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SSUMMARY 3 1.1 INTRODUCTION 7 1.2 SIGNIFICANT ACCOMPLISHMENTS IN THE LAST DECADE 8 Interior 8 Quiet Sun 8 Quiet Hel iosphere 9 Active Sun 10 Active Hel iosphere 1 0 Distant Heliosphere 12 1.3 SCIENCE THEMES FOR THE COMING DECADE 12 Exploring the Solar Interior 1 2 U nderstand i ng the Qu let Su n 1 4 Exploring the Inner Heliosphere 1 6 Understanding the Active Sun and the Heliosphere 17 Exploring the Outer Heliosphere and the Local Interstellar Medium 20 A Prioritized Set of Science Questions for the Coming Decade Requiring Major New Research Initiatives 23 1.4 EXISTING AND ANTICIPATED PROGRAMS 23 Operational Programs and Missions 23 Programs in Development 26 Approved Programs 26 1 .5 RECOMMEN DED N EW I N ITIATIVES 29 Primary Recommendations (Prioritized) 29 Future Missions (Unranked) Requiring Technology Development 36 1.6 NEW RESEARCH OPPORTUNITIES (NOT PRIORITIZED) 39 Instrumentation to Observe the Solar Atmosphere at 300 to 1,000 Angstroms AlGaN Solid-State Detectors for Solar Ultraviolet Observations 40

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2 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Low-Frequency Helioseismology 40 Radar Studies of the Quiet and Active Solar Corona 41 Instrumentation and Techniques for Imaging and Mapping the Global Heliosphere 41 Spectral-Spatial Photon-Counting Detectors for the X-ray and EUV Regions 41 Miniaturized, High-Sensitivity Instrumentation for In Situ Measurements 42 1.7 CONNECTIONS TO OTHER PHYSICS DISCIPLINES 42 Atomic Physics 42 Nuclear Physics 43 Plasma Physics 43 1.8 RECOMMENDATIONS (NOT PRIORITIZED) 43 Policy and Education 43 Other 45 ADDITIONAL READING 45

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PANEL ON THE SUN AND HELIOSPHERIC PHYSICS SUMMARY A revolution in solar and heliospheric physics is in progress. A variety of measurements, together with theory and numerical simulations, have created fresh insights into phenomena that occur in the Sun and the heliosphere and have sharpened our basic understand- i ng of the u nderlyi ng physical processes. Powerfu I mod- ern computing capabilities now allow us to examine these physical processes and predict their observable signatures in considerable detai 1. To continue this revo- lution, the Panel on the Sun and Heliospheric Physics has formed an aggressive plan for solar-heliospheric research in the coming decade. Its plan is built on a systems approach to this broad yet strongly coupled domai n. RESEARCH THEMES The prime guiding principle behind the major re- search issues and challenges for the next decade is to understand the processes that link the Sun-heliosphere- Earth system. The panel's recommended new programs are centered on five basic themes that stretch from the solar interior to the outer heliosphere and beyond: Exploring the solar interior, Understanding the quiet Sun, Exploring the inner hel iosphere, Understanding the active Sun and the helio- sphere, and Exploring the outer heliosphere and the local in- terstellar medium. SCIENCE QUESTIONS FOR NEW RESEARCH INITIATIVES IN SOLAR-HELIOSPHERIC PHYSICS FOR THE COMING DECADE (PRIORITIZED) Within the foregoing themes the panel has identi- fied and prioritized those science questions requiring new initiatives to continue present progress in solar- heliospheric physics across a broad front: 1. What physical processes are responsible for coro- nal heating and solar wind acceleration, and what con- trols the development and evolution of the solar wind in the i nnermost hel iosphere? 2. What determines the magnetic structure of the Sun and its evolution in time, and what physical pro- cesses determine how and where magnetic flux emerges from beneath the photosphere? 3 3. What is the physics of explosive energy release in the solar atmosphere, and how do the resulting helio- spheric disturbances evolve in space and time? 4. What is the physical nature of the outer helio- sphere, and how does the heliosphere interact with the galaxy? OPERATIONAL PROGRAMS AND MISSIONS If the current pace of progress in solar and helio- spheric physics is to continue over the next decade, it is essential that key capabilities of the current space pro- gram in solar-heliospheric physics continue at least until they are replaced by missions in development, by ap- proved missions awaiting development, or by the new initiatives recommended in this report. These capabili- ties are needed for a variety of high-priority science objectives and for the routine monitoring of the Sun and heliosphere that is critical for accurate specification and prediction of short-term space weather and longer-term space climate. In particular, it is essential that NASA maintain capabilities to image the corona at x-ray and extreme ultraviolet (EUV) wavelengths, to image coro- nal mass ejections in white light, to do helioseismology, and to measure the solar wind plasma, magnetic field, and energetic particle variations in near-Earth interplan- etary space. The panel supports the continued active tracking of the Wind mission for targeted research topics and as a backup to the Advanced Composition Explorer (ACE) as a 1 -AU monitor of heliospheric conditions. The panel also specifically recommends continuation of two missions that are uniquely sampling difficult-to-reach heliospheric regions: Ulysses, as long as it is technically possible to do so; and Voyagers 1 and 2, as long as they are capable of providing measurements necessary to characterize the location and nature of the termination shock and heliopause. The panel also recognizes that extended, continu- ous, well-calibrated observations from space are criti- cally important for detecting and measuring the now indisputable variability of the Sun's irradiance and strongly recommends that continuous irradiance mea- surements from both the ground and space be contin- ued indefinitely. Ground-based solar observatories carry out a vari- ety of research programs and also provide valuable long- term synoptic observations. Each of these observatories contributes to one or more of the panel's research priori- ties, as do those of the ground-based neutron monitor network. The panel has not attempted to prioritize the ongoing programs of these institutions.

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4 PROGRAMS IN DEVELOPMENT The panel's recommendations for new initiatives presume that the missions and programs presently under active development and l isted below wi l l become op- erational within the coming decade to address the high- priority science objectives in solar-hel iospheric physics for which they are designed. Solar Terrestria/ Relations Observatory (STEREO). A two-spacecraft mission with identical in situ and remote sensing instrumentation on both spacecraft. STEREO is designed to study the origin and heliospheric propagation of disturbances driven by coronal mass ejections and their products in the ecliptic plane out to 1 AU. So/ar-B. A joint Japanese-U.S.-U.K. mission that provides coordinated optical, EUV, and x-ray measure- ments to determine the relationship between changes in the photospheric magnetic field and changes in the structure of the chromosphere and corona. Synoptic Optical Long-term Investigation of the Sun (SOLIS). A suite of three National Solar Observatory (NSO) instruments at Kitt Peak, Arizona, designed to make sustained and well-calibrated observations relat- ing to long-term solar variabi I ity. Global Oscillations Network Group (GONG++~. Includes identical Michelson Doppler imaging instru- ments at six sites around the world to allow nearly unin- terrupted full-disk observations of solar oscillations and magnetic fields. It is anticipated that the GONG experi- ment, which is in the process of being upgraded, will be operated for at least a solar cycle in order to study how solar interior dynamics evolves over the solar cycle at a wide range of depths. APPROVED PROGRAMS The following approved programs, which are not yet under full development, are prerequisites to the pan- el's recommended new programs. Solar Dynamics Observatory (SDO). A NASA Liv- ing With a Star (LOOS) mission to study the Sun from the subsurface layers of the convection zone to the outer corona. It will carry an array of telescopes to image the inner solar atmosphere over a wide temperature range, an advanced Doppler package to image subsurface structures and detect sunspots developing on the far side of the Sun, an EUV irradiance monitor to study both short- and long-term variations in the solar irradiance THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS that arise in response to changes in the solar magnetic field, and one or more coronagraphs to image the solar corona out to ~15 Rs. Instrumentation proposals for this mission have been submitted and are awaiting selec- tion. tSee note, p. 45.] Advanced Technology Solar Telescope (ATST). A ground-based National Science Foundation (NSF) pro- gram to provide precise, sensitive, high-resolution (0.1 arcsec) measurements of the solar magnetic and veloc- ity fields with a broad set of diagnostics over a wave- length range from 0.3 to 35 micrometers. The telescope will have a very large aperture (4 m) and employ adap- tive optics to attain these measurement goals and will be used to study solar magnetic fields from the density scale length of the photosphere up through the 5,000,000 K coronal plasma. This program is in the definition phase and is a major tech n ical chal lenge. RECOMMENDATIONS FOR MAJOR NEW INITIATIVES (PRIORITIZED) 1. A solar probe mission to the near-Sun region (4- 60 RsJ to determine the origin and evolution of the solar wind in the innermost heliosphere via in situ sam- pling. The region inward of 0.3 AU is one of the last unexplored frontiers in our solar system, the birthplace of the hel iosphere itself. Remote sensing observations and in situ sampling of the solar wind far from the Sun have provided tantalizing glimpses of the physical na- ture of this region. However, to understand how the solar wind originates and evolves in the inner helio- sphere requires direct in situ sampling of the plasma, energetic particles, magnetic field, and waves, as close to the solar surface as possible the panel's top science priority for the coming decade. Such measurements will determine how energy flows from the interior of the Sun through the surface and into the solar atmosphere, heat- ing the corona and accelerating the wind, and will also reveal how the wind evolves with distance in the inner heliosphere. These measurements will revolutionize our basic understanding of the expanding solar atmosphere. The panel therefore strongly recommends a solar probe to the near-Sun region that emphasizes in situ measure- ments of the innermost heliosphere. The generic solar probe recommended by the panel is not necessarily identical to the Solar Probe mission for which NASA released an Announcement of Opportunity in Septem- ber 1999 that placed equal emphasis on both in situ and remote sensing observations. For a first solar probe, the panel strongly believes that the in situ measurements are of the highest priority and should not be compro-

