This chapter summarizes our current understanding of the various ionospheres in the solar system. The order of presentation of the planetary ionospheres follows their position with respect to the Sun, that is, it starts with Mercury and ends with Pluto. The amount of information currently available varies widely, from a reasonably good description for Venus to just a basic guess for Pluto. In the last section of this chapter, the ionospheres of the various moons and that of Comet Halley are described. Here again the existing data are extremely limited and, with the exception of Titan, practically no new information will be forthcoming in the foreseeable future.

Mercury

Mercury does not have a conventional gravitationally bound atmosphere, as indicated in Section 2.4. The plasma population caused by photo and impact ionization of the neutral constituents, which is present in the neutral exosphere, is an ion exosphere, not a true ionosphere. No quantitative calculations of the plasma densities have been carried out to date. The global Na+ production rate was estimated to be a few times 1023 ions s–1, but no other studies have been published and there are no observations concerning the thermal plasma densities.

Venus

Of all the non-terrestrial thermospheres and ionospheres in the solar system, those of Venus have been the most studied, mainly because of the Pioneer Venus Orbiter (PVO) spacecraft, which made measurements over the 14-year period from 1978 to 1992.

Plasma physics is a subject where advanced mathematical techniques are frequently required to gain an understanding of the physical phenomena under consideration. This is particularly true in studies involving kinetic theory and plasma transport effects, where scalars, vectors, and multi-order tensors are needed (Chapters 3 and 4). Therefore, it is useful to briefly review some of the required mathematics.

A scalar is a single number that is useful for describing, say, the temperature of a gas. However, in order to describe the velocity of the gas, both a magnitude and direction are required (e.g., a vector). A vector is defined relative to some orthogonal coordinate system and three numbers, corresponding to the components of the vector, are required to define the vector. In a Cartesian coordinate system, the vector a is given

where e1, e2, and e3 are unit vectors along the x, y, and z axes, respectively. In index notation, the vector a is simply represented by aα where α varies from 1 to 3.

The plasma parameters in the Earth's ionosphere display a marked variation with altitude, latitude, longitude, universal time, season, solar cycle, and magnetic activity. This variation results not only from the coupling, time delays, and feedback mechanisms that operate in the ionosphere–thermosphere system, but also from the ionosphere's coupling to the other regions in the solar–terrestrial system, including the Sun, the interplanetary medium, the magnetosphere, and the mesosphere. The primary source of plasma and energy for the ionosphere is solar EUV, UV, and x-ray radiation; but magnetospheric electric fields and particle precipitation also have a significant effect on the ionosphere. The strength and form of the magnetospheric effect are primarily determined by the solar wind dynamic pressure and the orientation of the interplanetary magnetic field (IMF), i.e., by the state of the interplanetary medium. Also, tides and gravity waves that propagate up from the mesosphere directly affect the neutral densities in the lower thermosphere, and their variation then affects the plasma densities. The different external driving mechanisms, coupled with the radiative, chemical, dynamical, and electrodynamical processes that operate in the ionosphere, act to determine the global distributions of the plasma densities, temperatures, and drifts.

As noted in Section 2.3, the ionosphere is composed of different regions and, therefore, it is instructive to show the regions in which the different external processes operate. Figure 11.1 indicates the altitudes where the various external processes are most effective.

Collisions play a fundamental role in the dynamics and energetics of ionospheres. They are responsible for the production of ionization, the diffusion of plasma from high to low density regions, the conduction of heat from hot to cold regions, the exchange of energy between different species, and other processes. The collisional processes can be either elastic or inelastic. The interactions leading to chemical reactions are discussed in Chapter 8. In an elastic collision, the momentum and kinetic energy of the colliding particles are conserved, while this is not the case in an inelastic collision. The exact nature of the collision process depends both on the relative kinetic energy of the colliding particles and on the type of particles. In general, for low energies, elastic collisions dominate, but as the relative kinetic energy increases, inelastic collisions become progressively more important. The order of importance is from elastic to rotational, vibrational, and electronic excitation, and then to ionization as the relative kinetic energy increases. However, the different collision processes may affect the continuity, momentum, and energy equations in different ways. For example, ionization of neutral gases by solar radiation and particle impact are the main sources of plasma in the ionospheres and these processes must be included in the continuity equation. On the other hand, ionization collisions are very infrequent compared to binary elastic collisions under most circumstances, and therefore, the momentum perturbation associated with the ionization process is generally not important and can be neglected in the momentum equation.

