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.

The solar wind is the extension of the solar corona to very large heliocentric distances. As we shall see later in this chapter the solar wind exists because of the huge pressure difference between the hot plasma at the base of the corona and the interstellar medium.

The existence of a continuous solar wind was first suggested by Ludwig Biermann based on his studies of the acceleration of plasma structures in comet tails. The detailed mathematical theory of the solar wind was put forward by Eugene Parker. The solar wind was first sporadically detected by the Soviet space probes Lunik 2 and 3, but the first continuous observation of the solar wind was made with the Mariner 2 spacecraft.

In this chapter we shall describe the “classic” theory of the solar wind, which is based on the fluid approximation of coronal and interplanetary plasmas.

Hydrostatic Equilibrium — It Does Not Work

The simplest theoretical description of the solar corona is based on the assumption of a spherically symmetric, steady-state hydrostatic corona. Single-fluid equations can be applied since the gas is assumed to be a fully ionized, quasineutral proton—electron plasma. The effects of magnetic field and heat conduction are neglected in this simple approximation.

Maxwell's velocity distribution function together with Clausius's mean free path concept were the key elements that made it possible to develop connections between the motions of microscopic molecules (molecules are defined in a very broad sense here, referring to any neutral or charged particle composing the gas) and the macroscopic properties of gases (observable by the methods of classical physics).

In this chapter we introduce some fundamental concepts of kinetic theory. For more details we refer the reader to books on classical kinetic theory (cf. Gombosi 1994).

Collisions

First, we introduce several statistical quantities such as the mean free path, collision frequency, collision rate, collision cross section, and differential cross section. We shall also see that these quantities are closely related to each other and to other fundamental molecular quantities. We will use very simple physical models to emphasize the basic concepts behind these new statistical quantities.

Mean Free Path

The free path is the distance traveled by a molecule between two successive collisions. The mean free path is the average distance between two successive collisions of a single molecule.

Consider a single molecule with velocity v. Assume that this particle suffers a collision with another molecule at a distance of s = 0. Let Ppath(s) denote the probability that this molecule survives a distance s without suffering a second collision.

INTRODUCTION

The close of the Hadean and opening of the so-called Archean eon is defined and characterized by the oldest whole rock samples found on Earth, 4.0 billion years old. At the opening of the Archean, Earth had an atmosphere, rich in carbon dioxide, with perhaps some nitrogen and methane but little molecular oxygen, and liquid water was stable on its surface. Mantle convection had begun producing oceanic basalts and continental-type granitic rocks. The rate of impacts of asteroidal and cometary fragments had decreased significantly. The Moon, formed from Earth at the end of accretion some half billion years before, could be seen in the terrestrial sky.

By 3.5 billion years ago, rocks were present that recorded definitive evidence for life; less definitive evidence exists back to almost 3.9 billion years. Layered formations in ancient limestones contain concentric spherical shapes, stacked hemispheres and flat sheets of calcium carbonates (calcite), and trapped silts. These stromatolites are best understood as the work of bacteria from 3.5 billion years ago, precipitating calcium carbonate in layers as one of the byproducts of primitive photosynthesis. If the interpretation is correct, life on Earth was present then and somewhat earlier as well, because such bacteria constitute already reasonably well-developed organisms.

It therefore appears that, as Earth settled down from the chaos of accretion, core formation, and impacts, life was able to exist on its surface (figure 12.1).

THE EXPANDING HUMAN POPULATION

Overpopulation is the root cause of human-induced global warming and depletion of resources for future generations. From the beginning of humankind to just over 100 years ago, the world's human population was less than one billion. Our planet now holds between 5 and 6 billion persons with a growth rate that will take us over 10 billion by the middle of the next century (figure 23.1). The present net increase in population amounts to about 90 million people a year. Growing population is a twoedged sword. Increasing numbers of people, supported in adequate living standards by advancing technology, represent an expanding reservoir of personalities, innovative ideas, and the creative seedcorn for future developments in both technological and humanistic spheres of existence. On the other hand, unbridled population growth that outpaces technological developments designed to stem its negative impacts could push humanity into a downward spiral of resource depletion, decreased overall living standards, and ever more profound alteration of natural systems by human activities.

