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119 role of magnetic-field geometry and the mutual exclusivity between wind flux and bright x-ray emission regions may provide important insights into the physics of stellar winds. Furthermore, the heating of the solar corona is clearly magnetic in character. Solar Nonradial Pulsations and Seismology The unexpected discovery that the solar atmosphere has radial wave motions with a 5-min period led to major theoretical studies of the generation, propagation, and damping of acoustic, gravity, and magnetic waves in stel- lar atmospheres. Theory indicates that the oscillations observed at the solar surface are manifestations of stand- ing, nonracial p-mode oscillations of the outer convection zone and predicts a well-defined observable relation between the temporal and spatial power spectra of the 5-min oscillations. The observational confirmation of the existence of such a relation is another triumph for theoretical research in solar plasma dynamics and rivals in importance the earlier confirmations of predictions of sunspot magnetic fields and the solar wind. m e 5-min solar oscillations are now being used to probe solar interior structure in a manner analogous to seismological probing of the core structure of the Earth. These studies have already resulted in improved models of the solar convection zone and in a measure of the rotation of the upper convection zone. Longer time sequences of precise solar surface velocities at the 5-min periods, as well as at longer periods (160-min periods have been reported), will extend this technique for "viewing" the astrophysical processes in the solar interior to regions closer to the solar core. Together with the solar neu- trino observations, these seismological studies are begin- ning to define the conditions of the interior of the Sun and to provide truly unique insight into stellar structure and evolution. The proposed "Star Probe n mission to the Sun would provide sensitive new measurements of the solar gravitational field and add profoundly to our knowledge of the solar interior, with important additional implica- tions for studies of stellar interiors generally. III . SCIENCE OPPORTUNITIES FOR THE 1980' s A. Introduction The 1970's have been exciting years for astronomy. Increasing instrumental sensitivity and resolution over a

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120 wide range of wavelengths has led to the discovery of amazing objects, mechanisms, and interrelations. During the 1980's we will build on and extend the programs that have proved to be so fruitful during the 1970's. The Very Large Array (VLA) radio telescope, con- structed in stages over the past decade, is now becoming fully operational; two new 4-m optical telescopes have gone into operation at Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory; and Space Tele- scope (ST), together with several highly sensitive IR space observatories, will begin to return data by the middle of the present decade. These present and planned facilities will require increasing levels of ground-based optical and infrared support work if their scientific return is to be maximized in the years ahead. A balanced program for astronomical research in the coming decade will thus require at least one new optical/IA telescope larger than any yet constructed, together with a network of smaller (yet still substan- tial) telescopes both at the National Astronomy Centers and at university observatories. Such a large telescope-- which was recommended in each of the two earlier decade reviews of astronomy published by the National Academy of Sciences--will be used to pursue frontier problems requir- ing difficult observations of faint objects. The develop- ment and testing of instrumentation, on the other hand, is more advantageously carried out on smaller telescopes in the 2.5-5-m class, which offer ample opportunities for experimentation and modification with relatively short lead times; many of these will be university-based facil- ities. Equipped with modern instrumentation and detec- tors, these more modest telescopes will be able to attack an exciting array of research problems during the 1980's, as well as provide important supporting observational data for larger, forefront instruments. The new capabilities to be presented by ST, the forthcoming series of IR space observatories, and the 15-m-class optical/IA telescope proposed here will be extraordinary; however, demands for the precious observing time afforded by these powerful facilities will be correspondingly great. Therefore, observational programs requiring large commitments of telescope time will, for the foreseeable future, continue to demand the capabilities provided by substantial numbers of ground-based telescopes in the 2.5-5-m class. The present difficulty of access to optical/IA tele- scopes of aperture large enough to address frontier prob- lems in astronomy has reached crisis proportions. If

