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Solar and Space Physics: Milestones and Science Challenges

The fundamental goal of solar and space physics research is to discover, to explore, and ultimately to understand the activity of a star—the Sun—and the often complex effects of that activity on the interplanetary environment, the planets and other solar system bodies, and the interstellar medium. This enterprise involves the study of an exotic and dynamic world of ionized gases (plasmas), magnetic and electric fields, and small- and large-scale electrical currents. It is motivated by the deep-seated human impulse to know and understand the workings of Nature. Its province is that portion of the universe dominated by the Sun’s activity. Although solar and space physicists focus their inquiry primarily on the behavior of magnetized plasmas in the solar system and on the interactions of these plasmas with each other and with electrically neutral matter, the processes that they seek to understand are fundamental, so that the lessons of solar and space physics are often relevant to our understanding of astrophysical objects lying well beyond the reach of the Sun’s influence.

Intellectual inquiry into fundamental processes often yields utilitarian benefits of considerable value to society.1 In the case of solar and space physics, utility is found principally in three areas. First, solar activity produces disturbances in Earth’s space environment that can adversely affect certain important technologies and threaten the health and safety of astronauts. Knowledge obtained through solar and space physics research is essential to the development of means and strategies for mitigating the harmful effects of such disturbances. Second, global climate change is an issue of great scientific complexity and profound societal significance. Recent studies suggest that solar variability may have been responsible for a large fraction of the changes in global mean surface temperature that occurred prior to 1900 and that it continues to have a significant influence today. Understanding variations, both long- and short-term, in the Sun’s magnetic activity and radiative output is one of the necessary conditions for



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1 Solar and Space Physics: Milestones and Science Challenges The fundamental goal of solar and space physics research is to discover, to explore, and ultimately to understand the activity of a star—the Sun—and the often complex effects of that activity on the interplanetary environment, the planets and other solar system bodies, and the interstellar medium. This enterprise involves the study of an exotic and dynamic world of ionized gases (plasmas), magnetic and electric fields, and small- and large-scale electrical currents. It is motivated by the deep-seated human impulse to know and understand the workings of Nature. Its province is that portion of the universe dominated by the Sun’s activity. Although solar and space physicists focus their inquiry primarily on the behavior of magnetized plasmas in the solar system and on the interactions of these plasmas with each other and with electrically neutral matter, the processes that they seek to understand are fundamental, so that the lessons of solar and space physics are often relevant to our understanding of astrophysical objects lying well beyond the reach of the Sun’s influence. Intellectual inquiry into fundamental processes often yields utilitarian benefits of considerable value to society.1 In the case of solar and space physics, utility is found principally in three areas. First, solar activity produces disturbances in Earth’s space environment that can adversely affect certain important technologies and threaten the health and safety of astronauts. Knowledge obtained through solar and space physics research is essential to the development of means and strategies for mitigating the harmful effects of such disturbances. Second, global climate change is an issue of great scientific complexity and profound societal significance. Recent studies suggest that solar variability may have been responsible for a large fraction of the changes in global mean surface temperature that occurred prior to 1900 and that it continues to have a significant influence today. Understanding variations, both long- and short-term, in the Sun’s magnetic activity and radiative output is one of the necessary conditions for

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distinguishing the human influence on global climate from the background of natural variability (see sidebar, “The Sun and Climate”). Third, as evidenced by the enthusiastic reception that audiences have given the documentary SolarMax,2 the subject matter of solar and space physics, like that of their sister discipline, astronomy, exercises a powerful hold on the interest and imagination of the public. Solar physics and space physics, along with astronomy, are thus particularly well suited to contribute to the strengthening of science education and to the development of a scientifically literate public and a technically trained workforce. THE DOMAIN OF SOLAR AND SPACE PHYSICS The domain of solar and space physics is that region of our galaxy known as the heliosphere (Figure 1.1). The heliosphere is the cavity formed within the warm plasma of the local interstellar medium by the solar wind, the Sun’s ionized, supersonically expanding atmosphere. Situated inside this bubblelike cavity, and immersed in the solar wind flow, are the nine planets of the solar system, the asteroids, the comets, and the icy trans-Neptunian objects of the Kuiper Belt. At its center is the Sun, an ordinary main-sequence star that, with an age of 4.5 billion years, has reached the midpoint of its stellar life. The boundaries of the heliosphere have not yet been surveyed because they are farther out than the most distant deep-space probe, Voyager 1, which in May 2002 was located some 86 astronomical units (AU)—more than 12.5 billion kilometers—from the Sun.3 The size and structure of the heliosphere are determined by the relative pressures of the solar wind and the interstellar medium and will change as these pressures vary. At this point in the history of the solar system, most of the changes in the dimensions of the heliosphere probably result from variations in solar wind ram pressure over the course of the 11-year solar cycle and are likely to be comparatively minor (a few percent). There is reason to believe, however, that the size of the heliosphere can vary dramatically with sufficiently large changes in the density of the local interstellar medium. Recent computer simulations show that an encounter with an interstellar cloud whose neutral hydrogen density is 50 times that of the heliosphere’s present galactic environment could reduce the size of the heliosphere by as much as 80 percent, producing changes in the inner solar system that could affect Earth’s space environment and climate.4 The wind that inflates the heliosphere, the solar wind, blows continuously. It originates in the several-million-degree solar corona and is accelerated to supersonic speeds near the Sun. Like its coronal source, the solar

