Retiring from paid employment at the RAE in May 1988 proved to be the prelude to two years of unpaid attendance part-time, clearing up the loose ends. During the last twenty of my forty years at the RAE I was able to adopt the most efficient procedure of filing away all working papers and reports received, by subject, in filing cabinets that remained in place and just increased in numbers. No time was wasted in going through them on throwing-away sprees. It was ‘onward undaunted’ continuously, with everything undisturbed, in the same office. All that had to end in 1988. In a long and traumatic series of evening massacres at home, I ploughed through the 25,000 neatly-filed letters in my ‘general correspondence’ and threw away about 98%. My wife with great forbearance allowed the smallest bedroom in the house to be lined with shelves and converted into an archive room, to store the papers needed for this book, and some of my books and reports on space topics. A second round of massacres is now in prospect among those archives …

At the RAE, meanwhile, I was obliged to vacate my office in Q134 Building in May 1988, and took a suitcase-full of selected papers each day across to a new office in R14 Building, reducing the bulk to a mere six filing cabinets. The Table of satellites was taken over by Doreen Walker and Alan Winterbottom: Geoffrey Perry, uniquely knowledgeable in current space activities, continued to supply the basic data under contract.

In 1961 a clear ocean of scientific research seemed to have opened up, ready to sail into and explore. The climate seemed set fair too. This optimism – fearing ‘nor sea rising, nor sky clouding’ – was justified by events: the 1960s proved to be a decade of fairly easy achievement, exploiting techniques already devised.

The RAE research on the upper atmosphere had so far been received in deafening silence by the Meteorological Office, which regarded anything at heights above about 20 km as rather ‘way out’ and of no interest to weather forecasters. This hardline attitude by meteorologists was slowly softening, and the Royal Meteorological Society invited me to give the Symons Memorial Lecture on 1 March 1961: the title was ‘Satellites and the Earth's outer atmosphere’, and I ranged more widely than in previous talks, discussing the history of ideas on the atmosphere and also venturing further outwards, above 1000 km height, into the exosphere and magnetosphere.

A month later came the most important scientific meeting I ever attended, the 1961 COSPAR Symposium at Florence. For this occasion we gathered all the data on air density for an updated picture of the variations with height, with solar activity and between day and night. Fig. 4.1 shows the graph of density versus height obtained from twenty-nine different satellites launched before 1961, as presented at Florence.

The morning was sunny and serene, the day was Monday 12 September 1948, and I was travelling by train to begin a new life working at the Royal Aircraft Establishment at Farnborough in Hampshire. As the steam-engine puffed along the last few miles from Guildford to the curiously-named North Camp station, I had no idea what was in store, never having ventured into Hampshire before (unnecessary travel had been frowned on during the Second World War). During the previous two years I had been working for a mathematics degree at Cambridge, and it was in the garden of the Cambridge Appointments Board in May that I was interviewed by two ‘Men from the Ministry’ and offered a post in the Guided Weapons Department at the RAE, as an alternative to three years of military service. My interviewers were very pleasant and persuasive, and the alternative was also persuasive: I accepted the post as a temporary Scientific Officer at the excellent salary of £340 a year, though with various deductions.

At first sight, the Royal Aircraft Establishment created a favourable impression, because I had seen nothing like it before. It covered about three square miles and seemed like a small town. Some of the buildings were rather scruffy, but some were quite presentable, and the built-up area was balanced by the extensive airfield. There were about 10,000 people working at the RAE then, and the whole place seemed to be buzzing with activity, the noisiest buzzing being produced by the frequent take-offs and landings of jet aircraft.

In 1970 a new world beckoned, the realm of resonance, with prospects of fresh and fertile fields of research. A satellite experiences resonance when longitudinal variations in gravity cause changes in the orbit that build up continually, day after day and month after month. Orbital changes that are basically very small then magnify themselves until they are large enough to be accurately determined: thus resonance creates a powerful technique for measuring the gravity field.

In earlier chapters the Earth's gravity has been taken to be composed of a series of zonal harmonics dependent only on latitude, and independent of longitude. This is an over-simplification, because in reality gravity varies with longitude: the variations are small, but detectable. The zonal harmonics discussed in previous chapters can be regarded as longitude-averaged, and each of them needs to be supplemented by a teeming family of harmonics that are dependent on longitude as well as latitude,‘tesseral harmonics’ as they are called, after the tesserae of varied shapes in a Roman mosaic floor.

The variation of a tesseral harmonic with longitude is specified by its order. A tesseral harmonic of order 15 gives rise to 15 undulations as you go round the equator (or any other line of latitude), as shown in Fig. 5.1. The symbol m is used to denote the order of a tesseral harmonic: it is helpful to think of m as specifying the variations between one meridian and another. (The zonal harmonics, being independent of longitude, are tesseral harmonics of order zero.)

This book is a personal account of the researches based on analysis of satellite orbits between 1957 and 1990 at the Royal Aircraft Establishment, Farnborough, work in which I played a leading role. The book is most definitely not an impartial history of the subject world-wide: contributions by other groups are mentioned only when necessary. Nor is the book an autobiography, though the science is punctuated – and perhaps enlivened – by some personal experiences.