Solar extreme ultraviolet (EUV) radiation and particle, mostly electron, precipitation are the two major sources of energy input into the thermospheres and ionospheres in the solar system. A schematic diagram showing the energy flow in a thermosphere/ ionosphere system caused by solar EUV radiation is shown in Figure 9.1. Relatively long wavelength photons (≥900 Å) generally cause dissociation, while shorter wavelengths cause ionization; the exact distribution of these different outcomes depends on the relevant cross sections and the atmospheric species. The only true sinks of energy, as far as the ionospheres are concerned, are airglow and neutral heating of the thermosphere. Even the escaping photoelectron flux can be reflected or become the incoming flux for a conjugate ionosphere. The specific distribution of the way that energy flows through the system is very important in determining the composition and thermal structure of the ionospheric plasmas. This chapter begins with a discussion of the absorption of the ionizing and dissociating solar radiation and the presentation of information needed to calculate ionization and deposition rates. This material is followed by a description of particle transport processes. The chapter ends with a presentation of electron and ion heating and cooling rates that can be used in practical applications.

Absorption of Solar Radiation

Radiative transfer calculations of the solar EUV energy deposition into the thermosphere are relatively simple because absorption is the only dominant process.

The 13-moment system of transport equations was introduced in Chapter 3 and several associated sets of collision terms were derived in Chapter 4. However, a rigorous application of the 13-moment system of equations for a multi-species plasma is rather difficult and it has been a common practice to use significantly simplified equation sets to study ionospheric behavior. The focus of this chapter is to describe, in some detail, the transport equations that are appropriate under different ionospheric conditions. The description includes a clear presentation of the major assumptions and approximations needed to derive the various simplified sets of equations so that potential users know the limited range of their applicability.

The equation sets discussed in this chapter are based on the assumption of collision dominance, for which the species velocity distribution functions are close to drifting Maxwellians. This assumption implies that the stress and heat flow terms in the 13-moment expression of the velocity distribution (3.49) are small. Simplified equations are derived for different levels of ionization, including weakly, partially, and fully ionized plasmas. A weakly ionized plasma is one in which Coulomb collisions can be neglected and only ion-neutral and electron-neutral collisions need to be considered. In a partially ionized plasma, collisions between ions, electrons, and neutrals have to be accounted for. Finally, in a fully ionized plasma, ion and electron collisions with neutrals are negligible.

This chapter describes the various measurement techniques that are directly applicable to the determination of ionospheric parameters. This discussion is restricted to the most commonly used methods, which measure the thermal plasma densities, temperatures, and velocities. In general, these techniques can be grouped as remote or direct (in situ) ones. Topics related to direct measurement techniques are described in the first four sections and the rest of the chapter deals with remote sensing. The remote, radio sensing methods rely on the fact that the ionospheric plasma is a dispersive media (Section 6.8) while the relevant radar measurements use the reflective properties of the plasma. The direct in-situ measurement techniques discussed here are restricted to those that are applicable to altitudes where the mean-free-path is greater than the characteristic dimension of the instrument.

Spacecraft Potential

In situ measurements of ionospheric densities and temperatures are based on the laboratory technique developed and discussed by Irving Langmuir and co-workers over seventy years ago. These so-called Langmuir probes, or retarding potential analyzers (RPAs), had been used for many years in laboratory plasmas before they were adopted for space applications. On a rocket or a satellite, the voltage applied to an instrument has to be driven against the potential of the vehicle, and therefore, it is appropriate to begin with a discussion of the factors that affect the value of this potential.

Background and Purpose

The ionosphere is considered to be that region of an atmosphere where significant numbers of free thermal (≤1 eV) electrons and ions are present. All bodies in our solar system that have a surrounding neutral-gas envelope, due either to gravitational attraction (e.g., planets) or some other process such as sublimation (e.g., comets), have an ionosphere. Currently, ionospheres have been observed around all but two of the planets, some moons, and comets. The free electrons and ions are produced via ionization of the neutral particles both by extreme ultraviolet radiation from the Sun and by collisions with energetic particles that penetrate the atmosphere. Once formed, the charged particles are affected by a myriad of processes, including chemical reactions, diffusion, wave disturbances, plasma instabilities, and transport due to electric and magnetic fields. Hence, an understanding of ionospheric phenomena requires a knowledge of several disciplines, including plasma physics, chemical kinetics, atomic theory, and fluid mechanics. In this book, we have attempted to bridge the gaps among these disciplines and provide a comprehensive description of the physical and chemical processes that affect the behavior of ionospheres.

