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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
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
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.
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-
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
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;
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
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.
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.
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.
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.
1 2 DISTANT HELIOSPHERE · The radio signature of the interaction of globally merged interaction regions with interstellar plasma just beyond the heliopause provided the first direct measure of the size of the heliosphere. · Interstellar pickup OH, 3He, N. O. and Ne were discovered and the cosmologically significant 3He/4He ratio was measured. The composition of the local inter- stellar material was deduced from pickup ion and anom- alous cosmic ray measurements. · A gradual deceleration of the solar wind associ- ated with mass loading by pickup ions was detected in the distant heliosphere. · The termination shock was confirmed to be a powerful accelerator and found to energize some par- ticles up to GeV energies. · Cosmic rays were discovered to have limited ac- cess to the heliosphere over the poles of the Sun, con- trary to expectations, wh i le the mod u ration of cosmic rays was observed to extend to distances beyond 100 AU. · A hydrogen "wall" was predicted and detected at the nose of the heliosphere in the direction of the solar system's motion through the local interstellar medium. 1.3 SCIENCE THEMES FOR THE COMING DECADE The panel's recommendations for new initiatives in solar and heliospheric physics in the coming decade are based on science questions arising in five basic research themes; these research themes and the underlying sci- ence questions are discussed below in some detail. At the end of this chapter, the panel prioritizes the science questions requiring new research initiatives in the com- ing decade. EXPLORING THE SOLAR INTERIOR The complex turbulence within the solar convec- tion zone exhibits remarkable properties that have largely defied theoretical explanation. One fundamental q uestion concerns how convection red istri buses angu I ar momentum to produce a differential rotation that varies with radius and latitude. A second question concerns local and global magnetic dynamo processes occurring within the convection zone. The panel believes local THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS chaotic processes cause small-scale magnetic fields, while global processes create the cyclic magnetic active regions that have very well-defined rules for field polar- ity and emergence latitudes. These two issues are inti- mately I i n Led, for the global dynamo action yield i ng the 22-year cycles probably is very sensitive to the Sun's internal angular velocity profiles and meridional circu- lations. Little is known in detail about the operation of the global dynamo, partly because it is a difficult region to observe and partly because of problems in simulating the complex and highly nonlinear dynamics in the con- vection zone. A major challenge for the coming decade is to understand the complex operation of this deep convective shell that is responsible for solar magnetism. Helioseismology, the study of the acoustic p-mode oscillations of the solar interior, has provided a remark- able new wi ndow for study) ng dynamical processes deep within the Sun. Nearly continuous helioseismic observations from SOHO and GONG have revealed that the deeper radiative interior rotates as a solid body, pos- sibly owing to the existence of primordial magnetic fields, while the convection zone exhibits prominent differential rotation. These two regions are joined at a complex shear layer, the tachocline (Figure 1.41. Near the surface a thin but pronounced shear boundary layer, in which the angular velocity increases with depth at intermediate and low latitudes, has been discovered. Local domain helioseismic studies have begun to probe that near-surface shear layer, revealing large-scale flow patterns that are modulated by surface activity, evolving meridional circulations, propagating banded zonal flows, and complex, evolving flow patterns that may be associated with the largest scales of deep convection (Figure 1.5~. It is anticipated that the upgraded GONG experiment will operate for at least a solar cycle in order to study the evolving solar interior dynamics over a wide range of depths for an extended period. In addition, the helioseismology experiment on the Solar Dynamics Ob- servatory (see "Operational Program and Missions," in Section 1.4) will provide an unprecedented examina- tion of the near-surface convection zone region. What Is the Origin of the Solar Cycle? Observations and simple models suggest that strong, organized, toroidal magnetic fields, generated from ex- isting poloidal fields in the tachocline, emerge through the photosphere to form active regions. The weaker poloidal field is then thought to be regenerated either by turbulence throughout the convection zone or by the breakup of twisted active regions near the surface. Re-
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS 1 3 FIGURE 1.4 Cutaway images of solar rotation in January (left) and July (right) 1996 illustrating changes in internal rotation with time. Near the surface, faster rotation is indicated by red, slower by green, and intermediate by yellow; below 0.85 R. faster rotation is indicated by red and slower by blue.The left-hand side of each sphere shows the surface view.The white line indicates 0.71 R. the base of the convection zone that appears to coincide with the position of the tachocline. Courtesy of the National Solar Observatory. cent mean-field dynamo models have shown that a dy- namo that generates toroidal and poloidal fields in sepa- rate regions can produce field strengths similar to those i nferred from observations. Hence, d inferential rotation, cyclonic turbulence, and magnetic buoyancy al I must play important roles in the generation of the observed large-scale magnetic fields associated with the 22-year activity cycle. Rapid advances in massively parallel computer architectures are now enabling detailed stud- ies of the processes thought to be crucial to the global dynamo. Although fully self-consistent MHD simulations of the dynamo will be computationally infeasible into the near future, many elements of solar dynamo pro- cesses can and should be tackled in the next decade within a focused theory initiative (see "Focused Theory/ Modeling/Simulation Mission: AVirtual Sun," in Section 1 .5). What Is the Structure Beneath the Convection Zone? The radiative zone spans the inner 70 percent by radius of the solar interior, with the innermost 25 per- cent occupied by the nuclear burning core. The stratifi- cation and rotation of much of this deep interior have been probed with helioseismology using p-mode data, and the results have greatly improved stellar structure models. The helioseismic inversions also imply that the nuclear core probably has not experienced recent dy- namic overturning events that would have mixed the core composition. The now well-determined models yield estimates of neutrino generation that are about threefold greater than what has been detected experi- mentally. The recent discovery of neutrino oscillations may reconcile neutrino observations with structural models. In addition, there are hints of interesting 1.3- year variations in rotation rates that are out of phase above and below the tachocl i ne, suggest) ng dynamic magnetic links between the convection zone and the radiative interior (Figure 1.4~. Our knowledge of the in- nermost Sun remains very incomplete, largely because few p-modes can penetrate into these central regions. The critical issues summarized above call for renewed efforts to seek to detect internal gravity (g-modes) asso- ciated with the deep interior, with astrometry possibly providing new routes to be explored (see "AlGaN Solid- State Detectors for Solar Ultraviolet Observations," in Section 1.6~.
14 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS . ~ 3; . as ~20 ~ 30 60 90 180 ~~. C~x^:rington Longitude ~ ~ '0 SOU ' ~ :~` ~ ~ SeD : - ~llF FIGURE 1.5 Local helioseismic probing of subsurface flows with ring-diagram methods reveals solar subsurface weather (SSW). Shown are synoptic maps of horizontal flows at a depth of 10.2 Mm for Carrington rotations 1 923,year 1997 (bottom),and 1 974,year 2001 (top), underlaid with surface magnetic field patterns (red and green indicate field polarity). In 1997, when the Sun is quiet magnetically, SSW involves meandering jets and wavy flow structures; the low-order undulations in longitude are reminiscent of jet stream flow in Earth's atmosphere. In 2001, when the Sun is active, the large-scale patterns of SSW are strikingly different, with the evolving flows exhibiting major deflections in the vicinity of active complexes.The Northern Hemisphere in 2001 shows the reversed circulation of a second meridional cell directed equatorward at midlatitudes, whereas in the Southern Hemisphere the meridional flows are consistently poleward. SOURCE: D.B. Haber, B.W. Hindman, J.Toomre, R.S. Bogart, and R.M. Larsen, 2002, Evolving submerged meridional circulation cells within the upper convection zone revealed by ring-diagram analysis,Astrophys.J. 570: 855-864. UNDERSTANDING THE QUIET SUN The "quiet Sun" is a misnomer, as a decade of in- tense scrutiny has revealed. The magnetic field inside and outside active regions emerges through the photo- sphere, spreads, and cancels constantly, accompanied by ceaseless dynamic and radiative manifestations of ongoing energy release on a wide range of spatial and temporal scales. Even long-lasting structures like promi- nences are far from static. Some fraction of the energy emerging through the photosphere heats the solar atmo- sphere to temperatures far above the 6000 K photo- sphere. We know that heating the corona requires only a small fraction of the energy flux heating the chromo- sphere, yet both regions still present major theoretical and observational challenges to understanding the heat- ing processes. A coherent program of observations and model development for the evolving plasma and mag- netic field properties in the chromosphere and corona is needed to address the following unsolved problems dur- ing the next decade.
