and significant impact on our understanding of our environment on Earth. Although the Sun emits only about one millionth of its total luminosity in the form of x- and gamma-ray photons and energetic particles, these radiations carry much of the information we can obtain about the complex process of the solar corona as well as vital clues to the structure and dynamics of the under- lying chromosphere and photosphere and the deeper, invis- ible layers. Consequently, high-energy solar astronomy has assumed a central role in the effort to understand and predict the phenomena of the Sun on which the con- ditions of life on Earth depend. The Sun is the only star close enough to permit obser- vation and measurement of its surface activity at all wavelengths with spatial, temporal, and spectral resolu- tions that could, in principle, reveal the finest signifi- cant details. Moreover, the properties of matter ejected from the corona as solar wind and energetic particles can be determined directly by measurements in interplanetary space. Thus, in the study of the Sun, astronomy can go far beyond the limitations of other stellar observations, in which the nature of stellar processes must be inferred from analysis of spatially unresolved radiations and can reveal in detail how an apparently typical main-sequence dwarf star works. And in this study our observing sta- tions can be located about as close as they can safely be to a dense, hot cosmic plasma exhibiting many of the phenomena of magnetohydrodynamics that must be understood

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70 before other compact sources of x-ray photons and cosmic rays, both Galactic and extragalactic, can be completely comprehended. The broad features of the Sun's structure are well established. In particular the temperature profile of the Sun along a radius from its center falls from an estimated 15 X 106 kelvins in the core to a minimum value near 4200 K at a height of 560 km above the photo- spheric surface, the level at which the optical depth to space is unity. Energy released by fusion of hydrogen into helium in the core is transported outward by radiative diffusion to the base of the convective zone, which is believed to be at about 0.86 solar radius (1 solar radius is approximately 7 X 105 km). From there the energy is carried primarily by turbulent convection to the photosphere, the 400-km-thick layer from which most of the solar power is radiated as visible photons with a spectrum of which the continuum is like that of a blackbody at 5770 K. Continuing outward through the chromosphere, the temperature rises, at first gradually to 9000 K at the top of the chromosphere, 2000 km above the surface, then steeply to 105 K in a narrow transition region with a radial thickness of only 700 km, and on to values of several millions of degrees in the corona, which extends to several solar radii during periods of moderate solar activity. The solar spectrum is a composite of the emissions from all the regions above the photospheric surface. Nonuniform rotation and convective motions of the highly conductive material within the Sun generate mag- netic flux, which is continually carried to the surface by turbulent diffusion and magnetic buoyancy. From there the field emerges into the solar atmosphere with a strength that ranges from a few gauss in quiet regions to 100 gauss in active regions. Variations in this field, caused by motions of its anchor points in the dense plasma of the photosphere, give rise to a variety of phenomena on size scales ranging from less than a few hundred kilo- meters, the limit of currently available optical resolu- tion, to millions of kilometers. The corresponding time scales range from seconds in the case of explosive flares, through days for major changes in specific active regions, to years and decades for the general activity variations that are caused by reconfigurations of the entire internal solar magnetic field. On a scale of centuries the sunspot activity cycle and the attendant coronal phenomena wax and wane, sometimes ceasing almost completely for several decades.

