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This textbook describes physical conditions in the upper atmosphere and
magnetosphere of the Earth. This geospace environment begins 70 kilometres
above the surface of the Earth and extends in near space to many times the
Earth's radius. It is the region of near-Earth environment where the Space
Shuttle flies, the aurora is generated, and the outer atmosphere meets particles
streaming out of the sun. The account is introductory, at a level suitable for
readers with a basic background in engineering or physics. The intent is to
present basic concepts, and for that reason the mathematical treatment is not
complex. SI units are given throughout, with helpful notes on units where
these are likely to be encountered in the research literature. Each chapter has
a reading list also.
There are three introductory chapters that give basic physics and explain the
principles of physical investigation. The principal material contained in the
main part of the book covers the neutral and ionized upper atmosphere,
the magnetosphere, and structures, dynamics, disturbances and irregularities.
The concluding chapter deals with technological applications.
This textbook is suitable for advanced undergraduate and new postgraduate
students who are taking introductory courses in upper atmospheric, iono-
spheric, or magnetospheric physics. It is a successor to The Upper Atmosphere
and Solar-Terrestrial Relations by J. K. Hargreaves, first published in 1979.
Cambridge atmospheric and space science series
Editors
Alexander J. Dessler
John T. Houghton
Michael J. Rycroft
Titles in print in this series
M. H. Rees, Physics and chemistry of the upper atmosphere
Roger Daley, Atmospheric data analysis
Ya. L. Al'pert, Space plasma, Volumes 1 and 2
J. R. Garratt, The atmospheric boundary layer
J. K. Hargreaves, The solar-terrestrial environment
Sergei Sazhin, Whistler-mode waves in a hot plasma
S. Peter Gary, Theory of space plasma microinstabilities
Ian N. James, Introduction to circulating atmospheres
Tamas I. Gombosi, Gaskinetic theory
Martin Walt, Introduction to geomagnetically trapped radiation
B. A. Kagan, Ocean-atmosphere interaction and climate modelling
The solar-terrestrial
environment
An introduction to geospace - the science of the terrestrial
upper atmosphere, ionosphere and magnetosphere.
J. K. Hargreaves University of Lancaster
CAMBRIDGE
UNIVERSITY PRESS
Contents
Preface
1 The Earth in space
Introduction The sun and the solar wind The atmosphere and the ionosphere Geomagnetic field and magnetosphere Nomenclature Summary
2 The physics of geospace
Useful units and fundamental constants Properties of gases Gas laws Thermal equilibrium Continuity Collisions Diffusion Magnetoplasma Electric and magnetic energy Gyrofrequency Betatron acceleration Plasma frequency Debye length Frozen-in field E x B drift Fermi acceleration Waves Phase velocity Refractive index Group velocity Polarization Energy density
1 1 2 3 3 4 4 6 6 9 9
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viii Contents
x Contents
5.6 Particles in the magnetosphere 164 5.6.1 Principal particle populations 164 5.6.2 The plasmasphere and its dynamics 164 5.6.3 The plasma sheet 169 5.6.4 Boundary layers 173 5.7 Van Allen particles 174 5.7.1 Discovery 174 5.7.2 Trapping theory 175 5.7.3 Sources, sinks and morphology 183 5.8 Magnetospheric current systems 189 5.8.1 The magnetopause current 190 5.8.2 The tail current 191 5.8.3 The ring current 191 5.8.4 Birkeland currents 194 5.9 Substorms in the magnetosphere 196 5.9.1 Consequences of intermittent merging 196 5.9.2 Substorm triggering and the influence of the IMF 200 5.9.3 Substorm currents 201 5.10 Magnetospheres of other planets 202 Further reading 205
6 Principles of the ionosphere at middle and low latitudes 208 6.1 Introduction 208 6.2 Physical aeronomy 209 209 210 213 218 220 222 222 225 227 229 233 236 236 237 240 242 243 247
249 249 250 253 253 261 261
Principles The Chapman production function Ionization by energetic particles Principles of chemical recombination Vertical transport Chemical aeronomy Introduction E and Fl regions F2 region and protonosphere D region Principles of airglow Charged particle motions and electrical conductivity Introduction Particle motion in a magnetic field in the presence of collisions Responses to a neutral-air wind Response to an electric field Conductivity Further reading
7 Ionospheric phenomena at middle and low latitudes
7.1. 7.1. 7.1. 7.1. 7.1.
