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Humanity has always been fascinated by atmospheric sources of light. The first records of auroras date back thousands of years to biblical, Greek, and Chinese documents. The name aurora borealis (latin for northern dawn) was coined by the French mathematician and astronomer, P. Gassendi, who described a spectacular event observed in southern France on September 12, 1621. Airglow was discovered in 1901 by Newcomb who explained it as light from stars too faint to be seen individually. It was not before the 1930s that scientists realized that the source of the faint “light of the night sky” must be zodiacal light and atmospheric luminescence. Figure 9.1 shows a photograph of a spectacular aurora. Both aurora and airglow are caused by excitation of atmospheric species followed by subsequent radiation of photons. They are, however, quite different in terms of excitation mechanisms, temporal and spatial characteristics, intensity, and dominant emissions. Airglow is the amorphous, faint optical radiation continuously emitted in wavelengths from the far UV to near infrared (but excluding thermal emissions in the long wavelength infrared). The Earth's airglow mainly originates from discrete atomic and molecular transitions (an exception is a weak continuum in the green). Airglow is mainly caused by three fundamental processes: direct scattering of sunlight, emissions associated with ionization and recombination, and radiation associated with neutral photochemistry.

Understanding the fundamental properties of the energetic particle population in the heliosphere is very important for two reasons: These particles represent considerable hazard for both humans and radiation-sensitive systems in space, because they can penetrate through large amounts of shielding material. They carry information about the large-scale properties of the heliosphere and the galaxy. High-energy cosmic ray particles carry a large amount of kinetic energy. The deposition of this energy can cause permanent effects in the material through which the cosmic ray particle passes. In the case of biological materials or miniature electronic circuits, these effects can be very serious. In order to provide adequate shielding for radiation-sensitive systems, we need to know the basic properties of the high-energy particle radiation, including its elemental composition, energy spectrum, and temporal variations. A significant portion of our present knowledge about the global structure of the heliosphere comes from energetic particle observations. These particles travel through space at velocities considerably higher than the characteristic velocities of the local plasma population. Because the propagation of the energetic particles is greatly affected by various physical properties of the medium, energetic particles sample regions of the heliosphere and the galaxy that are currently not accessible to spacecraft.

So far we have considered gases which were in kinematic equilibrium. It was always assumed that the molecules did not have any organized motion (bulk motion), and that the gas temperature and the number density of the molecules were constant everywhere. Under such conditions there is no net particle transport from one place to another. In this chapter we relax our earlier assumptions and let the macroscopic quantities (gas bulk velocity, temperature, pressure, concentration, etc.) have slow spatial variations. These configuration space gradients result in macroscopic transport phenomena, such as diffusion or heat flow. Fluid dynamics describes these transport phenomena using empirical transport coefficients, such as viscosity, diffusion coefficient or heat conductivity. In this chapter we shall use the elementary mean free path method to describe macroscopic transport phenomena and to calculate approximate values for the transport coefficients. This simple and remarkably successful method is quite general because it does not depend on the form of the distribution function and, therefore, does not assume equilibrium. The mean free path method is based on the assumption that the gas is not in equilibrium, but the deviation from equilibrium is so small that locally the distribution of molecular velocities can be approximated by Maxwellians. However, the macroscopic parameters of the local Maxwellians slowly vary with configuration space location. Mathematically this assumption can be translated into two basic conditions. The first condition is that the variation of all macroscopic quantities is small within a mean free path.

Gaskinetic Theory is an introductory text on the molecular theory of gases and on modern transport theory. It is suitable for upper division undergraduates in physics and first year graduate students in aerospace engineering, upper atmospheric science and space research. The first part introduces basic concepts, including the distribution function, classical theory of specific heats, binary collisions, mean free path, and reaction rates. Transport theory is used to express coefficients such as viscosity and heat conductivity in terms of molecular properties. The second part of the book covers advanced transport theory. Generalised transport equations are derived from the Boltzmann equation. The Chapman–Enskog and the Grad methods are discussed to obtain higher order transport equations for low density gases. The aerodynamics of solid bodies is explored and the book concludes with the kinetic description of shock waves.