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.