A book of this kind, a hybrid of science and life, presents the author with many stylistic problems. I have ruthlessly gouged out as many ‘I's as possible, and have tried to avoid mentioning too many names (with apologies to all those who find themselves liquidated). I decided to use ‘we’ quite often: throughout the book we means ‘those of us at the RAE who were concerned with or working on the problem’. Individual names are mentioned too, of course, and often the we is defined by giving the authors of a paper in a note.

I have tried to make the book widely intelligible to readers without specialized knowledge. There is a light sprinkling of mathematical equations: but if you don't like them you can skip them without losing the thread.

Most spacecraft chatter continuously, sending back to the ground stations so much data that storage can be quite a problem. The satellites selected for orbit analysis, on the other hand, are usually dumb (and deaf and blind): but they can be seen from the ground as they cross the sky, and from the observations their orbits can be determined.

Dynamics of the polar ionosphere

Chapter 5 described how the magnetosphere circulates as two regions, an inner one rotating daily with the Earth, and an outer one circulating under the influence of the solar wind. The polar ionosphere is connected by the geomagnetic field-lines to this outer region, and – since the field-lines are (almost) equipotentials – its circulation is essentially a projection of that of the outer magnetosphere.

F-region circulation

In the F region, where the ion–neutral collision frequency is small relative to the gyrofrequency, the plasma moves with the magnetic field-lines. Alternatively, we can say that the electric field which the solar wind generates across the magnetosphere (Section 5.5.3) is mapped into the F region along the equipotential field-lines. The polar-cap electric field so created (as measured by a stationary observer) then acts as the driving force for the F-region plasma (Sections 2.3.7 and 6.5.4). The integral of the electric field gives the total electric potential across the polar cap.

Vertical structure

Nomenclature of atmospheric vertical structure

The static atmosphere is described by the four properties, pressure (P), density (ρ), temperature (T) and composition. Between them these properties determine much of the atmosphere's behaviour. They are not independent, being related by the universal gas law which may be written in various forms (Equations 2.5–2.7). For our purposes the form

P = nkT, (Equation 2.7)

where n is the number of molecules per unit volume, is particularly useful. The quantity ‘n’ is properly called the concentration or the number density, but density alone is often used when the sense is clear.

The regions of the neutral atmosphere are named according to various schemes based in particular on the variations with height of the temperature, the composition, and the state of mixing. Figure 4.1 illustrates the most commonly used terms. The primary classification is according to the temperature gradient. In this system the regions are ‘spheres’ and the boundaries are ‘pauses’. Thus the troposphere, in which the temperature falls off at 10 K/km or less, is bounded by the tropopause at a height of 10–12 km. The stratosphere above was originally thought to be isothermal, but in fact is a region where the temperature increases with height. A maximum, due to heating by ultra-violet absorption in ozone, appears at about 50 km and this is the stratopause.

Introduction

The ionized part of the atmosphere, the ionosphere, contains significant numbers of free electrons and positive ions. There are also some negative ions at the lower altitudes. The medium as a whole is electrically neutral, there being equal numbers of positive and negative charges within a given volume. Although the charged particles may be only a minority amongst the neutral ones they exert a great influence on the medium's electrical properties, and herein lies their importance.

The first suggestions of electrified layers within the higher levels of the terrestrial atmosphere go back to the 19th century, but interest was regenerated with Marconi's well known experiments to transmit a radio signal from Cornwall in England to Newfoundland in Canada in 1901, and with the subsequent suggestions by Kennelly and by Heaviside (independently) that, because of the Earth's curvature, the waves must have been reflected from an ionized layer. The name ionosphere was coined by R. Watson-Watt in 1926, and came into common use about 1932.

Since that time the ionosphere has been extensively studied and most of its principal features, though not all, are now fairly well understood in terms of the physical and chemical processes of the upper atmosphere. Typical vertical structures are as shown in Fig. 6.1.

Introduction

Science and engineering are related activities with different objectives. The purpose of science is to gain knowledge, and the essence of scientific achievement is intellectual rather than practical. To a scientist the knowledge and the ideas are what matter most. Engineering, on the other hand, is all about practical things. The result of successful engineering endeavour is a machine, a device or a scheme for performing some specific task. What matters to the engineer is that the machine, device, etc., should work well, and he/she will draw on any area of knowledge or experience to achieve this. Some of that knowledge might be science based; some might not.

Having drawn the distinction, we should at the same time recognize that there are strong links between these activities. Science relies on instruments and computers, the products of the engineer, and it should be abundantly clear from Chapter 3 how much the progress of geospace has depended on the development of techniques. And although there is no law that engineering must be science based, it draws heavily on scientific knowledge in practice. It would be an unusual engineer who relied entirely on historical practice or intuition. It is the purpose of this chapter to consider the impact of geospace science on practical activities within the province of the engineer.

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.