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THE SUN AND CLIMATE The Sun is the ultimate driver of the climate system, and it is reasonable to suspect that there might be a link between solar variability and changes in climate. Despite many claims of correlations between solar-activity indicators and climate variables, the existence of such a link has been controversial. Part of the difficulty has revolved around the question of a physical mechanism to couple solar variability to the lower atmosphere and the surface of Earth. Variations in irradiance are an obvious and plausible mechanism for solar influence on climate. The Sun’s total irradiance (the solar “constant”) varies on time scales at least as long as the 11-year solar activity cycle, but the variations directly observed have been too small (0.1 percent of the total irradiance) to have a significant impact. If larger variations occur, they can either weaken or amplify anthropogenic effects and thereby increase or reduce the time available to address these effects. It is of the utmost importance to estimate their magnitude. Part of the variation in the Sun’s irradiance takes place in the ultraviolet (UV) region of the spectrum that is responsible for both the formation and the destruction of ozone in the stratosphere. The relative UV variations are much larger than total luminosity variations and, as shown by recent modeling work, can have a significant effect on tropospheric dynamics and hence on climate. Other, less direct mechanisms have been suggested whereby the Sun might influence climate, such as those in which solar-induced variability in cosmic-ray ionization affects cloud formation. Clouds have a major influence on climate, so if these links exist, they could provide a powerful means of amplifying the effects of solar variability. An alternative suggestion involves changes in the global electric field influencing the freezing of supercooled water droplets and the release of latent heat. Some observations have been quoted as support for these mechanisms, but they remain unsubstantiated and controversial, indicating the need for further study.

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Two model-based estimates of the globally averaged solar radiative flux entering the atmosphere, shown as departures from the 1980 value of about 240 W m-2. The Lean et al. estimate (Lean, J., J. Beer, and R. Bradley, Reconstruction of solar irradiance since 1610: Implications for climate change, Geophysical Research Letters 22, 3195-3198, 1995) is based on the known irradiance variation on the time scale of the 11-year solar magnetic cycle together with a long-term variation derived from cosmogenic isotope variations and the observed characteristics of sunlike stars. The Hoyt and Schatten estimate (Hoyt, D.V., and K.H. Schatten, A discussion of plausible solar irradiance variations, 1700-1992, Journal of Geophysical Research 98, 18895-18906, 1993) uses a long-term variation based on the length of the 11-year cycle rather than its amplitude, together with other parameters of solar variability. While there are significant differences between the two plots, their gross similarity can be taken to imply that the overall features of solar irradiance variability are known. A word of caution is necessary, however, since the mechanisms responsible for long-term irradiance variability are not well understood and could in principle involve as yet unknown changes in the transport of radiation through the Sun’s convective zone. Courtesy of G.C. Reid.

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wind is structured and variable. It varies in density, speed, and temperature, and in the strength and orientation of the magnetic field embedded in its flow (the interplanetary magnetic field, or IMF). At solar minimum, the heliosphere is dominated by a fast solar wind from high latitudes, while during the approach to and at solar maximum it is dominated by a slow and variable wind from all latitudes. This change in the structure of the solar wind reflects the dramatic reconfiguration of the corona that takes place as the polarity of the solar magnetic field reverses. During this period, the solar wind is also increasingly disturbed in its flow by coronal mass ejections (CMEs), which occur over ten times more often at solar maximum than at solar minimum. CMEs are transient releases of huge quantities of coronal plasma and magnetic fields into the heliosphere, sometimes at initial speeds in excess of 1,000 kilometers per second. Fast CMEs drive powerful shock waves, which accelerate solar wind ions to energies high enough to penetrate a space suit or the hull of a spacecraft5 and can cause severe disturbances in the geospace environment when they encounter Earth’s magnetic field. Developing the ability to predict CMEs is thus an important goal for solar and space physicists. The eruption of CMEs and flares, the heating of the corona to temperatures several hundred times that of the Sun’s visible surface, the acceleration of the solar wind—all of these processes, whose detailed workings are still poorly understood, are driven or mediated by energy provided by magnetic fields generated within the upper third of the Sun’s interior, in the so-called convection zone. In this region, the rotational and turbulent convective motions of the electrically conducting plasma drive a magnetic dynamo that generates and maintains the Sun’s global magnetic field as well as smaller-scale local fields. The magnetic fields thus generated emerge through the photosphere, forming sunspots and other active regions and creating the complex and dynamic coronal structures revealed in such stunning detail in recent images from the Transition Region and Coronal Explorer (TRACE) spacecraft (Figure 1.2). The last decade has seen considerable progress in theoretical and modeling studies of solar magnetism. However, important questions remain—for example, about the distribution, emergence, and evolution of magnetic flux on the Sun; about the storage and release of energy in solar magnetic fields; and about the detailed workings of the solar dynamo and the origins of the solar cycle. In particular, the astonishing fibril state of the magnetic field at the visible surface needs to be understood, along with the degree to which the field is in a fibril state far below the surface.

