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Polar caps form one of the ionospheric sources of magnetospheric plasma. This is due to the so-called polar wind, first suggested from theoretical arguments (Banks and Holzer, 1968; Axford, 1968). The proposed mechanism for this "classical" polar wind, ambipolar electric field, separates it from other ion outflow events. The defining classical polar wind characteristics are that it is cold, field-aligned (out of the ionosphere), and the velocities are inversely correlated with ion mass, favouring lighter ions, i.e., H+ and He+ (Banks and Holzer, 1969; for a recent review of polar wind, see Ganguli, 1996).
Later observations have revealed some new features in the polar wind. For example, there are clear day-night asymmetries in the ion and electron features, and also O+ outflows to occur (Abe et al., 1993). Furthermore, the outflow velocities increase monotonically with altitude, and are supersonic at high altitudes (Abe et al., 1993). These features may be explained by photoelectron effects (Tam et al., 1998).
For more observations see, e.g., Nagai et al. (1984) and Chandler et al. (1991).