This book is an introduction to the physics of the solar wind and magnetosphere. These regions of space are filled with charged particle gases called plasmas. The study of solar system plasmas is commonly called space physics. This book started as lecture notes for courses that I have taught at the University of Michigan and the University of Kansas. The book is an introductory textbook aimed at advanced undergraduate and graduate students who possess an undergraduate physics background but have not taken any plasma physics courses. An introduction to plasma physics, including the topic of magnetohydrodynamics, is included in order to make the book self-contained. Undergraduate-level electromagnetic theory and mechanics are extensively used, and the Appendix provides a very brief review of the first topic.

The book can be divided into three parts. The first part, consisting of Chapters 1 through 4, provides an introduction to plasma physics. In particular, Chapter 1 gives a brief introduction to space plasma physics, kinetic theory is discussed in Chapter 2, and Chapter 3 is concerned with single particle motion in electric and magnetic fields. Chapter 3 also contains material on energetic particle motion in the radiation belts. Magnetohydrodynamics (MHD) is introduced in Chapter 4. Examples dealing with phenomena in the solar wind and magnetosphere are provided. Students who have already taken a standard plasma physics course can skip over much of the first part of the book.

INTRODUCTION

THIS CHAPTER DEALS with the magnetopause and the magnetotail, the two regions within the earth's magnetosphere where relatively thin sheets of electric current separate regions of distinctly different magnetic fields. Such current sheets are common features in the solar system. Examples are found from the solar corona to the heliopause; so by extension we expect that they are common throughout the space-plasma universe. Yet these two current sheets, the magnetopause and the tail current sheet or neutral sheet, have been, by far, the most extensively studied, as they are by far the most accessible to direct observation.

A current sheet can be defined as a thin surface across which the magnetic-field strength and/or direction can change substantially. From Ampere's law it follows that the surface must carry a substantial electric current. “Thin” is a relative term, but here we mean that the sheet thickness is very much smaller than the other dimensions of the sheet, or than the sheet radius of curvature. This means that the sheet can be described locally as a plane. The thickness of the two magnetospheric current sheets typically is several hundred kilometers, while they extend for tens of earth radii.

As we shall show later, current sheets arise naturally out of the frozen-in-flux concept introduced in Chapter 2. Collisionless plasmas do not mix easily; instead, they tend to form cells of relatively uniform plasma permeated by a magnetic field. These cells are separated by thin current sheets through which little or no magnetic flux crosses. The interaction between different plasma regimes occurs at these thin boundaries - hence their importance and the importance of the physical processes that occur there. Magnetic reconnection is one of these processes, arguably the most important, certainly in a magnetospheric context. This chapter will focus on magnetic reconnection and how it occurs at the magnetopause and in the geomagnetic tail.

Before we begin our description of magnetic reconnection and the theories that have been developed to describe it, we introduce the magnetopause and magnetotail in rather more detail than was possible in Chapter 6. Throughout these sections, basic physics is stressed and is used to derive the properties of the regions described. We begin with the magnetopause.

SOLAR-TERRESTRIAL PHYSICS is principally concerned with the interaction of energetic charged particles with the electric and magnetic fields in space. In the vicinity of the earth, most of these charged particles derive their energy ultimately from the sun or from the interaction of the solar wind with the earth's magnetosphere. These interactions are complex, because the magnetic and electric fields that determine the motion of the particles are affected in turn by the motion of these charged particles. Some solar-terrestrial research is carried out on the surface of the earth with cameras, photometers and spectrometers, and magnetometers and other devices sensitive to the processes occurring high in the upper atmosphere and magnetosphere, but today most of this research is carried out using rockets and satellites that enable measurements to be obtained directly in the regions of strongest interactions. In recent years, these in situ data have resulted in explosive growth in our knowledge and understanding of solar-terrestrial processes. Nevertheless, the field has had a long history of investigation, starting well before the advent of satellites and rockets. We shall briefly review that history in order to provide a context for our later, more physically oriented presentation of the processes occurring in the solarterrestrial environment.

