A gas consisting of charged particles is called a plasma, although the use of the term is often restricted to charged particle gases in which collective phenomena, such as plasma oscillations, are more important than collisional phenomena. Collisions generally involve the short-range interactions of discrete particles, whereas collective phenomena involve large numbers of particles working in unison. The charged particle species in most plasmas are positive ions and negative electrons, although negative ions are also present in the D-region of the terrestrial ionosphere. Fully ionized plasmas contain only charged particles, whereas partially ionized plasmas also contain neutral gas. The solar wind plasma – that is, the interplanetary medium – is a fully ionized plasma; the ionosphere is a partially ionized plasma. A variety of methods have been developed to describe plasmas. Kinetic theory uses particle distribution functions to describe plasmas, whereas fluid theory (which includes magnetohydrodynamics or MHD) only uses a few macroscopic quantities derived from the full particle distribution functions. Because the subject of kinetic theory is largely outside the scope of an introductory book on space physics, this book will primarily use fluid theory to explain plasma phenomena in the solar system. However, a short introduction to kinetic theory and the derivation from kinetic theory of the fluid equations is provided in this chapter. More detailed treatments of kinetic theory can be found in the references listed at the end of the chapter.

We learned in the previous chapter that the solar wind is an almost collisionless plasma consisting mainly of protons and electrons flowing outward from the Sun supersonically and super-Alfvénically at several hundred kilometers per second. The interplanetary magnetic field is carried out into the solar system by the solar wind. Planets and other solar system bodies act as obstacles to the flow of the solar wind, but the nature of this interaction strongly depends on the characteristics of the planet. Chapter 7 deals with the solar wind flow around planets and other objects. A very brief introduction to this topic was given in Chapter 1. Further reading material on this topic can be found in the bibliography at the end of this chapter. Chapter 8 will deal with the internal dynamics of the terrestrial magnetosphere as well as with the magnetospheres of the outer planets.

Types of solar wind interaction

Nature of the obstacle

The manner in which the solar wind interacts with objects, or bodies, in the solar system depends, naturally, on the characteristics of that object. Relevant characteristics include its heliocentric distance (r), its size, whether or not it has an atmosphere and ionosphere, and the strength of its intrinsic magnetic field. Table 7.1 lists some relevant characteristics for all the planets and for other solar system bodies.

The Sun is a star. As stars go, the Sun is rather cool and small and has the gross characteristics listed in Table 5.1. The Sun is the source of virtually all energy in our solar system, including the Earth. Solar radiation heats our atmosphere and provides the light needed to sustain life on our planet. The Sun is also the source of space plasmas throughout the solar system. For example, solar extreme ultraviolet (EUV) radiation is largely responsible for the existence of planetary ionospheres via the photoionization of atoms and molecules in the upper atmospheres of the planets. The solar wind plasma is really an extension of the solar corona out into interplanetary space. The Sun is also, naturally, the source of solar activity. Solar activity refers to both short-term and long-term temporal variations in the solar atmosphere (and hence in the solar wind) that create changes in the Earth's plasma environment (i.e., geomagnetic activity). We will deal with the effects of solar activity on the Earth later.

The field of solar physics has advanced dramatically during the past few decades, due to observations made by increasingly sophisticated ground- and space-based observatories, including NASA's OGO, Skylab, and Solar Maximum missions and the NASA/ESA SOHO (Solar and Heliospheric Observatory) mission, and due to theoretical developments in the areas of stellar nuclear physics, stellar radiative transfer, and solar MHD.

The intrinsic magnetic field of the Earth acts as an obstacle to the solar wind and shields a volume of space, called the magnetosphere, from direct access of the solar wind. In Chapter 7, we considered the role of the magnetosphere as an obstacle to the solar wind and were mainly concerned with the region “external” to the magnetopause. The details of the internal dynamics of the magnetosphere do not seriously affect, at least to about the 95% level, the external solar wind plasma flow, but the solar wind does strongly affect the internal dynamics of the magnetosphere and ionosphere, as we will see in this chapter. This chapter will strongly emphasize macroscopic or fluid aspects of magnetospheric physics rather than the microscopic physics operating in the magnetosphere. Some aspects of the inner magnetosphere (i.e., the ring current and radiation belts) were already considered in Chapter 3.

The terrestrial magnetosphere has been extensively studied over the past 35 years with dozens of Earth orbiting satellites. The International Sun Earth Explorer (ISEE), Dynamics Explorer (DE), and AMPTE missions have been especially important, and in the near future we can expect useful information from recently launched spacecraft such as Geotail and Polar. The volume of observational and theoretical literature that exists, mainly in the Journal of Geophysics Research–Space Physics, has become immense. Much has been learned about how the magnetosphere works, although many key processes remain poorly understood.

