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A Science Strategy for Space Physics: Chapter 2 A Science Strategy for Space Physics 2 The Physics of the Solar Wind and the Heliosphere SCIENTIFIC BACKGROUND The Sun's outer atmosphere, or corona, is extraordinarily hot (over a million degrees) compared to the visible outer surface. The solar wind and the heliosphere exist because the pressure of the hot corona is too great to be contained by the combination of solar gravity, magnetic forces, and the pressure of the interstellar medium. Observations of stellar x-rays show that coronas and winds are pervasive features of late-type stars. As the only stellar-scale plasma system that permits a close-up view and in situ measurements, the Sun and heliosphere must be the guideposts for understanding those general astrophysical phenomena. Researchers do not yet understand the physical mechanisms that heat the solar corona, although it is generally accepted that magnetic fields must play a REPORT MENU central role. X-ray images acquired from space show that much of the corona is NOTICE dominated by arch-like structures of plasmas confined by the solar magnetic field. MEMBERSHIP The variety and superposition of magnetic structures visible at any time compound SUMMARY PART I the difficulty of resolving issues associated with coronal heating mechanisms. The PART II x-ray observations also show a strikingly filamentary and restless corona (Figure CHAPTER 1 6); flares and mass ejections (see Chapter 1, "Mechanisms of Solar Variability") CHAPTER 2 are but one component of the pervasive variability. In addition, there are regions, CHAPTER 3 known as coronal holes, that appear as dark areas in x-ray images of the Sun. The CHAPTER 4 magnetic fields from coronal holes are carried into interplanetary space by the CHAPTER 5 solar wind. X-ray observations also reveal widespread tiny active regions, called PART III coronal bright points, that may play a role in heating the corona and accelerating APPENDIX the solar wind. On the other hand, observations at 1 AU show the preponderance of magnetohydrodynamic waves, in particular, Alfvén waves, of solar origin. Such waves may play a very important role in the acceleration of the solar wind. file:///C|/SSB_old_web/strach2.html (1 of 9) [6/18/2004 2:19:34 PM]
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A Science Strategy for Space Physics: Chapter 2 FIGURE 6 Clockwise from upper left, the development of coronal structure observed by Yohkoh's soft x-ray telescope on November 12, 1991. A bright magnetic arcade forms at location A1 over a period of a few hours; a transient jet-like feature appears at location J. The lower left panel is a ground-based image (negative print) of neutral helium absorption that reveals structural connections between the chromosphere and the corona-for example, the "footpoints" of the arcade extending to the right of A2 as seen in x-rays (upper right panel) form a bright band in the helium image. (Courtesy Yohkoh team and K.L. Harvey, National Solar Observatory.) In the early 1970s coronal holes were identified as the source of the high- speed, quasi-stationary wind, but the solar source of the low-speed, quasi- stationary wind is still being debated. The speed and the fluxes of mass and energy of the low-speed wind are consistent with acceleration by thermal pressure gradients in the corona, but some additional push is required to generate the high- speed wind from coronal holes. Theories abound, but the relative importance of waves and of jets of particles, perhaps generated in bright points or in myriad tiny flare-like events, is not yet known. It is considered likely by some theorists that the unknown mechanisms that heat the corona and that accelerate the wind in coronal holes are very closely linked. This quasi-stationary solar wind is intermittently interrupted by the passage of coronal mass ejections (CMEs). CMEs are distinct plasma structures in interplanetary space that open and carry out magnetic fields from previously closed regions of the corona. Since the magnetic field is not observed to build up in interplanetary space, there must be some process closing off open flux; this opening and closing is an active area of research. About one-third of all CMEs are file:///C|/SSB_old_web/strach2.html (2 of 9) [6/18/2004 2:19:34 PM]
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A Science Strategy for Space Physics: Chapter 2 sufficiently fast to drive transient interplanetary shocks; it is these fast CMEs and the draped and compressed magnetic fields ahead of them that cause nearly all of the largest geomagnetic storms at Earth. Unfortunately, it is not yet possible to predict the beginning of CMEs from signatures in the preexisting coronal structure. The highly variable ion composition of the solar wind is another unresolved puzzle. The relative abundance of helium in the solar wind ranges from essentially zero to ~40%. In some types of solar wind flows the abundances of the elements depend on their first ionization potentials, whereas that effect is less discernible in the wind from coronal holes. The heavy elements are also observed to flow with speeds exceeding the proton speed, indicating the presence of a poorly understood physical mechanism that preferentially accelerates heavy ions relative to protons. Taken together, the