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FIGURE 1.1 Artist’s concept of the heliosphere, the cavity formed in the local interstellar medium by the solar wind. The dimensions of the heliosphere are not known, but its diameter is estimated to be on the order of 200 to 250 AU. A boundary referred to as the heliopause separates the solar wind from the magnetized interstellar plasma. Inside this boundary is a shock wave, the termination shock, where the speed of the solar wind changes from super- to subsonic. A shock wave may also form upstream of the heliosphere if the motion of the heliosphere through the interstellar medium is supersonic. Shown in the figure are the trajectories of the four deep-space probes—Pioneers 10 and 11 and Voyagers 1 and 2—that are headed out of the solar system and that may soon encounter the termination shock. However, only the two Voyagers are still performing science operations. Courtesy of the Jet Propulsion Laboratory.

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FIGURE 1.2 The Sun’s corona imaged in the extreme ultraviolet (171 Å) by the TRACE telescope. The emissions are from Fe IX/X at a temperature of ~1,000,000 K. The magnetic field emerging from the photosphere structures the coronal plasma in an intricate and dynamic architecture of loops, arcades, and filaments. Courtesy of NASA and the Stanford-Lockheed Institute for Space Research.

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As the solar wind flows away from the Sun and fills the heliosphere, it interacts in various and complex ways with the planets and other solar system bodies that it encounters. The nature of this interaction depends critically on whether the object has an internally generated magnetic field (Mercury, Earth, the giant outer planets) or not (Venus, Mars, comets, the Moon). For example, Mars has no strong global magnetic field, and the solar wind impinges directly on a significant fraction of its thin carbon dioxide atmosphere.6 The erosion of the atmosphere resulting from this interaction may have played an important role over the last 3 billion or so years in the evolution of Mars’s atmosphere and climate.7 In contrast, the terrestrial atmosphere is protected from direct exposure to the solar wind by Earth’s magnetic field, which forms a complex and dynamic structure—the magnetosphere—around which most of the solar wind is diverted (Figure 1.3). The solar wind’s interaction with the magnetosphere, effected principally through a temporary merging of the interplanetary and terrestrial magnetic fields, stirs and energizes the magnetospheric plasmas and leads to periodic explosive releases of magnetic energy. In these events, known as magnetospheric substorms, powerful electrical currents flow between the magnetosphere and the ionosphere, injecting several billion watts of power into the upper atmosphere and producing often quite spectacular displays of the aurora borealis and australis—the Northern and Southern Lights (Figure 1.4). Unfortunately, at times of extreme magnetospheric disturbance, the beauty of the auroras can be accompanied by less benign phenomena— radiation belt enhancements, for example, and ionospheric disturbances— that can disrupt the communications, navigation, and power systems on which modern society so extensively depends. Earth’s aurora is ultimately powered by energy produced in thermo-nuclear reactions within the Sun’s core and conveyed to Earth by the Sun’s magnetized wind.8 The last half century has seen remarkable advances in our knowledge and understanding of this stellar wind and its solar source, and of its interactions with Earth, other planets, and the ionized and neutral gases of the local interstellar medium. The beginnings of this understanding reach farther into the past, however, to the middle of the 19th century, when evidence for connections between solar activity and terrestrial phenomena began to accumulate rapidly and the work of James Clerk Maxwell unified the foundations for the theoretical description of the electromagnetic forces that govern the behavior of matter in the plasma state.

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FIGURE 1.3 Artist’s conception of Earth’s magnetosphere, the volume of space around Earth dominated by the geomagnetic field and populated with plasmas of both ionospheric and solar wind origin. These plasmas are organized in distinct structures or regions characterized by different properties and separated by extremely thin boundary layers. A current sheet known as the magnetopause separates the magnetospheric plasma from the solar wind. In the upstream direction, this boundary is located at an average distance of 10 Earth radii (1 Earth radius = 6,378 kilometers) from Earth’s center. Farther upstream, a standing shock wave (the bow shock) forms as the supersonic wind is slowed and heated by its encounter with the magnetosphere. On the nightside, the interaction with the solar wind stretches the terrestrial field into an elongated, tail-like structure that extends hundreds of Earth radii—well beyond the orbit of the Moon—in the antisunward direction. The ionized gases that populate the magnetosphere are remarkably dilute: The densest magnetospheric plasma is 10 million times less dense than the best laboratory vacuum! Nevertheless, the motions of these highly tenuous plasmas drive powerful electrical currents, and during disturbed periods, Earth’s magnetosphere can dissipate well in excess of 100 billion watts of power—a power output comparable to that of all the electrical power plants operating in the United States.