ANCIENT AURORAL SIGHTINGS

The emerging field of solar-terrestrial physics began with a growing appreciation of two phenomena: the aurora and, later, the geomagnetic field. Because it can be observed visually, the aurora was the first of these phenomena to be recorded. Most other phenomena of this emerging field awaited the advent of new technology, such as the compass in the case of geomagnetism, before they were discovered. References to the aurora are contained in the ancient literature from both East and West. Several passages in the Old Testament appear to have been inspired by auroral sightings, and Greek literature includes references to phenomena most likely to have been auroral phenomena. For example, Xenophanes, in the sixth century B.C., mentions “moving accumulations of burning clouds.” Chinese literature also describes possible auroral sightings, of which several occurred prior to 2000 B.C.

INTRODUCTION

NUMEROUS NATURALLY OCCURRING celestial phenomena have been observed and admired since the dawn of human history, but few have stirred human imagination, curiosity, and fear as much as the aurora. The aurora (also called the northern lights and polar lights) is certainly one of the most spectacular of nature's phenomena (Figure 14.1a-d).

When we search for records of the northern lights dating from more than 1,000 yr ago, we find that most come from the Mediterranean countries, that is, from low latitudes. Yet auroral displays are seen in that area only after unusually strong solar activity. The time lapse between such large auroral events can be 50-100 yr. Furthermore, an aurora seen at such low latitudes is significantly less dramatic and colorful than those at the higher latitudes of common auroral displays (see Section 14.3). Nonetheless, the ancient low-latitude events were dramatic enough to strike fear into the hearts of those who saw them.

At much higher latitudes, the aurora borealis (Le., in the north) and aurora australis (in the south) routinely appear in the so-called auroral zones, far from most population centers. Even today, the southern auroral zone (roughly around Antarctica) is inhabited only intermittently. The northern auroral zone, which crosses Alaska, northern Canada, northern Scandinavia, and Siberia, has always been accessible to frontiersmen (hunters and fishermen) living in the polar region. More recently, the area under the northern lights has become permanently, although sparsely, populated.

In earliest historical times, inhabitants of Greenland and the Nordic countries interpreted the northern lights as omens from the gods portending disaster, as signs from deceased relatives, as signs of a battle among the gods, or as weather signs. From more recent and more scientific Scandinavian records (The King's Mirror, written about 1230 A.D.), it appears that the regions of auroral activity have shifted significantly during the past 1,000 yr. For those interested in the history ofthe aurora, monographs by Brekke and Egeland (1994) and Eather (1980) are available.

Those who appreciate the beauty of nature may find nothing compara- ble to a night with a magnificent auroral display. It is just as beautiful to watch today as it was in the earliest days of human history.

INTRODUCTION

IN THE LABORATORY, we can modify the conditions in our experiments and apparatus and record how the process changes. We would like to be able to make such modifications in studying the solar-terrestrial system, but we cannot. By and large, solar-terrestrial physics is an observational science, rather than an experimental science. There are few active experiments we can perform to determine how the system works, either in the magnetosphere or in the laboratory. We cannot create magneto spheres with quantitatively scaled parameters to see how they work. Nor can we simulate accurately all the processes with our computers. We are restricted to working with the magneto spheres that we have found in place. Fortunately, these magneto spheres differ sufficiently that comparisons among them can give some insight into the governing processes at work. Unfortunately, we have limited data on the magneto spheres of the outer planets. As shown in Figure 15.1, there have been five passes through the Jovian magnetosphere, three at Saturn, and one each at Uranus and Neptune.

As discussed in Chapter 6, Mercury provides an important contrast to the terrestrial magnetosphere because it has no significant ionosphere or atmosphere. The outer planets provide us with other comparisons. First, as we move outward in the solar wind, the properties of the solar wind change. This may affect the coupling of energy flux from the solar wind into the magnetospheres. Also, the sizes of the magneto spheres are different. Those of the outer solar system, the “gas giants,” are much larger than that of Earth. The rapid rotation of all four of the gas giants leads to important centrifugal forces that far exceed those in the earth's magnetosphere. In the Jovian magnetosphere there is a significant source of plasma from the moon 10. Although the other Galilean moons provide some mass, and though there are weak mass sources in the magnetospheres of Saturn, Uranus, and Neptune, none approaches the magnitude of the 10 source.