heavy ion abundances, their charge states, and their speeds and temperatures must contain clues, currently undeciphered, to the processes responsible for heating the corona and accelerating the wind. As the wind flows out into the heliosphere (Figure 7), the properties such as the densities, velocities, temperatures, anisotropies, heat fluxes, beams, and other nonthermal characteristics of the ions and electrons evolve. Several factors have been invoked to explain why the temperature of the solar wind cools off with solar distance less slowly than adiabatically. Conservation of the particles' magnetic moments during the expansion, the pickup of newly ionized interstellar gas, and other processes result in unstable configurations that lead to the generation of waves and wave-particle interactions. The cooling rate may be controlled by a turbulent cascade of energy from longer to shorter scales. A related debate concerns the source of interplanetary turbulence: Is it caused principally by velocity shears or by the nonlinear evolution of magnetohydrodynamic waves? The very large scale of the heliosphere provides a unique opportunity to study those specific plasma microprocesses in relative isolation and with minimal confusion by boundary effects. FIGURE 7 A diagram showing current ideas about the three-dimensional structure of the heliosphere and its interaction with the interstellar medium. Neither the termination shock, the heliopause, nor the bow shock has yet been directly observed. (Courtesy of the Jet Propulsion Laboratory.) Although it has been known for 40 years that the flux of galactic cosmic rays in the inner solar system varies roughly out of phase with the solar activity file:///C|/SSB_old_web/strach2.html (3 of 9) [6/18/2004 2:19:34 PM]
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A Science Strategy for Space Physics: Chapter 2 cycle, the dominant cause of the modulation is still being hotly debated. There are sets of observational data that support a diffusion/convection mechanism, and other sets of observational data suggest at other times and other places within the heliosphere the principal mechanism may be particle drifts in the large-scale interplanetary magnetic field. Finally, heliospheric physicists are poised to enter a new and exciting era of exploration and discovery-namely, direct observation of the interaction of the heliosphere with the local interstellar medium, whose properties are very poorly known. As shown in Figure 7, two or three boundary structures are expected in the region where the solar wind interacts with the interstellar plasma: a heliopause separating the two plasmas, a termination shock where the solar wind is decelerated from a supersonic to subsonic state, and perhaps a bow shock in the interstellar plasma. The Voyager spacecraft have detected low-frequency radio signals not seen in the inner heliosphere. One interpretation is that those waves may be generated by plasma processes at the heliopause. Neutral interstellar atoms are free to stream into the heliosphere where they become ionized and are then carried outward as pickup ions in the solar wind. Acceleration of the pickup ions leads to a population of energetic particles called anomalous cosmic rays, but it remains to be seen how much of the acceleration occurs at the termination shock versus how much occurs upstream of the termination shock, either stochastically or by interplanetary shocks. Determination of the strength of the termination shock and the extent to which it accelerates particles to high energies will also serve as a test of theories that some of the galactic cosmic radiation may be accelerated in the termination shocks of certain types of stars. It is hoped that the Voyager spacecraft will cross the termination shock in the next few years to a decade and will leave the heliosphere sometime during the next few decades. In summary, the goals of heliospheric physics are to understand why the Sun and many stars have coronas and winds, what physical processes occur in the large regions dominated by the winds, and how the winds interact with the interstellar medium. Specific research questions raised by recent observations of the solar corona and the heliosphere are as follows: How does the energy transported from the solar interior heat the corona? How is the magnetic structure of the corona related to its thermal and dynamic evolution? Are coronal bright points an important source of heat? Is coronal heating caused by a theoretically predicted spectrum of microflares and nanoflares that are too small to be seen with current observational capability? Is the absorption of magnetohydrodynamic wave energy important, and if so, how and where are the waves generated? What are the three-dimensional topologies of coronal structures? How and where do the magnetic fields in the heliosphere open and close? What processes are responsible for the acceleration of the quasi- stationary solar wind? What are the roles of jets, hydromagnetic waves, and bright file:///C|/SSB_old_web/strach2.html (4 of 9) [6/18/2004 2:19:34 PM]
