Diurnal variations of the Fe layer in the mesosphere and lower thermosphere: Four season variability and solar effects on the layer bottomside at McMurdo (77.8S, 166.7E), Antarctica

Abstract

We provide one of the first reports on diurnal variations of the Fe layer in the mesosphere and lower thermosphere (MLT) using nearly 1000h of data collected with an Fe Boltzmann lidar at McMurdo, Antarctica from December 2010 through 2011. The Fe layer undergoes significant diurnal variations on the bottomside in autumn and spring with daytime downward extension and nighttime upward contraction. Such variations are absent in winter under 24-h darkness. Summertime Fe density perturbations exhibit downward tidal phase progression. The bottom growth/contraction of Fe layer is shown to be a solar effect that is closely correlated with the solar elevation angle. The bottom transition corresponds to the solar elevation between -9 and -1 when sunlight passes through the lower atmosphere before reaching the MLT. Once the solar elevation is above -1, the layer bottom remains nearly flat at its daytime altitude (∼70-73km). When the solar elevation is below -9, the layer bottom stays at its nighttime altitude (∼78-80km). The descending transition rate at dawn is faster than the ascending rate at dusk. The time delay between appreciable Fe density and sunrise/sunset at the MLT increases with decreasing altitude and varies with season. We describe qualitatively how both neutral Fe chemistry with H, O and O 3 and photolysis of Fe-containing molecular species may play important roles in Fe diurnal variations. Accurate rate coefficients for Fe neutral and photochemical reactions are essential to quantitatively explain the solar effects in diurnal variations of the Fe layer. © 2012. American Geophysical Union. All Rights Reserved.