Observed behaviour of the mid-latitude ionosphere E region and sporadic-E Fl region F2 region and its anomalies D region Effects of solar flares
Contents xi
Preface
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. Since the book is introductory (though intended for readers who already have a background in basic physics or engineering), the picture is painted with a broad brush. Explanations are placed in a physical context as far as possible - which means that there have to be equations - but the mathematical treatment is kept to an elementary level. Some of the material is descriptive. The intention is to inform the reader about the basic concepts and methods and to leave him or her with a good idea of what 'geospace' is all about and why it is important, and of the general state of knowledge. The book should be suitable for undergraduates after the first couple of years and for fresh graduate students, and should enable them to move on to the advanced books and the scientific literature. Professionals qualified in other fields who need
xiii
xiv Preface
information about the ionosphere, or about the effects of solar activity, for instance, should also find it useful. The increased sophistication and greater depth of knowledge in the subject, compared with 10 years ago, have made this book more difficult to write than was my first effort. Bearing in mind the readers for whom it is mainly intended, I have constantly had to compromise between keeping the text at a suitably introductory level and being sufficiently up to date. Critics should also remember, please, that the task has to be completed within a reasonable length - or the product would come out too expensive for them to buy. It will be for the reader(s?) to judge whether the result has the right balance. One deliberate change is that SI units are now taken as the primary system. We must still remember, nevertheless, that the enormous literature already published in c.g.s. units is not about to self-destruct, and therefore the older system has been included in a secondary role. An introductory book should lead the reader on to the advanced books and the relevent scientific papers, and this includes help with the units. With the same thought in mind, suggestions for further reading are given after each chapter. The reading lists are in two parts: the firstof books or sections of books where the presentation will be tutorial and from which the reader may verify and amplify what I have said; the second comprises mainly review papers which treat the topics in greater detail and which present the state of knowledge at the time they were written. I expect someone's favourite review will have been omitted; if so I can only plead that the lists have to be limited in length and that the selection is necessarily a personal one and in no sense definitive. The principal material is contained in Chapters 4 to 9, which between them discuss the neutral and the ionized upper atmosphere, the magnetosphere, structures, dynamics, disturbances and irregularities. Chapter 2 summarizes points of basic physics which may or may not have been encountered before, but which are particularly important for the comprehension of the succeeding material. Chapter 3 describes the methods of geospace investigation, dwelling on the physical principles rather than the hardware. Practical applications are discussed in Chapter 10. Some paragraphs have been set in smaller type, and these can be omitted at a first reading without loss of continuity. I have often been surprised by the degree of cooperation that goes on between scientists, who so often seem actually pleased to assist one another, expecting nothing other than reciprocation in return. I have benefited from that attitude in preparing this text. In particular I wish to thank Sa. Basu, Su. Basu, K. Bullough, M. J. Buonsanto, J. D. Craven, M. A. Clilverd, R. F. Donnelly, G. Enno, H. Gough, C. Haldoupis, M. A. Hapgood, G. Heckman, R. A. Heelis, R. B. Home, R. D. Hunsucker, U. S. Inan, J. D. Mathews, M. H. Rees, P. H. Reiff, A. S. Rodger, H. H. Sauer, A. J. Smith, H. C. Stenbaek-Nielsen, R. D. Stewart, D. M. Willis and K. C. Yeh, each of whom has helped in some specific way, for example by providing an unpublished diagram or by helping me with some scientific point. Last, but certainly not least, I thank the members of my family for their relative patience on the many occasions when I disappeared to the 'office' to talk to the computer.
2 The Earth in space
to say that reusable vehicles (the Space Shuttle) are well established, and space station technology (Mir) is already highly developed. We may expect to see further developments in shuttle/space station technology over the next few years in each of the major space centres, and one day we shall perhaps see these competing efforts growing together into a single global enterprise. All of this depends on a knowledge and understanding of geospace. But in addition to its importance in applications, the science is important in its own right for fundamental studies such as of the properties of tenuous atmospheres and their photochemistry, of wave propagation and of plasma physics. The medium of near space and its physics are not readily reproduced in earth-bound laboratory conditions, and to a large extent geospace provides its own laboratory.
We shall be concerned with three broad regions:
The space between Sun and Earth, across which solar-terrestrial influences propagate; The terrestrial atmosphere, neutral and ionized, with which the solar emissions react; The geomagnetic field external to the solid Earth, which influences the ionized atmosphere and controls the Earth's outermost regions.
