COMPARISON OF KINETIC AND HYDRODYNAMIC DESCRIPTIONS OF THE PROTON POLAR WIND IN THE TRANSITION TO COLLISIONLESS FLOW

We have compared kinetic and fluid model descriptions of the proton po lar. wind outflow, from the collision-dominated, subsonic regime at lo wer altitudes, through the transition to supersonic flow, and well int o the supersonic how regime. The kinetic model is based on the Fokker- Planck collision operator, and the two fluid models employed are based on the 8-moment expansion and the 16-moment bi-Maxwellian expansions, respectively. We find excellent agreement between the kinetic descrip tion and the fluid models for the proton density and flux, even in the transsonic and supersonic flow regimes. The models are also in qualit ative agreement for the temperature and heat flux moments, although ne ither fluid model reproduces the negative (downward) kinetic heat flux es found at high altitudes. The 16-moment fluid model gives a temperat ure anisotropy similar to the anisotropy derived from the kinetic solu tion. The assumed forms for the velocity distribution, on which the fl uid expansions are based, do not agree with the kinetic velocity distr ibution, except in the subsonic region where the departure from a Maxw ellian distribution is small. Near the fluid critical point the kineti c model develops a double-hump distribution, with an isotropic, low-en ergy core and an anisotropic, high-energy tail, and at higher altitude s the distribution function develops a ''kidney bean'' shape.