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A Science Strategy for Space Physics: Chapter 2 points? What effect does the magnetic structure have on the acceleration? Where does the slow solar wind originate, and why is it slower than the fast wind from coronal holes? What clues about the mechanisms responsible for coronal heating and solar wind acceleration exist in the charge distributions and the elemental composition of solar wind ions? How do the properties of the fields and particles in the heliosphere depend on solar distance, latitude, and longitude (with respect to the Sun's motion through the local interstellar medium) and on solar activity? What are the origins, structure, and evolution of plasma waves and turbulence? What is the role of pickup ions for the dynamics and properties of the solar wind? Where and how are the pickup ions accelerated to become the anomalous cosmic rays? How do shocks develop and interact and accelerate particles? How do energetic particles propagate throughout the heliosphere, and what are the relative roles of diffusion/convection and drifts? What are the natures and locations of the heliospheric boundary structures? What are the properties of the local interstellar medium? CURRENT PROGRAM The principal approach to studying coronal heating and solar wind acceleration is to make coordinated multiwavelength observations at different radial distances along the same streamline, starting from the source region at the solar surface and extending into interplanetary space. Such observations are the goal of several current and planned efforts. Yohkoh images the multimillion-degree coronal plasmas at x-ray wavelengths. Spartan 201 is a shuttle payload with instruments for remote diagnosis of the properties of the solar wind in the region within 10 solar radii of the solar surface. SOHO will map the solar disk and the corona out to 30 solar radii. The space-based data are supplemented and extended by rocket, balloon, and ground-based observations of the solar surface, the inner white-light corona, solar radio emission, and the scintillation of radio sources (both natural and spacecraft) caused by fluctuations in the corona and the solar wind. The properties of the solar wind are being and will be measured by near- Earth spacecraft (IMP-8, followed by Wind, and then ACE) and by Ulysses at high solar latitudes. Although ICE (ISEE-3) is available for low-latitude measurements at a longitude far removed from Earth, its operations and data analysis have been terminated. Four current and planned spacecraft (Ulysses, Wind, SOHO, and ACE) carry new-generation instruments capable of determining the mass and charge of solar wind ions. Combining those in situ measurements with remote sensing of conditions at the coronal source of the wind will help address questions about the file:///C|/SSB_old_web/strach2.html (5 of 9) [6/18/2004 2:19:34 PM]
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A Science Strategy for Space Physics: Chapter 2 wind's acceleration and elemental fractionation. Heliospheric plasma processes and the three-dimensional structure of the heliosphere are addressed by Pioneer 10 (in the ecliptic near 58 AU, headed down the heliospheric tail at 2.6 AU/year), Voyagers 1 and 2 (~54 and 40 AU, headed into the interstellar wind at midlatitudes with speeds of >3 AU/yr), Ulysses (in a high-inclination eccentric orbit reaching out to Jupiter and passing close to the solar poles), and near-Earth spacecraft such as IMP-8, Wind, and ACE (see Figure 7 and, in Chapter 3, Figure 10). The near-Earth fluxes of energetic particles are also monitored by additional Earth satellites (e.g., SAMPEX, GOES, and DMSP) as well as by ground-based neutron monitors sensitive to galactic cosmic rays and the highest-energy solar particles. The Voyagers are well instrumented for studying the evolution of waves and turbulence, the modulation of galactic cosmic rays, and the acceleration of anomalous cosmic rays and how those processes depend on distance from the Sun and on changes driven by the solar activity cycle. If current estimates of the heliospheric structure are correct, the Voyagers are expected to survive long enough (until ~2015) to observe the termination shock and perhaps also the heliopause. Ulysses adds a third spatial dimension to heliospheric physics. First, the three-dimensional magnetic and plasma structure of the heliosphere will finally be revealed. The minimization of effects due to velocity shears and stream interactions allows the study of processes such as the generation and evolution of turbulence, wave-particle interactions, and energetic-particle transport. Ulysses data will also be used to choose between different theoretical models of the modulation of galactic cosmic rays. FUTURE DIRECTIONS The present configuration of the Voyagers, Pioneer 10, Ulysses, and near- Earth spacecraft is unique, and there are no plans to deploy a second set of such well-distributed heliospheric observatories. Because many of the critical questions listed above concern the large-scale structure of the heliosphere over the full range of solar distances, latitudes, and activity, operating the Voyagers and Pioneer 10 as long as technically feasible and operating Ulysses throughout its second polar passage at the solar maximum are the committees' highest priority for heliospheric research. In addition, Cassini cruise science would allow the first measurements of some solar wind parameters (e.g., pickup ions and electron heat flux) beyond the orbit of Jupiter. Vigorous coordination of the different heliospheric data sets, with each other and with remote sensing of the solar corona by Yohkoh, Spartan 201, SOHO, and ground-based methods, together with the inclusion of guest investigators with different backgrounds and insights, would greatly enhance the scientific yield. Many of the current and planned studies of the corona not only image its topology, but also provide data from which some properties of the coronal plasma file:///C|/SSB_old_web/strach2.html (6 of 9) [6/18/2004 2:19:34 PM]