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PANEL ON THE SUN AND HELIOSPHERIC PHYSICS mised. In general, the panel did not find various ration- ales given for including remote sensing instrumentation on a first mission to the near-Sun region to be compel- ling. Nevertheless, the panel appreciates that a solar probe mission provides perhaps the first opportunity to measure the photospheric magnetic field in the polar regions of the Sun via remote sensing. Such measure- ments will address the panel's second science priority and should be the secondary objective of a solar probe mission. 2. The Frequency Agile Solar Radiote/escope (FASRJ to image the Sun with high spatial and spectral resolu- tion over a broad frequency range (0.1 to 30 GHzJ. Radio imaging and radio spectroscopy provide unique insights into the solar chromosphere and corona. Com- bining radio imaging with radio spectroscopy provides a revolutionary new tool to study energy release in flares and coronal mass ejections (CMEs) and the thermal structure of the solar atmosphere in three dimensions. Moreover, radio imaging spectroscopy provides a range of powerful techniques for measuring magnetic fields in the corona. For example, measurements of gyroreso- nance emission can be used to determine the magnetic field strength i n active regions at the base of the corona, observations of gyrosynchrotron radiation from mildly relativistic electrons can be used to probe the coronal magnetic field in solar flares, and multiband Stokes-V observations of solar free-free emission can be utilized to provide a measure of the longitudinal field to strengths as low as a few gauss. In addition, observations of radio depolarization and Faraday rotation can be used to mea- sure even smaller magnetic fields in particular source regions (e.g., CM Es) or along particular lines of sight within the outer corona. FASR will probe both the quiet and the active solar atmosphere and is uniquely suited to measure coronal magnetic fields, nonthermal emis- sions from flares and CMEs, and the three-dimensional thermal structure of the solar atmosphere. Thus it di- rectly addresses aspects of the panel's top three science . . . priorities. 3. Virtual Sun, a focused, interagency theory/mod- eling/simulation program to provide physical under- standing across the Sun-he/iosphere-Earth system. U n- derstanding the physical connections between the Sun, the heliosphere, and Earth is the prime guiding principle behind the major research issues and challenges for the next decade. The system is strongly coupled and highly nonlinear, linking spatial scales from current sheets to the size of the heliosphere and varying on time scales from fractions of a second to millennia. Its complexity has long been an obstacle to a full understanding of key 5 mechanisms and processes, let alone to the construction of global models of the entire system. However, during the last decade we have broadened considerably our theoretical u nderstand i ng of the Su n-hel iosphere-Earth system, have col lected a rich observational base to study it, and have witnessed a rapid development of super- computing architectures. Together, these developments suggest that the time is ripe to complement the U.S. Observational program in solar and space physics with a bold theory and modeling initiative that cuts across dis- ciplinary boundaries. The Virtual Sun program will in- corporate a systems-oriented approach to theory, mod- eling, and simulation and ultimately will provide continuous models from the solar interior to the outer heliosphere. The panel envisions that the Virtual Sun will be developed in a modular fashion via focused at- tacks on various physical components of the Sun- heliosphere-Earth system and on crosscutting physical processes. Two problems that appear ready for such a concentrated attack are the problem of the solar dy- namo and that of three-dimensional magnetic reconnec- tion in the solar atmosphere and heliosphere. Approxi- mately 10 years will be required to achieve the goals of this mission. The panel envisions that the program will require both continuity and community oversight to meet such ambitious goals. In particular, individual com- ponents should be competitively selected and reviewed periodically to assess quantitative progress toward completion of a worki ng Vi rtual Su n model . 4. U.S. participation in the European Space Agency's (ESA'sJ Solar Orbiter mission for a combined in situ and remote sensing study of the Sun and heliosphere 45 Rs from the Sun. Solar Orbiter is a natu- ral successor to SOHO to explore the Sun and its inter- action with the heliosphere. Selected by ESA for launch in the 2008-2015 time frame, Solar Orbiter will use a unique orbital design to bring a comprehensive payload of imaging and in situ particle and field experiments into an elliptical orbit with a peribelion of 45 Rs. At this distance Solar Orbiter will approximately co-rotate with the Sun. An overall goal of the mission is to reveal the magnetic structure and evol ution of the solar atmosphere and the effects of this evolution on the plasma, energetic particles, and fields in the inner heliosphere. The orbital plane will increasingly become tilted with respect to the ecliptic plane, so that near the end of the mission the spacecraft will attain a solar latitude of 38 degrees. Thus, over the course of the mission Solar Orbiter wi l l provide data on the magnetic field and convective flows at high latitude that are essential for understanding the solar dynamo. The panel finds that U.S. participation in this

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6 mission would be a cost-effective way for U.S. scientists to address various aspects of the top three science pri- orities. The mission will be particularly attractive if it includes instrumentation to investigate particle accel- eration close to the Sun. 5. A multispacecraft heliospheric mission to probe in situ the three-dimensional structure of propagating heliospheric disturbances. Solar wind disturbances driven by CMEs are inherently complex three-dimen- sional structures. Our understanding of the evolution and global extent of these disturbances has largely been built on single-point in situ measurements obtained at and beyond 1 AU, although some multispacecraft ob- servations of heliospheric disturbances have been ob- tai ned an d STE REO wi I I provi de stereoscop i c i magi n g and two-point in situ measurements of CME-driven dis- turbances. The panel believes that a multispacecraft heliospheric mission consisting of four or more space- craft less than 1 AU from the Sun, separated in both radius (inside 1 AU) and longitude and emphasizing in situ measurements, promises a significant leap for- ward in our understanding of global aspects of the evo- lution of solar wind disturbances. A mission of this kind will illuminate the connections between solar activity, heliospheric disturbances, and geomagnetic activity and will directly addresses the third science priority; it is an essential element of NASA's Living With a Star program. 6. A reconnection and microscale (RAMS probe to examine the solar corona remotely with unprecedented spatial (~10 kmJ and temporal (millisecond to second resolution. Observations and theory have long indicated that magnetic reconnection plays a key role in rapid energy release on the Sun. Although magnetic reconnec- tion and its repercussions have long been studied inten- sively i n Earth's magnetosphere via both observations and theory, many questions remain about its operation in the solar corona, where physical conditions are con- siderably different from conditions in the magneto- sphere. Moreover, in situ sampling deep in the Sun's atmosphere is clearly out of reach. High-resolution spa- tial and temporal observations of the solar atmosphere are required to make further progress for understanding how magnetic reconnection operates in the solar atmo- sphere, in particular for understanding its role in the magnetic restructuring and rapid energy release charac- teristic of solar disturbances. This mission thus addresses aspects of the second and third science priorities for the coming decade.The panel finds that a RAM mission will also provide data extremely useful for understanding how wave transport and dissipation occur in the solar atmosphere, an aspect of the first science priority. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS 7. An interstellar sampler mission for the remote exploration of the interaction between the heliosphere and the local interstellar medium. The bou ndary be- tween the solar wind and the local interstellar medium (LISM) is one of the last unexplored regions of the helio- sphere. Very little is currently known about the shape and extent of this region or the nature of the LISM. The physical nature of these regions will be studied by an interstellar sampler mission using a combination of re- mote sensing and in situ sampling techniques at helio- centric distances between about 1 and 4 AU. The panel finds that such a pioneering mission would reveal new properties of the i nterstel lar gas and the transport of pickup ions in the heliosphere and would thus directly address the fourth science priority. This mission is a natural precursor to a more ambitious probe to pen- etrate the interstel lar medium directly. PROGRAMS REQUIRING TECHNOLOGY DEVELOPMENT (NOT PRIORITIZED) Several missions have been identified that address the panel's high-priority science questions, but as pres- ently conceived, these missions require further technol- ogy development. The panel recommends that in the comi ng decade NASA develop the necessary technolo- gies (for example, propulsion, power, communications, and instrumentation) to prepare for the following solar- heliospheric missions: (1) an interstellar probe, to pass through the boundaries of the heliosphere and penetrate directly into the interstellar medium with state-of-the-art instrumentation; (2) a multispacecraft mission to obtain a global view of the Sun, to reveal the Sun's polar mag- netic field and internal flows, to provide three-dimen- sional views of coronal mass ejections, and to observe internal flows, surface magnetic fields, and the birth of active regions everywhere; and (3) a particle accelera- tion solar orbiter to investigate particle acceleration in the innermost heliosphere and in solar flares at an obser- vation point 0.2 AU from the Sun. NEW RESEARCH OPPORTUNITIES (NOT PRIORITIZED) The panel recognizes several opportunities for new solar and heliospheric measurements that could provide breakthroughs in understanding, and recommends spe- cifical Iy that the fol lowing measurements and/or devel- opments be pursued with vigor: Instrumentation to observe the chromosphere- corona transition region in the 300-1,000 A band;