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121 healthy advances in astronomical science are to be sus- tained in the years ahead, we must respond positively to the challenges that confront us now. We now have the opportunity to build, using only a very small fraction of our national wealth, those facilities that are needed to continue to expand the golden age of astronomical discov- ery in which we live--an age unmatched in richness and vitality by any other since the time of Galileo, some 350 years ago. B. Scientific Programs 1. Galactic Astronomy Galactic Structure We know that we live in a spiral galaxy, although its dimensions and detailed morphology remain a mystery. We do not know how far our Sun is from the center, nor do we know the rotational velocity about the center with an accuracy sufficient to determine the Galactic distance scale to within 20 percent. During the coming decade, the variety of approaches available for the detailed study of our Galaxy should produce a more coherent pic- ture of the stellar system in which we live. Utilizing observations from ST and the recommended 15-m facility, dynamical studies of faint halo stars, distant globular clusters, and outlying satellite galaxies will delineate the extent and the mass of the Galactic system; spectro- scopic studies will determine chemical evolution as a function of age and of position; and radio, millimeter, and submillimeter observations of molecular clouds will identify regions of star formation and help us to learn their histories and dynamics. Sophisticated theoretical models will probe the stability of disk systems. From these studies, astronomers should learn what effects warps in the outer disk have on stability. The Magellanic Stream--that extensive band of neutral hydrogen reaching from our Galaxy to its nearest satellite neighbors--may be understood in this context. From x-ray observations of nearby galactic nuclei, we will study events relevant to the energetics of our own Galaxy's nucleus. Our near- est spiral neighbor, M31, now has 17 x-ray sources iden- tified in its nucleus; their nature is not yet understood. We should ultimately piece together a detailed picture of our own Galaxy from future studies, both internal and external.

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122 Dynamical studies of external galaxies have led to the conclusion that disks of spiral galaxies may be stabilized by the presence of a low-luminosity halo. Only for our own Galaxy can we currently hope to observe both stars and globular clusters in the halo and to determine its composition. Broadband photographic photometry down to magnitude 24.5 would isolate halo dwarfs and identify them to distances of 60 kpc, as well as detect giants throughout the total extent of the halo. Crude but still useful abundance indicators could be obtained out to 25 kpc, together with radial velocities accurate to about 60 km/sec. For dwarfs, giants, and globular clusters, a detailed map of the halo, complete with distances, compo- sition, abundances, and space motions will be forthcoming and will be the ultimate test of models of halo formation and the possible presence of nonluminous matter in the outer regions of our Galaxy. m e composition of the inner spheroidal bulge; star-by- star studies of the nuclear bulge; and details of the physics, chemistry, and dynamics of interstellar molecules in the Galactic nucleus will also emerge from observations in the optical, W. IR, and radio spectral regions. A rough rotation curve of the nucleus has recently been de- termined from the N II line at 12 m. Continued studies of the composition and dynamics of the Galactic center from a 15-m ground-based telescope will produce exciting results and perhaps even tell us if there is a black hole at the center of our Galaxy. b. Star Formation Star formation is a process on which almost all aspects of Galactic evolution hinge. Present knowledge only scratches the surface of problems such as the processes leading to star formation, the initial mass functions, and how and why the rate of star formation varies with time and location in the Galaxy. An inter- play between observation and theory is needed in order to understand the star-formation process; the processes by which planets form; and the structure, chemical evolution, and stellar populations in our own and other galaxies. Infrared-radiation processes dominate the evolution of interstellar gas and dust from an equilibrium temperature of a few kelvins in molecular clouds to stellar surface temperatures of several thousand kelvins. Infrared obser- vations are thus vital to the study of the energy balance and physical conditions that occur during the transition from diffuse gas to nuclear-burning star. The structure

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123 and sizes of protostars and the dense dust clouds in which they are immersed require high-resolution observa- tions at all IR wavelengths, especially beyond 30 Am. High spectral and high spatial resolution will enable us to determine composition, density, temperature, and mass of both the dust and gas within the molecular clouds and in the immediate vicinity of the protostar. With a spec- tral resolution of 105 it will be possible to measure the radial velocity of individual components within the system; measurements of polarization in the IR region may give information on magnetic fields in the clouds. Recent work has shown that there is an interplay between the dynamics of spiral structure in a galaxy and the processes of star formation within the disk. Star formation occurs over large regions of space, possibly because the suspected triggers of star formation (spiral density waves, shock fronts from supernovas or H II regions) are large-scale phenomena. While observations on small spatial scales are important in learning the details of the processes involved, large-scale radio observations, together with the W studies of the inter- stellar medium discussed below, are also important for an understanding of these processes c. Interstellar Medium Our picture of interstellar matter has undergone sub- stantial changes within the past decade. It now appears that the two major components are a hot intercloud medium and the dense molecular clouds discussed above. Observa- tions of diffuse soft x rays made with rockets and with HEAD-1 and observations of W absorption lines made with Copernicus demonstrate that a substantial fraction of the local interstellar medium is filled with hot (T = 10V K), low-density gas, whose scale height is so great that it merges with a Galactic corona and wind. During the 1980's, we will study the degree of homo- geneity of the interstellar medium with respect to tem- perature, pressure, chemical composition, and cloud size. Much of this information will come from high-resolution spectroscopy with ST. The one-dimensional structure of the clouds will be fully revealed; specific clouds will be assigned distances, masses, and temperatures. The mass spectrum of clouds as a function of velocity, known for the first time, will permit detailed studies of cloud formation and destruction. Such a program combines ground-based work, ST observations, far- W spectroscopy, and IR studies. .

