Ion acoustic waves and Langmuir waves
In Section 2.8 the phase velocities of ion and electron acoustic waves were used in a qualitative description of the incoherent scatter spectrum. A simple derivation of the dispersion equations of these two wave modes is given in this appendix.
B.1 Ion acoustic waves
Ion acoustic waves propagating in plasma are nearly similar to ordinary sound waves in neutral gas: they are longitudinal waves consisting of compressions and rarefactions progressing in the medium. The role of ions is the same as that of neutral atoms in ordinary sound waves. A difference is that, unlike sound waves, ion acoustic waves can also propagate in collisionless medium, because the charged ions interact at long distances via their electromagnetic field. A second difference is that plasma also contains electrons which have their effect on the wave dispersion equation. Due to their small mass the electrons are very mobile and they quickly follow the ion motion trying to preserve the charge neutrality. The electron motion is caused by a small electric field internally generated by the plasma as a result of variations in the local ion density.
B.2 Langmuir waves
One might think that a thermally excited wave mode similar to the ion acoustic wave, with its phase speed determined by the electron mass and temperature, could propagate in the electron gas of the plasma. In thermal equilibrium, however, the electron motion is strongly controlled by ions, and therefore no such wave with a significant amplitude is generated. The situation will change if energetic photoelectrons or energetic electrons caused by particle precipitation are present in the plasma. Then the electrons may have a sufficient energy to create density enhancements violating the local charge neutrality and electron plasma waves with associated electric fields may be generated. The frequency of these waves is so high that the massive ions are not affected by them and therefore the ion gas can only be regarded as a constant positive charge density taking care of average charge neutrality in the medium. One should notice that, due to the significant electric field, the wave mode thus generated has characteristics different from those of the ion acoustic wave.
New Zealand, the Far North on the North Island (below)
New Zealand, the North Island (two above) / Nov. 20, 2016 http://go.nasa.gov/2gcgt1E
http://go.nasa.gov/2gcfOxq / I have never seen this here before…are they ‘ice nucleating’ an big engineered winter storm to make people think the earth is cooling? Insane…
The Great Lakes USA (two above) http://go.nasa.gov/2gccvWR
Japan (two above) Nov.20, 2016 http://go.nasa.gov/2gchBlU