ABSTRACT
In this chapter we survey the arena within which the electron acceleration to be explored in the following chapters takes place. We look at its nature, its configuration and its electron content. The medium is an unfamiliar one and yet at the same time the one that is most common in the universe. The arena is vast. It ranges through the Earth’s outer environment, into interplanetary space, and on to the Sun and interstellar space. The electrons populating it span more than ten orders of magnitude in energy from the lowest currently measurable to the highest theoretically attainable. Phase-space densities encompass more than 40 orders of magnitude.