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71 The corona is heated with energy supplied from the interior region and transported across the cooler under- layers against the temperature gradient by processes other than diffusion or convection. It loses energy steadily by emission of E W and soft x-ray photons, both inward and outward by thermal conduction inward and by expulsion of hot gas outward in the form of the solar wind. The flux of this energy is small in comparison with the luminosity of the photosphere. Nevertheless, the coronal emission and the solar wind have profound effects on the atmospheres and magnetospheres of the Earth and other planets. The origin, transport, and conversion of internal solar energy into coronal heat and kinetic energy of the solar wind are central problems of solar astronomy. With the recent discovery that hot coronas are present in stars of all spectral types and nearly every luminosity class the problem of coronal heating has become a general problem of stellar astrophysics, and the results of detailed studies of the solar corona have acquired a broad significance for all of astronomy. Solar flares are exDlORiOn-R in the mAan-hi~ nlA~mA of the lower corona. . . ~ ~ , . . _ _ ~ = ~ Their frequency and magnitude vary with the sunspot activity that currently has two maxima in a cycle of quasi-periodic magnetic field variations with a period of about 22 years. Flares occur in the vicinity of sunspots and last up to 20 min. They are powered by 1028-1032 ergs of energy stored in unstable configurations of the magnetic fields associated with sunspots, and it is believed that they occur when this energy is suddenly released by reconnection of field lines at the boundaries of regions where they are oppo- sitely directed. Flares result in acceleration of high- energy particles, intense local heating of the chromo- sphere, and in eruption of chromospheric material into the corona. Their effects are manifest over the entire electromagnetic spectrum from radio to hard x rays and MeV gamma rays, in energetic electrons and nuclei of all the elements in the corona, all ejected into interplane- tary space, and detectable by suitable means. Undoubtedly MeV neutrons are produced along with gamma rays when energetic nuclei collide with coronal or photospheric matter, but they mostly decay before they reach the distance of the Earth. As the most readily observable examples of the class of cosmic explosions resulting from sudden conversion of magnetic energy stored at compara- tively high concentrations into high energy particles and heat, flares are of great interest both in themselves and

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72 in the opportunity they present to develop a deeper under- standing of plasma physics for application to other areas of astronomy. The discovery of flares on other stars, and the observation that some of them are enormously more energetic than the largest solar flares, have given the study of solar flares a broad significance for stellar astronomy. Here again, as in the problem of stellar coronal heating, systematic investigations of the cor- relations between stellar flares and other stellar char- acteristics have shed significant new light on the mechanisms of flares on the Sun. The focus of solar astronomy on the phenomena of a single star, and the necessity of close coordination in strategy and timing of solar observations over the entire spectrum, make it desirable to consider the status and future of high-energy observations in solar astronomy as a whole rather than separately as parts of x-ray, E W. gamma-ray, or cosmic-ray astronomy. Moreover, the bright- ness of the Sun in the spectral range from the visible to the x-ray region places special engineering requirements on instrumentation that cannot be readily made compatible with the requirements for extrasolar observations. m e scientific status and future scientific objectives of solar astronomy are discussed in detail in the report of the Astronomy Survey Committee's Working Group on Solar Physics. We present here only a brief summary of recent progress and future prospects in high energy solar astron- omy as background for our discussion of new programs. Solar neutrino observations are considered separately in the section on neutrino astronomy. B. Progress during the 1970's 1. General Features of the Solar Atmosphere Among the most important developments in understanding the Sun's atmosphere during the decade of the 1970's was the discovery that it is divided into regions of two distinct kinds that are either magnetically "open" or magnetically "closed." In closed regions the field lines have the form of loops with both ends anchored in the dense plasma of the photosphere; in open regions the field lines are anchored at one end in the photosphere and stream outward into interplanetary space. Closed regions occupy typically 80 to 90 percent of the solar surface. Magnetic force confines the plasma in

