Collisionless solar wind protons: A comparison of kinetic and hydrodynamic descriptions

Kinetic and hydrodynamic descriptions of a collisionless solar wind proton gas are compared. Heat conduction and viscosity are neglected in the hydrodynamic formulation but are automatically included in the kinetic formulation. The fact that the results of the two models are very nearly the same indicates that heat conduction and viscosity are not important in the solar wind proton gas beyond approximately 0.1 AU. It is concluded that the hydrodynamic equations provide a valid description of the collisionless solar wind protons and, hence, that future models of the quiet solar wind should be based on a hydrodynamic formulation.