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.

The beginnings

The term ‘science fiction’ was first used by one of the founders of the modern genre, Hugo Gernsback. Gernsback, after whom the annual science fiction 'Hugo’ awards are named, was the founder of the Amazing Stories magazine in April, 1gz6.The slogan on the title page proclaimed its mission: 'Extravagant Fiction Today … Cold Fact Tomorrow'. Of course, few of the stories published in Amazing Stories lived up to this claim, but science fiction does have some notable successes to its credit over its relatively brief history. Two of the founding fathers of science fiction – H. G. Wells and Isaac Asimov – have already been mentioned in earlier chapters. In this final chapter, we shall examine the interplay between relativity and science fiction. We begin with Johannes Kepler, arguably the first writer of the genre.

Kepler was born in south-west Germany in a small town called Weil-der-Stadt. Kepler's first great work, A New Astronomy, was published in I 609, and it remains a landmark in the history of science. In it, Kepler formulated the first ‘natural laws’ – precise, verifiable statements about natural phenomena expressed in terms of mathematical equations. Arthur Koestler, in his marvellous book The Sleepwalkers, claims that it was Kepler's laws that 'divorced astronomy from theology and married astronomy to physics'. Unlike Copernicus, Galileo or Newton, Kepler did not attempt to disguise the way in which he arrived at his conclusions -all his errors and sidetracks are faithfully recorded along with his final revelation.

Geometry and gravity

The discovery of ‘non-Euclidean’ geometry in the nineteenth century came as a great surprise and was greeted by disbelief. One of the pioneers of this new geometry, Janos Bolyai, a Hungarian army officer, expressed his joy with the words:

'Euclidean’ geometry is the geometry we learn in school, with its familiar apparatus of points, straight lines, circles, ellipses and triangles. In particular, we are all brought up to believe that the three angles of a triangle add up to 180 degrees and that parallel lines never meet. Such Euclidean geometry is the geometry of the plane – technically called a ‘flat’ space. By contrast, non- Euclidean geometry describes a ‘curved’ space. What do we mean by these terms?

Some idea of a curved space can be gained by considering geometry on the surface of the Earth. The Earth is approximately spherical, and on its surface it is easy to construct triangles whose angles add up to more than I 80 degrees (Figure 9. I). Similarly, lines of longitude start out parallel at the equator but converge and cross at the poles. The surface as a whole does not obey Euclid's rules. Since such a familiar example of a surface is non- Euclidean, why are such geometries so unfamiliar to most of us?

The momentous day in May

In May of 1905, Einstein was twenty-six years old, and his ten-year struggle with the problems of relativity was about to come to a triumphant climax. About a year before this, he had begun to feel that the velocity of light must be universal – independent of the motion of the source. If this were true, then there was no need to worry about motion relative to any mythical aether, and the null result of Michelson and Morley became obvious: the speed of light is the same in both arms of the apparatus, whatever direction they are pointing relative to the Earth's motion. But the Earth does move round the Sun – so something was wrong with the ‘relativity’ of Galileo and Newton and their familiar addition of velocities, at least where light is concerned. As we asserted in chapter 2, and as we shall show in the next chapter, in Einstein's relativity speeds do not add up in the expected way. We are also forced to re-think our notions of space and time. This new vision of space and time is what we shall look at in this chapter. Let us start by recalling what Galileo and Newton believed, before looking at Einstein's version of the relativity principle.

In the sixteenth century, it seemed natural to believe that, if the Earth was moving, neither an arrow shot straight up nor a stone dropped from a tower would follow the same straight-line path.

Einstein's revolution

The famous Russian scientist Lev Landau used to keep a list of names, in wich he graded, physicists into ‘leagues’. The first division contained the names of physicists such as Niels Bohr, Werner Heisenberg and Erwin Schroedinger, the founding fathers of modern quantum physics, as well as historical ‘giants’ such as Isaac Newton. He was rather modest about his own classification, grading himself 2½, although he later promoted himself to a 2. Most working physicists would be happy even to make it into Landau's fourth division: David Mermin, a well-known and perceptive American physicist, once wrote an article entitled ‘My life with Landau: homage of a 4½ to a 2’. What is the point of this story? The point is that any book about relativity is inevitably also about Albert Einstein, and Einstein was a remarkable physicist by any standard. Landau, in fact, created a special ‘superleague’ containing only one physicist, Einstein, whom he classified uniquely as a½. Thus, the popular opinion that Einstein was the greatest physicist since Newton is widely shared among professional physicists.

When Einstein wrote about ‘The wonderful events which the great Newton experienced in his younger days…’, and commented that, to Newton, Nature was ‘an open book’, he could well have been writing about himself.