WAVES IN PLASMA
Due to collective behavior of plasma (because of the electric and magnetic fields) it can be developed in the plasma an ample range of waves and oscillations in the acoustic, radio and optical frequencies. Roughly speaking these waves can be divided into two groups. The first group is related to ion oscillations (in the figure, it is the red curve). These waves have a low frequency. It is about acoustic waves influenced by the presence of a magnetic field. Sometimes we call them magnetoacoustic waves. Their typical frequency is the ionic plasma frequency:
ωpi = (ne2/miε0)1/2.
The second group depends on the electron oscillations (the blue curve in the figure). These waves have radio or optical frequency and are related with electromagnetic waves that propagate in the plasma. The typical frequency is the electronic plasma frequency:
ωpe = (ne2/meε0)1/2.
When an explosion occurs in a homogeneous medium [and assuming the source of the explosion is itself homogeneous and isotropic N. of T.], the acoustic shock wave expands with a shape of a spherical surface. The sound in the plasma behaves in a different form. Plasma itself is an anisotropic medium. This is because of the presence of the magnetic field. The acoustic wave is influenced by the magnetic field, and also it expands within an anisotropic medium. The surface of the wave is not spherical and has a more complicated shape:
In the figure above are schematized the wavefronts of the propagating acoustic wave, which is modified under the presence of the magnetic field (and therefore it is called magnetoacoustic wave). The individual modes of the wave are called: Alfvén Wave (AW), Slow magnetoacoustic wave (S) and Fast magnetoacoustic wave (F). In the radial axis of the polar diagram is expressed the phase velocity vf = ω/k, according to the axial angle α between the direction of the expansion and the direction of the magnetic field B.
Similarly as in other anisotropic media, even in plasma are manifested the electromagnetic waves O (Ordinary) and X (eXtraordinary). As it is evident from the introductory diagram, the ordinary wave propagates only at frequencies higher than the electron plasma frequency. At lower frequencies the plasma is opaque to light (electromagnetic radiation). This is because the electrons at lower frequencies get to perceive and follow the external stimuli, vibrate and absorb the energy of the electromagnetic wave. This phenomenon is very well known for the radio wave in our ionosphere. The higher frequency waves penetrate the ionosphere, for them it is “transparent”, whereas the lower frequency waves do not penetrate in any way. For the extraordinary waves the question about penetrability and transparency within the plasma is even more complicated.
Propagation of the ordinary wave (O wave)
Propagation of the extraordinary wave (X wave)
Translation: Arturo Ortiz Tapia, 2005