Plasma Waves

Abstract

Plasmas can support a large variety of wave modes. In an unmagnetized collisionless plasma, electromagnetic waves exist only for frequencies that are higher than the electron plasma frequency. Measuring the refractive index of a plasma with a laser interferometer is a suitable diagnostic method for the plasma density. Longitudinal electrostatic waves are found near the electron plasma frequency, or as ion-acoustic waves below the ion plasma frequency. A magnetic field makes the plasma anisotropic. Waves propagating along the magnetic field are subject to Faraday rotation. Resonances are found at the cyclotron frequencies. Transverse waves show bi-refringence as ordinary and extraordinary mode. The latter has resonances at the upper and lower hybrid frequency. All these wave modes can be used for diagnostic purposes, e.g., in radio astronomy, where the Faraday effect yields the product of plasma density and magnetic field strength, or in ionospheric physics, where ionosondes measure the vertical density profile.

“What is the use of a book”, thought Alice,

“without pictures or conversations?”

Lewis Carroll, Alice in Wonderland