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS How Does Magnetic Structure Emerge and Evolve? Solar magnetic flux emerges on virtually every scale of convection. In active regions, where the flux distribu- tion as a function of size is now well characterized, the fl ux turnover time appears to be days. In ephemeral regions, which collectively contain over 95 percent of the photospheric flux, the turnover time is much shorter, on the order of several hours. Strong magnetic fields of mixed polarity, which have spatial scales at or below the resolution limit of current instrumentation, collect in the chromospheric network, which as a result is poorly characterized. An active topic of investigation is to de- termine how convection distorts and twists these flux concentrations as they rise through the convective zone and penetrate the photosphere. Existing models are highly simplified and generally treat one or two discrete flux tubes instead of the complex, continuous distribu- tion present both above and beneath the Sun's surface. The large-scale distribution of magnetic flux on the Sun can store large amounts of energy, which may be released gradually or explosively, and ultimately deter- mines the coupling from the Sun to the heliosphere. The small-scale, mixed-polarity fields, on the other hand, may provide most of the energy that heats the "quiet" corona and powers the solar wind. To understand the evolving distribution of fields over the Sun and into the heliosphere throughout the solar cycle, we must identify and understand the sources of magnetic flux, the flux emergence process, mechanisms for spreading the flux, and the manner in which flux disappears from the sur- face. Progress has been made on all fronts over the last decade, due to new observations, ideas, and models as well as to advances in computer capabilities, but more work in this area is needed. The panel recommends two pertinent observational quests for the coming decade: measurements of the time-varying magnetic field from photospheric to coronal heights and detai led measure- ments of the spatial and temporal scales on which mag- netic flux emerges and dissipates through the solar cycle (see subsections "Frequency Agile Solar Radiotele- scope," "Focused Theory/Modeling/Simulation Mission: A Virtual Sun," and "A Reconnection and Microscale Probe," in Section 1.5, and "Instrumentation to Observe the Solar Atmosphere at 300 to 1,000 Angstroms," in Section 1 .6~. These observations wi I I not on Iy open a window onto the governing processes beneath the sur- face but will also establish the fundamental properties that any successful model of the Sun's magnetic field must reproduce. The panel also recommends that a sub- stantial effort be made to introduce greater realism into 1 5 models of the subphotospheric rise and resulting emer- gence of magnetic flux with a wide range of spatial scales, as a logical extension of theoretical advances in understanding the solar dynamo (see subsection "Fo- cused Theory/Modeling/Simulation Mission: A Virtual Sun," in Section 1.51. What Heats the Solar Atmosphere? Many fundamental aspects of the chromosphere are unknown: for example, the heating mechanisms), the source of the highly inhomogeneous flows, and the rela- tionship between the thermal and magnetic topologies. Substantial improvements in observational techniques and modeling are needed to determine which of several promising mechanisms transports subsurface energy into the chromosphere and dissipates it there. The chromo- sphere poses a special challenge to simulations, because the presence of neutral atoms and molecules, the com- parable energy content of plasma and magnetic field, and the effects of radiative transport cannot be ignored there. The detailed thermal and dynamical structuring also remains puzzling: for example, static, one-dimen- sional semiempirical models do not predict the observed amount of cool gas or the thick vertical structure and do not incorporate important dynamical effects. Sorely lack- ing are temporally and spatially resolved observations of the thermal state of the inhomogeneous and dynamic chromosphere (see subsections "Frequency Agile Solar Radiotelescope" and "U.S. Participation in ESA's Solar Orbiter Mission," in Section 1.5), as well as physics- based models that i ncl ude al I relevant thermodynamic processes, radiative transport, and realistic three-dimen- sional topologies. The corona is thought to be heated either by wave resonance processes or by a distribution of discrete events ("nanoflares"), although alternative models exist. Wave heating models specifically predict where and on what time scales energy deposition occurs in coronal holes and loops. To discriminate among various models, we need to derive the rate of energy deposition in coro- nal structures as a function of position and time from observed histories of the plasma temperature, density, and magnetic field. In addition, the observed wave spec- trum must be extended to higher frequencies to deter- mine whether the waves enter the corona from below or are generated in the corona by particle beams or recon- nection (see "Primary Recommendations (Prioritized)," in Section 1 .5~. Numerical studies of wave heating are only beginning to include appropriate three-dimensional
1 6 magnetic geometries and enough of the important phys- ics to enable comparing calculations and observations. The contribution of nanoflares to coronal heating and dynamics depends sensitively on several factors, including their rate of occurrence and the energy output per event. At present, the corresponding properties of observed x-ray bursts, which are orders of magnitude more energetic, have been extrapolated to small ener- gies in order to determine the occurrence rate and en- ergy content of nanoflares. It is thus critically important to obtain more sensitive, high-cadence observations of the smallest energy release events on the Sun. More- over, we need to understand the mechanism responsible for energy release in nanoflares, currently believed to be recon nection and its by-products. Di rect observations of the signatures of reconnection as it operates in the co- rona (subsection "A Reconnection and Microscale Probe," in Section 1 .5), coupled with greater theoretical insight into the physics of reconnection (subsection "Fo- cused Theory/Modeling/Simulation Mission: A Virtual Sun," in Section 1.5), would enable substantial progress to be made on the long-standing problem of coronal heating. EXPLORING THE INNER HELIOSPHERE The most important unexplored region of the helio- sphere is within ~0.3 AU of the Sun, where the solar wind originates. The processes operating in this near- Sun region create the heliosphere and, apart from solar irradiance variations, determine the influence of the Sun on Earth and its magnetosphere. It is in the near-Sun region that the corona is heated and the plasma acceler- ated to supersonic speeds in both a quasi-steady form and in the episodic coronal mass ejection events. This is also the region where solar wind turbulence arises and where coronal structures, such as plumes, streamers, and holes, evolve into solar wind structures of varying speed, density, kinetic temperature, composition, and magnetic field strength. To understand the processes that heat and accelerate the solar wind and that determine the evolution of coronal structure into solar wind struc- ture, this region must be explored directly as close to the Sun as a spacecraft can survive. To date the solar corona has only been observed remotely, through imaging, radio sounding, spectros- copy, and downstream in situ sampling. These observa- tions have improved dramatically over the past decade and provide tantal izing gl impses of the physical nature of the innermost heliosphere. To discriminate among THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS competing theories and to make a major breakthrough in our understanding of the coronal origins of the solar wind, however, we need to know physical parameters that are difficult or impossible to measure remotely, including the structure, strength, and fluctuation spec- trum of the ambient magnetic field, the hydromagnetic waves/turbulence down to scales comparable with the thermal ion gyroradi i, phase-space distribution func- tions of both thermal and suprathermal particle popula- tions, and the manner in which these quantities evolve with distance from the Sun in the innermost helio- sphere. Thus, there is a compelling need for direct in situ measurements in the inner heliosphere close to the Sun (see subsection "A Solar Probe Mission," in Section 1.5~. Our exploration of this heretofore unsampled environment almost certainly will also yield unanticipated resu Its that wi l l modify our understand- ing of the Sun and the heliosphere. What Is the Origin of the Solar Wind? Since the initial prediction of the solar wind by Parker in 1958, numerous theories have been devel- oped for mechanisms) by which the corona is heated and the solar wind is accelerated. Because in situ mea- surements at 1 AU and remote observations of UV emis- sion from the corona by SOHO show indirect evidence for ion heating by resonance with ion-cyclotron waves, most recent theories assume the introduction of hydro- magnetic waves at the base of the corona. Various sources have been suggested for this wave energy. The wave excitation may be stationary or episodic in nature. Measurement of the intensity spectrum of magnetic fluc- tuations in the corona and its variation is required to discriminate between possible wave origins. Measure- ment of ion distribution functions in or near the solar wind acceleration region will establish whether the ions are indeed heated and accelerated by ion-cyclotron waves, or perhaps by an entirely different mechanism, such as plasma jets from nanoflares. Solar wind acceleration is closely linked to coronal heating. Although some early models suggested that the energy flux responsible for solar wind acceleration is supplied by electron heat conduction and exospheric models of the solar wind expansion are still being ex- plored, it is now general Iy thought that the electrons do not carry sufficient energy to drive the coronal expan- sion. Instead, the solar wind acceleration is thought to be associated with the ion thermal pressure gradient and with the outward pressure gradient of Alfven waves. Model-independent in situ measurements are required
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS to settle this issue and to determine if the acceleration process is fundamental Iy the same or different in high- and low-speed wind, respectively. Direct measurements of ion and electron distribution functions and the wave fields present in the innermost heliosphere (see "A Solar Probe Mission," in Section 1.5) should answer the fun- damental questions about coronal heating, solar wind acceleration, and inner heliosphere evolution that have dominated much of solar and heliospheric physics since the discovery of the corona. Exploring the Fundamental Physics of Collisionless Plasmas The solar wind is a superb laboratory for direct in- vestigation of processes in collisionless plasmas that have general application to other space plasmas and to astrophysical domains where the plasma can not be di- rectly sampled. These processes include those that gov- ern the basic evolution of plasma particle distribution functions; the development and evolution of waves and turbulence; the generation and structure of collisionless shocks and other discontinuities; particle acceleration to relativistic energies by shocks, turbulence, and mag- netic reconnection; and plasma instabilities associated with various forms of phase space "free energy." These processes occur throughout the solar wind, and much of our present understanding of these processes and phe- nomena is derived from past heliospheric investigations. However, the near-Sun region in the innermost helio- sphere is unique. Here the ions are subject to scattering on ion-cyclotron waves, solar gravity, and magnetic fo- cusing, and their electron distribution function is ex- pected to change dramatically with increasing radial distance. The distribution function should reveal the development of the field-aligned beam component that carries the electron heat flux and its modification by plasma microinstabilities and wave-particle scattering. The wave power spectrum will reveal clearly for the first time the onset of hydromagnetic turbulence. Collision- less shocks driven by CMEs are expected to peak in strength in this region, with high intensities of proton- excited waves and effective acceleration of ions to GeV energies. The inner heliosphere is also the region where inner source pickup ions are created from neutrals emit- ted by interplanetary dust. The pickup process and modi- fications of pickup ion distributions are best studied in these regions. Solar energetic particle fluxes should be intense in the very inner heliosphere, and their accelera- tion mechanisms should not be obscured by interplan- etary transport. The electromagnetic emissions due to solar energetic particle interactions with the solar atmo- sphere are also more intense in the inner heliosphere, and secondary neutrons can be observed at low ener- gies before they decay. These secondary photons and particles provide high-resolution information on solar energetic particles trapped in solar active regions that are not otherwise di rectly observable. UNDERSTANDING THE ACTIVE SUN ANDTHE HELIOSPHERE Stressed magnetic fields are thought to provide the free energy for a wide range of transient energetic phe- nomena on the Sun, ranging from the smallest microjets and microflares to the largest flares and CME/filament eruptions. The largest of these energetic bursts drive so- lar wind disturbances, accelerate particles to high ener- gies, and contribute to variations in the solar irradiance, all phenomena directly affecting the near-Earth environ- ment. A primary goal of Living With a Star and the National Space Weather Program is to understand the underlying physics of these events sufficiently well to predict geoeffective solar disturbances. In part, this re- quires that we establish the precise relationship between flares and CMEs as well as the relationship of flares and CMEs to newly emerging magnetic flux, active promi- nences, transient coronal holes, and the global waves that propagate away from some flare and CME initiation sites. Some of the fundamental questions that must be answered to understand the physics of eruptive events are summarized below. How Is Magnetic Energy Stored and Explosively Released? Solar magnetic fields can accumulate and store ex- cess energy through twisting, either before or after emer- gence through the photosphere. These stresses are trans- mitted to the coronal field, which, according to most models, then serves as the reservoir to be tapped by explosive energy release. Consequently, measuring and understanding the degree to which the coronal field is nonpotential is one prerequisite to understanding solar eruptive activity. To date, the energy content of non- potential coronal magnetic fields has been estimated through extrapolations of photospheric vector magneto- grams and through field morphologies inferred from EUV and soft x-ray images and/or white-light coronagraph data. Although such estimates have demonstrated that ample magnetic free energy is available to drive ob- served energetic phenomena, we need direct, quan-
1 8 titative information about the coronal magnetic field strength and topology to form a complete picture of the physical origins of these events. Prospects for the com- ing decade are encouraging in this regard: Both micro- wave imaging spectroscopy and coronal magnetography in the optical/infrared (JR) regime may provide reliable measurements of the coronal magnetic field for the first time (in Section 1.4, see "Ground-Based Programs," and in Section 1.5, see "Frequency Agile Solar Radiotele- scope"). Theory and observations by YoLkoh, SOHO, and TRACE indicate that magnetic reconnection plays a key role in rapid energy release on the Sun. On macroscopic scales, results from the first generation of three-dimen- sional MHD models of reconnection demonstrate that two-dimensional models are of I imited appl icabi I ity to the real solar atmosphere. Efforts are under way to con- struct physics-based models for many forms of solar ac- tivity. For example, new models of CMEs have been developed in which magnetic reconnection operates in different ways: as a pre-eruption trigger, as a "tether- cutting" mechanism, or as a posteruption path to relax- ation. The scenario actually preferred by the Sun in the initiation and evolution of CME eruptions remains con- troversial, requiring more realistic models and definitive observational tests. Because competing CME models assume distinctly different initial conditions, a pressing observational issue for the coming decade is to deter- mine the pre-eruption magnetic field and plasma condi- tions. We now know from in situ observations that at least 30 percent of CMEs in the solar wind have a mag- netic flux rope structure, yet it remains uncertain how and where these flux ropes are generated. In addition, we do not yet understand the origin of observed mix- tures of closed, open, and (occasionally) disconnected field lines within CMEs in the solar wind, how those mixtures relate to the problem of magnetic flux buildup i n the hel iosphere, or how the observed anomalous ion ic compositions within CMEs arise. Finally, models capable of exploring the macroscopic consequences of recon- nection cannot address the microscopic means by which field lines are first severed and then reconnected, nor can they predict the kinetical Iy determined by-products of reconnection, such as particle acceleration or plasma wave generation. To understand the reconnection pro- cess and its links to solar and heliospheric activity, we need a combined and well-focused theoretical/simula- tion, observational, and experimental effort (see "Fre- quency Agile Solar Radiotelescope," "Focused Theory/ Modeling/Simulation Mission: A Virtual Sun," and "A Reconnection and Microscale Probe," in Section 1.5~. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS When magnetic energy is released, particles can be accelerated to high energies in several ways: as a direct consequence of reconnection-associated electric fields, by shocks, or by stochastic processes such as wave- particle interactions. Models of stochastic acceleration in flares successfully account for certain properties of energetic electrons and ions derived from both remote and in situ data, yet full closure between theory and observation remai ns el usive. Precisely when and where particle acceleration occurs in a flaring volume, as well as the properties of the waves on which they resonantly scatter, have not been determined. There are also ques- tions about the relative roles of acceleration and trans- port in producing the charge state and composition char- acteristics of solar particle events that have been observed by ACE, Wind, and SOHO during the current solar maximum. Further progress in this area will re- quire observations of ion charge state and composition at high energies; observations of low-energy neutrons and particles close to the Sun, where transport effects are minimized; and sensitive, high-resolution, hard x- ray, gamma-ray, and radio observations (see "Primary Recommendations (Prioritized)," in Section 1.5~. How Do Heliospheric Disturbances Evolve? The recognition that CMEs are the primary drivers of heliospheric disturbances with the greatest impact on the near-Earth space environment was one of the most significant highlights of the past decade. Their intrinsic variety and evolving three-dimensional structures have been made acutely apparent by observational advances (e.g., SOHO) in detecting these events both close to and relatively far (30 Rs) from the Sun (Figure 1.6~. In addi- tion, in situ heliospheric observations and simulations have demonstrated that the effect of a given CME on the heliosphere depends critically not only on its inherent mass and momentum but also on the nature of the flow regime into which it is launched. Indeed, a particular CME can produce radically different effects at different hel iocentric distances, latitudes, and longitudes (Figure 1.71. The physical processes involved in CME-distur- bance propagation in the heliosphere are reasonably well understood in principle, but the application of these principles to a specific event is constrained by our lack of knowledge of initial conditions within the CME and the state of the ambient plasma and magnetic field into which the CME propagates. In order to test and validate models of disturbance propagation in the heliosphere, we need accurate measurements of CME characteristics close to the Sun and in situ measurements of helio-
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS 19 FIGURE 1.6 A coronal mass ejection observed in white light with the wide field of view coronagraph on SOHO. Courtesy of the Large Angle and Spectrometric Coronagraph (LASCO) science team. spheric conditions before and during the disturbances as functions of heliocentric distance, latitude, and longi- tude (see subsection "A Multispacecraft Heliospheric Mission," in Section 1.51. To date, CMEs have been detected near the Sun through imaging (e.g., SOHO), while heliospheric dis- turbances have been identified solely through their plasma, magnetic field, and energetic particle signatures, currently being provided by ACE and Ulysses. Despite key advances in analysis techniques over the past de- cade, unambiguous identification of CME material in the solar wind far from the Sun remains difficult. The disconnect between solar and heliospheric observations has frustrated many attempts to relate coronal and solar wind disturbances, particu larly at times of frequent solar eruptions. Missions such as STEREO should begin to bridge this gap and will help provide a comprehensive picture of the entire disturbance process from birth at the Sun to heliospheric response. However, full under- standing of disturbance propagation in the solar wind, an essential component of both the National Space Weather and the Living With a Star programs, will re- quire multipoint observations at different heliocentric distances and longitudes (see subsection "A Multispace- craft Hel iospheric Mission," in Section 1 .5~. Acceleration at CME-driven shocks produces the most dramatic solar energetic particle events observed within the heliosphere. The physics of particle accelera- tion at shocks is understood in principle, but a number of crucial questions remain unanswered. For example, we do not know what seed particles are accelerated in these events, whether shocks acting alone are capable of accelerating particles to the GeV energies sometimes observed, and whether scattering near the shocks is pri- marily produced by ambient or self-generated waves. There is a pressing need for self-consistent numerical
20 CAS E = 1 DAY = 10 2 AL ::~ i< ~ , ~ . /~ ~'1 to ~ ACJ 44 RADIAL VELOCITY (~m/s) 800 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS i=4 /. DAY = 1C 1 A INvJFUTFO OF~C DAY = 1 Rio DENSITY (cm-3) 1 T 1 1 1 1 1 5.01 FIGURE 1.7 Simulated, color-coded, central-meridian (green line) plots of radial velocity and density for three different heliospheric disturbances initiated 10 days earlier at 0.14 AU.The solid red circle in each panel indicates the location of the initial disturbance relative to a tilted low-latitude band (orange) of slow solar wind,which is surrounded by faster wind extending up to the polar regions of the Sun. Plasma within the initial disturbance pulse, which lasted for 14 hours, had the same speed as the high-latitude wind and an internal pressure eight times greater than that within the ambient wind at all latitudes. Note the considerable latitudinal distortion of the injected material as it propagates out into the heliosphere in all three cases. Courtesy of D. Odstrcil, Astronomical Institute, Ondrejov, Czech Republic, and V. Pizza, SEC/NOAA, Boulder, Colorado. models of the complex interplay between particles and waves i n the vici n ity of the shock front and for i ncorpo- ration of fluid and kinetic effects into a coherent picture of CME-driven particle acceleration. Observations de- signed to probe acceleration sites with both in situ and remote sensing instrumentation (see, in Section 1.5, sub- sections "U.S. Participation in ESA's Solar Orbiter Mis- sion," "A Multispacecraft Heliospheric Mission," and "A Particle Acceleration Solar Orbiter") will play a critical role in deciphering the physics of shock acceleration. EXPLORING THE OUTER HELIOSPHERE AND THE LOCAL INTERSTELLAR MEDIUM As the solar wind expands through the solar system it eventual Iy reaches a point where it can no longer hold off the pressure of the interstellar medium, and it under- goes a shock transition, forming the solar-wind termina- tion shock (Figure 1.8~. We have only limited knowl- edge of the termination shock, the interface between the solar wind and the interstellar plasma (the heliopause),
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS (K) 7.0E+03 1.6E+04 3.8E+04 8.7E+04 2.0E+05 4.7E+05 1.1 E+06 AU 400 200 o 200 400 -500 -300 -1 00 1 00 300 AU (cm~3) o.oo 0.04 0.08 0.12 0.16 0.20 0.24 0.28 21 FIGURE 1.8 Model calculations simulating the interaction of the heliosphere with the interstellar medium. Prominent features include the termination shock, the heliopause, the hydrogen wall immediately upstream from the heliopause, and the upstream bow shock, which exists only if the motion of the heliosphere through the interstellar medium is supersonic. Courtesy of G. Zank, University of California, Riverside. and the local interstellar medium beyond. Although high-energy cosmic rays, neutral interstellar gas, and large interstellar dust grains are able to cross the heliopause, the solar system is effectively shielded from the interstellar plasma, magnetic fields, low-energy cos- mic rays, and small dust grains. Thus the outer helio- sphere and its dynamics affect the space environment of Earth. One of the great frontiers for space science during the 21 st century will be to explore the boundaries of the heliosphere and the interstellar medium beyond.
22 How Do the Sun and Heliosphere Interact with the Galaxy? The Sun is moving through the LISM with a relative velocity of ~26 km/s. As interstellar plasma encounters the blunt nose of the heliosphere its flow is diverted, much like the solar wind is diverted around Earth's mag- netosphere. The heliosphere and its boundaries provide a unique laboratory for studying plasma processes and the interaction of a star with its environment. The termi- nation shock and heliopause are expected to move by ~10 to 20 AU over the solar cycle in response to changes in the dynamic pressure of the solar wind. In- deed, those boundaries may never be truly stationary. Some of these changes are caused by large interplan- etary shocks created during violent outbursts of solar activity. The Voyager spacecraft have discovered in- tense and long-lasting radio emission from the direction of the nose of the heliosphere, occurring approximately a year after the largest episodes of solar activity. This radio emission is apparently excited as strong interplan- etary shocks interact with interstellar plasma in front of the heliopause. In the course of the Sun's journey around our galaxy it traverses a wide range of interstellar conditions. For the past few thousand years the Sun has apparently been immersed in a low-density (~0.2 cm-3) cloud with a temperature of ~7000 K. The portion of the interstellar gas that is neutral (including a large fraction of H. He, N. O. Ne, and Ar) is able to penetrate the heliosphere, where some of the material is ionized by solar UV or charge exchange with the solar wind and become pick- up ions. These pickup ions are convected into the outer heliosphere, where some fraction are apparently accel- erated to cosmic ray energies at the termination shock and become the anomalous cosmic rays. Some of the accelerated anomalous cosmic rays charge-exchange with neutral hydrogen to produce energetic neutral at- oms (ENAs). Charge exchange processes involving shock-heated solar wind beyond the termination shock produce lower-energy ENAs that may provide a means of imaging the global heliosphere. A combination of in situ and remote sensing measurements is needed to explore the many processes occurring in the dynamic boundary region. The location of the termination shock and helio- pause and the size of the heliosphere are somewhat uncertain and undoubtedly variable. As of September 1, 2001, Voyager-1 was at ~82 AU and movi ng outward at 3.6 AU/year, followed by Voyager-2 at ~65 AU. Al- though the termination shock has not yet been detected THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS d i rectly, evidence from several approaches i nd icates that the shock is currently between 85 and 100 AU from the Sun in the upstream direction, suggesting thatVoyager-1 will probably encounter the shock one or more times with i n the next few years. These encou nters wi I I estab- lish the scale-size of the heliosphere, provide a direct measure of the pressure of the LISM, and initiate a new era of studies of the interaction of a star with its environ- ment. Since it will be two decades or more before a more capable spacecraft can reach this boundary re- gion, it is critically important to keep the voyagers oper- ating as long as they are returning useful scientific data on the heliosphere's interaction with the LISM. What Is the Physical Nature of the Local Interstellar Medium? The nearby interstellar medium includes species that are predominantly ionized (e.g., C, S. and Si), those that are mostly neutral (H. He, N. O. Ne, and Ar), and others that are mainly locked up in grains (e.g., Al, Ca, and Fe). Although pickup ions and anomalous cosmic rays pro- vide information on species that are predominantly neu- tral in the LISM, we presently have no information on the elemental and isotopic composition of elements that are mostly ionized, nor do we know to what extent refractory elements have condensed into grains. In addi- tion, we have almost no knowledge of the direction and strength of the interstellar magnetic field, or of the inten- sity and composition of low-energy cosmic rays outside the heliosphere. Direct sampling of the composition of interstellar matter would provide a benchmark for com- parison with solar-system abundances and provide con- straints on galactic chemical evolution. Abundance mea- surements of isotopes that include 2H, 3He, ~ 3C, ~ SO, 22Ne, 26Mg, and 30Si would constrain cosmological and nucleosynthesis models and provide a more accurate picture of the evolution of the solar system, our galaxy, and the universe. To advance studies of the outer heliosphere and LISM over the coming years we need a program that includes (1 ) continuation of the Voyagers, (2) a mission to several AU that would measure the neutral LISM that penetrates the inner heliosphere and provide ENA and EUV images of the heliospheric boundaries, and (3) a technology development program leading to an inter- stellar probe carrying advanced instrumentation to ex- plore the boundaries of the heliosphere and our local interstellar environment. The panel believes that some of the intermediate goals of such a program can be achieved by an Explorer-class mission to several AU;
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS however, the panel has not prioritized Explorer-class missions in this report. A PRIORITIZED SET OF SCIENCE QUESTIONS FOR THE COMING DECADE REQUIRING MAJOR NEW RESEARCH INITIATIVES Some of the science questions discussed in this chapter will be addressed by existing missions and pro- grams or those in the active queue (see Section 1.4~. To continue progress across a broad front it is necessary to prioritize the outstanding science questions that will not be resolved by existing, in development, or approved missions and programs. With the above in mind, the panel has identified and prioritized the following out- standing science questions that are ripe for attack in the coming decade: 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. 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? 1.4 EXISTING AND ANTICIPATED PROGRAMS OPERATIONAL PROGRAMS AND MISSIONS Space-Based Missions The current fleet of operational space missions in- cludes a very capable array of remote-sensing and in situ instrumentation that has been responsible for many sig- nificant scientific advances over the past decade (see Section 1.2 for some highlights). Missions that are sched- uled to obtain solar and heliospheric observations be- yond FY 2002 are listed (alphabetically) in Table 1.1, along with the science themes that they address (the five 23 themes are introduced in Section 1.31. If the current pace of progress is to continue over the next decade, it is essential that key capabilities of the current program continue at least until they are replaced by missions in development (Table 1.2) or by the new initiatives (Sec- tion 1.5~. These capabilities include the ability to obtain continuous and redundant solar irradiance measure- ments, to image the corona in x rays and the EUV, to image CMEs in white light, to do helioseismology, and to measure the solar wind plasma, magnetic field, and energetic particle variations in near-Earth interplanetary space. Such capabilities are needed both for high-prior- ity science objectives and to achieve the goals of the Living With a Star program. They are also essential for the routine monitoring of the Sun and heliosphere, which is critical for accurate specification and predic- tion of short-term space weather and longer-term space climate by NOAA and DOD and for detecting and mea- suring the now indisputable variability of the Sun's irra- diance. With regard to the latter, the panel notes that it is only because of a sequence of overlapping, well-cali- brated experiments that we are beginning to understand why solar luminosity varies and how it could change sufficiently on human time scales to affect terrestrial climate. The panel also specifically recommends continua- tion of two missions that are uniquely sampling diffi- cult-to-reach heliospheric regions: Ulysses and Voyag- ers 1 and 2. Ulysses should be continued as long as it is technically possible to do so, and Voyagers 1 and 2 should be continued as long as they are capable of pro- vid i ng measu remeets necessary to characterize the lo- cations and natures of the termination shock and helio- pause. The panel strongly supports NASA's plan to keep Wind operating as a backup 1-AU monitor of helio- spheric conditions and to enable observational cam- paigns for targeted research topics. In developing its recommendations for the coming decade, the panel as- sumes that adequate resources will continue to be avail- able for mission operations and data analysis for RHESSI. Ground-Based Programs The ground-based solar observatories are leading laboratories for the development of new solar experi- ments and carry out a variety of research programs. They have also provided the long-term synoptic observations crucial to understanding the underlying mechanisms of solar variability. Without the interlocking efforts of many ground observatories, we would not have some of the essential framework on which our leading models of the
24 TABLE 1.1 Operational Space Missions THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Mission Description Themes Advanced Composition Explorer (ACE) A mission to study the elemental and isotopic composition of solar wind, solar energetic particles, and cosmic rays that also provides real-time solar wind data from L1. Genesis Discovery-class mission that will collect solar wind samples from L1 orbit and return 2, 4 them to Earth for laboratory analysis. Also provides solar wind data from L1. Geostationary Operational NOAA meteorological satellites that also provide real-time solar x-ray, solar particle, and 4 Environmental Satellites (GOES) magnetic field data. GOES-12 carries a new Solar X-ray Imager (SXI). Ramaty High Energy Solar Explorer mission to study explosive energy release in solar flares using gamma-ray and 4 Spectrographic Imager (RHESSI) x-ray imaging. Solar, Anomalous, and Magnetospheric Polar-orbiting mission that measures the composition of solar energetic particles, 4, 5 Particle Explorer (SAMPEX) anomalous cosmic rays, and trapped radiation. Solar and Heliospheric Observatory ESA-led mission to study the Sun's outer atmosphere, the solar wind, and solar internal 1, 2, 4 (SOHO) structure using helioseismology. Transition Region and Coronal Explorer mission with an EUV/UV telescope to study dynamic phenomena on the Sun 2, 4 Explorer (TRACE) with arc-second resolution. Ulysses ESA/NASA mission that will soon complete its second orbit over the poles of the Sun 2, 4, 5 carrying a payload composed mainly of particles and fields instruments. Voyager Interstellar Mission Voyager 1 and 2 are now speeding towards the solar wind termination shock with 5 hopes of eventually reaching the nearby interstellar medium. Wind ISTP mission that provides an extensive set of particle and field measurements of the 2, 4, 5 solar wind in the vicinity of and largely upstream of Earth. NOTE:The research themes are (1 ) exploring the solar interior, (2) understanding the quiet Sun, (3) exploring the inner heliosphere, (4) understanding the active Sun and heliosphere, and (5) exploring the outer heliosphere and the local interstellar medium. TABLE 1.2 Programs in Development or Awaiting Launch Mission Description Themes National Polar-orbiting A multiagency program for polar-orbiting satellites that will provide a variety of meteorological 2 Operational Environmental and climate data including the total solar irradiance. Satellite System (NPOESS) Solar-B An ISAS-led mission that will measure the full solar vector magnetic field on small scales along 2, 4 with coordinated optical, EUV, and x-ray measurements. Solar Mass Ejection Imager A multiagency mission led by the USAF with an all-sky camera to image CMEs as they propagate 3, 4 (SMEI) through the solar wind. Solar Terrestrial Relations Twin spacecraft that provide stereo imaging and in situ measurements of coronal mass ejections 3, 4 Observatory (STEREO) as they propagate from the Sun to 1 AU. Triana Earth-observing mission that will orbit L1; it includes solar wind and magnetic field instruments. 2, 4 NOTE:The research themes are (1 ) exploring the solar interior, (2) understanding the quiet Sun, (3) exploring the inner heliosphere, (4) understanding the active Sun and heliosphere, and (5) exploring the outer heliosphere and the local interstellar medium.
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS TABLE 1.3 Primary Ground-Based Solar Observatory Contributions 25 NSO/ NSO/ NSO/ Arecibo BBSO GONGa SOLIS Otherb HAG SOON MSO MWSO NRAO OVRO PSPTa RSTNa SFSO WSO Longitudinal Bfields S S S S S I S Vector B fields I S I S Ca II K-line photometry I S S S S H-alpha photometry S S S He photometry S S S Continuum photometry S S S Coronal emission S I Coronal continuum S Helioseismic data S I 10-cm radioflux I I I S Radio emission data I I I Primaryresearch themes' 3,4,5 2,4 1 NOTE: S. synoptic observing programs; I, intermittent observing programs. 2,3,4 2,3,4 1-4 4 1-4 1 3,4,5 3,4,5 1,2,4 3,4 2,4 2,3,4 aNetwork operated by several institutions. brother" is NSO's Dunn Solar Telescope, the McMath-Pierce Solar Telescope, and the Evans Solar Facility. 'The research themes are (1 ) exploring the solar interior, (2) understanding the quiet Sun, (3) exploring the inner heliosphere, (4) understanding the active Sun and heliosphere, and (5) exploring the outer heliosphere and the local interstellar medium. Key: BBSO, Big Bear Solar Observatory; NSO/GONG, National Solar Observatory/Global Oscillation Network Group; NSO/SOLIS, NSO/Synoptic Optical Long-Term Investigation of the Sun; HAO, High Altitude Observatory; SOON, Solar Optical Observing Network; MSO, Mees Solar Observatory; MWSO, Mt.Wilson Solar Observatory; NRAO, National Radio Astronomy Observatory; OVRO, Owens Valley Radio Observatory; PSPT, Precision Solar Photometric Telescope (network); RSTN, Radio SolarTelescope Network; SFSO, San Fernando Solar Observatory;WSO,Wilcox Solar Observatory. magnetic Sun and heliosphere have been constructed. The support of these observatories for present space mis- sions and their role in training future solar physicists are vital to the health of this field. Table 1.3, although not exhaustive, describes many of the principal components of the ground-based network for observing the Sun and also indicates roughly to which of the five research themes each observatory will contribute. In addition to the solar ground observatories, a network of neutron monitors spread around the world measures the inten- sity of and anisotropies in the cosmic ray flux arriving at Earth. Such measurements probe the magnetic structure of the heliosphere and contribute to research theme 4. The panel has not attempted to prioritize the ongoing programs of the ground-based institutions and facilities. NASA Supporting Research and Technology Program A large number of ideas, models, and instruments that are in common use in solar and heliospheric phys- ics can trace their origins to NASA's Supporting Research and Tech nology (SR&T) program. Th is wide-rang) ng pro- gram supports theory and modeling, data analysis and interpretation, instrument development, and suborbital flight opportunities, as well as ground-based and labora- tory studies in support of NASA's flight program. Many scientists in solar and heliospheric research had their graduate work supported in part by this program, and SR&T funding has contributed to many of the exciting discoveries and developments in this field over the past decade. The SR&T program has also contributed sub- stantially to the development of future instruments, pro- grams, and mission concepts considered in this report. Despite these successes, funding for the SR&T program remained relatively flat during the past decade. The panel is encouraged by reports that NASA is currently seeking to increase funding for this important compo- nent of its overall program. During the past year NASA's SR&T program has been reorganized into 11 research "clusters." In the 2001 SR&T Senior Review, chartered by NASA to review the effectiveness of the SR&T program and recommend pos- sible changes, the overall program was found to be very productive, and the Solar and Heliospheric Cluster was one of those judged to be deserving of increased fund- ing. The panel supports the Senior Review recommen- dation for increased SR&T funding for the Solar and Heliospheric Cluster.
26 Suborbital Programs The Suborbital program has long been an essential component of the solar/hel iospheric research program. For example, rocket experiments have obtained the highest resolution x-ray images of the Sun to date, while balloon experiments have detected gamma rays from the Sun and have investigated the solar modulation of galactic cosmic-ray electrons and positrons. The capa- bility of flying long-duration balloons in Antarctica promises to open new opportunities in solar and helio- spheric physics. In addition, suborbital programs pro- vide relatively inexpensive opportunities to test new detector technologies and to train students in detector design, development, and construction, and in cam- paign management and data analysis. Although the NASA, NSF, and DOD suborbital programs emphasize small projects, which are not explicitly considered in this report, the panel strongly recommends continua- tion of this highly cost-effective program. However, in spite of the importance of the suborbital program for all of solar and space physics, the current NASA manage- ment of the sounding rocket component through a com- mercial contractor has resulted in higher costs, less tech- n ical support, i Inefficient management, and a lower fl ight rate. The current budget and launch rate of a few per year cannot sustain a viable sounding rocket commu- nity. The panel recommends that NASA evaluate the management structure of the sounding rocket program and increase both the level of support and the flight rate. PROGRAMS IN DEVELOPMENT Space Missions The space missions listed (alphabetically) in Table 1.2 are either awaiting launch or scheduled for launch within the next few years. Major programs in develop- ment include the following: · So/ar-B. A joint Japanese-U.S.-U.K. mission that will obtain coordinated optical, EUV, and x-ray mea- surements to determine the relationship between changes in the photospheric magnetic field and changes in the structure of the chromosphere and corona. Solar- B will measure the full vector magnetic field in the photosphere down to 0.2 arcsec to isolate elemental flux tubes with a sensitivity to the transverse component of the field vector of about 100 G. · Solar Terrestria/ Relations Observatory (STEREO). THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS A two-spacecraft mission with identical remote sensing and in situ particle and field instrumentation on both spacecraft. It will study the origin and heliospheric propagation of coronal mass ejections in the ecliptic plane. The angular separation between the two space- craft in the ecliptic plane will gradually increase with time, with one spacecraft leading and one trailing Earth in its orbit about the Sun. Eventual Iy one spacecraft wil I be able to observe in situ CME-driven disturbances im- aged by the other spacecraft. Ground-Based Programs · Synoptic Optical Long-term Investigations of the Sun (SOLIS). The program is becoming operational at NSO on Kitt Peak. It consists of a suite of three new instruments to make optical measurements of processes bearing on solar variability whose study requires well- calibrated and sustained observations over a long pe- riod of time. The instruments are a vector spectromag- netograph (high-sensitivity, full-disk measurements of magnetic fields), a full-disk imager (for spectral images of solar disk activity), and a solar spectrometer (for accu- rate spectral line profiles of the Sun as a star). The ex- pected 2.3 TB of daily raw data will be processed in a manner that al lows selected products to be retrieved promptly via Web interfaces. · Global Oscillations Network Group. The group operates identical Michelson Doppler imaging instru- ments at six sites around the world to allow nearly unin- terrupted full-disk observation of solar oscillations and magnetic fields. This helioseismology experiment in- volves very active international scientific participation both in operating the sites and in the intricate analysis of the data to make inferences about structure and dynam- ics within the solar interior. The GONG instruments have just been upgraded to new CCD detectors with 1024 x 1 024 pixel arrays (as GONG+) and now await the imple- mentation of high-performance computing systems to allow full primary analysis of the 32-fold greater data stream (as GONG++. It is anticipated that the GONG experiment will be operated for at least a solar cycle in order to study for an extended period how solar interior dynamics evolves over a wide range of depths. APPROVED PROGRAMS The following approved programs, which are not yet under full development, are prerequisites for the panel's recommended new initiatives.
