Oxygen aurora during the recovery phase of a major geomagnetic storm

We have analyzed ultraviolet spectra measured in the postsunset auroral zone on 16 July 2000, during the recovery phase of the major geomagnetic storm of 14–16 July 2000. We find enhanced oxygen ion and neutral line emissions above 300 km in the postsunset sector of the auroral oval during the initial fast recovery phase of the storm, also called the Bastille Day storm. No comparable emissions are seen in simultaneous measurements of nitrogen and hydrogen emission features, indicating that these features are not related to electron or proton aurora. The enhancements are seen slightly equatorward of all other nitrogen line and band emissions that are presumed to comprise the nominal diffuse electron auroral oval. On the basis of the altitude profile, location, and timing of the enhancements, we interpret these oxygen emissions as a product of precipitating ring current oxygen ions in the auroral atmosphere, possibly after pitch‐angle scattering by electromagnetic ion cyclotron waves. These emissions may provide an important measure of the contribution of collisions and charge exchange interactions to the loss of oxygen ions and the recovery of geomagnetic storms and substorms.