Investigation of the upper mesospheric dynamics under late polar summer conditions by EISCAT and lidar

Abstract

In August 1989, a strong polar cap absorption event allowed extended measurements of both horizontal and vertical wind components in the upper mesosphere and lower thermosphere by means of the EISCAT UHF radar. A distance (130 km) away from the radar site, a Na lidar instrument simultaneously measured temperature profiles over several hours in the altitude range of 85–100 km. This combination of measurements gives detailed information about the behaviour of the polar mesopause region in the transition period from summer to autumn conditions. Below 87 km, the wind measurements reveal typical summer profiles of the mean horizontal components and a significant long-period wave which we interpret as the semi-diurnal tide. Between 87 and 90 km, we find a strong wind shear; there, we observe a strong damping of the tide. Higher up, the mean horizontal wind is nearly zero, which represents autumn rather than summer conditions. In particular the vertical wind, but also the horizontal components reveal short-period oscillations with a small or vanishing phase progression, mainly in the 75–85 km altitude region. Most probably these are gravity waves with very long vertical wavelengths as predicted by theory to be prominent in the vertical wind. The temperature measurements confirm the typical polar summer conditions with a marked temperature minimum of 120-130 K at around 87 km and partially very large positive temperature gradients higher up, especially before local midnight. The measurements suggest that this peculiar thermal and dynamical structure in late summer acts as a major barrier for tidal waves and short-period waves leading to strong turbulence slightly above the mesopause temperature minimum.