We have studied the escape of a minor ion from the terrestrial atmosphere by outflow along geomagnetic field lines that are open to the interplanetary medium. Coulomb collisions with the background ions are treated rigorously in the Fokker-Planck transport equation which is adopted in the transition region between the collision-dominated low-altitude regime (treated by moment equations) and the collisionless regime at 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 approximations, such as drifting Maxwellian distributions. For He+, while the densities and drift speeds are sensitive to the velocity distribution, the flux is a robust quantity that depends primarily on the altitude of the transition between the region of chemical equilibrium and no charge exchange loss. ¿ American Geophysical Union 1996