Almost everyone has heard about astronomy though they might not understand it, and almost everyone knows about meteorology even if they cannot spell it. This book is all about the bit in between. Primarily an introductory textbook for students with a background of basic classical physics, it endeavours to describe and explain the phenomena of the terrestrial outer atmosphere and the regions of ‘space’ nearest to the Earth.

As practitioners will know, this is not a part of the environment that is well known to the general public. The performance of the communications media when attempting to discuss an aurora, or describe the ionosphere, or report the effects of a magnetic storm, is ample testimony to that. Yet, while our subject is a branch of physics and also a branch of geophysics, it may properly be included amongst the environmental sciences as well. Though in the main an academic subject, it is also one which impinges on practical effects of the environment – for instance, communications technology and space activities.

The present book is a sequel to The Upper Atmosphere and Solar–Terrestrial Relations, which Van Nostrand Reinhold Co. Ltd. published in 1979. I would have liked to get away with merely inserting necessary corrections to the original text, but, unfortunately for me, the science of the upper atmosphere and near space has moved on apace. So I have had to add a good deal of new material, and the whole book has, in fact, been recast – though some of the original matter has been retained (with Van Nostrand Reinhold's kind permission) where it seemed appropriate.

Introduction

The solar–terrestrial environment, nowadays sometimes called geospace, includes the upper part of the terrestrial atmosphere, the outer part of the geomagnetic field, and the solar emissions which affect them. It could be defined as that region of space closest to the planet Earth, a region close enough to affect human activities and to be studied from the Earth, but remote enough to be beyond everyday experience. Clearly, it is not the familiar atmosphere of meteorology; nor is it the inter-planetary space of astronomy, though it interacts with both. The material found there is mainly terrestrial in origin and strictly a part of the atmosphere of the Earth, though it is greatly affected by energy arriving from the Sun. Starting some 50–70 km above the Earth's surface and extending to distances measured in tens of Earth radii, geospace is a region of interactions and of boundaries: interactions between terrestrial matter and solar radiation, between solar and terrestrial magnetic fields, between magnetic fields and charged particles; and boundaries between solar and terrestrial matter, and between regions dominated by different patterns of flow.

The purpose of this chapter is to summarize points of physics that will be needed in order to grasp the fundamentals of geospace science. It is assumed that the student is already familiar with basic physical concepts such as energy, temperature, quanta, waves, molecules, heat, and electric and magnetic fields – topics, it will be noted, which come mainly within the domain of classical physics. Most students of physics will have covered these areas in the first year or two of their university courses. But, like most specialities, upper atmosphere and space science have their own peculiar slant. We have to deal with a gas, and in particular with an electrified gas. We will be concerned with the propagation of waves – mainly electromagnetic waves, but some others too – in that gas. We shall need to know how a steady magnetic field affects the behaviour of gas and of waves. Energetic particles and photons will enter the gas, and their interactions have to be included. So the present chapter outlines the relevent background. Much of the material should be revision but some may be new.

It is up to the student whether to study this chapter thoroughly before tackling the subsequent ones, or merely to scan it through now in order to return for clarification later, if and when questions arise.

The importance of observations

It would be difficult to overstate the importance of observational techniques in the development of the science of geospace.

Science is about the real world – the world which, most of us believe, exists outside ourselves and would continue to exist even if we were not here to see it. Our perception of that real world, however, is subjective and depends entirely on how we sense it, on the data we collect by eye or ear. This is equally true whether we observe directly, have the assistance of an instrument (telescope, ear trumpet), or take our information from the readings of a sensor (oven thermometer, particle detector on a spacecraft). The scientist's job is to make sense of such data, to fit them into existing knowledge, and to try to build up a coherent picture of the nature and working of the real, external world. Without observations we know nothing and can understand nothing. This is true of life in general and of science in particular.

The history of science plainly shows that new methods and novel techniques nearly always bring in their wake fresh advances of knowledge. Geospace is no exception. Of such an inaccessible region of the Earth we can learn very little without instruments, and the great expansion of knowledge during the last 50 years is a direct result of technological advance.

The geospace environment is complex and subtle. The concepts of the previous chapter lay down the basis of ionospheric behaviour, but in the real world additional factors conspire to complicate matters. To a large extent the ionosphere varies in a regular and predictable manner, but these regularities may not always accord with simple theory. In addition, major perturbations called storms occur from time to time, and the spatial structure includes irregularities of various sizes. Indeed, it appears that the structure of the ionosphere includes all the scales of space and time that are accessible to observation.

