If a charged particle species of a collisionless plasma possesses a non-Maxwellian velocity distribution function, a short wavelength normal mode of the system may grow in amplitude. This is a microinstability; its theory is well described by the Vlasov equation. The purpose of this monograph is to describe in an accurate way the theory of damped normal modes and a limited number of microinstabilities that may arise in various space plasma environments.

The two words that best characterize the work described in this book are “limited” and “accurate.” In order to keep the discussion limited, I have chosen idealized, not observed, distribution functions. Many spacecraft have provided excellent observations of electron and ion distributions in the Earth's magnetosphere and nearby solar wind. The tremendous variety of these distributions makes it difficult to select a few for special representation. My choice here has been to use Maxwellian or bi-Maxwellian distributions with field-aligned drifts to represent some of the more important general free energy sources. Although the resulting instabilities may not correspond to any particular data set, I hope that each one represents the general properties of a very broad class of data.

To provide accuracy, I have followed the same procedure for each distribution function and plasma model. After assuming a zeroth-order distribution, I derive (or at least explicitly state) the associated dispersion equation without approximation. Because I deal with linear theory throughout this book, it is always straightforward to do this, although the algebra gets tiresome at times.