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PANEL ON THE SUN AND HELIOSPHERIC PHYSICS Solid-state detectors for solar UV observations; Low-frequency helioseismology measurements to search for g-mode osci I rations; rona; Radar studies of the quiet and active solar co- Instrumentation and techniques for imaging and mapping the global hel iosphere; Spectral-spatial photon counting detectors for x- ray and EUV wavelengths to study reconnection on the Sun; and Minaturized, high-sensitivity instrumentation for in situ measurements. POLICY ISSUES (NOT PRIORITIZED) The panel makes several policy recommendations, some of which parallel those in the 2001 NRC report U.S. Astronomy and Astrophysics: Managing an Inte- grated Program: The panel strongly encourages NASA, NSF, and other agencies that fund solar and heliospheric physics to continue interagency planning and coordination ac- tivities to optimize the science return of ground- and space-based assets. It encourages a similar high level of planning and coordination between NSF's Astronomi- cal Sciences (AST) and Atmospheric Sciences (ATM) Divisions. The panel recommends that NSF plan for and provide comprehensive support for scientific users of its facilities. This includes support for data analysis, related theory efforts, and travel. The panel recommends that NASA support in- strumentation programs, research programs, and soft- ware efforts at national and university ground-based facilities where such programs are essential to the sci- entific aims of specific NASA missions and/or the stra- tegic goal of training future personnel for NASA:s mis- sion. The panel recommends that NSF and NASA study ways in which they could more effectively sup- port education and training activities at national and university-based facilities. This support is particularly needed for training scientists with expertise in develop- ing experiments and new instruments. The national laboratories have capabilities that could be better ex- ploited by the universities. The panel recommends that both NSF and NASA study the idea of forming Centers of Excellence with strong university connections and tied to national facilities as a means of sustaining uni- versity-based research efforts and of educating and 7 training the scientists, technicians, and instrument builders of the next generation. These centers should have lifetimes of 10 to 15 years and should be reviewed every 2 to 3 years to ensure they remain on track. 1.1 INTRODUCTION The Sun is a magnetic star, while the solar wind is both the prototype stellar wind and the only stellar wind we can hope to sample directly with in situ measure- ments. Solar, hel iospheric, geomagnetic, and iono- spheric activity are all linked via the solar wind to the variability of magnetic fields that pervade the solar at- mosphere. Solar activity and resulting heliospheric dis- turbances can have profound impacts on our techno- logical society, while long-term variations in the Sun's total radiative output are thought to affect Earth's cli- mate. The Sun's magnetic field is generated by the mag- netic dynamo processes occurring within the turbulent convection zone that occupies the outer 30 percent (by radius) of the Sun. These fields emerge from beneath the photosphere on a wide range of scales, from small fibril concentrations in the intergranular lanes to large active regions. The Sun exhibits a 22-year cycle of global mag- netic activity, involving sunspot eruptions with well-de- fined rules for field polarity and emergence latitudes during the cycle. A major challenge is to understand the physical processes that produce the Sun's magnetic field, heat the corona, and accelerate the solar wind, and to understand the mechanisms that connect the solar inte- rior to the solar atmosphere, to the heliosphere, and to Earth's magnetosphere. It is also a challenge to measure changes in the solar irradiance, to relate irradiance changes to the evolving solar magnetic field, and to understand how changes in solar irradiance might affect Earth's climate. The totality of the interactions of the Sun with the heliosphere and Earth is the focus of NASA's Sun-Earth Connections Theme and its Living With a Star program, as wel I as of the National Space Weather Pro- gram, led by NSF. These interactions are also of consid- erable practical importance to agencies such as NOAA, DOD, and DOE. Throughout its study the Panel on the Sun and Helio- spheric Physics has considered the Sun and the helio- sphere as a strongly coupled system. In Section 1.2 of th is report the panel h igh I ights some of the sign if icant accomplishments in solar-heliospheric physics from the last decade, while in Section 1.3 it identifies five basic

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8 research themes that encompass most of the major un- solved problems and unexplored frontiers in solar- heliospheric physics for the coming decade. These themes relate to perceived opportunities for the future and extend from the solar interior to the outer helio- sphere: . Exploring the solar interior, Understanding the quiet Sun, Exploring the inner hel iosphere, UnderstandingtheactiveSunandheliosphere, and Exploring the outer heliosphere and the local in- terstellar medium. Within these themes the panel has identified the outstanding science questions that currently are at the cutting edge of solar-heliospheric physics, and at the end of Section 1.3 it prioritizes those questions requiring new initiatives to continue present progress across a broad front. Section 1.4 summarizes existing, in development, and approved programs that are continuing the revolu- tion in solar and hel iospheric physics but that alone wi l l not resolve the panel's prioritized set of questions. The panel's recommended new initiatives are all linked to that set of questions and are described in detail in Sec- tion 1.5. In Section 1.6, the panel identifies several opportu- nities for new measurements that could provide break- throughs in our understanding of solar and heliospheric processes. Section 1.7 discusses the links between solar- heliospheric physics and other physics disciplines. The panel's recommendations on policy and education are provided in Section 1.8, which also provides a final recommendation on program support. 1.2 SIGNIFICANT ACCOMPLISHMENTS IN THE LAST DECADE Recent years have witnessed an extraordinary and ongoing revolution in solar and heliospheric physics, as is evident in the following short, and necessarily limited, summary of some of the research highlights from the last decade. These highlights are spread across the wide THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS domain of solar-heliospheric research and form the ba- sis and framework for our recommended new initiatives for the coming decade. INTERIOR Differential rotation as a function of depth has been measured in the convection zone of the Sun, and surprisingly strong velocity shears were discovered at the base and top of the zone. These shear layers are likely birthplaces of the Sun's large- and small-scale magnetic field, respectively. Large-scale meridional flows from 10 to 30 m/s were discovered within the convection layer. Signatures of newly emerging active regions were detected beneath the solar surface, yielding a potential new tool for predicting future sites of solar activity. Improved pressure and temperature models of the solar interior provided by helioseismic measure- ments sti 11 appear to ru le out an astrophysical sol ution to the neutrino problem; neutrino osci I rations were discov- ered. The solar irradiance has been shown to vary in a complicated way with the advance of the solar activity cycle. The helium mass fraction abundance in the Sun has been sensitively measured to be 0.2468, which dis- agrees with estimates for the cosmic helium abundance. QUIET SUN Even the quiet Sun was found to be ceaselessly dynamic. More than 95 percent of the magnetic flux in the quiet Sun was discovered to emerge from beneath the surface in less than a day (Figure 1.1~. Many coronal loops are heated with i n ~1 0,000 km of their footpoints rather than uniformly or at the loop tops. Coronal plasmas were discovered to be highly inhomogeneous in the cross-field direction on scales of a few hundred kilometers or less, implying a correspond- ing degree of inhomogeneity in the coronal heating mechanism (Figure 1.21. Coronal ions were discovered to have large ther- mal anisotropies, suggesting that ion cyclotron waves may be a dominant source of ion heating in the corona. Microflares were revealed to be common in and near the chromospheric network. Coronal magnetography was pioneered in radio and i nfrared measu remeets.

