The variability of 558Â nm OI nightglow intensity measured over Adelaide, Australia

The intensity of the OI 558 nm nightglow emission from heights near 97 km has been routinely measured since 1995 at Buckland Park (34.9°S, 138.6°E) near Adelaide, Australia. The intensity exhibits spring and autumn enhancements, bright nights and clear seasonal and inter-annual periodicities. Like many other mid-latitude observations, the autumn enhancement is greater than the spring enhancement. A Lomb periodogram analysis of the intensity indicates the presence of annual, semi-annual, and quasi-biennial oscillations. The annual and semi-annual oscillations have about equal intensity at this latitude, with amplitudes of between 17(±5)% and 14(±5)% of the mean intensity, respectively. This is consistent with Adelaide being a transitional latitude between a dominant semi-annual oscillation observed at low latitudes and the dominant annual oscillation observed at mid-latitudes. The annual oscillation is observed to peak in summer while the semi-annual oscillation takes maximum values near equinox. The quasi-biennial oscillation has a smaller amplitude at about 5(±1)% of the mean intensity and takes a maximum value near the autumnal equinox. There is evidence of a solar cycle dependence of the intensity, with nightglow intensity tracking the solar cycle. A harmonic fit to this period yields an amplitude of about 20(±15)% of the mean intensity, but naturally, this result needs to be treated with some caution, as there is only about one period present.