MONTE-CARLO STUDY OF THE TRANSITION REGION IN THE POLAR WIND - AN IMPROVED COLLISION MODEL

A Monte Carlo simulation was used to study the steady state flow of th e polar wind protons through a background of O+ ions. The simulation r egion included a collision-dominated region (barosphere), a collisionl ess region (exosphere), and the transition layer embedded between thes e two regions. Special attention was given to using an accurate collis ion model, i.e., the Fokker-Planck expression was used to represent H - O+ collisions. The model also included the effects of gravity, the polarization electric field, and the divergence of the geomagnetic fie ld. For each simulation, 10(5) particles were monitored, and the colle cted data were used to calculate the H+ velocity distribution function f(H+), density, drift velocity, parallel and perpendicular temperatur es, and heat fluxes for parallel and perpendicular energies at differe nt altitudes. The transition region plays a pivotal role in the behavi or of the H+ flow- First, the shape of the distribution function is ve ry close to a slowly drifting Maxwellian in the barosphere, while a '' kidney bean'' shape prevails in the exosphere. In the transition regio n, the shape of f(H+) changes in a complicated and rapid manner from M axwellian to kidney bean. Second, the flow changes from subsonic (in t he barosphere) to supersonic (in the exosphere) within the transition region. Third, the H+ parallel and perpendicular temperatures increase with altitude in the barosphere due to frictional heating, while they decrease with altitude in the exosphere due to adiabatic cooling. Bot h temperatures reach their maximum values in the transition region. Fo urth, the heat fluxes of the parallel and perpendicular energies are p ositive and increase with altitude in the barosphere, and they change rapidly from their maximum (positive) values to their minimum (negativ e) values within the transition region. The results of this simulation were compared with those found in previous work in which a simple (Ma xwell-molecule) collision model was adopted. It was found that the cho ice of the collision model can alter the results significantly. The ef fect of the body forces was also investigated. It was found that they can also alter the results significantly. Both the body forces and col lision model have a large effect on the heat flux, while they have onl y a small quantitative effect on the lower-order moments (density, dri ft velocity, and temperature).