A brief history of ionospheric research is given later in this introductory chapter. An overview of the space environment, including the Sun, planets, moons, and comets, is presented in Chapter 2. This not only gives the reader a quick look at the overall picture, but also provides the motivation for the presentation of the material that follows.

Chemical processes are of major importance in determining the equilibrium distribution of ions in planetary ionospheres, even though photoionization and, in some cases, impact ionization are responsible for the initial creation of the electron-ion pairs. This is particularly apparent for the ionospheres of Venus and Mars because they determine the dominant ion species (Sections 13.2 and 13.3). The major neutral constituent in the thermosphere of both Venus and Mars is CO2, and yet the major ion is, as a result of ion-neutral chemistry. Therefore, a thorough knowledge of the controlling chemical processes is necessary for a proper understanding of ionospheric structure and behavior. The dividing line between chemical and physical processes is somewhat artificial and often determined by semantics. In this chapter the discussion centers on reactions involving ions, electrons, and neutral constituents; photoionization and impact ionization are discussed in Chapter 9.

Chemical Kinetics

The area of science concerned with the study of chemical reactions is known as chemical kinetics. This branch of science examines the reaction processes from various points of view. A chemical reaction in which the phase of the reactant does not change is called a homogeneous reaction, whereas a chemical process in which different phases are involved is referred to as a heterogeneous reaction. In the context of atmospheric chemistry, heterogeneous reactions involve surfaces and are significant in some of the lower atmospheric chemical processes (e.g., the Antarctic ozone hole), but do not play an important role in ionospheric chemistry.

Before discussing the various ionospheres in detail, it is necessary to describe the physical characteristics of the bodies in the solar system that possess ionospheres as well as the plasma and electric-magnetic environments that surround the bodies because they determine the dynamical processes acting within and on the ionospheres. It also is useful to give a brief overview of the characteristics of the different ionospheres, including those associated with planets, moons, and comets. This not only allows the reader to easily see the diversity of ionospheric characteristics and features, but also provides motivation for the fundamental physics and chemistry covered in later chapters. In what follows, the sequence of the discussion is the Sun, the interplanetary medium, the Earth, the inner and outer planets, and then moons and comets.

Sun

The Sun is a star of average mass (1.99 × 1030 kg), radius (696,000 km), and luminosity (3.9 × 1026 watts) whose remarkable steady output of radiation over several billion years has allowed life to develop on Earth. The Sun is composed primarily of hydrogen and helium, with small amounts of argon, calcium, carbon, iron, magnesium, neon, nickel, nitrogen, oxygen, silicon, and sulfur. The solar energy is generated from the nuclear fusion of hydrogen into helium in a very hot central core, which is about 16 million kelvins. This energy is first transmitted through the radiative zone and then the convective zone, which is the outer 200,000 km of the Sun.

Plasma waves are prevalent throughout the ionospheres. The waves can just have fluctuating electric fields or they can have both fluctuating electric and magnetic fields. Also, the wave amplitudes can be either small or large, depending on the circumstances. Small amplitude waves do not appreciably affect the plasma, and in many situations they can be used as a diagnostic of physical processes that are operating in the plasma. Large amplitude waves, on the other hand, can have a significant effect on the plasma dynamics and energetics. In general, there is a myriad of waves that can propagate in a plasma, and it is not possible, or warranted, to give a detailed discussion here. Instead, the focus in this chapter is on just the fundamental wave modes that can propagate in both magnetized and unmagnetized plasmas. First, the general characteristics of waves are presented. This is followed by a discussion of small amplitude waves in both unmagnetized and magnetized plasmas, including high frequency (electron) waves and low frequency (ion) waves. Next, the effect that collisions have on the waves is illustrated, and this is followed by a presentation of wave excitation mechanisms (plasma instabilities). Finally, large amplitude shock waves and double layers are discussed.

General Wave Properties

Many types of waves can exist in the plasma environments that characterize the ionospheres. Hence, it is useful to first introduce some common wave nomenclature before discussing the various wave types.