The Equinox Transition in Atomic Oxygen Concentration

The nighttime airglow emitted from the upper mesosphere and lower thermosphere is dominantly produced through reactions driven by the recombination of atomic oxygen to its molecular form. For this reason the atomic oxygen concentration may be monitored with ground-based or satellite measurements, the latter providing height profiles of the atomic oxygen number density. At mid-to high latitudes the atomic oxygen concentration is high in winter, and low in summer, reflecting the large-scale thermospheric circulation. More detailed recent observations have shown that the transition between the two is rather sudden, so that in the northern hemisphere the atomic oxygen is suddenly depleted near the end of March. This concentration remains low until near summer solstice, when it begins to recover slowly, with a sudden return to winter values soon after the fall equinox. In this presentation, ground-based observations and measurements from the WIND Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite are combined to describe the phenomenon of the equinox transition.