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124 Heating mechanisms for clouds should be quantitatively defined by this decade. Detection of weak molecules with ST will permit a complete grid of chemical reactions to be built up, so chemical heat input will be defined. X-ray heating will be determined from studies of ioniza- tion in H II regions. Definition of the velocity struc- ture in each cloud will allow the mechanical energy injected by shock waves to be evaluated. The thickness of halo gas at different temperatures will be determined, as will the rotation properties of the gas at high Galactic latitudes. The apparent rotation is tied to the magnetic-field strength and the mass inflow and outflow rates at high latitudes. The degree of "patchiness" of the halo gas will be determined. All of this information is relevant to the absorption lines seen in quasar spectra, because the likelihood of seeing ab- sorption lines through halos of foreground galaxies depends on these same properties in galaxies along the lines of sight to the quasars. ~ _ d. Emission Nebulas Ultraviolet spectroscopy is highly valuable for studies of supernova remnants (SNR's) and planetary nebulas. It not only allows sensitive measurements of abundances of certain elements, such as carbon, which are difficult to study from the ground, but it also provides sensitive determinations of electron temperature by virtue of accessibility of strong, high-excitation emission lines such as those of C IV, N V, and O VI. Sensitivity below 1150 ~ is required to observe important resonance lines such as those of O VI, S IV, and S VI. At present, only the very brightest H II regions have been observed with IUE, in the wavelength range above 1900 A. Further studies of H II regions and SNR's will require an optimized nebular spectrograph, if possible sensitive down to 950 A. ST and the W spectrographic telescope would be very valuable for detailed studies of planetary nebulas, since these objects are generally of higher surface brightness than are H II regions and SNR's. Because planetary nebulas are of generally small angular size, high spatial resolu- tion in imaging spectroscopy is necessary for their study. Of particular importance are determinations of small-scale temperature and composition variations, with angular resolution considerably better than achievable from the ground. It is also important to confirm (or otherwise) current indications of composition variations with posi-

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125 Lion of the planetary nebulas and H II regions in the Galaxy. To observe a sufficiently large number of objects, at sufficiently large distances to establish abundance gradients, will require both very high sen- sitivity and high angular resolution. e. Outer Atmospheres of Stars At the end of the 1970's, a revolution began in our appreciation of the range of phenomena that occur in the outer atmospheres of stars. The realizations that almost all stars have hot coronas, and that at least all luminous stars have massive winds, is changing our perspective on stellar evolution and the interaction between stars and the interstellar medium. Several important problems stand out as ready for concerted attack in the coming decade: . We need reliable measures of plasma character- istics and flow velocities and of the variability of these properties. The W spectrographs on IUE and ST will provide valuable information on species formed in chromospheres and winds at temperatures up to 2 X 10S K, but to probe the hotter regions of coronas and winds requires spectroscopic observations of ions formed at higher temperatures. The proposed far- W spectrograph in space with sensitivity down to at least 300 A is needed for this task. We need to understand the basic mechanisms respon- sible for the heating of stellar chromospheres and coronas and for accelerating stellar winds. Are the outer atmo- spheres of most stars heated by wave processes, the dissi- pation of magnetic fields of magnetohydrodynamic waves, or radiative instabilities? Also, why do the cool lumi- nous stars appear not to have hot coronas? Winds may be accelerated by thermal pressure gradients associated with a hot corona, radiation pressure on spectral lines and dust, or momentum deposition by waves of various types. It is important to learn which processes dominate in dif- ferent regions of the HR diagram and exactly how these processes work. m e far- W spectrograph in space will provide essential insight into these questions, but theoretical studies and, especially, detailed studies of the nearest star, the Sun, by SOT and the Solar Coronal Explorer are also essential for progress. Recent studies of the Sun point conclusively to the dominant ways in which magnetic fields control the heating of the chromosphere and corona and the geometry