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A Science Strategy for Space Physics: Chapter 2 can be inferred, including the temperatures, densities, and flow speeds of different species, such as electrons, protons, and heavy ions. The calculated parameters, however, are subject to large uncertainties because of the highly structured nature of the solar corona and the superposition of plasmas with different temperatures and densities along the line of sight. Only for long-lived coronal features can observations of the rotating Sun alleviate the problems involved with line-of-sight effects. One way to reduce the uncertainties and to obtain "ground truth" for the remote sensing methods is to make in situ measurements over the distance range from the 0.3 AU perihelion of the Helios spacecraft down as close as possible to the Sun (within a few solar radii). Making such observations poses formidable technical challenges. If a cost-effective method for exploring this region can be found, it would surely command very high priority. Simultaneous remote sensing of the corona would greatly enhance the value of the in situ measurements. Another approach to surmounting line-of-sight problems is to obtain tomographic, three-dimensional observations of the Sun by remote sensing from two or more solar longitudes. Observations of the solar corona from a longitude ~90¡ from Earth would also allow measurement of the masses and velocities of coronal mass ejections heading Earthward that may cause geomagnetic disturbances a few days later. Other applications of a longitudinally distributed set of spacecraft are discussed in Chapter 1, "Mechanisms of Solar Variability." Further observations are required to determine which among many hypothesized mechanisms transport energy from the solar surface into the chromosphere and corona. One set of ideas centers on continual magnetic reconnection and fine-scale activity ("nanoflares"). Magnetohydrodynamic waves represent another family of possibilities; theoretical studies suggest particular attention to waves with periods from 1 to 10 s. The coronal magnetic field is usually estimated by extrapolating the field measured at the solar surface. Direct measurement techniques are not well developed. Estimates derived from radio observations depend on assumptions about the emission mechanism and independent information about plasma density and temperature. Another approach is to infer the field from the linear depolarization (via the Hanle effect) of ultraviolet radiation by neutral hydrogen in the corona, again combined with independent estimates of thermodynamic plasma parameters. Given the importance of the plasma and field parameters in the corona for solar wind models, it is extremely important for future programs to reduce the uncertainties and the model dependence of such measurements. The instrumentation already launched on Galileo and under development for the Cassini mission is entirely appropriate to obtain data at solar distances intermediate to Earth and the Voyagers during those missions' interplanetary cruise phases; however, there are no current plans to do so. Such cruise data would substantially enhance synoptic coverage of the heliosphere, thus increasing the return on investments already made in those programs. Cruise science should also be included on other future missions that explore regions of the heliosphere file:///C|/SSB_old_web/strach2.html (7 of 9) [6/18/2004 2:19:34 PM]
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A Science Strategy for Space Physics: Chapter 2 remote from the Earth. Because the possibility of in situ observations of the interstellar medium was not considered in the design of the instruments on the Voyagers, those spacecraft cannot make many of the measurements required to fully characterize the properties of the local interstellar medium and its interaction with the heliosphere. A space mission optimized for the study of the outer heliosphere and the interstellar medium, including the plasmas, cosmic rays, and neutral gas, would greatly enhance our understanding of how the heliosphere interacts with its galactic environs. In addition, none of the interplanetary spacecraft mentioned above includes instrumentation for studying phenomena that may depend on the sign of the particle's electric charge. The technology to do so through the study of cosmic-ray electrons and positrons exists but awaits a spaceflight opportunity. Missions to the innermost heliosphere and the interstellar medium are technically very challenging. The mass that can be delivered for missions of reasonable duration is severely limited by the capability of propulsion systems. Thus, realization of those objectives can be advanced by a program to develop a new generation of very small but still highly capable spacecraft and instruments. file:///C|/SSB_old_web/strach2.html (8 of 9) [6/18/2004 2:19:34 PM]