Self‐consistent simulation of the photoelectron‐driven polar wind from 120 km to 9 RE altitude

Abstract

It has long been recognized that photoelectrons can enhance the ambipolar 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 altitudes, and the maximum photoelectron production rate occurs in the 130–140 km altitude range, it is essential to model this photoelectron‐driven polar wind self‐consistently from the E region to an altitude of several 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 RE, and a steady state collisionless semikinetic method for the altitude range 2–9 RE. We apply this model to investigate the photoelectron‐driven polar wind with ionospheric conditions ranging from solar minimum (F10.7 = 90) to solar maximum (F10.7 = 200). The O+ and H+ densities are found to increase by factors of approximately 5 and 2, respectively, from solar minimum to solar maximum below 3 RE altitude. However, the parallel bulk velocities display little variation with increased F10.7 for altitudes below 3 RE. An electric potential layer of the order of 40 V develops above 3 RE altitude, when the included downward magnetosheath electron fluxes (such as polar rain) are insufficient to balance the ionospheric photoelectron flux. Such potential layers accelerate the ionospheric ions to supersonic speeds at high altitudes, above 3 RE, 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 conditions when the magnetospheric electron density is increased from 0.05 to 2 cm−3.