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FIGURE 1.4 Earth’s aurora as seen in visible light from the ground (left) and in the far ultraviolet from the Earth-orbiting IMAGE spacecraft (right). Auroras are produced when energetic charged particles from the magnetosphere precipitate into Earth’s atmosphere. The charged particles collide with the atoms and molecules of the upper atmosphere, exciting them and causing them to emit light at various wavelengths. The ground-based photograph is courtesy of Jan Curtis. The space-based image is courtesy of the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) far-ultraviolet imaging team and NASA. MILESTONES: FROM STONEHENGE TO SOHO The Sun and the heavens have long occupied a special place in human culture, as is amply documented by archaeological and other evidence of the astronomical interests of many different “pre-scientific” peoples, from the Neolithic builders of Stonehenge to the Mayans, whose astronomer-priests prepared solar-eclipse prediction tables of amazing accuracy. It was not until the 19th century, however, some 200 years after the invention of the telescope and the discovery of sunspots, that the systematic scientific study of the Sun began, with the discovery of the sunspot cycle and the application of the new science of spectroscopy to the analysis of solar composition. During the second half of the century, correlations reported between solar activity (as manifested in the changing sunspot number and in flares), disturbances in the Earth’s magnetic field, and auroral activity clearly suggested the existence of a physical connection between the Sun’s activity and terrestrial magnetic and upper atmospheric phenomena. The

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nature of this connection—one of the central themes of space physics— became the subject of intense study and controversy during the first half of the 20th century. By midcentury, the prevailing theory involved ionized “corpuscular streams” from the Sun that traveled at speeds of 1,000 to 1,600 kilometers per second and within which the geomagnetic field formed a cavity.9 This picture was changed dramatically in the late 1950s when it was shown theoretically that the outer solar corona could not be static but must be continually expanding outward. The model of individual corpuscular streams was replaced by the modern concept of a continuous solar wind.10 The launch of the first Earth-orbiting satellites in the late 1950s and of the first interplanetary probes a few years later revolutionized the scientific community’s ability to study the Sun-Earth connection, the interplanetary environment, and the space environments of the other planets. The event that signaled the opening of the new era in the exploration of space was the startling discovery, in 1958, of belts of trapped energetic charged particles (the Van Allen belts) circling Earth’s middle and low latitudes at altitudes between 400 km and 60,000 km.11 Within a short time of this discovery, the theoretically predicted solar wind with its embedded magnetic field had been observed and measured, and the cavity within it—now named the magnetosphere—was being surveyed. In situ measurements from a series of Earth-orbiting spacecraft, operating both inside and outside the magnetosphere, mapped this region’s large-scale structure, establishing the existence of a boundary (the magnetopause) separating the magnetosphere from the solar wind and of a twin-lobed magnetic tail extending many hundreds of Earth radii in the antisunward direction. Theory had predicted that the supersonic nature of the solar wind would give rise to a collisionless bow shock upstream of the magnetosphere, and this prediction was verified by in situ satellite observations. These early space missions also confirmed the presence inside the magnetosphere of a ring current and of magnetic-field-aligned currents flowing between Earth’s high-latitude upper atmosphere and the magnetosphere.12 Of fundamental importance for the field of solar-terrestrial research were the prediction13 and discovery during the first decade of the space age of a link between geomagnetic activity and the orientation of the magnetic field embedded in the solar wind (the IMF). During the ensuing decades, space physicists made significant progress in understanding this link, which involves the merging of the interplanetary and terrestrial magnetic fields and the consequent transfer of energy, mass, and momentum from the solar wind into the magnetosphere, often resulting in major disturbances of Earth’s

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Sun and various heliospheric processes powered by the solar magnetic field remains incomplete. An understanding of magnetic field generation is also the key to understanding magnetic activity in whole classes of solarlike stars. The concentrated fibril bundles of emerging subsurface solar magnetic flux are mysterious and far from fully explored. Periodic reversals of magnetic field, variability of the level of activity, and the nature and origin of coronal holes, all features of the solar cycle, are fundamental drivers of the state of the entire heliosphere. In recent years, the lower corona has been observed at spatial and time resolutions that have revealed details of phenomena not yet explained in terms of basic physics. It is still not understood how extremely high temperatures, in excess of several million degrees, are maintained above the cooler solar surface. Although the energy that powers this heating must come ultimately from the convection zone and photospheric activity, we do not understand how it is transported through the chromosphere and into the corona and how it is converted to heat beginning within about a tenth of a solar radius above the photosphere. This intellectual deficit directly translates into an incomplete understanding of the origins of the solar wind and of the baseline conditions for establishing the entire extended solar atmosphere. Although the Ulysses mission established a firm distinction between two different states of the solar wind (fast, hot, rarefied and slow, cooler, denser), the origins of the two states remain important questions, and it is not known if the slow wind is quasi-stationary or results from transient processes. Determining the composition and evolution of solar wind plasma structures remains a major challenge. One of the most significant developments of the past decade has been the recognition of the importance of CMEs in driving geoeffective heliospheric disturbances. Many questions remain, however, about the initiation and evolution of CMEs—about the role of magnetic reconnection, for example, and about the origin of the magnetic flux rope structure observed in a number of CMEs. Similarly, the past decade has seen notable advances in our understanding of particle acceleration at the Sun and in the heliosphere; of particular importance is the recognition of the role of both flares and CME-driven shocks in the acceleration of solar energetic particles (SEPs). However, much remains to be learned about the spatial and temporal evolution of the SEP sources and about the basic SEP acceleration and transport processes.

