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 EWV. IV. 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. Solar radiation leads to ion–electron production and heating via photoelectron energy degradation, with EUV wavelengths dominating in the lower thermosphere (E and F1 regions) and UV and X-ray wavelengths dominating in the mesosphere (D region). These processes occur over the entire sunlit side of the Earth.

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. Next, in Chapter 3, the general transport equations for mass, momentum, and energy conservation are derived from first principles so that the reader can clearly see where these equations come from.

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 with 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.

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 is also 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 see easily 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 (6.96 × 105 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 2.00 × 105 km of the Sun. The Sun's surface is irregular because of the strong convection in this outer zone, displaying both small-scale and large-scale convective cells or granules. The small-scale cells are about 1000 km in diameter, with individual cells lasting for approximately 10 minutes.

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. Aschematic 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 energy 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.