In this chapter we will first consider the regular behaviour of the ionosphere at middle and low latitudes, including ionospheric electric currents, and will then discuss perturbations and irregularities.

Observed behaviour of the mid-latitude ionosphere

We will begin with the regularities of the ionosphere – those variations with altitude, time of day, latitude, season, and solar cycle which are repeatable and therefore predictable (at least to an extent), and also with the ionosphere's response to solar flares and eclipses. All of these relate to the larger scales of variation: vertical distances measured in tens and hundreds of kilometres, horizontal distances in hundreds and thousands of kilometres, times in hours to years.

It should not be assumed, though, that because behaviour is regular it is necessarily understood. Historically, ionospheric observations were compared with the Chapman theory of the production of an ionospheric layer (Sections 6.2.2 and 6.2.4), and major departures from the theory were christened ‘anomalies’.

Introduction

Beginning with the first tentative probes into space in the mid-1960s, the geological exploration of the solar system has revealed a remarkable diversity in the planets and their satellites. Each planetary body displays combinations of surface features that reflect unique geological histories and environments. Yet, when the surfaces of the terrestrial planets and satellites are analyzed in detail, we find that many of them have experienced similar geological processes in their evolution.

The discipline of comparative planetary geology has as its goal the definition of the fundamental processes that have shaped and modified the planets, satellites and other ‘solid surface’ bodies in the solar system. For simplification, we shall refer to all such objects simply as planets. The giant gaseous planets, such as Jupiter and Saturn, are excluded from study because they apparently lack solid surfaces and thus are not appropriate for geological analyses. The goal of planetary geology is achieved by determining the present state of planets, by deriving information of their past state(s) – or geological histories – and by comparing the planets to one another.

Comparative planetary geology has shown that nearly all of the planets have been subjected to major geological processes, including impact cratering, volcanism, tectonism (crustal deformation), and gradation. Gradation involves the weathering, erosion, and deposition of crustal materials through the actions of various agents, such as wind and water. This book deals with wind, or aeolian, processes (Fig. 1.1).

Introduction

It is possible, as we have seen in the last chapter, for a sea of windblown sand to create its own topographical features in the form of sand dunes. In this chapter we discuss the interactions which occur between windblown particles and topographical features such as hills, craters (particularly as related to craters on Mars), vegetation, and other obstructions to the boundary layer. In order to understand these interactions, it is necessary to discuss atmospheric circulation and the effects that obstructions have on wind flow near the ground.

Some of the primary indicators of topographical influences are various types of wind streaks. Mariner 9 and Viking images of Mars show a remarkable array of bright and dark surface markings that result from interactions between the wind and topography (Fig. 1.3(a)). Many of these streaks serve as surface wind direction indicators. The differences in streak geometry and their variability with time have sparked much discussion and controversy on their origin and evolution. We review the streaks of Mars and discuss their possible origins, as well as present some possible terrestrial analogs that may shed light on the atmospheric–topographical interactions involved in streak formation.

Atmospheric motions

General atmospheric circulation results primarily from planetary rotation combined with the variation of incident solar flux with position on the planet (see Chapter 2). On a smaller scale, atmospheric circulation is influenced by topographical features such as mountain ranges, and on Earth is further influenced by the oceans through the exchange of heat and water vapor with the atmosphere.

Ralph Bagnold – an engineer by training, a military man by profession, and in many ways a geologist at heart – melded his interests into an elegant study of aeolian processes that has spanned many decades. In 1941 Bagnold published the first edition of his book, The Physics of Blown Sand and Desert Dunes. Often referred to simply as ‘Bagnold's classic book’, it is indeed a classic in every sense of the word. The fact that nearly every subsequent paper dealing with aeolian processes refers to the Bagnold book bears testimony that the basic principles described by him are essentially correct and have withstood the test of time.

Our book deals with aeolian processes in the planetary context. It is not our intent to ‘replace’ Bagnold's book or the research it represents. We learned that was neither required nor possible early in our own research program! Instead, we have built upon the solid foundation laid by Bagnold, testing the relationships defined by him through different approaches, and extrapolating the results to other planetary environments by attempting to predict how aeolian processes operate on Mars, Venus and, perhaps, Titan, the largest of the saturnian satellites.