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Key questions include: How and where does the solar dynamo operate, and in what ways do the fields created by the dynamo move up through the visible surface? How do the magnetic structures in the corona follow? What physical processes are responsible for heating the active x-ray corona? What heats the coronal holes? What are the basic physical mechanisms for, and characteristics of, the acceleration of the fast and slow solar winds? What controls the development and evolution of the solar wind in the inner heliosphere? What is the physics of explosive energy release in the solar atmosphere? How are CMEs initiated? How and where are particles accelerated at the Sun? The Heliosphere and Its Components Challenge 2: Understanding heliospheric structure, the distribution of magnetic fields and matter throughout the solar system, and the interaction of the solar atmosphere with the local interstellar medium. The heliosphere is not static. Complex and time-dependent phenomena such as stream interfaces, multiple current sheets, and CMEs are pervasive. Flux tubes are set into motion by coronal and photospheric processes that are not well understood. Interplanetary spacecraft instruments have repeatedly identified turbulence in the solar wind, but as yet a complete picture of the origin, evolution, and distribution of turbulent fluctuations is lacking. Although the interplanetary magnetic field and the radial distribution of plasma are understood at a coarse level, there are important unanswered questions. The structure of the magnetic field at high latitudes is unclear. The winding of the spiral magnetic field at large distances is not consistent with models. The distributions of dust, anomalous cosmic rays, and pickup ions all promise to reveal new physical insights about the overall structure of the heliosphere. The propagation of solar energetic particles and galactic cosmic rays depends on effects at the boundaries and also on small-scale turbulence. Thus the problems of cosmic ray modulation and energetic particle transport are particularly challenging, as they require a systemic understanding of heliospheric properties. Although direct exploration of interstellar space remains an important future goal, at present the heliopause bounds the domain of space physics and presents a more immediate goal for exploration. The complex solar

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atmosphere engenders an equally complex interaction with the interstellar medium. Although not yet directly explored, this distant boundary produces remote effects that we have begun to observe and study. For example, interstellar neutral atoms do not respond to the interplanetary magnetic field and thus are able to penetrate into the heliosphere, producing observable effects. These neutrals become ionized by charge exchange or ultraviolet radiation and become pickup ions that respond to and influence the solar wind and its magnetic field. The pickup ions are accelerated by interplanetary shocks and again to even higher energies at the termination shock, becoming the anomalous cosmic rays observed at Earth. A fascinating plasma physics process in its own right, this series of events and the feedback associated with it are facets of the spatially distributed interaction between the interplanetary and interstellar media. The transition at the heliopause between magnetic fields of solar origin and those of interstellar origin is another such boundary effect. As we begin to understand these interactions better in the coming decade, we will also begin to learn more about the next frontier of exploration, the local interstellar medium and the nearby parts of the galaxy that are not in direct plasma contact with our Sun. Key questions include: How do coronal structures evolve into solar wind structures of varying speed, density, kinetic temperature, composition, and magnetic field strength? How do CME-driven disturbances evolve in space and time as they propagate through the heliosphere? What is the structure of the interplanetary magnetic field at very large distances from the Sun and as a function of the solar cycle? How are plasma, neutrals, heavy ions, turbulent fluctuations, solar energetic particles, and galactic cosmic rays distributed throughout the entire heliospheric volume? How do the solar wind plasma and magnetic field interact with the electromagnetic field, plasma, and neutrals in the nearby region of the galaxy? How and where is the boundary of the heliosphere established? How does it move in time, and how do such changes affect our space environment? What is the nature of the local interstellar medium? Space Environments of Earth and Other Solar System Bodies Challenge 3: Understanding the space environments of Earth and other solar system bodies and their dynamical response to external and internal influences. Of the magnetized solar system bodies, it is Earth’s space envi-

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ronment and its interaction with the solar wind that have been most extensively studied. As discussed above, more than 40 years of space- and ground-based observations have provided a good picture of the overall structure of the magnetosphere and its constituent plasma populations, of the solar wind’s role in driving magnetospheric dynamics, and of the electrodynamic coupling of the magnetosphere to the ionosphere. Nonetheless, important gaps in scientific understanding remain—concerning, for example, the configuration and dynamics of the magnetosphere under extreme solar wind conditions (i.e., during strong geomagnetic storms); many aspects of magnetic reconnection at the dayside magnetopause and in the magnetotail; particle energization in the inner magnetosphere; the complex structure of the magnetotail as it absorbs and releases energy extracted from the solar wind; and the temporal and spatial scales of the electrodynamical processes by which some of this energy, along with momentum, is transferred to and redistributed within the ionosphere-thermosphere system. Much remains to be learned about the dynamics and energetics of Earth’s middle atmosphere and its coupling to the ionosphere-thermosphere system. Poorly understood, too, are possible influences of the space environment on Earth’s weather and climate. Jupiter’s magnetosphere is the most thoroughly studied magnetosphere after Earth’s. Six flyby missions and Galileo’s successful 8-year tour, complemented by high-resolution auroral imaging from both ground- and space-based observatories, have yielded a wealth of information about the rotationally powered Jovian magnetosphere. However, Jupiter’s high-latitude magnetosphere, a region of crucial importance for understanding the transfer of planetary rotational energy to the magnetosphere and the transfer of magnetospheric energy back into the upper atmosphere, remains unexplored. The next decade will see extensive exploration of the magnetosphere of another giant planet, Saturn. Following its arrival in the Saturn system in 2004, the Cassini spacecraft will gather data on Saturn’s magnetospheric plasma sources (rings, icy satellites) and make observations needed to assess the relative contributions of solar wind energy and planetary rotational energy to dynamics. Of particular interest in the Saturn system is the interaction between the moon Titan, with its massive nitrogen-methane atmosphere, and the plasma contained in Saturn’s magnetosphere. Observations made during Cassini’s many flybys of Titan will deepen our knowledge of the processes involved in the interaction between an unmagnetized or weakly magnetized body and an externally flowing magnetized plasma and the effects of this interac-

