The broad picture

In the previous chapters, we have explored the conventional thinking of cosmologists and astrophysicists in their attempt to understand the structures in the universe. Some of these attempts have been very successful, while others must be still thought of as theoretical speculations. Since different aspects of structure formation were touched upon in different chapters of this book, it is worthwhile to summarize the conventional picture in a coherent manner.

The key idea behind the models for structure formation lies in treating the formation of small-scale structures like galaxies, clusters, etc. differently from the overall dynamics of the smooth background universe. This is linked to the assumption that, in the past, the universe was very homogeneous with small density fluctuations.

The evolution of the smooth universe is well described by the standard big bang model. Starting from the time when the universe was about one second old, one can follow its evolution till the time when matter and radiation decoupled – which occured when the universe was nearly 400 000 years old. During this epoch, the energies involved in the physical processes ranged from a few million electron volts to a few electron volts. This band of energies has been explored very thoroughly in the laboratory experiments dealing with nuclear physics, atomic physics and condensed matter physics. We understand the physical processes operating at these energy ranges quite well, and it is very unlikely that theoretical models based on this understanding could go wrong. In other words, we can have a reasonable amount of confidence in our description of the universe when it evolved from an age of one second to an age of 400 000 years.

The cosmic rainbow

In chapter 1, we did a rapid survey of the universe, listing its contents, and in chapter 4, we plan to discuss these objects in more detail. You may wonder how such a detailed picture about the universe has been put together. This has been possible because we can now observe the universe in a wide variety of wavebands of the electromagnetic spectrum, and virtually every cosmic object emits radiation in one band or another. In this brief chapter, we shall have a rapid overview of how these observations are made. While describing the observational techniques, we will also mention briefly the astronomical objects which are relevant to these observations. These objects are described in detail in the next chapter, and you could refer back to this cha46 pter after reading chapter 4.

It is rather difficult to ascertain when the first astronomical observation was made. Right from the days of pre-history, human beings have been wondering about the heavens and making note of the phenomena in the skies. The earliest observations, needless to say, were made with the naked eye. With the advent of the optical telescope, one could probe the sky much better and detect objects which were too faint to be seen with the naked eye. As the telescopes improved, the quality of these observations increased.

There is, however, an inherent limitation in these early observations. All these observations were based on visible light. We now know that visible light is an electromagnetic wave whose wavelength is in a particular range.

The subject of cosmology – and our understanding of how structures like galaxies, etc., have formed – have developed considerably in the last two decades or so. Along with this development came an increase in awareness about astronomy and cosmology among the general public, no doubt partly due to the popular press. Given this background, it is certainly desirable to have a book which presents current thinking in the subject of cosmology in a manner understandable to the common reader. This book is intended to provide such a nonmathematical description of this subject to the general reader, at the level of articles in New Scientist or Scientific American. An average reader of these magazines should have no difficulty with this book.

The book is structured as follows: chapter 1 is a gentle introduction to the panorama in our universe, various structures and length scales. Chapter 2 is a rapid overview of the basic physical concepts needed to understand the rest of the book. I have tried to design this chapter in such a manner as to provide the reader with a solid foundation in various concepts, which (s)he will find useful even while reading any other popular article in physical sciences. Chapter 3, I must confess, is a bit of a digression.

Probing the past

The discussion in the last chapter shows that most of the prominent structures in the universe have formed rather recently. In terms of redshifts we may say that galaxy formation probably took place at z < 10. Our understanding of galaxy formation could be vastly improved if we could directly observe structures during their formative phases. Remember that in the case of stars we can directly probe every feature of a stellar life cycle from birth to death; this has helped us to understand stellar evolution quite well. Can we do the same as regards galaxies?

Unfortunately, this task turns out to be very difficult. The life span of a typical star — though large by human standards — is small compared to the age of the universe. This allows one to catch the stars at different stages of their evolution. For galaxies, the timescale is much longer and so we cannot hope to find clear signals for galaxies of different ages. Secondly, the distance scales involved in extragalactic astronomy are enormously large compared to stellar physics. This introduces several observational uncertainties into the study.

In spite of all these difficulties, astronomers have made significant progress in probing the universe during its earlier phases. We saw in chapter 5 that the farther an object is the higher its redshift will be. But since light takes a finite time to travel the distance between a given object and us, what we see today in a distant object is a fossilized record of the past. Consider, for example, a galaxy which is at a distance of one billion light years.