We begin with an introduction to aeolian processes and a general overview of aeolian activity on the planets. We then discuss, in Chapter 2, the requirements for aeolian activity – a dynamic planetary atmosphere and a supply of particles capable of being moved by the wind – and describe in Chapter 3 the physical processes involved in particle movement by the wind. In Chapters 4 and 5 we describe wind-eroded and wind-deposited features and landforms.

Introduction

Sand dunes and deserts are intimately linked in the minds of most people and epitomize aeolian processes. The most impressive sand dunes occur in vast sand seas, often called ergs – an arabic word for wind-laid sands. Almost all ergs occur in desert basins, generally in areas downwind from terrain experiencing high rates of deflation. Sand then tends to collect where net transportation is low, as in topographical basins or traps.

Wilson (1971, 1973) surveyed all ergs on Earth that are larger than 12 000 km2 and identified 58 such areas, mostly within regions receiving less than 15 cm precipitation per year. Despite the link between sand dunes and deserts, the two need not occur together. Many deserts lack dunes entirely. And dunes may occur in non-desert areas, such as along coastlines and in river flood plains. Although dunes are aesthetically pleasing, they represent only one form of aeolian deposit.

Bagnold (1941) first described the various processes involved in dune formation and sand migration. His model has been modified slightly and amplified by Hunter (1977) and Kocurek & Dott (1981), who describe three basic processes of deposition by windblown sand: (1) tractional deposition (Bagnold's ‘accretion’ process), in which grains moving by saltation and impact creep come to rest in a sheltered position, (2) grainfall deposition, in which particles settle out of the air, usually in zones of flow separation; Bagnold distinguished (1) from (2) in that, during grainfall, the grains do not move forward when they reach the ground, whereas in tractional deposition, they may bounce along until they find a stable position among other grains, and (3) grainflow deposition, or avalanching, in which grains reach the brink of a dune then avalanche down the slip face (Fig. 5.1).

Introduction

Many images of Mars show sand dunes and wind-related surface features that are clear evidence of atmospheric processes. These remarkable pictures have sparked new interest in understanding the complex phenomena associated with windblown particles, which in turn has led to increased knowledge and understanding of aeolian processes on Earth. In this chapter we discuss some of the basic physics of particle motion, particularly in regard to the effects of widely different atmospheric densities on the terrestrial planets. Most of these phenomena, such as the characteristics which determine onset of motion and the transport rate of material in motion, are not yet completely understood because of their physical complexity and the difficulty of observation.

Much of the early research on the basic physics of windblown sand was performed by R. A. Bagnold. As an officer in the British Army, he led expeditions across the sand seas of Egypt in the 1930s. His observations provided the foundation for understanding sand motion and the formation of sand dunes and ripples. Later, in England, he built a wind tunnel in order to make quantitative measurements of threshold wind speeds and mass transport rates. His findings are reported in his delightfully lucid cornerstone book (Bagnold, 1941) and in many papers.

A parallel effort on the problems of soil erosion has been carried out by the US Department of Agriculture at a wind tunnel laboratory in Manhattan, Kansas. Most of the understanding of the physics of soil movement by wind, especially as related to agricultural applications, was derived by W. S. Chepil who reported his work in a large number of papers during a 25-year period beginning about 1940.

Introduction

Dust. It gets in our eyes, our shoes, even in our lungs, sometimes causing disease to those who breath it. Dust storms cause visibility problems on highways, resulting in many accidents and deaths each year. And soil erosion is a major worldwide problem where surfaces are disrupted through cultivation, overgrazing, mining, construction, vehicular traffic, or other activities which disturb the surface or destroy vegetation cover.

Dust storms are common on both Earth and Mars. Although many storms on Earth reach sizes that can be seen on photographs taken from orbit, they do not become global in extent, as some do on Mars. Nonetheless, the great North American dust storm of 12 November 1933, covered a region greater than the combined areas of France, Italy, and Hungary (Goudie, 1978), and for the people involved in the dust storm, it might just as well have been global.

The dust storm of November 1933 is an example of a storm caused by an area of extreme high pressure and strong barometric pressure gradient. It covered the region southward from the Canadian border, stretching from Montana to Lake Superior, and to the western Ohio and lower Missouri River valleys (Hovde, 1934; Miller, 1934). That year a severe drought was experienced over the northern and central Great Plains, and crop and pasture grass failures left the soil exposed to the wind. Maximum recorded wind velocities on 12 November were 25 m/sec at Bismarck, North Dakota, and at Davenport, Iowa.