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126 and energy balance of the corona and wind. The magnetic field in turn is stochastic but variable over the well- defined cycle and presumably regenerated by dynamo processes. We need to understand the various aspects of the solar and stellar activity cycles. Stellar observa- tions are critical in this regard because different values of the important variables, such as rotational velocity, depth of convection zone, and gravity, need to Since very high spectral resolution, a high be sampled. _ signal-to-noise ratio, and stability over decade-long observing programs are needed, this program requires a high-resolution spectrograph/telescope dedicated to solar-stellar studies. 2. Extragalactic Astronomy a. Galaxies and Clusters of Galaxies - The evolution of galaxies, the evolution of clusters of galaxies, and the interaction between these two areas of study offer outstanding opportunities for discovery and understanding during the 1980's. Individual come portents of external galaxies will be observable with unprecedented detail. HR diagrams of the old stellar population, colors and luminosity functions of globular clusters, and IR spectroscopy of the interstellar medium in external galaxies will result. These in turn will give ages and metallicity histories of the galaxies. Within our own Galaxy, observations of the composition and dynamics of the nuclear bulge and of stars in the halo, plus abundances among main-sequence stars in globular and open clusters, will reveal the history of nucleosynthetic enrichment in the Galaxy. Galactic populations among the Hubble sequence are now generally understood in terms of systematic variations in the rates of star formation. If we are to understand Hubble types, we must discover what factors control the gas content and star-formation rates both now and in the past. Strong observational evidence from the past decade indicates that individual galaxies are frequently im- mersed within optically undetected, massive halos. Major photometric programs aimed at detecting faint halo stars must be pursued, as well as new dynamical tests for unseen matter. Important facilities here include the 21-cm capability of VLA, as well as improved optical techniques involving multiplexing red-shift detectors, additional 3- to 5-m-class telescopes, larger optical telescopes,

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127 and high-efficiency two-dimensional detectors. Adequate theoretical support is also vital if we are to understand the origin of these halos, which may even precede the formation of visible galaxies. m e mass we see in individual galaxies is only 10 per- cent of the total mass contained in groups and clusters of galaxies. Unbiased samples of accurate galaxy red snorts out to a red split of about O.1 are necessary to measure both the seen and unseen mass in clusters and groups. Generous time allocations on telescopes of moderate size are required. It is likely that in the 1980's, the study of clusters of galaxies will accelerate, spurred on by the wealth of information acquired from x-ray observations. From clus- ters in a variety of evolutionary states, we must study the morphology of galaxies within the cluster, how galaxy evolution (i.e., mergers, tidal stripping, gas stripping, velocity dispersion) is influenced by the cluster evolu- tionary state, and how clustering itself has evolved since the epoch of formation. On still larger spatial scales, it is important to verify the recent tentative detection of x-ray emission from superclusters, since the implied mass involved would dominate the gravitational potential of the supercluster. X-ray detectors of higher sensitiv- ity and resolution are required to map these extremely diffuse sources of low surface brightness. Optical and IR studies of the morphology, dynamics, and structure of superclusters is needed to acquire the data necessary for an understanding of our own corner of the Universe. b. Quasars The fundamental problems for the 1980's are the struc- ture of the central energy source in quasars and active galactic nuclei and the physical processes involved in the energy release. The basic picture of the accretion of matter onto a black hole needs to be confirmed, and the physical process actually producing the emitted energy remains to be understood. Do the differences among quasi-stellar objects (QSO's), BL Lac objects, Seyfert galaxies, and N galaxies observed in the radio, optical, IR, and x-ray regions arise from structural differences? A clear delineation of galaxy types and environments in which nuclear activity occurs will tell us much about the duration of nuclear activity and the importance of cluster membership to galaxy activ- ity. Ultimately, spatial resolution in the visible and the W regions approaching that now possible with ground-

