0

In any introductory plasma physics text books, one will encounter a host of plasma waves. However, often in research articles one can see that generation of any plasma waves say e.g. Langmuir waves, acoustic waves etc.

So my question is what do you mean by the saying "the generation of different waves that is taught in the text books"? These waves are already existing in the medium, right?

Correct me if I am wrong.

sreeraj t
  • 155
  • In the presence of electeric or magnetic field or etc you can have waves in plasma without them, there is no wave. What do you mean they are already there? – Paradoxy Aug 15 '19 at 12:12
  • Like a sound wave in the air, it needs to be excited first; before it is excited it does not exist. – Maxim Umansky Aug 15 '19 at 18:32
  • @Paradoxy if you go through Chen's plasma physics textbook (e.g.), one can see the derivation of many plasma waves. But none of them are talking about the so called as "generation" of these waves. BTW, one can have plasma waves even without ambient electric and magnetic field. – sreeraj t Aug 16 '19 at 02:12

1 Answers1

1

In a real medium (not some unrealistic, idealized textbook example) like a plasma, there are always natural, thermal fluctuations of the system (e.g., see the article at https://ui.adsabs.harvard.edu/abs/2014PhPl...21a2902V/abstract). These are akin to what are often called normal modes of the system. In the case of a coherent wave, they tend to be an enhancement of a specific part of these thermal fluctuations due to an instability. An instability is just the mechanism that converts some free energy (in a plasma this is typically Gibbs free energy) into electromagnetic radiation of some sort.

A simple approximation typically used early in such books is called cold plasma theory. This is just the limit where the ion and electron temperatures go to zero. Then one has things like the R-mode and L-mode etc. While it is obviously wrong to assume a plasma has zero temperature, the theory is surprisingly accurate for some waves like whistler waves. That is, thermal effects do not significantly modify the dispersion relation.

These waves are already existing in the medium, right?

Kind of, yes. By that I mean that there are always thermal oscillations present but they need not be coherent or well defined if the plasma is marginally stable. If one, for instance, shot a beam of particles through the plasma, suddenly a specific part of the $\omega - \mathbf{k}$ space would absorb the free energy in the beam resulting in a coherent emission of electromagnetic energy. An example of this specific instability is called a two-stream instability (there are also bump-on-tail versions where the beam is much faster than the core thermal speed etc.).

So my question is what do you mean by the saying "the generation of different waves that is taught in the text books"?

This is slightly poor phrasing but such statements are meant to imply that the fluctuations are now above the thermal, background oscillations. That is, the signal-to-noise ratio exceeds unity by enough that the oscillations stand out in some way, either measured with in situ spacecraft/lab observations or simulation output.

honeste_vivere
  • 15,592