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS Solar Dynamics Observatory The Solar Dynamics Observatory tsee note, p. 451 is a Living With a Star mission that is designed to pro- vide essential data for understanding (1 ~ the near-surface region of the convection zone, (2) the emergence of magnetic fields from the convection zone, (3) the result- ing restructuring of the chromosphere and transition region, (4) the processes that reconfigure magnetic fields in the solar atmosphere while releasing energy gradu- ally or explosively, (5) the magnitude and cause of both short- and long-term variations in the full-disk solar irradiance spectrum, and (6) coronal events driven by variations in the solar magnetic field. The baseline SDO consists of the following instru- mentation: · A Helioseismic and Magnetic Imager (HMI) to study the origins of solar variability using solar oscilla- tions and the longitudinal photospheric magnetic field; · An Atmospheric Imaging Assembly (AIA) to study coronal energy storage and release in rapidly evolving coronal structures over a broad temperature range; · A spectrometer for irradiance in the EUV to study both short- and long-term variations in the full-disk solar irradiance spectrum in the EUV; and · A white-light coronagraphic imager to study tran- sient and steady-state coronal plasma emissions. SDO will obtain whole-Sun images and will fly in a geosynchronous orbit that allows very high data rates (~200 Mbps) almost continuously, obtaining: · Maps of the flows, temperatures, and magnetic fields in the solar interior; · Maps of the surface velocity pattern; · Images of the inner corona at temperatures from 50,000 to 5,000,000 K; · Irradiance maps of the solar surface; and · Images of the white light corona out to ~15 Rs. If a vector magnetograph is selected (currently a second-order priority), SDO will also obtain the mag- netic field vector over the solar surface to deduce the magnetic stresses and current systems in the photosphere associated with impulsive events and evolving magnetic structures. The SDO instrumentation represents a major ad- vance over instruments flown on previous missions such as SOHO and TRACE. For example, the field of view of AIA will be larger than the corresponding instrument on TRACE by a factor of 26 and will have a resolution that is 25 times better than the KIT instrument on SOHO and 27 a cadence that is 100 times faster. Moreover, AIA will obtain images in all wavelength bands simultaneously to separate temporal, thermal, and spatial evolution in the solar atmosphere. HMI wi 11 do time-distance hel io- seismology over the entire surface rather than in a box 10 arc minutes on a side as did SOHO. In addition, the continuous and very high SDO data rate (200 Mbps as compared with a maximum rate on SOHO of 160 kbps, available only 8 hours per day) allows transmission of h igh-resol ution i mages th roughout the day. The mission plan is to obtain the following: · Continuous high-resolution Dopplergrams for acoustic imaging of subsurface regions, · Full-disk solar and coronal imaging in multiple wavelengths, · Three-dimensional acoustic images that show the full history of active-region development in space and ti me, · Spatial spectroscopy to follow the connection of structures from the photosphere to the corona and to measure temperature and densities, · Irradiance monitoring to measure solar luminos- ity variations in the EUV, and · H igh-temporal-resol ution i mages of the solar co- rona in white lightto follow disturbance evolution in the inner and outer corona. The SDO measurements wi 11 complement those be- ing made by other solar missions operating at the same time. For example, SDO will act as STEREO's third eye, placing a third coronagraph, a magnetograph, and an x- ray imager between STEREO's twin spacecraft. STEREO in turn will provide the directivity, velocity, and topol- ogy of CMEs observed as "halo" events by SDO. In addition, SDO will contribute to the objectives of Solar- B by adding continuous observations of the upper co- rona and higher-cadence, continuous observations of the lower corona. Instrument proposals have been selected for S DO, and the mission is scheduled for launch in 2007. Esti- mated cost (panel's best guess based on information pro- vided by NASA) is $350 million. The panel strongly recommends that the Solar Dynamics Observatory pro- gram proceed to the full development stage. Advanced Technology Solar Telescope Upcoming solar space experiments will provide in- termittent observations of photospheric magnetic fields and EUV measurements of coronal plasma conditions
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS with angular resolution characterized by their 1-m- aperture-class telescopes. However, a larger-aperture telescope that operates in the infrared is needed to measure coronal magnetic fields above the solar limb. At optical and IR wavelengths, Zeeman splitting measurements are a model-independent remote-sensing technique for measuring magnetic fields. With current developments in infrared array technology, coronal Zeeman observations with sensitivities of a few tens of gauss have been achieved while observing regions tens of thousands of kilometers above the limb. Figure 1.9 illustrates an example of such measurements and dem- onstrates that coronal magnetic fields can be directly measu red with presently avai I abl e tech n iq ues. However, a telescope larger than any currently practical space solar telescope is required for such measurements to become a routine tool for understanding the solar atmo- sphere and to achieve high-angular-resolution measure- ments of the photospheric magnetic field. The Advanced Tech nology Sol ar Telescope is a so- lar-community effort led by the National Solar Observa- tory and funded by NSF (presently only for the design phase). The goal is to design and build a 4-m facility that employs adaptive optics and that is aimed at determin- ing the solar magnetic field from the photosphere up through the corona. In the lower atmosphere the tele- scope will achieve flux density sensitivity of a few gauss or less. ATST will provide observations of the solar at- mosphere at a high temporal cadence and with better than 0.1 -arcsecond resolution, which is sufficient to re- solve the pressure scale height and the photon mean free path in the solar atmosphere. Thus, it will enable critical observational tests of models of solar plasma processes. The telescope will be located at a site that offers superb seeing and sustained periods of clear weather and will replace existing NSO facilities at Kitt Peak and Sacramento Peak. It is anticipated that ATST will begin operations in the 2009-2010 time frame. The estimated cost is $70 million for construction (based on a prelimi- nary NSO-sponsored design study) and $14.0 million per year for 5 years of operations and science (opera- tional cost estimated to be 20 percent per year of the total construction cost). The panel believes that the Ad- vanced Technology Solar Telescope is an extremely promising avenue for extending our understanding of the Sun's magnetic field that takes full advantage of the research and technical capabilities of the NSO. It strongly recommends that the program proceed to the full development stage. 29 1.5 RECOMMENDED NEW INITIATIVES The panel's recommendations for new initiatives in this chapter presume that missions and programs al- ready under development or approved for development will become operational within the coming decade to address the high-priority science objectives in solar- heliospheric physics for which they are designed. PRIMARY RECOMMENDATIONS (PRIORITIZED) A Solar Probe Mission The region inward of 0.3 AU is one of the last unex- plored frontiers in our solar system, the birthplace of the heliosphere itself. Remote sensing observations and in situ sampling of the solar wind far from the Sun have provided tantalizing glimpses of the physical nature of this region. However, to understand how the solar wind originates and evolves in the inner heliosphere, the pan- el's top science priority for the coming decade, we need direct in situ sampling of the plasma, energetic particles, magnetic field, and waves as close to the solar surface as possible. Such measurements will determine how energy flows upward in the solar atmosphere, heating the corona and accelerating the wind, and will also reveal how the wind evolves with distance in the inner heliosphere. They will revolutionize our basic under- standing of the expanding solar atmosphere. The panel therefore strongly recommends a solar probe to the near- Sun region that emphasizes in situ measurements of the innermost heliosphere. The generic solar probe the panel recommends is not necessarily identical to the Solar Probe mission for which NASA released an An- nouncement of Opportunity in September 1999 and that placed equal emphasis on 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 compromised. In general, the panel did not find the arguments given for remote sens- ing measurements to be sufficiently compelling to in- clude such measurements as a top priority on a first mission to the near-Sun region. Nevertheless, the panel appreciates that a solar probe mission provides perhaps the first opportunity to measure the photospheric mag- netic field in the polar regions of the Sun via remote sensing. Such measurements will help address our sec- ond science priority and could be a secondary objective of a solar probe mission.