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PANEL ON THE SUN AND HELIOSPHERIC PHYSICS 9 FIGURE 1.1 The magnetic Carpet" fine-scale concentrations of magnetic flux with opposite polarities (white and black spots, respec- tively) that cover the solar surface and that emerge and disappear on a time scale of ~40 hours.The carpet is shown here superposed on an EUV image of the lower solar corona from the SOHO/EIT experiment. Field lines extending above the carpet as large loops are derived from models using SOHO/MDI measurements of the magnetic field in the photosphere. Courtesy of Stanford-Lockheed Institute for Space Research. QUIET HELIOSPHERE Fast solar wind from high latitudes was found to dominate the three-dimensional heliosphere at solar minimum (Figure 1.31; slow and variable wind from all latitudes was found to dominate the heliosphere in the years prior to and at the solar maximum. A significant portion of the slow solar wind ac- celeration was found to occur well away from the Sun, outtoatleast30Rs. Slow wind and fast wind were discovered to have consistently different ionic compositions and elemental abundances, providing keys to understanding their dif- ferent origins at the Sun. The global structure of corotating interaction re- gions was determined; these interaction regions have opposed north-south tilts in the opposite solar hemi- spheres. Co-rotating energetic particle events were dis- covered at very high solar latitudes near the solar activ- ity minimum, suggesting a new model for the helio- spheric magnetic field. A new source of pickup ions, thought to be solar wind deposited on and re-emitted from interplanetary dust grains, was discovered in the inner heliosphere. The open magnetic flux density was found to be nearly constant with latitude.

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1 0 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS FIGURE 1.2 Coronal loops typically exhibit filamentary structure with cross-field scales down to the resolution limit (several hundred kilometers) of the TRACE telescope, as shown in this image of extreme ultraviolet FelX emissions from coronal plasma at a temperature of ~1 MK. Courtesy of the TRACE team. ACTIVE SUN The vital role of magnetic reconnection was rec- ognized in most forms of solar activity. The notion that solar flares are the cause of CMEs and major space disturbances was seriously challenged; a new magnetic field- and CME-centered paradigm emerged. Blastlike global coronal waves were discovered propagating across the Sun in association with some large flares and CMEs. Two classes of solar energetic particle events were recognized in the heliosphere: impulsive events accelerated during flaring activity and the much larger gradual events accelerated in the solar wind by CME- driven shocks. Trans-iron nuclei with 36 < Z < 83 were found to be overabundant in some impulsive solar energetic par- ticle events by a factor of ~1,000, an important clue for understanding acceleration processes at the Sun. ACTIVE HELIOSPHERE CMEs were firmly established as the cause of tran- sient shock wave disturbances in the solar wind, nonre- current geomagnetic disturbances, gradual solar ener- getic particle events, and Forbush decreases of cosmic rays. The solar wind was discovered to be highly struc- tured at all latitudes during the approach to and at solar maximum.

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PANEL ON THE SUN AND HELIOSPHERIC PHYSICS 1 1 FIGURE 1.3 Solar wind speed and magnetic field polarity measured by Ulysses as a function of heliolatitude during its first polar orbit of the Sun on the declining phase of solar activity and near the solar activity minimum, overlaid with three concentric images of the corona obtained from the SOHO/EIT, the Mauna Loa coronagraph, and the C2 coronagraph on SOHO. Color coding of the speed profile indicates magnetic field polarity: red for outward pointing and blue for inward. Courtesy of the Ulysses solar wind plasma physics team. Mixtures of closed, open, and disconnected mag- netic topologies were discovered within CMEs in the sol ar wi nd. The discovery that suprathermal ions are always present in the slow solar wind provided direct evidence for persistent ion acceleration in the inner heliosphere. The discovery of rapid intensity variations in small solar energetic particle events provided direct evi- dence for the random walk of open field lines on the solar surface. A variety of seed populations for solar energetic and corotating particle events were discovered; the rela- tive importance of these seed populations varies from event to event.

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36 will be a focused, remote-sensing mission designed to meet these goals. Situated at the L1 point or in a polar Sun-synchronous orbit with a nominal lifetime of 3 to 5 years, RAM will provide multiwavelength EUV and soft X-ray observations of the Sun with the fol lowing charac- teristics: Spatial revel ution comparable to esti mated coro- nal current-sheet, nul l-point, and reconnection jet widths (21 0 km, or 0.01 arcsec when viewed from 1 AU); Temporal resolution comparable to the charac- teristic fast-mode wave crossing time of such structures and to typical wave periods (mi l l iseconds to seconds); Spectral resolution and field of view capable of measuring flow speeds into and out of reconnection sites (~10 to 1,000 km/s); Simultaneous imaging of plasmas from transition- region to flare temperatures; Sufficient i magi ng sensitivity to detect emission from wave and shock compressions; and Ful l-Sun context imaging of the surrounding mag- netic field and plasma conditions. By deciphering the evolving dynamics and energet- ics of fine-scale coronal plasmas, a RAM probe will make major breakthroughs on several of the outstanding problems in solar physics, including coronal heating, CME initiation, and solar wind acceleration, thus ad- dressing different aspects of the panel's top three sci- ence priorities. Most of the technologies required to build the above instruments are either direct extensions of wel I -tested methods or are bei ng adapted now from existing commercial/military applications. The estimated life-cycle cost of this Solar Terrestrial Probe (STP)-class mission is $275 million (based on estimates of individu- als advocating the mission). An Interstellar Sampler Mission The boundary between the solar wind and the LISM is one of the last unexplored regions of the heliosphere. Very little is currently known about the shape and extent of this region or the nature of the LISM. Certain aspects of these regions can be studied by a combination of remote sensing and in situ sampling techniques, thus addressing the panel's fourth science priority for the coming decade. An interstellar sampler mission (ISM), which will set the stage for a direct probe of the outer heliosphere boundaries and the LISM, will undertake the following: THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Measure directly the distribution functions of neutral interstellar H. He, N. O. Ne, and Ar and the corresponding pickup ions to establish the physical state of the interstellar gas and the heliospheric transport of . , . pICKUp Ions. Measure simultaneously the solar EUV emission and solar wind plasma properties to determine the ion- ization rates of the neutral gas and their time variations. Measure precisely the elemental and isotopic composition of the interstellar material (specifically, 2H, 3He, 22Ne, and 48O) to extract important information on the evolution of the early universe, galaxy, and Sun. Image the outer heliosphere to reveal its structure and dynamics, using both solar EUV emission back- scattered from interstellar O+ beyond the heliosphere and energetic neutral hydrogen created in the helio- sphere bou ndary region by charge-exchange i nteractions between neutral H and protons accelerated at the termi- nation shock. Measure energetic co-rotating ion events, the contribution of "inner source" dust to heliospheric pick- up ions, and the anomalous cosmic ray component that comes from pickup ions. The ISM orbit will be elliptical with aphelion and peribelion near 4 AU and 1 AU, respectively. The space- craft wi 11 be h igh Iy autonomous and solar-powered us- ing conventional propuIsion. The scientific objectives can be accomplished for $315 million (based on esti- mates of individuals advocating the mission). FUTURE MISSIONS (UNRANKED) REQUIRING TECHNOLOGY DEVELOPMENT Several new missions are needed to achieve high- priority science objectives, but they wi I I not be possi bl e unless new technology (or a new approach) is devel- oped. I n particu far, some missions await advanced pro- pulsion technology, which would also enable other ex- ploratory missions within the solar system. There are also cases where existing technology is available, but new tech nology cou Id provide en tranced capabi I ities or greater access to unexplored regions or new viewing perspectives. An Interstellar Probe Within the next decade the Voyagers will establish the size of the heliosphere and make fundamental dis- coveries about the termination shock and the region beyond, but their 25-year-old instruments will be un-

