Hydrodynamic Description of Space Plasma

Abstract

Because of its complexity, major difficulties are encountered in the use of the kinetic equation in the study of various processes in plasma. Thus, the class of problems that can be solved using the kinetic approach is quite limited. Additionally, for a large number of problems, the detailed kinetic description is excessive and only macroscopic parameters like density, bulk velocity, pressure, viscosity tensor, and heat flux are important. These quantities are defined as the corresponding moments of the velocity distribution function. The plasma kinetic description could thus be replaced by a system of velocity moment equations. These moment equations are usually called hydrodynamic or transport equations. The transport equation approach has received a lot of attention because it can handle the core plasma particle and energy flow conditions in the solar–terrestrial environment. Many of the highly nonequilibrium flows found in space plasma are characterized by appreciable temperature anisotropies, i.e., unequal species temperatures parallel and perpendicular to the magnetic field direction. Also, wave–particle interaction processes can dramatically contribute to the formation of these nonequilibrium plasma flows. Therefore, it is necessary to trace the derivation of the different sets of the transport equations in order to understand the applicability of space plasma hydrodynamic theory.