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73 the inner corona in these regions to motions along the loops. Maintained at comparatively high density by magnetic confinement, the plasma is heated to tempera- tures of millions of degrees by processes that derive their energy from the bulk motions in the photosphere and deeper layers. The relatively high density and high temperature cause the plasma to be highly luminous in x rays so that the closed regions appear in x-ray images as bright arches of various sizes that trace the loop struc- ture of the confining magnetic field. The open regions are relatively low in density and temperature and are correspondingly low in x-ray emis- sivity. In x-ray images they appear as dark regions called coronal holes. Spewing forth from these magneti- cally open regions, which occupy only 10 to 20 percent of the solar surface, coronal plasma spreads as it leaves the Sun and within a few solar radii is moving radially away over the entire solid angle to form the bulk of the solar wind. Adjacent solar-wind streams from different coronal holes may have magnetic fields with opposite directions so that current sheets form on their bound- aries, resulting in a sector structure of the solar wind that has been measured by plasma detectors and magneto- meters on satellites and space probes. Coronal holes, first observed in rocket x-ray photo- graphs, were studied in detail with instruments on the Apollo Telescope Mount (ATM) of the Skylab mission in 1973. Thousands of high-resolution x-ray and W images as well as x-ray and W spectra were recorded during a period of many months when the Sun was in the low part of its activity cycle. Many correlated ground-based obser- vations were also made. Much still remains to be done before the potential scientific value of the data archives from Skylab and the correlated studies is fully realized. X-ray bright points, discovered in the ATM x-ray observations, are very small transient regions of intense x-ray emission in the lower corona that are scattered over the Sun's surface in both magnetically open and closed regions. They subtend angles of only a few arcseconds or less and last for several hours. Their nature and cause are not well understood, though they appear to be magnetic loops over miniature bipolar features with diameters of the order of a few hundred kilometers. Ground-based spectrographic studies carried out with very high angular resolution revealed a remarkable property of the Sun's magnetic field in the photosphere

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74 where the foot points of the coronal loops and the open lines are anchored. In traversing the photosphere the magnetic field is concentrated in isolated, widely separated flux tubes with radii of the order of 100 km. In these tubes the field intensity is of the order of 103 gauss, a statically unstable condition that is apparently maintained by processes associated with the turbulent convective motions. Two central problems concerning the Sun's corona, previously thought to have been solved, were cast in a radically new light by high-energy observations during the 1970's, and the inadequacies of the prevailing theories were revealed. One of these problems is how th corona is heated. In 1970 it was generally believed that acoustic waves, generated in the turbulent convective zone, traversed the chromosphere and transition zone and dissipated their energy in the corona. ~ ~ On the basis of this theory it was believed that stars of spectral type A or earlier, being nonconnective, would not have coronas. Then E W observations of the Sun by OS0-8 demonstrated that the energy flux in such waves is inadequate by an order of magnitude to supply the power radiated by the corona. Ground-based optical observations and the IUE found evidence for photoionized gas in the winds of O stars, which do not have convective zones. And satellite x-ray observations beginning with SAS-3 and culminating with HEAD-1 and the Einstein x-ray observatory demon- strated that million-degree, x-ray emitting coronas, which may account for the photoionization in O stars, are found among stars of all spectral types, including, in particular, at' types ot nonconvect~ve stars. Emus the acoustic wave theory of coronal heating was proved to be inadequate to account for the Sun's corona and failed to predict the coronal properties of other stars. New clues to the nature of the heating mechanisms have since been found in a correlation between stellar rotation and coronal x-ray luminosities in late-type stars. The second central problem of the corona is how it gives rise to the solar wind. Here, again, the conven- tional view of the wind as a free expansion of plasma heated by thermal conduction was shattered by the dis- covery that the solar wind originates primarily in the coronal holes. The thermal conduction mechanism was already strained by an observed lack of an adequate temperature gradient to sustain the energy flux, averaged over the solar surface, that appears in the kinetic energy of the wind. It failed by more than an order of