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PANEL ON THE SUN AND HELIOSPHERIC PHYSICS FIGURE 1.12 The heliosphere from the Sun's center to its outer boundaries, shown on a logarithmic scale stretching from 1 o-3 to 1 03 AU. Courtesy of R. Mewaldt, California Institute of Technology. 37 able to measure many key properties or answer many basic questions. A new mission, an interstellar probe, is needed to carry instruments specifically designed for comprehensive study of the heliospheric boundaries and exploration of our local galactic environment (Figure 1 .12~. Once beyond the heliopause, an interstellar probe will discover the properties of interstellar gas, dust, the interstel lar magnetic field, and low-energy cosmic rays unaffected by the heliosphere. Direct measurements could be made of the composition of interstellar dust and of the elemental and isotopic abundances of ion- ized and neutral gas and low-energy particles, including key species such as 2H, 3He, TIC, and 26Mg. While this mission primarily addresses the panel's fourth science priority, objectives beyond solar and heliospheric phys- ics can also be addressed, including outstanding ques- tions in planetary physics, astrophysics, and cosmology. Although the scientific importance of an interstellar probe has been recognized by previous National Acad- emy of Sciences and National Research Counci I studies and by the NASA Strategic Plan, this mission is unlikely to happen without advanced propulsion technology. The principal scientific goals of an interstellar probe are as follows: Explore the outer heliosphere and the nature of its boundaries. Explore the outer solar system in search of clues to its origin. Explore the interaction of our solar system with the interstellar medium. Explore the nature of the nearby interstellar me- dium. To achieve these broad, interdiscipl inary objectives requires advanced instruments designed for comprehen-

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38 sive, in situ studies of plasma, energetic particles, fields, and dust in the outer heliosphere and LISM, as well as neutral atom and UV imaging instruments to map the large-scale structure and dynamics of the outer solar system and the global hel iosphere. The core instruments for this mission could be built today, but new concepts should also be considered, such as a molecular analyzer to identify organic compounds, a small infrared spectrometer to map dust distributions and the cosmological infrared radiation background, and a small CCD camera to survey Kuiper-Belt objects >1 km in size. This mission would require a radioiso- tope power system, and it would benefit from advanced communications and I ightweight spacecraft and instru- ment technologies. To penetrate significantly into the LISM, an interstel- lar probe shou Id reach at least 200 AU. To reach th is far i n ~1 5 years requ i res a velocity of ~1 4 AU/year (about four times the velocity of Voyager 1), for which ad- vanced propulsion is required. Solar sails and nuclear- electric propulsion appear promising, but neither has been tested in space. The estimated mission cost, in- cluding 1 5 years of mission operations and data analy- sis, is ~$500 million. (This is a very rough guess made by the panel; it assumes a solar sail mission and is based on a previous JPL study.4 When last updated by JPL, the cost estimate for a solar sail interstellar probe was $483 mi 11 ion, rough Iy consistent with the cost quoted here. The panel believes the estimate does not include the cost for developing the solar sail technology, which pre- sumably wi l l be used by other future missions as wel l.) Sending a spacecraft beyond the heliopause to ex- plore our local galactic neighborhood will be one of the grand scientific enterprises of this century. Developing technology to enable our first venture into the space between the stars should have very high priority. A Global Solar Mission Studies of the Sun's corona, activity, and interior are greatly hindered by the fact that almost all observations to date have been made from near Earth. The Sun's rotation hides much of the surface from our view for two weeks at a time, during which substantial changes can occur. Furthermore, the Sun's poles are not completely visible from anywhere in the ecliptic, so our knowledge of the magnetic field, thermal structure, and dynamics ~ Gavit, S.A. 1999. Interstellar Probe Mission Architecture and Tech- nology Report, I eternal document J PL-D-1841 0. Jet Propu lsion Labo- ratory, Pasadena, Cal if., October. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS of the polar regions is incomplete at best. A global solar mission, combining spacecraft on the farside of the Sun (or distributed in longitude about the Sun) with observa- tions from Earth and spacecraft viewing from over the solar poles, would enable a complete instantaneous pic- tu re of the Su n and its activity. Among the accompl ish- ments expected from such a comprehensive mission are these: Measuring for the first time the Sun's evolving polar magnetic field and subsurface polar motions; Probing three-dimensional structures deep be- neath the surface using two-position helioseismology techniques, and predicting when and where active re- gions will emerge over the entire Sun; Probing coronal magnetic fields with x-band and Ka-band Faraday rotation measurements; Examining three-dimensional thermal and mag- netic structures from a polar perspective; Tracking the complete life cycle of active regions and coronal holes; Linking variations in the high-latitude heliosphere to surface conditions; and Measuring the global effects of dynamic events with complementary stereoscopic imaging and in situ observations. These observations would address the panel's sec- ond and third science priorities by exploring the role of polar convection in solar magnetic field evolution, un- derstanding the mechanisms by which magnetic field reversal occurs, exploring the azimuthal and latitudinal structure of the corona and streamer belt, and under- standing the three-dimensional structure of CMEs and polar plumes. An orbital encounter with Venus would place a spacecraft on the far side of the Sun. The more difficult task, however, will be putting a spacecraft into a polar orbit. Preliminary studies of a polar mission, the solar polar orbiter, have evaluated placing imaging and in situ instruments into a circular polar orbit at 0.5 AU, where it would circle the Sun from pole to pole 3 times a year, in 3:1 resonance with Earth. This approach would achieve the desired orbit within 3: years via solar sail propulsion. Solar sails have not yet been tested in space, however, so other advanced propulsion methods should also be considered. A polar perspective could also be reached with conventional propulsion using a Ulysses- type trajectory and, while the orbital period is neces- sarily greater, the key objectives could be met by ~150 to 300 days of observations at latitudes from 60 to 90

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PANEL ON THE SUN AND HELIOSPHERIC PHYSICS degrees. Motivated by the unique promise of these new perspectives for add ressi ng outstand i ng sol ar- heliospheric problems, the panel recommends that a plan be formulated to achieve our first coordinated po- lar and farside views of the Sun. The panel believes that each of the missions envi- sioned for a global view of the Sun will be in the STP class and estimates their cost to be on the order of $350 million each. A Particle Acceleration Solar Orbiter The past decade has witnessed remarkable progress in our understanding of particle acceleration at the Sun and in the heliosphere. While it is now recognized that solar energetic particles (SEPs) are accelerated both in solar flares and at CME-driven shock waves, the details of these particle acceleration processes remain elusive. The 1-AU separation between the primary SEP accelera- tion region in the corona (for both flare and shock SEPs) and the near-Earth satellites that observe SEPs makes it difficult to disentangle acceleration and propagation ef- fects. Thus a mission must travel close to the Sun to observe SEPs in their infancy, before contamination by propagation effects. Such a mission should occur near the peak of the solar activity cycle to maximize the number of events observed. A particle acceleration solar orbiter (PASO) with a 0.2-AU perihelion passage, hover capability, and a suite of high-energy (UV, x-ray, gamma-ray) imagers and particle (electron, neutron, ion composition) detectors, would enable us to relate acceleration signatures at the Sun directly to particles observed in space, one aspect of the panel's third science priority. The mission will undertake the following: Delineate the spatial and temporal evolution of SEP sources. Elucidate basic SEP acceleration and transport processes. Determine the relative importance of electrons and ions for the flare energy budget. Extend the size distribution of flares to lower in- tensities and lower energies to gain insight on coronal heating. Detect low-energy solar neutrons for the first ti me. The estimated cost of the mission is $400 million (based on estimates of individuals advocating the mis- sion). Solar sail technology will be required to reach 0.2 AU, although many of the key science objectives could ~1 ,9 be obtained at lower cost but with reduced sensitivity, with conventional propulsion, and a 0.3- to 0.5-AU or- bit (e.g., as a component of Solar Orbiter). A PASO mis- sion is the next step required to advance our under- standing of solar energetic particles and ultimately will lead to improved predictions of this key space weather hazard. 1.6 NEW RESEARCH OPPORTUNITIES (NOT PRIORITIZED) The panel recognizes several opportunities for new solar and heliospheric measurements that could provide breakthroughs in understanding, and it recommends specifically that the following measurements and/or de- velopments be pursued with vigor. INSTRUMENTATION TO OBSERVE THE SOLAR ATMOSPHERE AT 300 TO 1,000 ANGSTROMS Full characterization of the transition between the upper chromosphere and corona is critical to under- standing how nonthermal heating occurs in the solar atmosphere. These regions are difficult to model be- cause the plasma beta drops rapidly with increasing height from greater than unity to less than 1 off. Obser- vations in these regions are also difficult, for two funda- mental reasons: first, the key physical processes occur very rapidly, thus requiring optical systems with high efficiency; second, the materials commonly used for solar imaging have low reflectivity, between 300 and 3,000 A, where most of the strong spectral lines in the upper chromosphere and transition region, as well as many coronal lines of interest, are formed. With suitable development, improvements upon two existing tech- nologies offer potential avenues for spectroscopic imag- ing of the Sun in this challenging, yet crucial, wave- length range. Most metallic films do not have high reflectivity be- low ~1,000 A. The traditional materials used with broad- band reflectance down to 500 A are silicon carbide, platinum, and osmium. To make good imagers in the sub-1,000-A region, the optical surfaces must be more uniform by a factor of 6 to 10 than for the visible spec- bum. Based on recent developments in fabrication of large silicon carbide optics and chemical vapor deposi-

