THE HELIOSPHERE

THE DOMAIN OF SOLAR AND SPACE PHYSICS

The Sun is the source of the heat and light that maintain Earth’s habitable environment, and the ultimate source of energy for life itself. It is also the origin of an invisible influence—in the form of a continual outflow of ionized gas, the solar wind—that affects not only Earth but also the other planets, moons, asteroids, and comets of our solar system. The region within which the Sun exerts this unseen influence is called the heliosphere, after “helios,” the Greek word for the Sun. The heliosphere can be thought of as a giant bubble inflated by the solar wind within the mixture of electrically neutral gas, ionized gas, and interstellar dust that form our solar system’s local galactic environment. Its boundaries are still uncharted but are thought to lie some 9 billion to 10 billion miles from the Sun, well beyond the orbit of Pluto, the outermost planet. As the solar wind flows away from the Sun and fills the heliosphere, it interacts in various ways with the planets and other solar system bodies that it encounters. The nature of this complex 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). Mars today, for example, has no strong global magnetic field, and so the solar wind impinges directly on a significant fraction of its thin carbon dioxide atmosphere. The erosion of the atmosphere resulting from this interaction over the last 3 billion years may have played an important role in the evolution of Mars’s atmosphere and climate. In contrast, Earth’s atmosphere is protected from direct exposure to the solar wind by the terrestrial magnetic field, which forms a complex and dynamic structure—the magnetosphere—around which most of the solar wind is diverted. The interaction with the solar wind drives a flow of extremely dilute ionized gases within the magnetosphere, powers the northern and southern lights, and is responsible for sporadic but sometimes quite severe disturbances in Earth’s space environment. Such space weather disturbances can interfere with communications and navigation systems, disrupt power systems, and pose a health and safety threat to astronauts.



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Understanding the Sun and Solar System Plasmas: Future Directions in Solar and Space Physics THE HELIOSPHERE THE DOMAIN OF SOLAR AND SPACE PHYSICS The Sun is the source of the heat and light that maintain Earth’s habitable environment, and the ultimate source of energy for life itself. It is also the origin of an invisible influence—in the form of a continual outflow of ionized gas, the solar wind—that affects not only Earth but also the other planets, moons, asteroids, and comets of our solar system. The region within which the Sun exerts this unseen influence is called the heliosphere, after “helios,” the Greek word for the Sun. The heliosphere can be thought of as a giant bubble inflated by the solar wind within the mixture of electrically neutral gas, ionized gas, and interstellar dust that form our solar system’s local galactic environment. Its boundaries are still uncharted but are thought to lie some 9 billion to 10 billion miles from the Sun, well beyond the orbit of Pluto, the outermost planet. As the solar wind flows away from the Sun and fills the heliosphere, it interacts in various ways with the planets and other solar system bodies that it encounters. The nature of this complex 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). Mars today, for example, has no strong global magnetic field, and so the solar wind impinges directly on a significant fraction of its thin carbon dioxide atmosphere. The erosion of the atmosphere resulting from this interaction over the last 3 billion years may have played an important role in the evolution of Mars’s atmosphere and climate. In contrast, Earth’s atmosphere is protected from direct exposure to the solar wind by the terrestrial magnetic field, which forms a complex and dynamic structure—the magnetosphere—around which most of the solar wind is diverted. The interaction with the solar wind drives a flow of extremely dilute ionized gases within the magnetosphere, powers the northern and southern lights, and is responsible for sporadic but sometimes quite severe disturbances in Earth’s space environment. Such space weather disturbances can interfere with communications and navigation systems, disrupt power systems, and pose a health and safety threat to astronauts.