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tion on the structure and dynamics of the body’s atmosphere. Such processes, already extensively studied at Venus by the Pioneer Venus Orbiter, will also be investigated during the next decade at Mars by the Japanese Nozomi mission and the European Mars Express mission. Owing to the presence of localized crustal remanent magnetic fields, Mars’s interaction with the solar wind is expected to be more complicated than that of Venus with the solar wind and that of Titan with Saturn’s magnetospheric plasma. The smallest of the planetary magnetospheres is Mercury’s, for which only limited data, acquired by Mariner 10 during two passes in the mid-1970s, are available. These data indicate that Mercury’s magnetosphere, like Earth’s, is energized by the interaction with the solar wind and suggest that intense, substorm-like particle acceleration events occur in its magneto-tail. It is not understood, however, how Mercury’s magnetosphere, in the absence of a conducting ionosphere, dissipates the energy that it extracts from the solar wind. This question will be addressed by two Mercury orbiter missions, NASA’s MESSENGER mission and the ESA’s BepiColombo mission, both scheduled for launch within the next decade. Key questions include: Does the state of Earth’s magnetosphere under extreme solar wind conditions differ qualitatively as well as quantitatively from its state under more moderate conditions? How and where does magnetic reconnection occur on Earth’s dayside magnetopause? What processes are responsible for the rapid acceleration of charged particles to hundreds of keV at the onset of magnetospheric substorms and to MeV energies during geomagnetic storms? Do the active auroral displays during substorms arise from instabilities in the ionosphere, or do they simply mirror plasma motions in the outer magnetosphere? To what extent are the ionized and neutral gases of Earth’s upper atmosphere affected by mechanical and electrodynamic inputs from the lower atmosphere? How does angular momentum conservation alter the dynamics of reconnection in Jupiter’s rapidly rotating magnetosphere? How is Jupiter’s high-latitude ionosphere coupled to the plasma populations of the magnetodisk? How do field-aligned currents arising from the interaction of moons with Jupiter’s magnetospheric plasma produce the emissions observed in the high-latitude ionosphere? What process is responsible for the pulsating x-ray aurora at Jupiter?

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What is the effect of localized crustal magnetic fields on the solar wind interaction with Mars? How are outflowing ions on the nightside of Mars accelerated to keV energies? How does the absence of a significant ionosphere affect the interaction of Mercury’s ionosphere with the solar wind? How do substorms at Mercury differ from terrestrial substorms? Fundamental Space Plasma Physics Challenge 4: Understanding the basic physical principles manifest in processes observed in solar and space plasmas. The ultimate goal of solar and space physics research is not merely to produce a detailed phenomenological description of its various objects of study but also to understand the fundamental physical processes that operate in them. As these processes largely involve matter in the plasma state, solar physics and space physics can both be considered branches of plasma electrodynamics, to which field they have contributed significantly over the years. The heliosphere is a natural laboratory for the study of plasma physics, and the next decade of research can be expected to lead to advances in our understanding of such fundamental plasma physical processes as magnetic reconnection; turbulence; charged particle acceleration and scattering; generation, transport, and damping of plasma waves; and magnetic dynamo action. The study of these processes in naturally occurring solar system plasmas will be complemented by their investigation with increasingly sophisticated computer models and in laboratory plasma experiments (see Chapter 4). Key questions include: What controls the rate of collisionless magnetic reconnection? How is reconnection initiated? Does it occur spontaneously or episodically or must it be driven by external triggers? What are the roles of non-magneto-hydrodynamic14 processes (kinetic Alfvén waves, whistler-mode waves, electron inertial effects) in magnetic reconnection? What is the nature of turbulence in nonuniform plasmas? How does microturbulence couple to magnetohydrodynamic or fluid turbulence? How does turbulence propagate across plasma boundaries? What are the processes by which particles are accelerated to very high energies in the heliosphere, and what governs their transport? What are the conditions under which the electrodynamic interaction of a conducting body with an ambient magnetized plasma generates waves that can affect the particle population?