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40 tion techniques for making polishable high-reflectance coatings, it shou Id be possible to develop si I icon car- bide optics with surfaces sufficiently good to make high- q ual ity, broadband i magers down to ~5 00 A. However, maintaining the shape and surface quality in optics con- structed of these materials remains a problem. On the other hand, normal-incidence EUV coat- ings, which allow narrow-band spectral imaging, neces- sarily have high reflectivity over narrow spectral ranges and do not work wel I above 300 A. Recent research has shown that materials exist that allow fabrication of mul- tilayer coatings at wavelengths as long as 400 A, and there is promise of coating systems for even longer wave- lengths. Relatively modest research efforts on normal incidence coatings should thus allow the production of spectral imagers at wavelengths well above 300 A. AlGaN SOLID-STATE DETECTORS FOR SOLAR ULTRAVIOLET OBSERVATIONS Solid-state detectors made from aluminum-gallium nitride (AlxGa~_xN) materials have outstanding potential to become the detectors of choice for most UV applica- tions in the wavelength range 1,000 to 3,000 A. AlGaN detectors are lightweight and compact and can record photons at very high rates. AlGaN detectors can readily make use of CMOS technologies so that individual pix- els can be addressed randomly for highly versatile and rapid readouts. AlGaN is a wide-bandgap material, mak- ing the detector inherently solar blind at visible wave- lengths, operable at room temperatures without thermal backgrounds, and radiation hard. In addition, AlGaN devices offer very high UV detective quantum efficien- cies (DQE > 85 percent) that are very stable. AlGaN detectors that are solar blind and that have DQEs in excess of 60 percent have already been demonstrated in the laboratory. While AlGaN devices offer great promise as superb UV detectors, the technology is still in its infancy and wi 11 requ i re substantial and prolonged development. The primary issue for UV solar applications is a very large nonthermal background produced by material defects. Fortunately, there is considerable (multibillion dollar) commercial and military interest in this material, includ- ing the desire for a solar-blind UV image sensor. Tre- mendous strides have been made in recent years to re- duce the defects and dislocations responsible for these unwanted backgrounds. The time is ripe to take advan- tage of the enormous worldwide investment being made in AlGaN materials research to develop UV image sen- THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS sors that meet the unique set of requirements for solar and hel iospheric appl ications. LOW-FREQUENCY HELIOSEISMOLOGY During the past decade, acoustic helioseismology was highly productive as a probe of the solar interior. Nevertheless, our last unexplored region of the solar interior remains the innermost core the region ulti- mately responsible for defining the solar luminosity and the solar neutrino flux. Unfortunately, the inner 10 to 20 percent of the Sun's radius is nearly impenetrable to surface acoustic waves. A potential exists to sense the kinematic and ther- modynamic structure of the core remotely by measuring lower-frequency waves in the visible photosphere modes with periods of tens of minutes to many hours, in contrast to 5-min acoustic modes. While buoyancy waves (solar "g-modes") may still bear fruit for this prob- lem, the absence of progress on this front, despite two decades of effort, suggests that new techniques should be sought. 20 15 : ~ ~ : : : ::: : :: : : : ~ :: :: :: ~ ~ :: : ::: : ~ :: ~ : '. ~. : ~-~ -^~.~; Low-1 Velocity noise ~ il ~. ~ ~ 15 1n 10 ~ . 5: ~ .~ ~ Low-1 MDI astrometry noise 100 200 300 400 500 Frequency (microHz) 600 700 FIGURE 1.13 The solar background noise at low frequencies, a regime where g-modes or r-modes could be detected, is illus- trated here. Root-mean-square astrometric noise levels, ex- pressed in cm/s, decrease at lower frequencies (as computed from SOHO/MDI limb astrometry, lower panel), while the solar velocity noise increases with lower frequency (upper panel). Courtesy of T. Appourchaux, Solar System Division, ESA-European Space Research and Technology Center.

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PANEL ON THE SUN AND HELIOSPHERIC PHYSICS A promising avenue may be the use of solar astro- metric techniques, which are especially sensitive to low- frequency g-mode and inertial (r-mode) oscillations. Figure 1.13 shows how the solar noise background de- clines, and thus the modal sensitivity increases, with decreasing signal frequency using astrometric tech- niques. In contrast, current Doppler methods suffer from increasing noise with lower frequency. RADAR STUDIES OF THE QUIET AND ACTIVE SOLAR CORONA Although passive radio-wave observations of the Sun and interplanetary medium are well established as important observational tools, active radio ranging of the near-Sun environment has had little development since the pioneering work of J.C. James in the 1 960s, which demonstrated that radar echoes could be ob- tained under many conditions. The data contained a variety of puzzling features, including anomalously high radar cross sections and Doppler features. A key ele- ment of the interpretational difficulty was our primitive understanding of coronal structure and dynamic activity at that time: for example, neither CMEs nor coronal holes had yet been discovered. Moreover, synoptic solar mag- netic field observations are not available for that time period so it is impossible, even retrospectively, to relate the observed echoes with I ikely coronal structures. Now, however, detailed contextual observations of the corona are routinely available and radar ranging techniques are mature. Given these advances, any rea- sonable opportunity to explore the possibilities of solar radar as an observing tool should be exploited. Active sensing of the solar corona offers the possibility of map- ping coronal structures, of detecting CMEs and measur- ing their radial velocity components, and of exploring wave-wave interactions in the solar corona. INSTRUMENTATION AND TECHNIQUES FOR IMAGING AND MAPPING THE GLOBAL HELIOSPHERE Most in situ heliospheric observations are local in nature, and it is often difficult to understand the global context of such measurements. New technologies for detecting energetic neutral atoms (ENA) and certain UV lines show considerable promise for performing mea- surements that can directly determine key aspects of the three-dimensional structure of the heliosphere and, pos- sibly, its time dependence. Similarly, data analysis ap- proaches using tomographic techniques have demon- strated new capabilities for imaging the large-scale 41 structure of the inner hel iosphere. Continued progress in these areas will depend critically on integrating new observations with advanced models in an iterative man- ner. The panel recommends further development of the following new technologies for imaging the global hel iosphere. ENA imaging. Newly developed ENA-imaging in- struments and techniques have been demonstrated for magnetospheric applications by the IMAGE mission. The same imaging principles should apply to the remote sensing of energetic particle acceleration processes on the Sun, in interplanetary space, and at the outer bound- aries of the hel iosphere. Although current techniques have been limited by very small geometric factors, more sensitive designs using new detection methods and background-suppression techniques appear promising. TV imaging. It has recently been proposed that the global shape ofthe heliosphere can be mapped by measuring solar EUV line radiation that is back-scat- tered by galactic plasma beyond the heliopause. Map- ping of the heliopause can be carried out using the oxygen (O+) resonance line (83.4 nm) provided that sufficient improvement in detection sensitivity can be achieved. This technique may also provide an indepen- dent measu re of the ion ization state of the local i nter- stellar medium. He/iospheric tomography. Within the inner helio- sphere, tomographic techniques have been applied to white-light images of the corona and radio observations of interplanetary scintil ration in efforts to image large- scale solar wind structures. Considerable progress has been made in reconstructing images of quasi-steady co- rotating structures, and these techniques are now being applied to transient disturbances such as those driven by CMEs. Given the importance of understanding such structures on a heliospheric scale, the panel recom- mends the continued development of tomographic tech- niques to image structures in the inner heliosphere. SPECTRAL-SPATIAL PHOTON-COUNTING DETECTORS FOR THE X-RAY AND EUV REGIONS Localized plasma heating and jetlike Alfvenic flows are key signatures of magnetic reconnection that are difficult to detect with present-day instrumentation. To probe and understand reconnection in solar activity, a new type of instrument must be developed that meets the following stringent requirements: (1) the ability to resolve a wide range of plasma velocities from ~10 to 1,000 km/s both along and transverse to the line of sight;

