The Kinetic Treatment of Space Plasmas

Abstract

To describe transport in space plasmas hydrodynamic and kinetic approaches have been proposed and used. These approaches are appropriate for separate collisional regimes. Hydrodynamics is a valid approximation of the general transport equations in highly collision‐dominated fluids. In low‐density plasmas where the Knudsen number is larger than 1 like at high altitude in planetary and stellar exospheres, only a kinetic treatment is appropriate. Collisionless models of the high altitude planetary atmospheres have been first based on the solution of the Liouville‐Vlasov equation. They proved to be very useful zero‐order kinetic approximations since they have demonstrated the importance of the internal polarization electric field in the acceleration process of the solar wind and polar wind protons. Higher order kinetic approximations have been more recently developed. Numerical solutions of the Fokker‐Planck equation for the velocity distribution of the electrons in the solar wind and H+ ions in the polar wind have now been obtained. The major and key differences between the results of these different approaches are illustrated through solar wind and terrestrial polar wind applications with comparison of the particle velocity distribution functions found in the different models.