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75 magnitude when confronted by the demands on the local energy flux that are imposed by the fact that most of the power of the solar wind comes from coronal holes that occupy less than 20 percent of the total surface area. 2. Transient Events Rapid changes in the magnetic field, such as occur in flares, generate intense electric fields, which accel- erate particles to suprathermal energies. These par- ticles, confined to motion along magnetic field lines, deposit a portion of their energy in Coulomb collisions and thereby heat the plasma. In rarified regions the energetic electrons emit synchrotron radiation, and some particles may escape along open field lines into inter- planetary space as solar cosmic rays. High-energy solar charged particles accelerated in flares to several hundred MeV were observed at Earth in the 1950's as well as bursts of synchrotron radiation attributable to intense fluxes of high energy electrons moving outward through the corona. The Orbiting Solar Observatories (OSO's) of the 1960's and various rocket experiments recorded detailed high-resolution spectra of the soft x rays emitted by chromospheric plasma heated to temperatures of the order of 107 K, apparently by the impact of flare-accelerated particles traveling downward. Observations were also made of hard x rays generated directly by the impact of high-energy flare-produced electrons on the chromospheric plasma. During the 1970's such observations were greatly extended and refined by the ATM mission, which recorded high-resolution images of the W and x-ray phenomena associated with flares. Substantial progress was made in relating features of the x-ray images to details of the magnetic-field configuration and in utilizing W and x-ray spectral diagnostics to determine the properties of the flare-heated chromospheric plasma. In many flares, protons and heavier ions are accel- erated to high enough energies to produce gamma rays and neutrons in nuclear collisions. The first astronomical observations of nuclear gamma-ray line emission was achieved by a detector on the OS0-7 in observations of such solar flares, and further observations have been made by HEAD-3 and the Solar Maximum Mission (SMM). Gamma-ray observations made on the SMM satellite, with a highly sensitive spectrometer, have shown that many flares emit detectable gamma-ray line fluxes and that the

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76 expected neutron emissions may also have been detected following a giant limb flare in 1980. These results opened a new approach to the study of the composition of the solar atmosphere, which previously depended entirely on direct measurements of the elemental and isotopic composition of the solar wind and flare-accelerated nuclei. Observation of the solar cosmic rays have revealed many cases of an abundance of 3He/4He, which is enhanced by a factor of 103-104 above the solar abundance ratio. This effect is not likely to be the result of spallation of 4He since the observations are not accompanied by high abundances of the expected companion spallation products 2H and 3H. The 3He-rich flare effect is likely due to a selective plasma preheating or injection mechanism. Events with 3He/4He enrichment may also be accompanied by an enrichment of energetic Fe nuclei. mese and other unexpected results from the study of the nuclear radiation from solar flares, though not yet understood, illustrate how the study of elemental and isotopic abundances in solar cosmic rays yields unique information about the conditions in the solar atmosphere and about the mech- anisms of particle acceleration. It is expected that correlation of the observations of particles and gamma rays by the SMM will greatly clarify our understanding of the high-energy aspects of solar flares. 3. Long-Term Variability Historical research during the 1970's showed that the Sun has been inactive--without spots or an extended corona-- for long periods of which the most recent was the last half of the seventeenth century. The absence of solar activity is correlated with periods of cold in the Earth's climate. Thus the study of the nature and origin of solar activity, and of the causes of long-term solar variabil- ity, has significance not only for our understanding of the structure and evolution of convective stars but also for our understanding the evolution of life and human society on Earth. The solar output of high-energy photons and particles changes dramatically with the number of Sun spots because the latter, being major eruptions of magnetic flux gen- erated inside the Sun, are conspicuous symptoms of the conditions that produce magnetic variations and instabil- ities in the solar atmosphere and consequent particle

