On the physical interpretation of Thomson scattering in a plasma

The scattering of a wave by an individual particle is due to the fact that the particle oscillates in the field of the incident wave and these oscillations radiate the scattered wave. It is usually believed that scattering in a plasma, even though the cross section in it is on the order of the Thomson scattering cross section in a vacuum, takes place by plasma density fluctuations, which also involve ions, so that the total scattered radiation is not the sum of Thomson scattering by individual electrons. Although the scattering formulas widely used in processing observations are correct, their interpretation often is not. This note proves rigorously that scattering in a plasma is the sum of the scattering from the electrons and ions, with the total momentum difference between the incident and scattered waves being distributed among the electrons and ions, and that it is only based on this interpretation that we can obtain the conservation laws for waves and particles in the plasma. General physical, astrophysical, and other implications of the correct interpretation of scattering processes for radiation frequencies much larger than the plasma frequency are discussed.