Ionospheric origin of magnetospheric O + ions

Flows of thermal atomic oxygen (O + ) ions are deduced from topside ionospheric plasma density profiles. The mean flux within most of the polar cap is of the order of 10 12 m −2 s −1 , a figure which is consistent with both theoretical and experimental estimates of the light ion polar wind at greater altitudes. Larger flows (up to 6 × 10 12 m −2 s −1 ) are observed near the poleward edge of the night‐side statistical auroral oval, a feature not reproduced in the light ion flux. The implication is one of a low altitude acceleration mechanism, acting upon the O + ions at these latitudes and at heights above that at which the fluxes are observed. Such a process would enable ions to escape from the ionosphere because they do not exchange charge with neutral hydrogen. The observations are in general agreement with energetic O + ions as previously observed in various parts of the magnetosphere.