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Space Weather Challenge 5: Developing near-real-time predictive capability for understanding and quantifying the impact on human activities of dynamical processes at the Sun, in the interplanetary medium, and in Earth’s magnetosphere and ionosphere. Space weather describes the conditions in space that affect Earth and its technological systems. Space weather is a consequence of the behavior of the Sun, the nature of Earth’s magnetic field and atmosphere, and our location in the solar system. Through various complex couplings, the Sun, the solar wind, and the magnetosphere, ionosphere, and thermosphere can influence the performance and reliability of spaceborne and ground-based technological systems. Solar energetic particle events and geomagnetic storms are natural hazards, like hurricanes and tsunamis. Solar energetic particle events can disrupt spacecraft operation and present a radiation hazard to astronauts and to the crews and passengers of aircraft flying at high latitudes as well. Severe geomagnetic storms can interfere with communications and navigation systems, disturb spacecraft orbits because of increased drag, and cause electric utility blackouts over wide areas. Both our understanding of the basic physics of space weather and our appreciation of its importance for human activity have increased considerably during the past decade. Much remains to be learned, however, about processes—such as radiation belt enhancements—that affect the environment in which many satellites operate; about the variations in the properties of the ionosphere-thermosphere system that can adversely affect Global Positioning System navigation systems and high-frequency radio wave propagation; and, finally, about the solar drivers of space weather. During the coming decade these problems will be the focus both of pure space physics research and of targeted basic research activities such as those envisioned in NASA’s Living With a Star initiative, the National Science Foundation’s National Space Weather Program, and the Department of Defense’s Space Weather Architecture Study.15 An important aspect of space weather-related research is the development of specification and predictive models that can be used for system design, space operations, and both now-casting and forecasting. Such models will be powerful and indispensable tools for the mitigation of the harmful effects of space weather. Key questions include: What measurements need to be made to quantify the effects of space weather? Can a sustainable observing program provide the input needed

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for useful predictions? Can the physics of space weather phenomena be understood well enough to become predictive? What requirements does the need to predict space weather place on specification models and on physics-based data assimilation models? What risks does space weather present for human spaceflight outside the protective shield of Earth’s magnetosphere? THE ASTROPHYSICAL CONTEXT The above challenges outline, in broad brush strokes, major themes that characterize research objectives in solar and space physics. The committee recognizes that these disciplines ultimately belong to the broader intellectual enterprise of seeking to understand the universe at large and to comprehend our place in it. An additional far-reaching goal thus defines the larger context of all solar and space physics research: Understanding the Sun, heliosphere, and planetary magnetospheres and ionospheres as astrophysical objects and in an astrophysical context. It is inevitable that we have come to a more specific and detailed understanding of our own star than of other stars. We also observe space plasma physics processes in the solar system much more closely than in more distant astrophysical objects. The detailed observations and models developed in space physics can therefore help to develop physically motivated explanations of less-well-constrained astrophysical phenomena. The cross-disciplinary influences flow in both directions, of course. That is, although we may acquire detailed knowledge about the Sun and its environment, it is only one star. If we believe that we have achieved an accurate understanding of a particular solar phenomenon, such as coronal heating, then we should be able to compare our model results with observations of Sun-like stars. Similarly, the physics of magnetospheres should have implications for astrophysical phenomena such as pulsars and jets (and, in the case of the jovian magnetosphere, for our understanding of the transfer of angular momentum by hydromagnetic processes in a rotating system). Moreover, many theoretical models would provide a way to scale physical predictions to stars that are not similar to the Sun. In this way, astrophysical observations can be used to test solar and space physics theories, and vice versa (see Chapter 4).

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UNDERSTANDING COMPLEX, COUPLED SYSTEMS Solar system plasmas are complex systems. Complexity arises from nonlinear couplings, both within a single system and between two or more different systems. Both types of coupling occur in solar and space plasmas. Beyond 10 to 15 AU, for example, the dominant constituent of the heliosphere by mass is neutral interstellar hydrogen. Charge exchange reactions couple this neutral hydrogen population and the solar wind, yielding a highly nonequilibrated, nonlinear system in which the characteristics of both populations are strongly modified. In addition to couplings between multiple constituents, solar system plasmas are characterized by couplings across a multiplicity of spatial and temporal scales; the nonlinear, dynamical, self-consistent feedback and coupling of all scales determine the evolution of the systems through the creation of large- and small-scale structures.16 Examples of such cross-scale coupling are reconnection and turbulence, which involve the nonlinear interaction of large-scale, slow magnetohydrodynamic behavior and small-scale, fast kinetic processes. Finally, distinct plasma regions and regimes are coupled across boundaries in a highly nonlinear, dynamical fashion. Such cross-system coupling is exemplified by the coupling that occurs between the solar wind and Earth’s magnetosphere as a result of the merging of interplanetary and geomagnetic field lines and by the electromagnetic coupling of the magnetosphere and the ionosphere. The complex, nonlinear, coupled character of solar system plasmas presents significant challenges to both our observational capabilities and our theoretical understanding. The research initiatives recommended by the committee and presented in the following chapter will enable solar and space physicists to address those challenges and thereby to achieve new and deeper understanding of solar system plasmas and of the fundamental physical processes that govern them. NOTES 1.   Conversely, research and technology that are explicitly directed toward practical ends can make substantial contributions to “pure” scientific inquiry and the acquisition of fundamental knowledge. A classic example from the early days of the space age is the important role played by the Vela satellites in the exploration of the magnetosphere and the nearby interplanetary medium. Launched during the 1960s and in early 1970, the Velas were part of a joint program of the Department of Defense and the Atomic Energy Commission to monitor nuclear tests from space. (Besides their role in magnetospheric research, the Velas also made a major contribution to astrophysics, through their discovery of gamma ray bursts.) Operational satellites—those of the Defense Meteorological Satellite Program, the geosynchronous