Introduction

The previous chapter contains contributions to the history of science in the field of space plasma physics. It explains how the plasmapause, this peculiar and unexpected magnetospheric frontier, was discovered independently in the late 1950s and early 1960s by two scientists from the two leading countries involved in space exploration. The discoveries were made by using two totally different technical methods of measurement: in situ spacecraft observations and electromagnetic sounding of the magnetosphere. These techniques were both in their infancy at the time.

The main results of electromagnetic sounding of the plasmasphere, from the ground and from satellites, will now be described. In situ satellite particle observations will be outlined in Chapter 3.

In both this chapter and the next the most relevant results will be presented without emphasis upon technical aspects of the experiments. Such aspects are well described in the specialized literature, examples in the case of the whistler method being works by Smith (1961a); Carpenter and Smith (1964); Helliwell (1965); Carpenter and Park (1973); Rycroft (1974a); Y. Corcuff (1975); Tarcsai (1975); P. Corcuff (1977); P. Corcuff, Y. Corcuff and Tarcsai (1977); Park and Carpenter (1978); Bernhardt (1979); Daniell (1986) and Rycroft (1987). An extensive review of the use of whistlers for magnetospheric diagnostics is given by Sazhin, Hayakawa and Bullough (1992).

Initial results

As noted above, Storey (1953) used whistlers for the initial identification of the dense plasmasphere, and Carpenter (1962b) used evidence of unusually low whistler travel times to infer the occurrence of deep, factor-of-∼ 10 depressions in electron density during the severe magnetic storms of the IGY.

Introduction

Preliminary remarks

Besides whistler observations, direct particle measurements from spacecraft (especially from satellites with highly elliptical and geostationary orbits) have contributed significantly to our understanding of the plasmasphere and of its outer boundary, the plasmapause. In particular, such measurements permit us to investigate a number of topics that are not subject to direct observation by radio techniques, including low-energy ion composition, pitch angle distribution, and temperature.

Satellite instruments which have contributed to plasmaspheric studies involve both direct particle flux measurements as well as wave observations. We have already reported in Chapter 2 some results from wave experiments, and in the present chapter will discuss such observations only when they appear to be complementary to direct plasma measurements. Most of our attention will be focused on direct particle measurements, obtained with Langmuir probes, charged particle traps, retarding potential analyzers (RPA), and ion mass spectrometers of different types.

Several problems are inherent in measurements made with the above-mentioned devices. The most serious problems arise when the instruments operate in a very low-density plasma and/or when the energy of the measured particles is very low. Indeed, it is difficult in practice to eliminate all the factors distorting the direct measurements, in spite of the care taken by their developers, including extensive preflight tests and calibration.

Introduction

Thus far we have reviewed observations of the plasmasphere and its properties (Chapters 1, 2 and 3). In Chapter 4 an overall phenomenological description of these properties was attempted. It remains to review in this Chapter the main steps followed since 1960 in our theoretical description and understanding of the plasmasphere, of its connection to the topside ionosphere, and of its outer boundary, the plasmapause.

We start with one of the first theoretical problems encountered by the whistler community: what is the distribution of plasma along the geomagnetic-field-aligned filamentary plasma ducts within which VLF waves propagate? This question along with hydrodynamical and kinetic models of the plasmaspheric refilling mechanism will be discussed in the first part of this Chapter. In the second part we review the mechanisms of convection and of plasma interchange motion in a magnetic field. Following historical order, we examine the theories that have been proposed for the formation of the plasmapause and for the dynamics of cold plasma in the inner magnetosphere.

There are subsidiary or complementary theoretical aspects that will not be addressed in this Chapter, examples being questions of propagation and ray tracing of VLF waves in the plasmasphere and the theory of wave–particle interactions. Such interactions were mentioned in Chapter 4 as a potential mechanism for particle pitch angle scattering and for heating the outer plasmasphere; these important physical processes themselves would deserve an entire monograph.

The basic theoretical concepts and ideas which flourished during the past thirty years have been discussed in a number of contributed articles and review papers; we are not sure that we have properly quoted them all, and any omission must be taken as unintentional.

The plasmasphere is the vast ‘doughnut’ shaped region of the magnetosphere that is filled with trapped ions and electrons of ionospheric origin; their energy is less than 1–2 eV. These charged particles are trapped on geomagnetic field lines, forming a cold thermal plasma cloud around the Earth out to geocentric equatorial distances of 4–5 Earth radii (RE).

The outer boundary of the plasmasphere forms a rather characteristic ‘knee’ in the equatorial plasma density profile. This field-aligned surface is called the ‘plasmapause surface or region’, or more simply the ‘plasmapause’. The plasmapause was discovered in the 1960s independently from in situ space probe measurements and from ground-based whistler observations. A first-hand account of the history of this discovery of the boundary is presented in Chapter 1. In the first part of this chapter K. I. Gringauz reports the prevailing situation in the former USSR, his design of the first ion traps flown in outer space and how he had to fight to get his experimental findings published and accepted in his country as well as in the Western World. In the second part of Chapter 1, D. L. Carpenter describes the situation in the US and the history of his discovery of the plasmapause from dynamic spectrograms of whistlers.

