Gravity wave instability structures and turbulence from more than one and a half years of OH* airglow imager observations in Slovenia

Abstract. We analysed 286 nights of data from the OH* airglow imager FAIM 3 (Fast Airglow IMager) acquired at Otlica Observatory (45.93 °N, 13.91 °E), Slovenia between 26 October 2017 and 6 June 2019. Measurements have been performed with a spatial resolution of 24 m/pixel and a temporal resolution of 2.8 s. A two-dimensional Fast Fourier transform is applied to the image data to derive horizontal wavelengths between 48 m and 4.5 km in the upper mesosphere / lower thermosphere (UMLT) region. In contrast to the statistics of larger scale gravity waves (horizontal wavelength up to ca. 50 km) we find a more isotropic distribution of directions of propagation, pointing to the presence of wave structures created above the stratospheric wind fields. A weak seasonal tendency of a majority of waves propagating eastward (westward) during winter (summer) may be due to secondary gravity waves originating from breaking primary waves in the stratosphere. We also observe an increased southward propagation during summer, which we interpret as an enhanced contribution of secondary gravity waves created as a consequence of primary wave filtering by the meridional mesospheric circulation. Furthermore, observations of turbulent vortices allowed the estimation of eddy diffusion coefficients in the UMLT from image sequences in 45 cases. Values range around 103–104 m2s-1 and mostly agree with literature. Turbulently dissipated energy is derived taking into account values of the Brunt-Väisälä frequency based on TIMED-SABER (Thermosphere Ionosphere Mesosphere Energetics Dynamics, Sounding of the Atmosphere using Broadband Emission Radiometry) measurements. Energy dissipation rates range between 0.63 W/kg and 14.21 W/kg leading to an approximated maximum heating of 0.2–6.3 K per turbulence event. These are in the same range as the daily chemical heating rates, which apparently stresses the importance of dynamical energy conversion in the UMLT.