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42 (2) enough sensitivity to capture the onset of a CME, flare, or other reconnection-driven event; and (3) spatial resolution commensurate with the scale of the reconnec- tion site and the width of the jets, coupled with a suffi- ciently large field of view to capture the dynamic and topological effects of reconnection. A new class of detectors microcalorimeters, super- conducting tunnel junctions, and transition edge sen- sors, which simultaneously provide spatial, spectral, and temporal i Information shows promise as near-ideal sen- sors for studying magnetic reconnection. While these detectors differ in their modes of operation, all are pho- ton-counting detectors that have high (2 to 10 eV) en- ergy resolution in the soft x-ray range from 1 to 10 keV, temporal resolution of hundreds of microseconds to mil- I iseconds, and the capabi I ity of bei ng fabricated as pixel arrays. Astronomical experiments employing such de- tectors have already flown in space, and detectors in this class are the baseline for several contemplated fu- ture astronomical missions such as Constellation X. Recommended missions such as a reconnection and microscale probe (see, in Section 1.5, subsection "A Reconnection and Microscale Probe") would benefit greatly from full development of these imaging spectros- copy technologies. For solar physics applications these detectors must be adapted to handle higher count rates than are experienced in astronomical applications. Suit- ably modified, they will be able to detect and measure the highly time-dependent and directional flows that are the hallmarks of magnetic reconnection in the solar co- rona, both along and transverse to the line of sight and to record the associated thermalization of the ambient plasma. Experiments employing these detectors can pro- vide the first fully three-dimensional picture of recon- nection in energetic solar phenomena, from coronal heating to coronal mass ejections, thus answering some of the most fundamental questions in solar and helio- spheric physics (themes 2 and 41. MINIATURIZED, HIGH-SENSITIVITY INSTRUMENTATION FOR IN SITU MEASUREMENTS The scientific payloads for exploratory missions such as an interstellar probe will be allotted only limited mass and power resources. Although most of the core instru- ments envisioned for this mission have considerable flight heritage and could be built today, there is a need for miniaturized, low-power versions of existing designs if the full range of science objectives for this and other exploratory missions is to be achieved. In addition, new technology is needed to develop instruments such as a THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS smal I neutral gas spectrometer, a smal I mol ecu I ar ana- lyzer, and a small infrared spectrometer. Missions within the heliosphere, such as ISM, that will remotely sample the interstellar medium need a high-sensitivity neutral gas spectrometer to measure a variety of interstellar neutral species, a high-sensitivity pickup ion spectrometer to measure the isotopic com- position of the interstellar gas, and a high-energy spec- trometer to measure ionic charge states to higher ener- gies than is presently possible. 1.7 CONNECTIONS TO OTHER PHYSICS DISCIPLINES Solar-heliospheric physics shares with other physics disciplines an interest in nuclear fusion, magnetic dyna- mos, magnetoconvection and turbulence, collisionless shocks, magnetic reconnection, energetic particle ac- celeration and transport, emission and absorption of electromagnetic radiation, instabilities (kinetic, fluid, and MHD), neutrino detection, multiwavelength spec- troscopy, and other diagnostic techniques. Over the past several decades, cross-fertilization among these various disciplines has advanced our understanding of the Sun and heliosphere. The panel therefore recommends the continuation of this highly fruitful interchange through- out the next decade, with particular focus on three re- search areas: atomic physics, nuclear physics, and plasma physics. ATOMIC PHYSICS Spectroscopy is used to map the physical and dy- namic properties of the solar atmosphere and has pro- vided opportunities for fundamental discoveries such as the existence of the element helium. Recent advances in computational technology are enabling more accurate multilevel atomic-physics calculations, which will im- prove line identification and interpretation, and more detai led calcu rations of the ti me-dependent ion ization balance essential for deciphering the sources of chro- mospheric and coronal heating. As high-cadence, high- resolution observations become available, a substantial effort will be required to incorporate these new theoreti- cal capabilities and laboratory measurements into analy- sis and interpretation of the data. The interpretation of certain types of particle obser- vations depends critically on our knowledge of several

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PANEL ON THE SUN AND HELIOSPHERIC PHYSICS complex processes, including EUV and collisional ion- ization, recomb) nation, charge exchange, and electron stripping of high-energy particles. Accurate modeling of these processes, including the effects of charge-to-mass ratio dependent fractionation during particle accelera- tion and transport, is essential for interpreting the mea- sured ionic charge state and elemental composition of thermal, suprathermal, and more energetic particles in the solar wind. For example, comparisons between ionic charge-state observations and models of shock accel- eration near the Sun, i n interplanetary space, and at the termination shock have been used recently to interpret the time scale for such acceleration processes. The Sun- heliosphere system provides unique opportunities for observing these processes, but proper interpretation of energetic ion composition measurements requires cross sections that have been measured in the laboratory or calculated with high precision. NUCLEAR PHYSICS Attempts to resolve the "solar neutrino problem" have produced a very close interplay between particle and nuclear physics, stellar structure theory, and helio- seismology. Hel ioseismic i aversion of p-mode frequen- cies, coupled with theoretical models for stratification in the nuclear burning core, has placed fairly tight bounds on neutrino production, thus ruling out many novel sug- gestions for how that production could be reduced. Along the way, substantial improvements in the theo- retical calculation of opacities and equations of state for the high-temperature, high-density plasma at intermedi- ate depths within the solar radiative interior were de- vised in order to bring solar structure models and helio- seismic inferences into close agreement. The recent apparent detection of other solar neutrinos, possibly aris- ing from electron neutrinos changing flavor during flight, may offer another path for resolving this major chal- lenge. PLASMA PHYSICS Our solar system, from the Sun's core to the helio- pause, is mostly composed of magnetized plasma that is, ionized gas threaded by magnetic fields. Conse- quently the Sun and the heliosphere behave on micro- scopic scales largely as a collection of ions and elec- trons whose dynamical properties are shaped (if not dictated) by the magnetic field, and on macroscales as a magnetofluid. Our proximity to this vast and variable plasma laboratory has yielded enormous benefits for 43 both astrophysics and laboratory astrophysics. The Sun and heliosphere routinely "perform" experiments in pa- rameter regimes that cannot be enacted on Earth, pro- viding insights into the basic physical mechanisms that govern the behavior of plasmas. For example, magnetic reconnection (see, in Section 1.3, "Understanding the Active Sun and the Heliosphere" and in Section 1.5, "Primary Recommendations (Prioritized)") a process equally important to space plasmas and magnetic-con- finement devices was first hypothesized as a way to expl al n sol ar fl ares and magnetospheric convection. I n turn, our understanding of the Sun and heliosphere has been revolutionized over the past 50 years by an ongo- ing influx of plasma theory and numerical modeling techniques developed for and applied to fusion devices and other laboratory experiments. Laboratory experi- ments, such as the Princeton Plasma Physics Laboratory Magnetic Reconnection Experiment, can probe funda- mental MH D and kinetic phenomena with a combined temporal and spatial resolution that exceeds what can presently be achieved in space. The funding agencies have acknowledged the benefits of this mutually benefi- cial relationship by supporting interdisciplinary studies through such programs as International Solar-Terrestrial Physics (NASA) and the Science and Technology Cen- ters (NSF). The panel enthusiastically encourages the continuation and expansion of this cross-fertilization among all branches of plasma physics, which is making an essential contribution to our understanding of the physical laws governing the plasma universe. 1.8 RECOMMENDATIONS (NOT PRIORITIZED) POLICY AND EDUCATION Solar and heliospheric physics is a large and com- plex enterprise involving academic institutions, feder- ally funded centers, observatories, and space missions, as well as commercial interests. To sustain a vigorous research community it is necessary to achieve the proper level and balance of resource al location and investment in the above. The research community, in turn, is needed to attain long-term strategic goals and to maintain U.S. leadership in science and technology. Despite the pres- ent overall health and vitality of the solar and helio- spheric physics community and the impressive advances of the past decade, there are serious concerns about the