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128 based very-long-baseline interferometry (VLBI) would allow material surround- ~ng a central object to be studied even at large red shifts and would enable us to understand the relationship the structure and velocity field of the between the thermal and nonthermal components. Because ST will clearly resolve the galactic component of nearby quasars, it will be the single most important instrument for studying these associated galaxies. Both ST and large ground-based optical telescopes can contribute to investigations of cluster membership. Direct observational evidence on the nature of the central energy source at high spatial resolution can currently be obtained only with VLBI observations. Because of its much shorter working wavelength, optical interferometry offers significant potential gains over radio techniques for receivers operating on identical baselines. Ground-based optical telescopes do not cur- rently exploit the potential for resolving power because of blurring by atmospheric seeing. . . . . . Novel techniques Devised to overcome ants degradation are currently limited to fairly bright objects. However, it is clear that much of the information contained in the optical and IR spec- tral regions is currently being wasted. The 1980's may be an opportune time to consider whether ground-based or space-based interferometers should be pursued first and to undertake serious design studies and perhaps even prototype models for optical interferometers. Deep surveys for QSO's, Seyfert galaxies, and radio galaxies are likely to remain our only probe of the clus- tering properties of the Universe in the red-shift range z = 1.5 to 4. We need to know how the clustering of QSO's compares with the weak clumping among distant radio sources. Optical objective-prism surveys from space and from the ground will remain a powerful technique for these surveys. We also need to understand the relation between active galactic nuclei and apparently normal nuclei. Are dormant quasars lurking within many local galaxies, inac- tive at present because there is no current fuel supply? ST will be able to place much better limits on the mass and space densities in the very centers of nearby galactic nuclei and will allow us to study the physics of the sur- rounding, ejected, or intervening material. The direct- imaging capabilities of ST will be adequate as currently planned. However, to measure rotation curves and vel- ocity dispersions close to the cores of nearby galactic nuclei, a two-dimensional detector for spectroscopy will be required. Such an instrument must exploit the high angular (spatial) resolution along the slit. How~v.?r. to m - A~:~,r" rc~tati~n Rev - i; and v`?1

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129 Many of the absorption lines in quasar spectra appear to arise from intervening gas clouds along the line of sight. Analysis of the distribution of these lines may tell us about the evolution of clustering in the Universe and how and when gas accreted into galaxies. The clouds are so numerous that, in most quasars, the individual components are unresolved. Further detailed study is hampered by inadequate spectral resolution. Higher reso- lution requires larger ground-based telescopes. For spectral coverage in the W region, ST could provide the necessary wavelength range, if a high-resolution echelle spectrograph with ample spectral range were to become available. c. Cosmology Because the cosmic microwave background radiation is by far the most accessible probe of the physics of the early Universe, it is vital to improve the accuracy of the measurements of the spectrum and to place more strin- gent limits on its large- and small-scale anisotropy. The Cosmic Background Explorer (COBE) satellite will provide excellent measurements of the spectrum and of the large scale anisotropy (angular scales of about 5 deg or more). Better limits on the fine-scale anisotropy, achievable with a Large Deployable Reflector in space, are probably the single most important measurements yet to be made on the microwave background. Such observations are needed to establish with certainty that the background is not composed of point sources. They may also detect inhomo- geneities at decoupling, which foreshadowed the eventual formation of galaxies and clusters of galaxies. During the 1980's we will follow up on recent, tantalizing hints of galaxy evolution at large red shifts. Many faint gal- axies at red shifts beyond z = 0.6 surprisingly appear to be blue, suggesting widespread supernova events in young galaxies. Details of the evolution of isolated galaxies and the effects of environment on Hubble type, gas con- tent, and star-formation rates should emerge. ST will provide high-resolution photographs indispensable to the morphological study of distant galaxies. Deep galaxy counts and red-shift surveys are central to the measure- ment of the luminosity function, rates of evolution, and clustering properties of high-red-shift galaxies. Broad- band energy distributions will be sensitive to the aging of stellar populations. In addition to ST, larger ground- based optical telescopes, more efficient ground-based telescopes, multiplexing red-shift detectors, improved IR

