Abstract
Abstract. A new version of the Fast Airglow Imager (FAIM) for the detection of atmospheric waves in the OH airglow layer has been set up at the German Remote Sensing Data Center (DFD) of the German Aerospace Center (DLR) at Oberpfaffenhofen (48.09° N, 11.28° E), Germany. The spatial resolution of the instrument is 17 m pixel−1 in zenith direction with a field of view (FOV) of 11.1 km × 9.0 km at the OH layer height of ca. 87 km. Since November 2015, the system has been in operation in two different setups (zenith angles 46 and 0°) with a temporal resolution of 2.5 to 2.8 s. In a first case study we present observations of two small wave-like features that might be attributed to gravity wave instabilities. In order to spectrally analyse harmonic structures even on small spatial scales down to 550 m horizontal wavelength, we made use of the maximum entropy method (MEM) since this method exhibits an excellent wavelength resolution. MEM further allows analysing relatively short data series, which considerably helps to reduce problems such as stationarity of the underlying data series from a statistical point of view. We present an observation of the subsequent decay of well-organized wave fronts into eddies, which we tentatively interpret in terms of an indication for the onset of turbulence. Another remarkable event which demonstrates the technical capabilities of the instrument was observed during the night of 4–5 April 2016. It reveals the disintegration of a rather homogenous brightness variation into several filaments moving in different directions and with different speeds. It resembles the formation of a vortex with a horizontal axis of rotation likely related to a vertical wind shear. This case shows a notable similarity to what is expected from theoretical modelling of Kelvin–Helmholtz instabilities (KHIs). The comparatively high spatial resolution of the presented new version of the FAIM provides new insights into the structure of atmospheric wave instability and turbulent processes. Infrared imaging of wave dynamics on the sub-kilometre scale in the airglow layer supports the findings of theoretical simulations and modellings.
Highlights
The reaction of hydrogen and ozone produces molecular oxygen and vibrational–rotationally excited hydroxyl (OH)∗
It is interesting to note that a smaller-scale wave packet is propagating from the left corner of the field of view (FOV) to the right corner, apparently advected by the background wind
The larger wave structure extends nearly over the entire image from the lower left to the upper right corner of Fig. 2a–f, and the quite limited horizontal width of the wave fronts makes the wave appear to be trapped in a narrow corridor in the airglow layer
Summary
The reaction of hydrogen and ozone produces molecular oxygen and vibrational–rotationally excited hydroxyl (OH)∗. The hydroxyl forms a Chapman layer at the upper mesosphere and lower thermosphere and is the brightest component of the airglow phenomenon. The peak concentration of OH∗ is located at an altitude of ca. The OH∗ mean emission altitude exhibits low annual variability (e.g. von Savigny, 2015). Once the waves reach the airglow layer, they influence the intensity of the airglow emission due to temperature and density variations. This makes the airglow an established phenomenon for the investigation of atmospheric dynamics. Based on temperature data derived from a Na lidar, Gardner et al (2002) concluded that the vertical heat flux due to wave dissipation is maximum near the mesopause altitude region because of the reduced stability
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