Plasma waves are prevalent throughout the ionospheres. The waves can just have fluctuating electric fields or they can have both fluctuating electric and magnetic fields. Also, the wave amplitudes can be either small or large, depending on the circumstances. Small amplitude waves do not appreciably affect the plasma, and in many situations they can be used as a diagnostic of physical processes that are operating in the plasma. Large amplitude waves, on the other hand, can have a significant effect on the plasma dynamics and energetics. In general, there is a myriad of waves that can propagate in a plasma, and it is not possible, or warranted, to give a detailed discussion here. Instead, the focus in this chapter is on just the fundamental wave modes that can propagate in both magnetized and unmagnetized plasmas. First, the general characteristics of waves are presented. This is followed by a discussion of small amplitude waves in both unmagnetized and magnetized plasmas, including high frequency (electron) waves and low frequency (ion) waves. Next, the effect that collisions have on the waves is illustrated, and this is followed by a presentation of wave excitation mechanisms (plasma instabilities). Finally, large amplitude shock waves and double layers are discussed.

General wave properties

Many types of waves can exist in the plasma environments that characterize the ionospheres. Hence, it is useful to first introduce some common wave nomenclature before discussing the various wave types. It is also useful to distinguish between background plasma properties and wave induced properties. In what follows, subscript 0 designates background plasma properties, and subscript 1 designates both the wave and the perturbed plasma properties associated with the wave.