Atomic oxygen annual and semi-annual variations in the mesopause region for mid and equatorial latitudes

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Satellite volume emission rates of the hydroxyl airglow and the atomic oxygen greenline measured by the wind imaging interferometer (WINDII) on board the upper atmospheric research satellite (UARS) are used to infer atomic oxygen profiles in the mesopause region. The data extends during the lifetime of the instrument from late 1991 through to 1997. The atomic oxygen derived from each emission is combined to provide a climatology extending from 80 to 115 km which includes up to 15 km above and below the peak in atomic oxygen concentration. A spectral analysis is performed for each latitude/local time/altitude bin on the time series of the atomic oxygen where the measurements were taken. Annual and semi-annual variations were identified and their resulting amplitudes and phases showed strong variability as functions of altitude and latitude. A summary of the conclusions are as follows: At mid-latitudes, the atomic oxygen time series consists of both annual and semi-annual variations with amplitudes up to 50% from the mean. The maximum and minimum of the annual variation depended on the hemisphere and the altitude. At the atomic oxygen peak (about 96 km ), the amplitude of the annual variation minimized in the winter hemisphere with a change phase in 180° through the equator towards the summer hemisphere and is consistent with previous observations. The phase of these variations depends on height. Below the oxygen peak (about 87 km ), the phase of the annual variation changed by 180°. At the equator, the atomic oxygen is dominated by the semi-annual variation with maxima about a month after the equinoxes with an amplitude that is stronger than in the northern hemisphere. The mid-latitude semi-annual results are in contrast to previous observations. There is also evidence for a quasi-biennial oscillation in the mesopause region in atomic oxygen at altitudes above 95 km .