30 Solar Probe is currently listed as a part of NASA's Living With a Star Program but is not a funded mission. The panel recommends that the generic solar probe dis- cussed here replace Solar Probe in that program; how- ever, the overall orbital characteristics of a solar probe will be essentially the same as before. Over a 10-day period the probe will sample the corona and solar wind inside 60 Rs at both high and low heliographic latitudes. The primary goals of this mission are these: · Locate the sources and trace the flow of energy that heats the corona. · Determine the acceleration processes and find the source regions of the fast and slow solar wind. · Identify the acceleration mechan isms and locate the source regions of solar energetic particles, and use these particles as remote probes of physical conditions closer to the Sun. · Determine how the plasma, energetic particles, magnetic field, and waves evolve within the innermost hel iosphere. A secondary objective is to determine the magnetic structure of the Sun's polar regions. To meet these science objectives, a solar probe must approach as close to the solar surface as is possible and carry a combination of in situ plasma, energetic par- ticle, magnetic field, and plasma wave experiments that wi 11 accompl ish the fol lowi ng: · Measure with high spatial and temporal resolu- tion fundamental plasma parameters (e.g., velocity dis- tribution functions, density, temperature, velocity, com- position) and magnetic field parameters (field strength and direction, power spectra) and their evolution with distance from the Sun. · Characterize the nonthermal properties of ion and electron distribution functions. · Identify wave modes and turbulence present in the inner heliosphere, their evolution with heliocentric distance, and their effect on particle distribution func- tions. · Measure the composition, flux, and anisotropy of energetic ions and electrons with high spatial and tem- poral resolution. The panel appreciates that for the high-speed wind most of the plasma heating and acceleration probably occurs at altitudes below that accessible to a solar probe. However, strong signatures of the processes that heat and accelerate the wind should still be evident in the THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS data obtained at closest approach. Moreover, present observations indicate that the acceleration of the slow wind continues out to at least 30 Rs. If it does not compromise the in situ measurements or add unduly to the overall mission cost, it is also desir- able that a solar probe include a remote sensing experi- ment to measure the photospheric magnetic field in the Sun's polar regions. The solar corona and inner heliosphere achieve their simplest form near solar activity minimum, and the data returned from a first encounter with the near-Sun region during the years surrounding solar minimum would have the most straightforward scientific interpretation. How- ever, a mission flown at any phase of the solar activity cycle will return unsurpassed and almost certainly un- expected information on the origins of the plasmas, fields, and energetic particles that fill the heliosphere. The panel therefore believes that the launch of a solar probe should not be seriously constrained by timing relative to the solar activity cycle. Since the payload the panel recommends for a solar probe differs somewhat from that most recently studied by NASA, the panel urges NASA to begin immediately to study the technological issues and costs associated with a solar probe and with implementing this ex- tremely vital mission in the coming decade. Previous estimates for a solar probe mission, including one pass by the Sun at an altitude of 3 Rs, were ~$600 million (estimate provided by Jet Propulsion Laboratory). This first mission to the near-Sun region will be one of the great explorations of th is new centu ry. Frequency Agile Solar Radiotelescope Radio imaging and radio spectroscopy provide unique insights into the quiet and active Sun. Combin- ing 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 vari- ety 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
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS 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. The Frequency Agile Solar Radiotelescope (FASR) is a proposed (largely to NSF) ground-based, solar-dedi- cated, interferometric array designed to perform time- resolved, broadband imaging spectroscopy at radio fre- quencies from ~0.1 to ~30 G Hz, thereby probing the Sun's atmosphere from the middle chromosphere to the outer corona. It is optimized to exploit a wide variety of techniques to measure coronal magnetic fields by per- forming time-resolved, radio imaging spectroscopy across nearly three decades in frequency. It was en- dorsed by the NRC's Astronomy and Astrophysics Sur- vey Committee as a mid-size, ground-based project. Table 1 .4 provides the specifications for FASR. FASR will be a versatile and powerful instrument, providing unique data to solar and space physicists for studying basic physical processes operating in the solar atmosphere such as reconnection, plasma heating and acceleration, and electron transport. FASR will address aspects of the panel's top three science priorities by providing the following: · Measurements of coronal magnetic fields and currents; · Observations of nonthermal particle emission as- sociated with flares, CM Es, and coronal shocks; · Observations of the three-dimensional structure of the solar atmosphere from chromospheric to coronal heights; · Observations of sites and mechanisms of coronal heating and solar wind acceleration; and · Observations of coherent emission mechanisms and plasma wave-particle interactions. TABLE 1.4 FASR Characteristics Property Frequency range Frequency resolution Time resolution Antenna size Number of antennas Number of baselines Polarization Independent data channels Angular resolution Field of view Specification ~0.1 -30 GHz ~3% (2-30 GHz) <1% (0.1-2 GHz) <1 sec (2-30 GHz) ~0.1 sec (0.1-2 GHz) D=3-5 m ~100 ~5,000 full Stokes 4-8 pairs 20/f arcsec (f in G Hz) 1 1 25/(fD) arcmin (f in GHz; D in m) 31 FASR wi 11 contribute significantly to the Living With a Star and the National Space Weather Program by pro- viding a number of real-time or near-real-time data prod- ucts. It will also make important contributions to synop- tic studies of the Sun and will yield valuable proxies for solar activity and total solar irradiance. The estimated construction cost for FASR is $30 mil- lion (based on the estimates of individuals advocating the faci l ity). Operations and science support wi l l cost $6 million per year for 5 years (operational cost estimated to be 20 percent per year of the total construction cost), leading to a total cost of $60 million. This versatile and powerful instrument represents a major advance over any existing solar radio telescope (see Table 1.5), and is expected to remain the world's premier solar radio in- strument for two decades or more after completion. Focused Theory/Modeling/Simulation Mission: A Virtual Sun Understanding the physical connections between the Sun-heliosphere and the Earth is the prime guiding principle behind the major research issues and chal- lenges for the next decade. The Sun-heliosphere-Earth system is strongly coupled and highly nonlinear, linking spatial scales from current sheets to the size of the helio- sphere and varying on time scales from fractions of a second to millennia. Its complexity has long been an obstacle to attaining a full understanding of key mecha- nisms and processes as well interconnections within the system, let alone construction of global models of the entire system. However, during the last decade we have broadened considerably our theoretical understanding of the Sun-heliosphere-Earth system, have collected a rich observational base from which to study it, and have witnessed a rapid development of supercomputing ar- chitectures. 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 disciplinary bou ndaries. The panel recommends a multiagency, 1 0-year pro- gram to initiate development of a Virtual Sun a global numerical model of the Sun-heliosphere system consist- ing of interconnected modules that calculate the behav- ior of important structural components subject to the pertinent controlling physical processes. The Virtual Sun will be flexible enough to accommodate new modules, to allow improvements in current modules, and to run at different levels of sophistication and resolution. An es- sential goal is to continually integrate our ever-improv-
32 TABLE 1.5 Existing Solar Radio Capabilities THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Observatory Country Angular Resolution Frequencies Type Gauribidanur India 5' 40-150 MHz 2D mapping Nan~ay France arcmin 150-450 MHz 2D mapping RATAN-600 Russia 240--15" 1-20GHz fan beam OVRO United States 90"-5" 1-18 GHz 2D mapping Siberian SRT Russia 20" 6GHz 2D mapping Nobeyama Japan 15", 8" 17, 34 GHz 2D mapping Itapetinga Brazil 2' 48 GHz Multibeam SST Argentina 3'-1' 212, 410 GHz Multibeam Metsahovi Finland 4'-1' 22, 37, 90 GHz Single beam Brunylsland Australia 3-20 MHz Spectrograph Izmiran Russia 25-260 MHz Spectrographs Ondrejov Czech Republic 0.8-4.5 GHz Spectrographs Tremsdorf Germany 40-800 MHz Spectrographs Zurich Switzerland 0.1-8 GHz Spectrographs Espiunica Portugal 150-650 MHz Spectrographs Nan~ay France 10-40 MHz Spectrograph Culgoora Australia 18-1800 MHz Spectrographs Hiraiso Japan 25-2500 MHz Spectrographs ARTEMIS Greece 100-469 MHz Spectrograph Beijing/Yunnan China 0.7-7.3 GHz Spectrometers DRAG Canada 2800 MHz Fixed frequency Cracow Poland 410-1450 MHz 6 fixed frequencies SRBL United States 0.4-15 GHz Fixed frequencies Nobeyama Japan 1.0-86 GHz 7 fixed frequencies Hiraiso Japan 200, 500, 2800 MHz 3 fixed frequencies Trieste Italy 237-2695 MHz 6 fixed frequencies ing understanding of the basic physics into the different modules and their connecting links. The physics of each parameter employed in the individual modules should be clearly understood and precisely related by basic equations to the processes that it represents. The de- velopment of an overarching framework that joins the modules consistently, and permits straightforward incor- poration of observational i nputs/tests and scientific visu- alization, will be a particularly challenging but essential aspect of this mission. The general value of end-to-end modeling has been recognized in other programs, most notably the DOD MURI and CHSSI programs and the NSF Science and Technology Centers, and by the Living With a Star science architecture team. However, the Virtual Sun is envisioned to have a far more extensive scope than those efforts. Examples of Virtual Sun components include mod- els of the emergence of magnetic loops in the photo- sphere, the evolution of MHD turbulence in the solar wind, the evolution of CME-driven disturbances, includ- ing the acceleration of energetic particles, and the inter- action of the solar wind with the LISM. Two modules that the panel particularly recommends for immediate development are those pertaining to the solar dynamo and magnetic reconnection, the source of solar magne- tism and its prime mechanism for releasing stored en- ergy, respectively. Note that these research thrusts have been chosen because of their immediate readiness for substantial progress; other modules needed to build a Virtual Sun will be tackled when they are ready for a focused theoretical and numerical attack. Major advances in supercomputing are now en- abling high-resolution, three-dimensional simulations of those dynamical processes that are viewed as critical elements in the operation of the global dynamo (see "Exploring the Solar Interior," in Section 1 .31. Given the vast range of dynamical scales involved in the solar convection zone, it is unlikely that fully self-consistent MHD simulations of the global dynamo can be achieved in the very near future. However, a hierarchy of cutting- edge simulations that solve the nonlinear MHD equa- tions without recourse to major simplifications in the physics are now tractable, thus permitting study of the primary building blocks in full dynamical detail.