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130 sensitivity out to 5 Em, and high-efficiency two- dimensional detectors in the range 0.15-5 Em are impor- tant for the program. The determination of the Hubble constant to an accuracy of better than 10 percent will require extensive observa- tions of both old and new standard candles such as RR Lyraes, Cepheids, and novae--all of which can be studied in nearby galaxies with ST. Other, more novel techniques may also be exploited, among which are the supernovae and the diminution of the microwave background in the direc- tion of clusters of galaxies. Improvements in millimeter receivers are required, and perhaps ultimately a large (10-m aperture or larger) millimeter antenna in Earth orbit as well. 3. Astrometry a. Stellar Census The major program for the coming decade will be to continue the census of the solar neighborhood, where "neighborhood" now means within 500 parsecs rather than 50 parsecs. Parallaxes will establish the distance scale and permit the determination of luminosities of both ordi- nary stars and, for the first time, some fairly exotic stars. The much larger number of proper motions also obtained will improve the understanding of local Galactic kinematics. m e greater number of binaries coming under study will permit the determination of many more stellar masses. Several clusters can be investigated for dynam- ics and astrophysical properties. More and smaller unseen companions will be located. The very nearest stars will have their properties determined to high . prec IS Ion. To augment existing astrometric programs and provide more uniform coverage, an astrometric instrument (2-m class) will be required in the southern hemisphere, and improved detectors (focal-plane detectors and interfero- meters, for example) and high-grade measuring machines (at least three) will be required at existing observa- tories. By the end of the decade, 1-m-class telescopes designed to achieve the very highest accuracy possible will be required in both hemispheres. In addition, a dedicated astrometric satellite should be under development.

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131 b. Solar-System Model The model of the solar system will be improved. This will necessitate finding as many members as possible, identifying the appropriate gravitational theories and models of individual bodies, and determining the most accurate values of the numerical parameters for use in these theories. The establishment of new occultation and laser-ranging networks will contribute significantly to this effort. c. Inertial Reference System A more nearly truly inertial reference system will be established. This will include the creation of a new fun- damental catalog, of one or more high-precision secondary astrographic catalogs, of tertiary catalogs of extragalac- tic objects (in some cases tied to the radio reference system), and of at least one survey of inertial proper motions. Besides the intrinsic galactic-structure, navi- gational, and geodetic applications, this reference sys- tem is necessary as part of the other two programs. Development of instruments with improved capability to measure large angles and a better understanding of the effects of the atmosphere are particularly needed. 4. Solar Physics Solar physics is at present evolving from an intensive study of our Sun, the closest star, into a more broadly based inquiry into the large-scale behavior of ionized gases in gravitational and magnetic fields. Active phe- nomena similar to those seen on the Sun occur in many stars and in galaxies as well. The Sun provides the only accessible laboratory in which the processes governing these many effects can be discerned in sufficient detail to permit real progress in our understanding of them. Other classes of astronomical objects complement the solar laboratory by exhibiting more extreme activity over a wider variety of conditions. As seen from this broadened perspective, the major scientific problem areas in "solar physics" that can be attacked profitably in the 1980's are the following: (a) The fundamental properties of the solar core, in particular its rotation rate, chemical composition, tem- perature distribution, and details of the processes of nuclear energy generation.

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132 Direct tests of models of the solar interior can be made by three techniques: the observations of the energy spectrum and intensity of the solar neutrino flux; the observation of the mass distribution of the Sun by the measurement of perturbations on the orbit of a close gravitational probe (such as the proposed Star Probe Mission); and the observation of the global oscillations of the Sun. (b) The hydrodynamic structure of the solar convec- tion zone, with particular emphasis on the character of the solar dynamo, the nature of large-scale circulation, and the implications of very-long-period global oscilla- tions. Convection is an essential factor in the evolution of many stars because it ultimately controls the mixing of stellar material and thereby influences the types of thermonuclear processes occurring in the interior and the chemical composition at the surface. Moreover, convection drives the global circulation of cool stars and indi- rectly, through the dynamo mechanism, generates their magnetic activity cycles. The convection zone is thus a basic source of nonthermal energy that may heat stellar chromospheres and coronas and provide energy to accel- erate stellar winds. Only the Sun can be observed in sufficient detail to guide a theory of convection based on first principles. During the past decade, the rotation of the Sun has been studied extensively from both ground and space obser- vatories. Among the many interesting results still to be explained are a difference in rotation rate of 5 percent between the magnetic and nonmagnetic areas in the photo- sphere, details of the global solar rotation, details of magnetic and nonmagnetic areas, depth effects, latitude effects, and long-term variations. Measurements of global circulation patterns and of nonracial pulsations provide important clues to the structure of the solar convection zone. (c) The processes involving small-scale velocity and magnetic fields and the wave modes that determine the thermodynamic structure and dynamics of the solar photo- sphere, chromosphere, and corona and their implications for stellar atmospheres.