Approximately 30 countries – most of Europe along with Japan – have achieved a roughly zero population growth rate (actually, an annual growth rate of less than 0.3%, as defined by the Washington, DC-based Worldwatch Institute).

Understanding the fundamental properties of the energetic particle population in the heliosphere is very important for two reasons:

1. These particles represent considerable hazard for both humans and radiation-sensitive systems in space, because they can penetrate through large amounts of shielding material.

2. 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.

INTRODUCTION

Earth at the close of the Archean, 2.5 billion years ago, was a world in which life had arisen and plate tectonics dominated, the evolution of the crust and the recycling of volatiles. Yet oxygen (O2) still was not prevalent in the atmosphere, which was richer in CO2 than at present. In this last respect, Earth's atmosphere was somewhat like that of its neighbors, Mars and Venus, which today retain this more primitive kind of atmosphere.

Speculations on the nature of Mars and Venus were, prior to the space program, heavily influenced by Earthcentered biases and the poor quality of telescopic observations (figure 15.1). Thirty years of U.S. and Soviet robotic missions to these two bodies changed that thinking drastically. The overall evolutions of Mars and Venus have been quite different from that of Earth, and very different from each other. The ability of the environment of a planet to veer in a completely different direction from that of its neighbors was not readily appreciated until the eternally hot greenhouse of Venus' surface and the cold desolation of the Martian climate were revealed by spacecraft instruments.

However, robotic missions also revealed evidence that Mars once had liquid water flowing on its surface. It is tempting, then, to assume that the early Martian climate was much warmer than it is at present, warm enough perhaps to initiate life on the surface of Mars.

INTRODUCTION

We close the part of the book on techniques for discerning Earth's history with a conceptual tool. The concept of plate tectonics, whereby the outer layer of Earth is divided into a small number of distinct segments called plates which move relative to each other, represents a breakthrough in explaining a diverse range of geologic phenomena across our planet. Although the basic ideas are now 30 years old or more, this picture or concept of how Earth's geology works, in a unified way, continues to provide fresh insights into evolution of Earth, the stability of the gross climate of our planet, and the distinctions between Earth and the other planets. Because of its importance, we introduce the concept early to allow the reader to gain an understanding of the basic ideas. We come back to plate tectonics again and again as a fundamental process on Earth driving climate change, erosional processes, atmospheric chemistry, and even the nature of life.

EARLY EVIDENCE FOR AND HISTORICAL DEVELOPMENT OF PLATE TECTONICS

Revolutions in scientific thinking often take place when increasing numbers of observations challenge existing theories, which in many cases have become dogmatic over time in the face of conflicting data. Particularly satisfying is the synthesis of widely diverse data into a single framework that explains well all of the data.

As early as Sir Francis Bacon over 350 years ago, but mostly since the early nineteenth century when maps of the world became good enough to reveal the true shapes of the continents, the significance of the curious matching of the edges of distant continents has been pondered.

Nature produces many different kinds of waves and oscillations. In general, these periodic phenomena are very different from each other. However, certain physical and mathematical properties are common to small-amplitude waves almost irrespective of their nature. For instance, all small-amplitude waves can be characterized by dispersion relationhips and they transport physical quantities with the group velocity of the wave.

In this chapter we will examine some fundamental proprties of small-amplitude waves in neutral and conducting fluids. We will see that although neutral fluids exhibit a relatively small number of fundamental wave phenomena, conducting fluids (especially when they are magnetized) are a very fertile medium for the generation of a huge variety of plasma waves.