Whistler waves are audio frequency radio waves produced by lightning in the atmosphere. They propagate back and forth along field-aligned plasma density irregularities in the magnetosphere. Their travel time from one hemisphere to the magnetically conjugate point in the opposite hemisphere is mainly determined by the electron concentration in the distant magnetosphere where the VLF waves cross the geomagnetic equatorial plane.

This book is much more than a monograph about a scientific topic; it also provides a historical account of the growth of a new field of research by some of its pioneers. As such it is a case study of the long road from observations to phenomenological description culminating in true physical understanding typical for the geophysical sciences. It also illustrates the strong dependence on international collaboration, in this particular case the stimulus provided by the International Geophysical Year (IGY). This field of research grew out of ground-based whistler observations conducted during the IGY on the one hand and the first in situ measurements in the space environment surrounding the Earth made possible with the concurrent advent of the space age, on the other.

The theme of this book, the plasmasphere of the Earth, had its beginning in the study of the Earth's ionosphere, the thermal (cold) plasma originating from the interaction of solar ionizing radiation with the Earth's neutral atmosphere. As a long-time practicioner in this discipline, I often was frustrated in my early years when asked where the upper boundary of the Earth's ionosphere was, or when reading in publications some completely arbitrary altitudes assigned to such a boundary. With the new concept of the Earth's magnetosphere it first appeared probable that cold plasma originating in the ionosphere, because of the magnetic control of charged particles, might extend throughout the closed geomagnetic field lines up to the magnetopause.

The origin of this monograph is a thesis entitled ‘Frontiers of the Plasmasphere’ that I submitted in 1985 at the Université catholique de Louvain in fulfillment of the ‘Agrégation de l'Enseignement Supérieur’. This D.Sc. thesis described a new physical theory for the formation of the plasmapause.

As a result of this work, Professor M. J. Rycroft, Editor of the Cambridge Atmospheric and Space Science Series, asked me to prepare a monograph on the Plasmasphere. I was honoured by this proposal, but I wanted to decline it in view of the formidable effort that this project would involve, the time that it would take to review the large body of observations as well as the set of controversial theories put forward over twenty years, and then the time needed to compile a comprehensive synthesis all that material. But both the Editor and the Publishing Director of Cambridge University Press (CUP) argued that there was no topical monograph on the Earth's plasmasphere currently on the market and that such a book would be useful in Space Science Laboratories and their Libraries. Since the referees consulted by CUP were also very positive about such a project, I finally accepted.

Of course, a comprehensive monograph on the Earth's plasmasphere should not only describe theoretical aspects as in my thesis. It needed to contain a comprehensive review of the observations collected in the plasmasphere and at the plasmapause. These observations come mainly from whistler as well as from in situ satellite measurements. Only experimentalists who themselves had contributed to observations of the plasmasphere could be responsible for this important part of the monograph.

Introduction

In the previous chapters, we have described different methods of observations which have been used over three decades to study the plasmasphere, its shape, ionic composition, dynamics and deformations during geomagnetic substorms. In this chapter we wish to put together all pieces with the hope that an up-to-date global picture of the plasmasphere will emerge. We briefly mention theories and models proposed to explain various features of the plasmasphere, empirical models being presented where relevant. Discussion of theoretical aspects will be found in Chapter 5.

The ionosphere as a source and sink for plasmaspheric particles

The ionosphere of the earth has been divided into different layers, the D-, E-, and F-regions. Above ∼ 300 km altitude where the maximum ionization occurs in the F-region, the thermal ion and electron densities steadily decrease with altitude. This region, which is the base of the topside ionosphere, extends deep into the magnetosphere. It forms the plasmasphere at low and middle latitudes, the plasmatrough at high and mid-latitudes, and the polar wind at high latitudes (see Fig. 4.1).

Figure 4.2a illustrates the daytime ionospheric and atmospheric composition, based on early IQSY mass spectrometer measurements. It shows that, below 1000 km altitude, ions remain minor constituents of the earth's atmosphere. Below 500 km O+ is generally the dominant ionic constituent. Above 600 km, there is a transition level where H+ become the dominant ions. Under exceptional geophysical conditions the He+ ion density can exceed the density of O+ ions and the density of H+ ions in an intermediate altitude range (He+ belt).