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77 acceleration and coronal heating. m e discovery of coronal x rays from many nearby stars during the 1970's has made it possible to undertake comparative studies of how coronal activity is related to stellar structure and rotation. C. Scientific Objectives of High-Energy Solar Astronomy Efficient progress in solar astronomy requires a broad program of coordinated observations in many wavelength bands as well as in situ measurements of particles and fields within the heliosphere. Although we focus here on the objectives for which high-energy observations are specially important, we wish to stress the need for cor- relative studies in the optical, W. and radio portions of the spectrum. The following questions define major problems of solar astronomy that require high-energy observations: 1. What processes determine the structure and dynamics of the solar chromosphere, corona, and solar wind? 2. In solar flares and other transient solar phenom- ena, what are the mechanisms of energy storage and release, particle acceleration, and local heating? 3. How does the Sun influence the structure and dynamics of the interplanetary medium? 4. What is the mechanism of the solar activity cycle, and how can its future behavior be predicted? 5. Are the relative abundances of isotopes and elements different in the various regions of the atmo- sphere, and if so, what are the mechanisms of their separation? Within this broad context the specific objectives of high energy observations during the next decade will include: 1. Detailed studies of the E W. x-ray, gamma-ray, and neutron emissions from the solar atmosphere with the highest attainable spatial and spectral resolutions; 2. Coordinated three-dimensional x-ray studies of coronal structure and activity and simultaneous In situ three-dimensional measurements of high-energy particles in the interplanetary medium; 3. Synoptic studies of the long-term variations in coronal activity and its effects on high-energy particles

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78 in the solar system together with comparative analysis of variations in coronal activity in other stars. 4. Observations of solar high-energy nuclei, elec- trons, neutrons, plasma, and magnetic fields from satellites and space probes. These studies will be almost entirely dependent on obser- vations from space vehicles, though new information about particle acceleration in solar flares and the effects of solar plasma on the flux of cosmic rays over long times and at very high energies can be derived from ground- based monitors that detect the secondary neutrons produced by high-energy charged particles in the atmosphere. D. Inventory of Present or Approved Resources The inventory of resources for high-energy solar astronomy must begin with the archives of data obtained by Skylab, the OSO's, the SMM, and from such interdisciplinary mis- sions as ISEE, IMP-8, and other deep-space probes (e.g., the Pioneers and Voyagers) that contained particle-and- field experiments. These provide a rich data base for the study of many aspects of solar physics. Much analyti- cal and theoretical work remains to be done before the scientific value of these archives is exhausted. It is therefore essential that relevant studies based on these archives be adequately supported. A new facility in an advanced stage of development is the International Solar Polar Mission (ISPM), which is designed to obtain, for the first time, x-ray and E W images of the solar atmosphere from above the Sun's poles. The ISPM will also contain particle-and-field experiments. With two spacecraft the mission would obtain critically important "stereoscopic" observations of the optically thin loop structures that appear to be the dominant structural feature of the high-temperature corona. Two solar E W experiments have been approved for Shuttle missions. One is a stigmatic E W spectrohelio- graph for study of the solar transition region. The second is an E W resonance-line chronograph, which will be used in combination with a conventional white-light coronagraph for measuring the velocity, temperature, and density of the corona within S AU of the Sun.

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79 E. Capabilities of Present or Approved Resources The SMM achieved major advances in the observation of hard x rays and gamma rays associated with solar flares. It is clear, nevertheless, that the solution of fundamen- tal problems concerning the processes of coronal heating and the phenomena of flares and other transient phenomena will require facility-class instruments capable of spec- trophotometric and line-profile studies with angular resolutions that match the finest details of significant structures that are known to exist, namely, 1 arcsec or less. Such instruments will represent a considerable advance over the capabilities of the pointed SMM instruments. m e ISPM will have only modest angular (approximately 4 arcsec) and spectral resolutions owing to the limits of size and weight for interplanetary missions. Simultaneous observations by an x-ray (X W) telescope in Earth orbit would greatly enhance the scientific value of the ISPM data. Several experiments on the variation of the solar flux at a number of wavelengths have been approved for Space- lab. However, a comprehensive program for the study of solar variability should be undertaken on a time scale exceeding at least the fundamental 22-year cycle. NASA supports basic theoretical studies in solar plasma physics, which are essential to the interpretation of observations and to the planning of future missions. F. New Facilities and Programs for the 1980's Recent advances in the techniques of high-energy observa- tions have opened a broad range of opportunities for new investigations that would assure a continuation of the rapid growth in our understanding of the high-energy phenomena of the Sun. Among these advances are improve- ments in x-ray telescopes that make it possible to record spectrally resolved soft x-ray images with angular resolu- tions finer than 1 arcsec; coded aperture detectors for hard x-ray image detection with resolutions of the order of arcseconds; high spectral and spatial resolution x-ray and E W spectrometers; highly sensitive scintillation spectrometers and high-resolution nuclear gamma-ray solid- state spectrometers; and cosmic-ray detectors capable of resolving the isotopes of all the elements up to nickel and the element distribution for Z up through 26. These

