OUTFLOW OF HE- COMPARISON OF HYDRODYNAMIC AND KINETIC DESCRIPTIONS( FROM THE POLAR IONOSPHERE )

Singly ionized helium, a minor species in the polar ionosphere, is bei ng propelled out along open magnetic field lines by an outward polariz ation electric field. In the present study we compare the kinetic and hydrodynamic descriptions of this transonic outflow treating the singl y ionized helium as a test particle population in a static background of singly ionized oxygen and electrons. We find that the resultant He particle fluxes are equal in both the eight-moment hydrodynamic descr iption and in the kinetic description based on the Fokker-Planck equat ion. A five-moment hydrodynamic description gives a flux that is some 40% lower. The increase of the He+ energy flux with altitude is also e qual in the eight-moment hydrodynamic and in the kinetic descriptions.