A comparison of kinetic and hydrodynamic models of an expanding ion-exosphere

The results of calculations based on kinetic and hydrodynamic models of a collisionless ion-exosphere are compared, and it is found that the density and velocity of an ionic constituent flowing supersonically throughout the exosphere can be adequately described by ordinary hydrodynamic equations with isotropic pressure. This suggests that the basic flow characteristics of the polar wind can be determined by hydrodynamic equations throughout the polar ionosphere. We derive a set of CGL-type hydrodynamic equations (for anisotropic pressure) that include the effects of energy and momentum transfer processes (e.g., collisions), and we show that such equations are applicable to problems of polar wind and solar wind temperature anisotropies, as well as to related flow problems.