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80 and other advanced technical capabilities would be employed in the new programs and facilities of which the following is a summary. 1. Shuttle Facilities A comprehensive Solar Shuttle Observatory should be developed to encompass a number of Principal Investigator and/or facility-class instruments that provide major improvements in angular resolution, spectral-resolving power, and sensitivity over previous instruments. The first component of this Observatory, the Solar Optical Telescope (SOT) is already approved for construction. The principal components required for high-energy observations are the following: Solar Soft X-Ray Telescope Facility (SSXTF): A large soft x-ray telescope with interchangeable focal-plane instruments that include high-resolution image detectors and spectrographs. The SSXTF will be an 0.8-m nested Wolter Type I telescope with 1200 cm2 effective collect- ing area over most of the wavelength range from 1.7 to 300 A, and with an angular resolution of 0.5 arcsec. Major emphasis will be placed on high-resolution spec- troscopy in the focal-plane instrumentation. ~Pinhole" Telescope/Occulter: A coded-aperature telescope employing a remote mask mounted on an extended boom will be used in conjunction with a position-sensitive detector to record images of hard x-ray events associated with flares and other impulsive solar phenomena. The pin- hole telescope will achieve subarcsec angular resolution over the energy range from about 1 to 100 keV, with moder- ate spectral resolution. The mask will also serve as an occulting disk for coronal observations in the visible to E W regions of the spectrum. Grazing-Incidence Solar Telescope (GRIST): An X W facility that may be developed by the European Space Agency. Interchangeable focal-plane instruments would include high-resolution image detectors and spectro- graphs. The GRIST will provide (1) an order-of-magnitude improvement in sensitivity compared with Skylab, (2) high time resolution for the study of transient phenomena, and (3) improved angular resolution (0.5-1.0 arcsec) necessary for the study of fine structures known to be present in the transition region. Spectroscopic observations with the capability of measuring intrinsic line profiles will

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81 be the major emphasis of the scientific program of the GRIST. Solar Shuttle E W Facility (SSEF): A normal-incidence telescope of modest aperture but with a high-reflectivity mirror for use in the spectral region from 360 to 1500 A. It will provide ultra-high angular resolution at wavelengths shorter than can be efficiently observed with the SOT. The E W facility will consist of a normal- incidence telescope feeding stigmatic E W spectrohelio- graphs operating between about 360 and 1500 A. The design objective for spatial resolution is 0.1 arcsec and that for spectral resolution is between 103 and 105. This instrument will complement the SOT by extending ultra-high-resolution observations to the spectral region of radiation characteristic of temperatures as high as 2 X 106 K. It is intended to achieve an order-of- magnitude improvement in angular resolution and spectral resolution compared with any currently approved instru- ment capable of observing material on the solar surface hotter than 2 X 105 K. Gamma-Ray Spectroscopy Facility (GRSF): A high- . . . resolution and high-sensitivity gamma-ray spectrometer for the study of nuclear gamma-ray lines. Both scintil- lation and solid-state detectors are needed to achieve the desired performance over the entire range of energies. This instrument and the "Pinhole" hard x-ray telescope may also be useful in the study of transient phenomena in extrasolar sources. 2. Solar Coronal Explorer Investigations of the large-scale structure of the inner heliosphere, begun on Skylab, would be extended with an Explorer-class satellite carrying an x-ray/X W telescope having an angular resolution finer than 1 arcsec and with white-light and resonance-line coronagraphs. The purpose of such a mission would be to measure the density, composition, and flow velocities in the corona and inner heliosphere and to explore the relations of these parame- ters to the structure of the lower atmosphere. It would also facilitate detailed investigations of the dynamics of the coronal transients, which have an important effect on the terrestrial environment.

