ABSTRACT
In deriving the equations describing the fluid behaviour of collisionless plasma, some assumptions were necessary. The double adiabatic equations and were obtained by truncation of the hierarchy of moment equations at the equation describing the time evolution of the pressure tensor. In truly collisionless plasma, the only accelerations experienced by individual particles are the result of the electromagnetic forces. The force arising from the gradient of the pressure is a bulk force. The damping of the compressional wave for parallel propagation arises because the derivative of the plasma dispersion function is complex. The damping depends crucially on the nature of the unperturbed distribution function and on whether higher-frequency processes, which may exist in the plasma, scatter particles from their unperturbed orbits. In different circumstances, different theories may be appropriate.