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS A focused, coordinated initiative should also take advantage of existing and planned resources to make major breakthroughs in understanding the basic physics of three-dimensional magnetic reconnection from mi- cro- to macroscales and in identifying and quantifying how reconnection operates under varying circumstances within the solar system. We can now model reconnec- tion with kinetic codes that simulate particle behavior within the dissipation regions at the heart of the re- connection process, simulate the macroscopic evolu- tion of three-dimensional reconnection with stead) Iy improving resolution (Figure 1.10), and utilize hybrid codes that combine both kinetic and MHD approaches. An ongoing, concerted analytical and numerical effort to understand reconnection in the magnetosphere (the Geospace Environment Modeling tGEM] challenge) serves as a successful prototype for a much broader attack on the reconnection problem in the more diverse arena of the Sun and heliosphere. ~ . ~ A major effort by several critical-mass groups is re- quired to develop the theoretical underpinnings and the suite of complex numerical tools necessary to address the important physics and boundary conditions; adapt these tools to the specific environments of interest; re- fine the models to reflect and reproduce the latest obser- vations obtained by the facilities and missions discussed in Sections 1.4, 1.5, and 1.6; and link these models at common boundaries to form a working analogue of the Sun-heliosphere system. The panel notes also that a sim- plified version of the Virtual Sun will be invaluable for education and publ ic outreach. A coherent theory mission of this scope will require approxi mately 1 0 years to ach ieve its goals. The panel envisions that the program will require both continuity and community oversight to meet such ambitious goals. In particular, individual components should be com- petitively selected and reviewed periodically to assess quantitative progress toward completion of a working Virtual Sun model. The program will require ample ac- cess to national supercomputing facilities and funding for researchers and should therefore be a natural venue for interagency support by NASA, NSF, DOE, DOD, and NOAA. The cost will be on the order of $5 million per year (this is simply an educated guess by the panel). Supercomputing resources would be provided separately through high-performance computing programs at par- ticipating agencies. The panel contends that such an effort is crucial to achieving the goals of the substantial U.S. and international investments in ground- and space- based hardware recommended for the coming decade. 33 FIGURE 1.10 Three-dimensional MHD simulation (at 2563 reso- lution) of the reconnection of a pair of twisted magnetic flux tubes. The images shown are isosurfaces where the magnetic field strength ABE equals ~B~max/2.The tubes are initially oriented perpendicular to each other and are pushed toward each other at a small fraction of the Alfven speed (panels (a) and (b)).The flux tubes collide at two points (panel (b)) and then reconnect at these two points (panel (c)).This double reconnection allows the tubes to exchange the sections between the two contact points (panels (d) and (e)) and, as a result, by the end of the simulation the two tubes have effectively Tunneled" through each other. This topological change releases energy by converting a portion of the twist in the tubes into linking between the two tubes.The time frame shown here covers about 70 tube Alfven crossing times. Courtesy of Mark Linton, Naval Research Laboratory. U.S. Participation in ESA!s Solar Orbiter Mission Solar Orbiter is a natural successor to SOHO to explore the Sun and its interaction with the heliosphere.
34 Selected by the European Space Agency for launch in the 2008-2015 time frame, Solar Orbiter will use a unique orbital design to bring a comprehensive payload of imaging and particle and field experiments into an elliptical orbit with a perihelion of 45 Rs and an even- tual maximum heliospheric latitude of 38 degrees. This orbit will, at times, provide a unique co-rotating vantage point for a combined in situ and remote sensing investi- gation of the Sun and hel iosphere. The primary goals of this mission are to provide data essential for understanding the solar dynamo, the mag- netic structure and evolution of the solar atmosphere from the equator to the poles, and the effects of this evolution on the inner heliosphere. It thus directly ad- dresses certain aspects of our top three science priori- ties. To meet these science objectives, Solar Orbiter will carry a suite of instruments that will do the following: ~ Probe via helioseismology the flow patterns in and beneath the photosphere. · Capture high-resolution EUV images of the Sun's ti me-vary) ng atmosphere. · Measure x-ray, gamma-ray, and neutron fluxes from particle acceleration processes in the solar atmo- sphere. · Measure the in situ properties of the solar wind (solar wind distribution functions, velocity, density, tem- perature, composition, and magnetic field) and solar energetic particles. The goals for Solar Orbiter and the types of instru- mentation to be included on it are complementary to those of a solar probe. Solar Orbiter does not approach nearly as close to the Sun as does a solar probe and thus will not capture in situ the earliest phases of the solar wind expansion and evolution. On the other hand, it will remain inside 0.3 AU for a far longer period and will even co-rotate with the Sun for weeks at a time. Unlike a solar probe, it will also include a reasonably full complement of remote-sensing experiments to ob- tain a close-up view of the Sun, including measure- ments of the Sun's polar regions late in the mission. The panel highly recommends a strong NASA in- volvement in ESA,s Solar Orbiter mission, similar to its engagement in other successful joint ventures with ESA, such as SOHO and Ulysses. This would be a very cost- effective way for the U .S. sol ar and hel iospheric com- munity to address aspects of the panel's top three sci- ence priorities. The panel finds this mission would be particularly attractive if it includes both in situ and re- mote sensing instrumentation to investigate particle ac- THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS celeration close to the Sun. The expected cost to NASA depends on the level of U.S. engagement in this high- leverage mission. The panel's very rough estimate is that a minimum of ~$100 million will be required for mean- ingfu I U.S. participation in the mission. A Multispacecraft Heliospheric Mission Solar wind disturbances driven by CMEs are inher- ently complex three-dimensional structures. Our under- stand i ng of the evol ution and gl obal extent of these d is- turbances has largely been built on single-point in situ measurements obtained at and beyond 1 AU, although some multispacecraft observations of heliospheric dis- turbances have been obtained and STEREO will provide stereoscopic imaging and two-point in situ measure- ments of CME-driven disturbances. The panel believes that a multispacecraft heliospheric mission, consisting of fou r or more spacecraft separated i n both red i us (i n- side 1 AU) and longitude and emphasizing in situ mea- surements, promises to dramatically advance our under- standing of the global aspects of the evol ution of these events. A mission of this kind will illuminate the con- nections between solar activity, heliospheric distur- bances, and geomagnetic activity, will directly addresses the panel's third science priority, and will be an essen- tial element of NASA's Living With a Star program. The main scientific issues that should be addressed by a mu Itispacecraft hel iospheric mission i ncl ude deter- mination of the following: · How CME-driven disturbances evolve as func- tions of heliocentric distance and longitude in a struc- tured ambient solar wind; · The internal structure and magnetic topologies of CMEs as well as effects of external field draping; · How energetic particle populations accelerated by CME-driven shocks vary as functions of heliocentric distance and longitude; and · How the large-scale structure of the solar wind evolves in longitude and with distance in the inner helio- sphere in the ecliptic plane. The optimum multispacecraft heliospheric mission will comprise a small constellation of at least four space- craft separated in solar longitude and radius with at least one orbital peribelion at or within ~0.5 AU. The space- craft orbits will lie in and near the ecliptic plane, while their relative positions will drift with time. Each space- craft will carry the same complement of in situ magnetic field, plasma, energetic particle detectors, and radio
PANEL ON THE SUN AND HELIOSPHERIC PHYSICS wave receivers. The mission will thus provide a highly detailed two-dimensional observational slice through the ambient solar wind and through CME-driven helio- spheric disturbances propagating near the ecliptic plane. The scientific return from a multispacecraft heliospheric mission wi l l be considerably enhanced if it is timed to be concurrent with coronagraph observations of CME disturbances departing from the solar atmosphere. A multispacecraft mission of this nature is of para- mou nt i mportance i n u nderstand i ng and pred icti ng the form and intensity of heliospheric disturbances imping- ing on Earth's magnetosphere and hence should be an essential part of NASA's Living With a Star program. The costs associated with this mission have not been exten- sively studied. The panel believes the mission will be equivalent to the Solar Terrestrial Probe mission and estimates it will cost of the order of $350 million. 35 A Reconnection and Microscale Probe Observations and theory have long indicated that magnetic reconnection plays a key role in rapid energy release on the Sun. Although magnetic reconnection and its repercussions have long been studied intensively in Earth's magnetosphere via both observations and theory, many questions remain about its operation in the solar corona, where physical conditions differ consider- ably from those i n the magnetosphere. Moreover, i n situ sampling deep in the Sun's atmosphere is clearly out of reach. Understanding reconnection in the solar context re- quires imaging (Figure 1 .1 1 ~ and spectroscopy of the fine-scale plasmas affected by reconnection a chal- lenging but technologically feasible task for the next decade. The Reconnection and Microscale (RAM) probe FIGURE 1.1 1 Close-up of a coronal null point outlined by 1 95-A emission (T ~ 1.5 MK), as seen by the TRACE instrument. Courtesy of A. Title, Stanford-Lockheed Institute for Space Research, Palo Alto, California.
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-
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-
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
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-
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.
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;
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
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
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
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.
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.