AN IMPROVED KINETIC-MODEL FOR THE POLAR OUTFLOW OF A MINOR ION

We have studied the escape of a minor ion from the terrestrial atmosph ere by outflow along geomagnetic field lines that are open to the inte rplanetary medium. Coulomb collisions with the background ions are tre ated rigorously in the Fokker-Planck transport equation which is adopt ed in the transition region between the collision-dominated low-altitu de regime (treated by moment equations) and the collisionless regime a t very high altitudes. Sample solutions are presented for both H+ and He+ ions in an O+ background. In the transition region and above, the distribution functions are not well represented by low-order approxima tions, such as drifting Maxwellian distributions. For He+, while the d ensities and drift speeds are sensitive to the velocity distribution, the flux is a robust quantity that depends primarily on the altitude o f the transition between the region of chemical equilibrium and no cha rge exchange loss.