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Understanding the Sun and Solar System Plasmas: Future Directions in Solar and Space Physics Solar and space physics is the scientific discipline that seeks to understand the inner workings of the Sun, the acceleration of its outer atmosphere into a supersonic wind, and the interaction of the solar wind with planetary upper atmospheres and magnetospheres, comets and other small bodies, and the local interstellar medium. Solar and space physics is a branch of plasma physics. Everyone is familiar with the three most common states of matter—solid, liquid, and gas. But there is also a less well known fourth state, plasma, which occurs when a gas becomes so hot that electrons separate from the atoms that make up the gas, resulting in a mix of negatively charged electrons and positively charged ions. Unlike ordinary gases (the air we breathe, for example), plasmas can carry electrical currents, and so their motion is subject to the influence of electrical and magnetic fields. Solar and space physics studies the behavior of plasmas in our solar system and their interactions with each other and with non-plasmas such as interstellar dust grains or the neutral gases of planetary atmospheres. It seeks to identify the composition of solar system plasmas and to understand the magnetic fields, electric fields, currents, and waves through and by which energy is transferred within them. Astrophysics, too, is concerned with matter in the plasma state. However, the objects that it studies lie far outside the boundaries of our solar system and can be studied only remotely, through the detection of the electromagnetic radiation that they emit or that passes through them. Solar and space physics is unique among the space sciences in that, since the beginning of the space age, it has been able to sample solar system plasmas and their associated electric and magnetic fields directly, through in situ measurements from satellites. Such measurements have been an invaluable source of knowledge about the physics of plasmas. What scientists have learned about the behavior of plasma within our own solar system can help them understand similar processes in remote astrophysical environments, inaccessible to direct measurement. Artist’s rendering of the heliosphere, showing its structure and boundaries.

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Understanding the Sun and Solar System Plasmas: Future Directions in Solar and Space Physics Solar and space physics is also unique among the space sciences in that what it learns about solar system plasmas can be of practical as well as purely scientific value. That is, the better scientists understand the workings of the Sun, the greater our capability will be to anticipate and protect against the adverse effects of space weather—whether in Earth’s own magnetosphere or in the space environments of those planets on which we hope to live and work in the future. Solar and space physics is a relatively mature discipline in which the question what is increasingly replaced by the questions how and why. Why does the Sun’s magnetic activity exhibit an 11-year cycle? How is the solar atmosphere heated to a million or more degrees, while the temperature of the Sun’s visible surface measures only around 6,000 degrees? How is the solar wind accelerated to supersonic velocities? How do magnetic fields interconnect, causing the rapid conversion of magnetic energy to charged-particle kinetic energy in solar flares and magnetospheric substorms? How do the heliosphere and our galaxy interact? These are but a few of the major questions that solar and space physicists will seek to answer during the coming decade. These questions and the research initiatives needed to address them were the subject of an 18-month study organized by the National Research Council’s Space Studies Board and its Committee on Solar and Space Physics and carried out in 2001 and 2002 by five ad hoc study panels under the oversight of the Solar and Space Physics Survey Committee. The Survey Committee’s findings and recommendations with respect to scientific goals for solar and space physics and an integrated research strategy for the next decade are reported in The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics. That 177-page report forms the basis for this booklet, which offers the general reader an overview of the field of solar and space physics, along with a summary of the key problems that solar and space physics research will target during the next decade and the principal research initiatives recommended by the Survey Committee. SOLAR AND SPACE PHYSICS The fundamental goal of solar and space physics research is to discover, to explore, and ultimately to understand the activity of a star—our Sun—and the often complex effects of that activity on the interplanetary environment, the planets and other solar system bodies, and the interstellar medium. The lessons of solar and space physics are often relevant to our understanding of astrophysical objects lying beyond our solar system. Solar and space physics, though “pure” science, also yields important practical benefits in the area of space weather.

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Understanding the Sun and Solar System Plasmas: Future Directions in Solar and Space Physics MAJOR THEMES IN SOLAR AND SPACE PHYSICS RESEARCH In surveying the current state of ground-based and space-based research in solar and space physics and recommending directions for future research, the Solar and Space Physics Survey Committee organized the outstanding questions that will be the focus of solar and space physics investigations during the next decade in terms of the following five themes or challenges: THE SUN’S DYNAMIC INTERIOR AND CORONA Understanding the structure and dynamics of the Sun’s interior, the generation of solar magnetic fields, the causes of solar activity and the origin of the solar cycle, and the structure and dynamics of the corona CHARTING THE HELIOSPHERE 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 SPACE ENVIRONMENTS OF EARTH AND OTHER SOLAR SYSTEM BODIES Understanding the space environments of Earth and other solar system bodies and their dynamical response to external and internal influences FUNDAMENTAL SPACE PLASMA PHYSICS Understanding the basic physical principles manifest in processes observed in solar and space plasmas SPACE WEATHER 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