In Chapter 3, we studied how single charged particles move in specified electric and magnetic fields, and we then applied our knowledge of single particle motion to the radiation belt and ring current plasma. However, the fields in some situations depend too much on the particle distributions to be readily specified and must be found self-consistently using the charged particle distribution functions. Often, it is not necessary to have complete information about the distribution functions in a system. In fact, it is usually sufficient to know only a few of the velocity moments of the distribution function, as derived in Chapter 2. In Chapter 4, we will adopt the “fluid” picture of a plasma, introduced in Chapter 2, and further refine it to obtain an analytical tool useful for studying space plasma phenomena. This analytical tool is called magnetohydrodynamics (or MHD for short). We cannot adequately cover in one chapter all the material that would be desirable to know about this subject and so the reader is encouraged to consult one or more of the references listed in the bibliography at the end of this chapter.

Two-fluid plasma

Let us consider a plasma consisting of two species: electrons (e) with mass me and a single ion species (i) with mass mi.

Most of the visible matter in the universe exists as a fluid composed of electrically charged particles rather than as a gas made of neutral atoms or molecules. Gas mixtures of electrically charged particles, such as electrons and ions, are called plasmas. Plasmas are found in the following solar system environments: the solar atmosphere, the interplanetary medium, planetary magnetospheres, and planetary ionospheres. Most of the interstellar medium is also plasma, as are most other regions of our galaxy.

Most of the plasma found in our own solar system is accessible to in situ measurements made by instruments onboard spacecraft. Since the advent of the space age in the late 1950s, space probes have visited Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and comets Giacobini–Zinner, Halley, and Grigg–Skjellerup. The space environment surrounding the Earth has also been extensively studied by experiments onboard rockets and satellites. The Sun and astrophysical plasma environments outside our own solar system are not subject to direct measurements but must be observed remotely with sophisticated instruments located either at ground-based observatories or on orbiting observatories. An exception to this are the very energetic particles called cosmic rays, which can be observed using Earthbased or balloon-borne experiments. Solar cosmic rays have energies up to about 100 million electron volts (100 MeV) and originate in the solar corona.

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.