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    spacecraft instrumented by the Los Alamos National Laboratory, and the meteorological satellites of the National Oceanic and Atmospheric Administration—continue to provide space physicists doing basic research with invaluable data about the terrestrial space environment. A recent example of the adaptation of an applied, real-world technology for the purpose of fundamental research is the use of data from the magnetometers on the Iridium communications satellites to prepare global maps of Earth’s field-aligned current systems. 2.   SolarMax, a 40-minute documentary in giant screen format, was written, directed, and produced by John Weiley. Chicago’s Museum of Science and Industry is the film’s executive producer and international distributor. NASA, the NSF, and the European Space Agency all assisted in the production of SolarMax. 3.   The interface between the solar wind and the local interstellar medium is complex. It consists of three main structures: the heliopause, which separates the solar wind plasma and the interstellar plasma; the termination shock, a shock wave inside the heliopause where the solar wind speed changes from supersonic to subsonic; and the heliosheath, a region of shocked solar wind between the termination shock and the heliopause. The distances to the termination shock and the heliopause are not known and are most certainly variable, as described in the text. However, if estimates placing the shock between 85 and 100 AU from the Sun are correct, Voyager 1, traveling at a speed of 3.6 AU per year, may soon encounter it. (An astronomical unit is the mean distance between the Sun and Earth, roughly 150 million kilometers.) Depending on the distance to the heliopause, the spacecraft may also reach the boundary of the heliosphere—and perhaps even cross into the interstellar medium—before it no longer has sufficient power to operate its instruments (around 2020). Voyager 2 is currently at 67 AU and traveling at 3.3 AU per year; it will thus encounter the shock later than Voyager 1. Like Voyager 1, Voyager 2 will have enough power to operate its instruments until 2020. 4.   Zank, G.P., and P.C. Frisch, Consequences of a change in the galactic environment of the Sun, Astrophysical Journal 518, 965-973, 1999. 5.   See the discussion of solar energetic particle events in the National Research Council’s Radiation and the International Space Station: Recommendations to Reduce Risk, National Academy Press, Washington D.C., 2000. 6.   Partial protection of the martian atmosphere from the solar wind is provided by the strong, localized remnant crustal magnetic fields recently discovered by the Mars Global Surveyor magnetic field experiment. Because of the presence of these fields, the solar-wind/ atmosphere interaction is considerably more complex at Mars than at Venus. 7.   Solar-wind-driven atmospheric erosion has certainly influenced the evolution of Venus’s atmosphere as well. However, the few studies of the implications of this loss process for the early history of the two planets have focused on Mars rather than Venus (e.g., Luhmann, J.G., R.E. Johnson, and M.H.G. Zhang, Evolutionary impact of sputtering on the Martian atmosphere by O+ pickup ions, Geophysical Research Letters 19, 2151, 1992, and Kass, D.M., and Y.L Yung, Loss of atmosphere from Mars due to solar wind-induced sputtering, Science 268, 697, 1995). It should be emphasized that such studies are subject to a number of important uncertainties—for example, those surrounding the level of the solar EUV flux, the planetary dynamo and global magnetic field at an earlier stage of Mars’s history, and the effect on the solar-wind/atmosphere interaction of the remnant crustal fields. 8.   Not all planetary magnetospheres are powered by the solar wind interaction. Jupiter’s giant magnetosphere (the largest object in the solar system) draws its power primarily from the rotational energy of the planet. Saturn’s magnetosphere, too, is thought to be rotationally driven. The relative roles of planetary rotation and the solar wind interaction in the dynamics of the Uranian and Neptunian magnetospheres are not known. 9.   Chapman, S., and J. Bartels, Geomagnetism, Oxford University Press, 1940. 10.   Parker, E.N., Interplanetary Dynamical Processes, Interscience Publishers, New York, N.Y., 1963.

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11.   In 1989, J.A. Van Allen (University of Iowa) was awarded the Crafoord Prize by the Royal Swedish Academy of Sciences for his discovery of the radiation belts. Van Allen describes the discovery of the radiation belts and the early days of magnetospheric research in Origins of Magnetospheric Research, Smithsonian Institution Press, Washington, D.C., 1983. 12.   Useful surveys of the development of magnetospheric physics before and after the International Geophysical Year are given by D.P. Stern in his articles “A brief history of magnetospheric physics before the spaceflight era,” Reviews of Geophysics 27, 103-114, 1989, and “A brief history of magnetospheric physics during the space age,” Reviews of Geophysics 34, 1-31, 1991. 13.   Dungey, J.W., Interplanetary magnetic field and the auroral zones, Physical Review Letters 6, 47-48, 1961. 14.   Magnetohydrodynamic theory incorporates the effects of magnetic fields in the hydrodynamic description of ionized gases (plasmas). Hannes Alfvén, whose work profoundly influenced space physics, was awarded the 1970 Nobel Prize in physics “for fundamental work and discoveries in magnetohydrodynamics with fruitful applications in different parts of plasma physics.” 15.   National Security Space Architect, Space Weather Architecture Study Transition Strategy, March 1999. Available online at <http://schnarff.com/SpaceWeather/PDF/Reports/P-IIB/02.pdf>. 16.   The organization of plasmas into large- and small-scale structures separated by thin boundaries has given rise to the picture, associated with Hannes Alfvén, of the “cellular structure” of solar system and astrophysical plasmas. Cf. H. Alfvén, Cosmic Plasma, D. Reidel, Dordrecht, The Netherlands, 1981.