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82 3. Interplanetary Laboratory (IPL) The proposed IPL Satellite in the Origin of Plasma in the Earth's Neighborhood (OPEN) series of satellite missions will carry advanced instrumentation for solar plasma measurements, magnetic fields, and instrumentation to measure the isotopes, especially the rare isotopes, of elements ejected from solar flares. 4. Advanced Solar Observatory m e Skylab results demonstrated the value of simultaneous observations over a range of wavelengths with high spec- tral and spatial resolutions over long periods of time. The Skylab experience, together with the Shuttle solar program, will provide a strong basis for development of the central element of the strategy for solar astronomy in the 1980's, namely the Advanced Solar Observatory (ASO), which will contain facility-class instruments operating with subarcsecond resolution at optical, X W. soft x-ray, and hard x-ray wavelengths, together with gamma-ray and neutron detectors with the highest attain- able angular resolution. The ASO will be a long-lived facility designed to be refurbished in orbit and periodi- cally brought back to Earth for overhaul and replacement of instruments. Since important solar problems can be addressed effectively with facility-class instruments on Shuttle sorties, the development of the instrument package can be carried out stepwise by development and use of individual facility-class Shuttle sortie instruments, which will ultimately be integrated into the ASO, pos- sibly aboard a Scientific Space Platform. The x-ray components of the ASO will be the SSXTF and the Pinhole telescope. The X W components of the ASO will be the GRIST and the SSEF telescope facility. For gamma-ray observations, the ASO will either incorporate the GRSF or carry out studies in coordination with the Gamma- Ray Transient Explorer. 5. Other Missions and Programs of Significance to Solar Physics The solar atmosphere and the interplanetary medium will be studied by the Star Probe, which will approach close to the Sun to measure directly conditions inside the

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83 region of solar-wind acceleration and to carry out ultra-high-resolution optical and x-ray observations. Both gamma-ray and neutron measurements could, in principle, be made on the Star Probe. The latter measure- ments must be made as close to the Sun as possible to enhance the low-energy neutron flux, which is greatly depleted by decay. These combined measurements, which are highly desirable at the next solar maximum, can establish the spectral shape of both the energetic par- ticles accelerated at the Sun and the products of their nuclear interactions with solar matter (e.g., gamma rays and neutrons). An alternative mission to accomplish this important solar objective would a lightweight Mercury orbiter. This mission would have the advantages not only of long-term observations but would make possible the observation of solar neutrons in the important low-energy range before they decay and simultaneous gamma-ray observations. An important next step in high-energy stellar astronomy will be detailed spectroscopic studies of stellar x-ray emissions and their relations to surface temperature, gravity, rotation, and magnetic fields. Such studies will provide critical new tests of theories of stellar activity and will therefore have important indirect benefits for solar physics. A strong program of supporting research and technology will continue to be essential to the continued vitality and health of solar physics. In this connection we note that Shuttle-launched "Experiments of Opportunity," which extend the observation time available to "rocket" payloads to 24 hours, and longer-duration balloon flights offer promising prospects for testing new instrumentation con- cepts and for exploratory research. And we emphasize once more that extended postmission data analysis and exhaustive study of existing archival data must be adequately supported in order to derive the full scientific value from the missions that have been flown. G. Summary and Principal Recommendations High-energy solar astronomy has achieved fundamental advances in our understanding of the Sun's outer atmo- sphere. At present, high-energy solar astronomy stands at the threshold of major new developments based on obser- vation and analysis of the microstructure of the processes that heat the corona and cause flares and other transient