Plasma waves - last update: 24 November 1998, 1440 UT (RR)

One of the most obvious features of the plasma state is the rich variety of wave motions which plasmas can support. Waves of a particular kind are said to be in a particular wave mode. The idea of a wave mode is familiar from other contexts. For example, in a compressible gas there are sound waves and, if there is a gravitational field present, there are also internal gravity waves. The study of the waves in plasmas is very useful for plasma diagnostics, since the wave modes of a plasma depend on the plasma properties.

Some important wave parameters in general are: Closer analysis of the waves reveals some additional parameters. There are two main different methods of approach used in analysing the plasma waves. One uses the dielectric tensor of the plasma, the other dispersion relation. The latter one is often simpler. The way the dispersion relation is calculated depends on the adopted plasma theory, which also determines the available wave modes. Important things to consider in plasma theories from the wave point of view are:
  1. What is the proper truncation level for the moments of Boltzmann's equation
  2. What plasma particles are taken into account
  3. Is the external magnetic field important or not
  4. Are collisions between the particles important
  5. What is the amplitude of the waves
The dispersion relations calculated are typically divided into two basic modes, longitudinal and transverse, according to the angle between the electric field component of the wave and the propagation vector k. However, to make things unnecessarily complicated, the nomenclature for the various plasma wave modes derived is far from systematic, being a mixture of historical names (e.g., Langmuir, Alfven, Bernstein), of names descriptive of the wave motion (e.g., ion acoustic waves, electron cyclotron waves, transverse waves), and of names characteristic of the theory used to derive the wave properties (e.g., magnetoionic waves, MHD waves, drift waves).

For more information about the wave theories, see:

For more information about the ULF frequency wave types found in the magnetosphere, see geomagnetic pulsations.

See also: