The purpose of this monograph is to formulate a quantitative and self-consistent theoretical approach to wave–particle interactions occurring in space plasmas, and present a logical development of the subject. In the Earth's magnetosphere, Nature has given us a plasma laboratory that is accessible to observations made by radio, magnetic and electric instruments on the ground, and a great variety of instruments aboard rockets and Earth-orbiting satellites. Spacecraft are making similar observations in the more distant solar system.

To understand such observations as fully as possible, with colleagues around the world we have been challenged to produce a rigorous description of the energetic charged particle distribution function interacting with electromagnetic waves across a wide frequency spectrum. The space plasma is, as a rule, a non-equilibrium system with sources and sinks of energy and charged particles. As such, electromagnetic waves are generated via the process of the stimulated emission of radiation. Together with the electrodynamic properties of the space plasma, determined by variations of the magnetic field and plasma density, this constitutes a maser system. It exerts a strong influence on the state of the space plasma.

Cyclotron masers (CMs) are a shining example of such maser systems operating in the Universe. Whether in the Earth's magnetosphere or Jupiter's, in the solar corona or in the laboratory, CMs are exciting systems to marvel at, to wonder about and to investigate in detail. Such is the theme of this book.