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44 way in which solar and heliospheric physics is currently supported. Ground-based national research faci I ities for so- lar and heliospheric physics are funded primarily by NSF, which is divided between the AST and ATM divi- sions under the Mathematical and Physical Sciences and the Geosciences Directorates, respectively. On the other hand, the ground-based solar facilities operated by uni- versities are primarily supported by NASA through its SR&T program, with secondary support from NSF. Space-based programs are funded mostly by NASA, with much smaller roles played by NOAA and DOD. Data analysis and theory efforts are mostly supported by rela- tively modest grants at NSF and by mission-specific pro- grams, the SR&T program, and the Sun-Earth Connec- tion theory program at NASA. Solar and heliospheric physics, perhaps more than any other discipline, relies extensively on multiwavelength observations from both ground-based facilities and space-based missions that are funded by multiple agencies. Interagency planning and coordination is therefore of critical importance. The excel lent coord i nation between NSF, NASA, and other agencies, developed in recent years for joint ventures such as the National Space Weather Program, needs to be mai ntai ned i n the futu ret NSF has a large investment in ground-based fa- cilities that it builds and operates. It invites scientists to use those ground-based facilities but does not routinely sponsor the data analysis activity required to maximize the science return from its facility investment. Those analysis efforts often represent the largest share of an individual scientist's research effort. In addition, in many cases NSF does not fully fund the travel required for a scientist to use its facilities. NASA generally has been effective in supporting mission-specific research, although support for theory associated with missions has been uneven. To enable the systems approach of the LWS program, NASA has recognized the need to integrate theory and modeling into all phases of mission development and deployment and has created a targeted theory, modeling, and data analysis "mission" within LOOS. The panel encourages similar approaches to integrate theory and modeling into future mission programs. Ground-based solar and heliospheric facilities routinely provide critical data that enable space-based missions to meet their science goals and/or enhance their scientific return. They also serve as important train- ing grounds for the scientists, technicians, and instru- ment builders upon which NASA relies. Yet NASA's sup- THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS port of and investment in ground-based facilities through its SR&T program is fragile and uneven despite the fact that a number of present and future NASA programs depend on auxi I iary support from ground-based faci I i- ties for mission success. Both NASA and NSF rely heavily on university pro- grams as training grounds for scientists, engineers, tech- nicians, and instrument builders. The continued success of space-based missions and ground-based facilities re- quires that university programs thrive. In fact, however, university-based programs have faltered in recent years, at least in part owing to short funding cycles, lack of outreach to promising students, and a shift of some tech- nical activities, for example instrument building and development, to commercial enterprises and NASA cen- ters. Increasingly the universities find it difficult to come up with the resources to develop new instruments and concepts. The net effect has been a decreased rate of production of educated professionals having solar and space physics expertise, with the shortfall of trained ex- perimentalists being particularly acute. It is widely rec- ognized thatthe national shortfall of qualified personnel will intensify during the next 5 years. For example, in that period over half of NASA center scientists and engi- neers will become eligible for retirement. The scientific community and the funding agencies need to be proac- tive in attracting qualified students to the scientific en- terprise. As a consequence of the above concerns, the panel makes several recommendations. Some of these recom- mendations, although independently arrived at by the panel, closely parallel recommendations in the 2001 N RC report U.S. Astronomy and Astrophysics: Managing an Integrated Program. The panel strongly encourages NASA, NSF, and other agencies that fund solar and heliospheric physics to continue interagency planning and coordination ac- tivities that will optimize the science return of ground- and space-based assets. It encourages a similar high level of planning and coordination between the AST and ATM divisions of NSF. The panel recommends that NSF plan for and provide comprehensive support for scientific users of its facilities. This includes support for data analysis, related theory efforts, and travel. The panel recommends that NASA support in- strumentation programs, research programs, and soft- ware efforts at both national and university ground- based facilities where such programs are essential to

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PANEL ON THE SUN AND HELIOSPHERIC PHYSICS the scientific aims of specific NASA missions and/or to the strategic goal of training future personnel critical to NASA,s mission. The panel recommends that both NSF and NASA study ways in which they could more effectively sup- port education and training activities at national and university-based facilities. This support is particularly needed for training scientists with expertise in develop- ing experiments and new instruments. The national laboratories have capabilities that could be better ex- ploited by the universities. The panel recommends that both NSF and NASA study the idea of forming Centers of Excellence with strong university connections and tied to national facilities as a means of sustaining uni- versity-based research efforts and of educating and training the scientists, technicians, and instrument builders of the next generation. These centers should have lifetimes of 10 to 15 years and should be reviewed every 2 to 3 years to ascertain they remain on track. OTHER The past decade has witnessed an explosion in col- laborative efforts aimed at understanding the Sun- heliosphere-Earth connection. A most positive develop- ment has been the growth of broad community organizations, such as RISE, CEDAR, GEM, and, more recently, SHINE, which is concerned almost exclusively with the panel's third science priority. These organiza- tions span what had been a deep gulf between pure scientific research and applied space weather studies. Although all of these efforts originated as grass-roots coalitions of researchers, they have flourished with sup- port from NSF. The panel strongly recommends that NSF continue its support for these groups, in particular SHINE. ADDITIONAL READING A strategy for the conduct of space physics research has been set down in a number of reports by the NRC's Space Studies Board and its predecessor the Space Sci- ence Board. These reports i nc I ude the fo I I owi ng: Note added in proof: As a result of selections made in August 2002, SDO is now in development. 45 Space Science Board, National Research Counci 1. 1985. An Implementation Plan for Priorities in Solar-System Space Physics. National Academy Press, Washington, D.C. Space Science Board, National Research Counci 1. 1983. The Role of Theory in Space Science. National Academy Press, Washington, D.C. Space Science Board, National Research Counci 1. 1980. Solar-System Space Physics in the 1980's: A Research Strategy. National Academy of Sciences, Washington, D.C. Space Studies Board, National Research Counci 1. 1995. A Science Strategy for Space Physics. National Academy of Sciences, Washington, D.C. Space Stud ies Board and Board on Atmospheric Sciences and Climate, National Research Council. 1991. Assessment of Programs in Solar and Space Physics1991. National Academy Press, Washington, D.C. The research in this field is summarized in both textbooks and conference proceedings, including the fol lowi ng: J.L. Kohl and S.R. Cranmer (eds.~. 1999. Coronal Ho/es and Solar Wind Acceleration. Kluwer Academic Publishers, Dordrecht. S. Habbal feds. 1997. Robotic Exploration Close to the Sun: Scientific Basis. AIP Conference Proceedings 385, Woodbury, New York. T. Bastian, N. Gopalswamy, and K. Shibasaki (eds.~. 2001. Solar Physics with Radio Observations. NRO Report 479. N.U. Crooker, J.A. Joselyn, and J. Feynman (eds.~. 1997. Coronal Mass Ejections. AGU Monograph 99. American Geophysical U n ion, Wash i ngton, D.C. R.A. Mewaldt, J.R. Jokipii, M.A. Lee, E. Mobius, and T. H. Zurbuchen (eds.~. 2000. Acceleration and Transport of Energetic Particles Observed in the He/iosphere. AIP Conference Proceedings 528. American Institute of Physics, Melville, N.Y. A. Balogh, R.G. Marsden, and Ed. Smith (eds.~. 2001. The He/iosphere Near Solar Minimum: The Ulysses Perspective. Spri nger, Ch ichester, U . K. S.R. Habbal, R. Esser, J.V. Hollweg, and P.A. Isenberg. 1999. Solar Wind Nine. AIP Conference Proceedings 471. American Institute of Physics, Woodbury, N.Y.

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46 P.C.H. Martens and D.P. Cauffman (eds.~. 2002. Multi- Wavelength Observations of Coronal Structure and Dynamics. COSPAR Colloquia Series Vol. 13. Pergamon, NewYork, N.Y. O. Engvold and J.W. Harvey (eds.~. 2001. Physics of the Solar Corona and Transition Region. Proceedi ngs of the Monterey Workshop, August 1999. Kluwer Academic Publishers, Dordrecht. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS P. Song, H.J. Singer, and G.L Siscoe (eds.~. 2001. Space Weather. Geophysical Monograph 1 25. American Geophysical Union, Washington, D.C. E.R. Priest and T. Forbes. 2000. Magnetic Reconnection: MAD Theory and Applications. Cambridge University Press, Cambridge, U.K.