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).

Introduction

Until the early 1950s no one suspected that the ionosphere of Earth extends far into the geomagnetic field and forms what is now called the protonosphere. The discovery of this region, filled with low-energy charged particles of ionospheric origin, is an interesting one from a historical perspective. It started with the pioneering whistler wave investigations by Storey, and continued with the theoretical work of Dungey, who developed the idea of a Chapman–Ferraro cavity. Then, towards the end of the decade, the first in situ measurements of the high-altitude plasma and of a steep density dropoff at L ∼ 4 were made by Gringauz and his colleagues in the USSR. Meanwhile, Carpenter was beginning the series of whistler studies that led to the identification of what is called the ‘knee’ in the equatorial electron density profile, and eventually to the description of the worldwide structure and first-order dynamics of what he called the plasmasphere. The historical account of these early discoveries is not generally well known by the younger generation of space physicists; some of its aspects are unknown even to the older. Since it is interesting to record for future studies of the history of science the paths followed by the pioneers, we devote this first chapter to events that happened in the early days of the space age.

In reading what follows, one should realize that in the middle and late 1950s the world space physics community was small and widely scattered.

Although we are aware that the theories reviewed in Chapter 5 neither address nor explain all aspects of currently available observations, the authors hope nevertheless that the Chapter has described the state of the art in this field of investigation. We have outlined, in a historical perspective, the successive steps in modelling the plasmasphere and its outer boundary, the plasmapause. As has happened in many other fields of investigation, the first model is the best known of all, but several successive generations of models were proposed later on to improve or replace the initial picture. The successive improvements on the preceding work have been outlined, as well as the limitations of each of the successive models. The authors will be rewarded for their efforts in writing this last Chapter and the whole book if it succeeds in setting the stage for the generation of future theories, if it stimulates new ideas and if it produces a revival of interest for the plasmasphere and plasmapause region.

The history of the discovery of the plasmapause by Gringauz and Carpenter, respectively, in the former Soviet Union and United States, has been reported in Chapter 1. Those who worked in this field in the 1960s will remember some of the episodes, but probably will have discovered other aspects of the story that are not published anywhere else.

Sky aficionados, whether professional astronomers or amateurs, always have two preoccupations. One is aesthetic: they want to capture, in a memorable way, the beauty of the sky. The other is, of course, scientific: they would love to quantify their observations, compare them with others', and verify or discover new effects.

Everyone knows that an astronomical observation uses a complex system. The telescope is an essential element, but is not unique: there is also the choice of site, shielded from light interference and turbulence, the construction and thermal stabilization of the dome, and, of course, the light detector that controls the quality of the final image. It would be more appropriate to speak of the ‘observing system’, whose every link is essential.

In its time, J. Texereau and G. de Vaucouleurs’ famous book L'Astrophotographie d'amateur inspired generations of amateurs when photography was the best way to capture photons. Today, modern light detectors are charge-coupled devices, commonly known as CCDs. If their format does not reach that of a photographic plate, still unequaled in the number of pixels it offers, their sensitivity is several dozen times better. And since we all know that the time needed to reach a given signal-to-noise ratio (which is directly linked to the possibility of detecting a possible astronomical source) varies as the inverse square of the sensitivity, it is easy to understand the incredible leap forward CCDs will enable observers to make.

Choice of optical combinations

In order to develop a strategy that will make you a true specialist in CCD observation, it is advisable as a first step to set up the telescope's optical assembly to obtain the desired field and resolution.

The resolution The maximum resolution we can achieve is determined by the telescope's diameter, the intrinsic quality of the images, or ‘seeing’, turbulence, and sampling. The first limitation comes from the phenomenon of diffraction caused by the instrument's diameter: the larger the instrument's diameter is, the better the resolution. For instance, a 12 cm diameter telescope cannot resolve better than 1 arcsecond, whereas a 50 cm telescope can reach 0.25 arcsecond. It is physically impossible to reach a better resolution at the diffraction limit of a given instrument.

The second limitation comes from the observation site's atmospheric turbulence. Unfortunately, atmospheric turbulence is often larger than the diffraction limit. We can assume that anything over 1 second of exposure time and with a diameter greater than 10 cm, the resolution limit caused by turbulence completely masks that caused by the diffraction limit. In terms of long exposures (above 1 second), classical amateur sites have seeing in the order of 5 arcseconds, with the better ones going as low as 2 or 3 arcseconds.

The third limitation comes from sampling by the CCD detector. The physical dimensions of the CCD's pixels limit the resolution by dividing the image into tiny tiles.

We hope that these pages have convinced you of the performance and simplicity of CCD imagery. We also hope that they have shown you what a CCD camera is and given you the essential ideas to equip yourself and use that equipment well.

This book is not an end in itself. Rather, it is one of the pieces of a puzzle which has barely begun to assemble itself. It represents only an introduction to amateur CCD astronomy. The subsequent chapters of this story will be written by the observers themselves. The ADAGIO association hopes to participate in this adventure, through the organization of new workshops and symposia, through its observational work and publications. It hopes to maintain links with the readers of this book to exchange results and information and to continue guiding if necessary.

At the time of this book's publication, amateur CCD astronomy is at its beginnings. It will blossom in the years to come. It will then be time to take stock and examine what actions will enable the amateur observer community to take full advantage of this new tool.