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133 A thorough understanding of chromospheric heating is necessary both for its own sake and to understand stellar coronas and stellar winds. Acoustic waves generated by photospheric turbulence provide sufficient energy to pro- duce the initial chromospheric temperature rise, but they probably fail to heat the upper chromosphere and corona. Research in the 1980's should extend current work and, most importantly, initiate fundamental new treaments of the problem, such as detailed studies of the effects of magnetic fields on the dissipation of all possible wave modes. Recent measurements by OS0-8 indicate that acoustic waves generated in the solar convection zone carry insuf- ficient energy to heat the corona. This discovery, as well as the observation of a strong correlation between atmospheric heating and the strength and configuration of the coronal magnetic field, have stimulated theoretical investigations into other heating mechanisms. (d) The physical processes that drive the solar activ- ity cycle, the variations on various time scales in the solar radiation output, and the effect of this variabil- ity on the Earth's upper atmosphere; the relation of this activity to the variability of other stars. The solar activity cycle is basically a magnetic cycle, produced by the arrival at the surface of the Sun of mag- netic fields generated in the convection zone below. The complex surface effects produced by the field involve sun- spots, plages, quiescent and active prominences, coronal arches and loops, flares, and enhanced emission of EUV radiation, x rays, and fast particles, as well as strong disturbances and turbulence in the solar wind. Weir study is of interest for the astrophysics of magnetic field-plasma interaction on stellar scales and also is of practical value because of the effects of long-term solar changes on the terrestrial environment. Evidence for solar effects on our climate is becoming increasingly persuasive. (e) The basic plasma-physics processes responsible for metastable energy storage, magnetic reconnection, particle acceleration and energy deposition in solar flares and related nonthermal phenomena; the implications for high-energy processes in the Universe.

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134 The most difficult phenomenon of plasma and high-energy astrophysics to understand is probably the transformation of slow, large-scale motion and magnetic stress into im- pulsive, energetic, small-scale events. me solar flare represents the -best-observed model of this mechanism. A concentrated theoretical program, in which magnetic recon- nection and particle acceleration are major topics, coup- led with the development of better methods for measuring the accelerating electric field directly as a function of position on the solar disk, is needed for the 1980's. These objectives can be met with the proper complement of instruments on an Advanced Solar Observatory. Flares on dMe flare stars have temperatures, x-ray light curves, and radio-emission properties similar to those of solar flares but have x-ray luminosities and coronal emission measures up to 10,000 times larger than is typical for large solar flares. During the next dec- ade, simultaneous observing programs involving gamma-ray, x-ray, W. optical, and radio observations are needed to explore what additional types of stars flare and to deter- mine whether the basic physical properties and flare mech- anisms of the Sun are similar to those of other stellar types. (f) The structure and plasma dynamics of the solar corona, including the processes involved in heating various coronal structures and initiating the solar wind; the origin of coronal transients; the three-dimensional structure of the interplanetary medium and its implica- tions for cosmic-ray modulation; and the implications for stellar coronas and winds and other astrophysical flows e Two outstanding discoveries of solar physics during the past decade are (1) the determination that coronal holes are the source of recurrent high-speed solar-wind streams of solar-terrestrial importance and constitute a major fraction of the solar wind and (2) that coronal transients are frequent mass-ejection events with large energies (1029-103~ ergs/sec~l) that are associated with a restructuring of the outer corona. The resonance-line and white-light chronographs, x-ray telescope, and mag- netograph on the proposed Solar Coronal Explorer satellite will provide the means of determining the plasma proper- ties of these structures as well as directly measuring the acceleration of the solar wind deep in the corona. Another important problem for the next decade is the determination of the three-dimensional structure of the