Finally, the outer planets have rings. Some of these are quite tenuous, such as Jupiter's, and have little effect on the magnetosphere. However, others, like that of Saturn, are extensive and quite thick and absorb the trapped radiation efficiently. The varying rates of absorption in the different magneto spheres cause significant differences in the particle populations in the innermost parts of these magnetospheres.

THE IONIZED GASES of the solar-terrestrial environment interact in very complex and sometimes counterintuitive ways. Our intuition about gases is trained in situations in which collisions are important, but in most of the ionized gases in the solar system the magnetic and electric fields control the motion of the particles, with collisions and gravitational fields being less important. In an introductory text such as this it is difficult to decide where to begin to discuss these interactions. One could start with the simplest systems and then add complexity; one could order the material by spatial location, discussing the sun first and then proceeding to follow the energy flow outward past all the planets; or one could follow a chronological approach, according to the order of discovery. There is much to justify a spatial approach, because the sun is the energy source for most of the plasma we encounter, either through coupling with the solar wind or through photoionization. On the other hand, the chronological approach follows the way scientists originally learned about how the solar terrestrial environment behaves. This approach has the advantage that the earliest concepts were simple and grew gradually in complexity, but it has the disadvantage that some of the early ideas were wrong and that sometimes science progresses in convoluted ways. Thus, this approach can be quite inefficient.

In this book we shall attempt to combine the three approaches. We shall try always to reduce topics to their basics before introducing the complications. The overall ordering of the book will follow the energy flow, starting with the sun, but first, Chapter 1 will provide some historical perspective. The historical approach is interesting, and it allows us a quick overview of the entire field before becoming too involved with the details. We shall begin with ancient observations of the “northern lights,” which we now refer to as the aurora borealis, or simply the aurora, and work our way up to the era of space exploration.

Chapter 2 covers the physics of the plasmas we encounter in space. In this chapter we describe some of the most basic physical processes that occur in space plasmas and the equations that govern them. In particular we introduce the magneto hydrodynamic (MHD) approximations that are so useful in describing the solar-terrestrial environment.

INTRODUCTION

THE SOLAR WIND IS a flow of ionized solar plasma and a remnant of the solar magnetic field that pervades interplanetary space. It is a result of the huge difference in gas pressure between the solar corona and interstellar space. This pressure difference drives the plasma outward, despite the restraining influence of solar gravity. The existence of a solar wind was surmised in the 1950s on the basis of evidence that small variations in the earth's magnetic field (geomagnetic activity) were produced by observable phenomena on the sun (solar activity), as well as from theoretical models for the equilibrium state of the solar corona. It was first observed directly and definitively by space probes in the mid-1960s.

Measurements taken by spacecraft-borne instruments since that time (and until recently) have yielded a detailed description of the solar wind across an area from inside the orbit of Mercury to well beyond the orbit of Saturn. Solar-wind observations and theory have been the subjects of several books (Parker, 1963; Brandt, 1970; Hundhausen, 1972) and a continuing series of conferences summarizing new research (e.g., Pizzo et aI., 1987). Why has this distant and tenuous plasma been the subject of sustained interest in the scientific community? The answer to this question probably stems from two important aspects of solar-wind research.

The first of these concerns the role of the solar wind in the interdisciplinary subject known as solar-terrestrial relations. As we shall see later in this chapter, the solar wind is significantly influenced by solar activity (or, in physical terms, by changes in the solar magnetic field) and transmits that influence to planets, comets, dust particles, and cosmic rays that are immersed in the wind. Some of these effects will be described more extensively in later chapters. The origin of the solar influence through interaction of the solar magnetic field with the expanding coronal plasma has become a major topic in present-day solar-wind research and will be heavily emphasized in this chapter.

The second important aspect of solar-wind research that helps to explain the sustained interest in the subject concerns the physical processes that occur in its formation and expansion from the hot solar corona to the cool and far more tenuous regions of the outer solar system.