Recent in situ observations have revealed novel features in the polar wind. Measurements between 5000 and 9000 km altitude by the Akebono satellite indicate that both H and O ions can have remarkably higher out¯ow velocities in the sunlit region than on the nightside. Electrons also display an asymmetric behavior: the dayside dierence in energy spread, greater for upward-moving than downward-moving electrons, is absent on the nightside. Here, we discuss the further development of a theory by

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... et al. that can explain most of these observed peculiar properties by properly taking into account the global, kinetic, collisional eects of the sunlit photoelectrons, on the background polar wind and the electric ®eld. Quantitative comparisons of the calculated results with actual data will be described. In addition, transition from the daytime photoelectrondriven polar wind to the night-time polar wind will be discussed.