Here we shall concentrate on small-amplitude waves, when the wave equations can be linearized. This does not mean that nonlinear phenomena are unimportant — they are just too complicated for this introductory text. Also, we will limit our discussions to single species gases (or in the case of plasmas, to single ion plasmas). The results can be generalized to multispecies plasmas, when needed.

Linearized Fluid Equations

First of all, let us consider the linearized version of the ideal MHD equations. We choose to use the MHD equations, because mathematically the Euler equations represent a subset of these equations (one just has to set B to zero everywhere at all times). Let us assume that we have a solution of the full equation set, that is, ρm0, u0, p0, and B0 represent a steady-state solution of Eqs. (4.89).

INTRODUCTION

The period from the formation of Earth, some 4.56 billion years ago, to the time when the oldest rocks still in existence today were formed, roughly 3.8 billion to 4.0 billion years ago, is called both the Hadean era and Priscoan eon of Earth. The term Hadean, referring to the classical Greek version of hell, is well chosen, because all evidence that we have is that the Hadean Earth was very hot and extremely active, with widespread volcanism and frequent impacts of debris left over from planetary formation. This time encompasses the assemblage of Earth from the smaller planetesimals, dramatic internal rearrangements such as core formation, the creation of the ocean and earliest atmosphere, and the origin of Earth's Moon. Forces that acted on Earth were essentially the same as those acting on Mars and Venus, and a traveler visiting Earth would have seen little to distinguish it from the two neighboring terrestrial planets.

Each planet initially had a molten, or nearly molten, silicate surface, followed by cooling and establishment of a solid crust. Each had an atmosphere dominated by carbon dioxide (CO2), with little free molecular oxygen (O2). Evidence exists that each planet had liquid water on its surface during a portion of the Hadean era. Most important, no sign of life could be seen on any of these three planets – conditions were too severe and variable to allow life-forms to survive except near the end of the Hadean on Earth, and perhaps at about the same time on Mars.

In this chapter we will briefly consider some of the basic theoretical tools used in describing the transport of superthermal particles. By superthermal particles we mean a very small fraction of the total particle population with energies far exceeding the average thermal energy. These superthermal particles contribute negligibly to the particle density and bulk velocity (due to their very small number compared to the total number of particles), but in some cases they may represent a significant contribution to the pressure and heat flow.

We will consider the basic transport equations describing two kinds of superthermal particles: energetic solar particles and photoelectrons. Since our goal is to provide an introduction to the theoretical tools of space physics, we will constrain our derivations to the most fundamental processes. More sophisticated treatments can be found in the literature.

Transport of Energetic Particles

As in most cases, we start from the Boltzmann equation describing the evolution of the particle distribution function. The main difference this time is that because superthermal particles can be relativistic, we need to derive a transport equation that is valid for relativistic particles as well. To achieve this we use the form of the Boltzmann equation given by Eq. (2.36), where the variables of the distribution function are time, location, and full (inertial) velocity.

The beginnings of this second international colloquium on Astronomy Teaching, eight years after the famous one in Williamstown, came during a meeting of Commission 46 in August 1994, in the Hague. It was then submitted as an IAU Colloquium by the President of Commission 46, John Percy, with the support of the newly born European Association for Astronomy Education.

When I was asked to chair the Scientific Organising Committee, I considered this proposal to be a great honour, that I acknowledge, and also an exciting way to learn more about the new developments in astronomy education that you are performing, so many of you, all around the world.

Then came a hard work! Step by step the programme was built, thanks to the help and suggestions from the SOC members, and I would like to mention more particularly Julieta Fierro, Andy Fraknoi, Barrie Jones, Derek McNally, John Percy.

It was my great pleasure, each day, to read your mails on my computer, or on the fax machine a pleasure mixed with some increasing anxiety, when their number began to grow rapidly! The Internet gives this beautiful possibility to interact so easily with people spread out all over the world – you have just to take account of the time zones, which could be also considered as a good astronomical exercise.