1.2 The Sun and the solar wind
The rather ordinary star at the centre of the solar system establishes for each planet a radiation environment which controls its temperature and determines the rate of evolution of that planet, the composition of its atmosphere, and its suitability for life. It is our good fortune - though if it were not we should not be here to complain about it - that planet Earth is intermediate between the extreme heat of the planets closer to the Sun and the extreme cold of the outer planets. The Earth's surface temperature permits water to exist in all three phases. Life emerged in the liquid phase and proceeded to alter the composition of the atmosphere, adding oxygen to the nitrogen and carbon dioxide already present. The presence of water as vapour also provided, and continues to provide, a source of hydrogen, which, as we shall see, is important at the atmosphere's higher levels. Thus the general level of solar radiation, combined with the distance between Sun and Earth, has largely determined the nature of the Earth's atmosphere. While long term change in this energy output may be responsible for slow climatic changes such as produced the Ice Ages, short term changes over days, weeks or a few years appear to have little climatic effect - despite strenuous efforts to discover some. At the higher levels of the atmosphere, though, the changes that accompany variations of solar activity may be large and rapid. The upper atmosphere, where most of the more energetic solar radiations are stopped, and which is heated by them, is very responsive to solar activity variations in general, as well as to the short-lived, intense and localized outbursts known as solar flares. In addition to radiation the Sun also emits a stream of matter. We think of planets like the Earth as stable, self-contained bodies that do not evaporate into space to any significant extent. Not so the Sun, which is not in equilibrium and continuously loses matter as well as radiation into space. This stream of matter is the solar wind, which
1.4 Geomagneticfieldand magnetosphere 3
forms the second vital connection between Sun and Earth. Also important is the weak magnetic field, the interplanetary magneticfield, which is embedded in the solar wind and is carried with it past the Earth, where it largely determines how strongly the solar wind couples with the matter of the remote terrestrial atmosphere. Although the solar wind does not penetrate down to the ground it is^ highly significant in geospace; indeed, some of the most remarkable behaviour is directly attributable to the variations of the solar wind and its magnetic field. The interactions are subtle ones and we shall spend some time dealing with them.
1.3 The atmosphere and the ionosphere
Less is known about the Earth's atmosphere than many people imagine. Near the ground the atmosphere is a relatively dense gas, mainly composed of molecular nitrogen and oxygen with smaller amounts of carbon dioxide, water and various trace gases. With increasing altitude the pressure and density decline. At 50 km 99.9% of the mass of the atmosphere is below, and at 100 km all but 1 part per million. Into these rarified upper levels penetrate the ultra-violet and X-ray emissions emanating from the Sun, photons which are sufficiently energetic to dissociate and to ionize the atmospheric species, thereby altering the atmosphere's composition and heating it. The heating creates a hot upper region called the thermosphere^ which is less turbulent than the lower regions, and in which gases of different density may separate. Thus the composition of the atmosphere changes with altitude, the lighter gases, particularly hydrogen, becoming progressively more dominant.
Because of the low pressure above about 100 km, ionized species do not necessarily recombine quickly, and there is a permanent population of ions and free electrons. The net concentration of ions and free electrons (generally in equal numbers) is greatest at heights of a few hundred kilometres, and although the electron concentration may amount to only 1 % of the neutral concentration the presence of these electrons has a profound effect on the properties and behaviour of the medium. This ionosphere is electrically conducting and can support strong electric currents. The ionized medium also affects radio waves, and as a plasma it can support and generate a variety of waves, interactions and instabilities that are not found in a neutral gas.
The upper atmosphere and ionosphere sit on the lower atmosphere, the domain of the meteorologists. We shall see that some of the behaviour of the higher regions is similar to that taught in meterology, but that there is much more besides.
1.4 Geomagnetic field and magnetosphere
As William Gilbert, physician to Queen Elizabeth I, realized 400 years ago, the Earth is itself a magnet. The geomagneticfield is generated by electric currents flowing
deep within the solid Earth and to afirst^ approximation may be represented as though due to a short bar magnet at the centre of the Earth. As a dipolefield^ it extends beyond the planetary surface, through the troposphere on which it has no effect, and into the ionized atmosphere where its effects are considerable. The geomagnetic fieldaffects the motions of ionized particles, and thus modifies ionospheric electric currents and the
1.6 Summary
vvwwvwwv-
E.M. radiation
Energetic particles
Magnetosphere
Upper atmosphere and ionosphere
Fig. 1.1 Summary of the solar-terrestrial environment. (After a sketch by J. C. Hargreaves).
classical physics, though some knowledge of chemistry and, of course, mathematics is also needed. In Chapter 2 we shall summarize some aspects of basic physics that are particularly important for an appreciation of the more specialized material to follow.
2
The physics of geospace
Someone told me that each equation I included in the book would halve the
sales. S. W. Hawking, A Brief History of Time (1988)
(Health warning - This chapter has more equations than any other.)
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.
2.1 Useful units and fundamental constants
SI (Systeme International) units were internationally adopted in 1960, though in the
guise of MKS (metre-kilogramme-second) they had already been creeping into use for
10 or 15 years previously. The school teachers, never slow (dare one say?) to embrace
an innovation, have done their duty, taught in SI, and probably no longer even
mention the older systems. Therefore - and not just because the Royal Society says so
- this book is in SI. It is what the readers will expect, and properly so.
So one might think that by 1990 the changeover would have been complete and that
centimetres, grammes and seconds (c.g.s.) would have followed feet, pounds and
seconds (f.p.s.) into the archives of science. But not so! These changes take a long time
to work through. University teachers are more conservative, and much of the delay is
also due to the professional scientist no less, the very person who has created all this
