SELF-CONSISTENT SIMULATION OF THE PHOTOELECTRON-DRIVEN POLAR WIND FROM 120 KM TO 9 R-E ALTITUDE

It has long been recognized that photoelectrons can enhance the ambipo lar electric fields affecting polar wind outflows [e.g., Axford, 1968; Lemaire, 1972]. Since ionospheric ions and electrons are produced in large part by photoionization of the neutral atmosphere at lower altit udes, and the maximum photoelectron production rate occurs in the 130- 140 km altitude range, it is essential to model this photoelectron-dri ven polar wind self-consistently from the E region to an altitude bf s everal Earth radii, Here we describe a new steady state coupled fluid- semikinetic model to efficiently couple the source region to the high- altitude regions, This model couples a fluid treatment for the 120-800 km altitude range, a generalized semikinetic (GSK) treatment for the altitude range 800 km to 2 R-E, and a steady state collisionless semik inetic method for the altitude range 2-9 R-E. We apply this model to i nvestigate the photoelectron-driven polar wind with ionospheric condit ions ranging from solar minimum (F-10.7 = 90) to solar maximum (F-10.7 = 200). The O+ and H+ densities are found to increase by factors of a pproximately 5 and 2, respectively, from solar minimum to solar maximu m below 3 R-E altitude. However, the parallel bulk velocities display little variation with increased F-10.7 for altitudes below 3 R-E. An e lectric potential layer of the order of 40 V develops above 3 R-E alti tude, when the included downward magnetosheath electron fluxes (such a s polar rain) are insufficient to balance the ionospheric photoelectro n flux, Such potential layers accelerate the ionospheric ions to super sonic speeds at high altitudes, above 3 R-E, but not at low altitudes, We also found that the potential layer decreases from 40 to 8.5 V for solar minimum conditions and from 46 to 12 V for solar maximum condit ions when the magnetospheric electron density is increased from 0.05 t o 2 cm(-3).