COMPARISON OF HYDRODYNAMIC AND SEMIKINETIC TREATMENTS FOR A PLASMA-FLOW ALONG CLOSED FIELD LINES

Hydrodynamic and semikinetic treatments of plasma flow along closed ge omagnetic field lines are compared. The hydrodynamic treatment is base d on a simplified 16-moment set of transport equations as the equation s for the heat flows are not solved; the hear flows are treated heuris tically. The semikinetic treatment is based on a particle code. The co mparison deals with the distributions of the plasma density, flow velo city, and parallel and perpendicular temperatures as obtained from the two treatments during the various stages of the flow subject to certa in assumed boundary conditions. In the kinetic treatment, the appropri ate boundary condition is the prescription of the velocity distributio n functions for the particles entering the flux tubes at the ionospher ic boundaries; those particles leaving the system are determined by th e processes occurring in the flux tube. The prescribed distributions a re half-Maxwellian with temperature T(o) and density n(o). In the hydr odynamic model, the prescribed boundary conditions are placed on densi ty (n(o)), flow velocity (V(o)), and temperature (T(o)). We found that results from the hydrodynamic treatment critically depend on V(o); fo r early stages of the flow this treatment yields results in good agree ment with those from the kinetic treatment, when V(o) = (kT(o)/2pim)1/ 2, which is the average velocity of particles moving in a given direct ion for a Maxwellian distribution. During this early stage, the flows developing from the conjugate ionospheres show some distinct transitio ns. For the first hour or so, the flows are highly supersonic and pene trate deep into the opposite hemispheres, and both hydrodynamics and k inetic treatments yield almost similar features. It is found that duri ng this period heat flow effects are negligibly small. When a flow pen etrates deep into the opposite hemisphere, the kinetic treatment predi cts reflection and setting up of counterstreaming. In contrast, the hy drodynamic treatment yields a shock in the flow. The reasons for this difference in the two treatments is discussed, showing that in view of the relatively warm ions, the coupling of ion beams and the consequen t shock formation in the off-equatorial region are not likely due to t he enhancements in the beam temperatures. The counterstreaming in the kinetic treatment and the shock in the hydrodynamic treatment first ad vance upward to the equator and then downward to the ionospheric bound ary from where the flow originated. The transit time for this advancem ent is found to be about 1 hour for the perspective models. After 2 ho urs or so, both models predict that the flows from the ionospheric bou ndaries are generally subsonic with respect to the local ion-sound spe ed. At late stages of the flow, when a substantial fraction of ions en tering the flux tube begin to return back in the kinetic treatment, th e hydrodynamic treatment with the boundary condition V(o) = (kT(o)/2pi m)1/2 yields an overrefilling, and the choice of V(o) becomes uncertai n.