Gravity waves are a significant driver of middle atmosphere dynamics with various excitation sources, e.g. the jet stream, convection zones, flow over orography and natural hazards such as tsunamis. OH-airglow measurements allow continuous night-time observations of gravity waves and various other wave types including singular events like bores and wall events at an altitude of about 86 km.Their respective signals are subject to the measurement system “Observations of Airglow with Spectrometer and Imager Systems” (OASIS). Imager systems allow the derivation of wave parameters such as the horizontal wavelength and the propagation direction. Data from spectrometers complement this information with wave amplitudes derived in temperature and absolute OH radiance.Since November 2022, the measurement system OASIS started routine observations at the Very Large Telescope (VLT) in the Atacama Desert at Cerro Paranal, Chile (24.6°S, 70.4°W) in cooperation with the European Southern Observatory (ESO). It is composed of two Fast Airglow Imagers (FAIM) and one Ground-based infrared P-branch Spectrometer (GRIPS) with high temporal resolution (1 image every 1/2 seconds, 1 spectrum every 15 seconds). Currently, over three years of data with nearly 100% night-time data coverage have been acquired. One of the goals of the observation site beside the general investigation of atmospheric dynamics is the investigation of tsunami-induced signals in OH airglow.Monitoring the OH airglow provides a unique opportunity to make continuous night-time observations of the middle atmosphere with high temporal and spatial resolution. However, the OH airglow causes noise in ground-based astronomical observations in the short-wave infrared like performed with the VLT due to its emissions in this spectral range. The project AirMon-VLT (“Airglow Monitor at the VLT”) brings together the interests of atmospheric scientists to understand middle atmosphere dynamics even better and astronomers who want to precisely know about the OH airglow variability and radiance. With this detailed knowledge an improved scheduling of deep sky observations for example at times with low OH airglow variability and radiance could be achieved. Also, precise and highly temporally resolved information about the change of OH airglow radiance can help to improve the correction of astronomical spectra.Within AirMon-VLT, the short and medium-term variability of OH airglow is investigated with statistical methods answering questions like which changes in airglow radiance could typically be expected within minutes/hours/days/etc., e.g. due to infrasound, gravity waves, tides, planetary waves, and by singular events like bore or wall events. With methods from the field of artificial intelligence predictions of the airglow variability will be made into the near and medium future (nowcasting and forecasting) to allow for a better scheduling of the astronomical targets. Also, additional data like ERA5 reanalysis data will be investigated for a more comprehensive understanding of causes of the variability from lower atmospheric layers.We present the project AirMon-VLT and the measurement system OASIS. We show first results of statistical evaluations about typical changes of airglow radiances related to various wave phenomena, including singular events like a potential wall event with an exceptional high radiance change of 60% within only one hour.

Unique observations of co-seismic infrasound waves and disturbances in the ionosphere recorded by continuous Doppler sounding systems (CDSS) in Czechia and Slovakia during geomagnetically quiet period and associated with the Kamchatka M8.8 earthquake on 29 July 2025, are analysed and discussed. It is shown by simultaneous ionospheric sounding by a digisonde that the co-seismic infrasound waves were detected by the CDSS at a record height of about 340 km over Czechia, which is much higher than in previous observations, at distances greater than 8000 km from the epicentre. The Doppler shift oscillations caused by ionospheric plasma quasiperiodic movement induced by the infrasound waves had a frequency around 0.005 Hz and were observed approximately 12 min after the arrival of causative Long period surface seismic waves in Czechia. The frequency spectrum of the vertical ground surface motion that generated the infrasound waves was much broader, including more intense fluctuations with frequencies around 0.05 Hz. However, the higher frequency infrasound waves were attenuated during their propagation upward and did not reach the observation altitude, which is confirmed by numerical simulation that is in a good agreement with the CDSS observation. The numerical simulation also proves that it is necessary to consider air/plasma compression when calculating air particle velocities from the measured Doppler shift values. Combination of the numerical simulation with measurements using the CDSS and ionosonde is therefore a useful tool for better understanding and monitoring of co-seismic infrasound that propagates up to the altitudes of ionospheric F2 layer.

Abstract The ionosphere is a dynamic environment regularly affected from above and from below. This study identifies periods within the years 2000–2023 when the impact from above, driven by space weather, was minimal. The quiet space weather periods have wide usage for two types of ionospheric studies. At first, studies of isolated space weather events that disrupt quiet periods. Second, studies of the lower atmosphere effects on the ionosphere when space weather drivers are negligible. For the first time, we provide examples of both such studies by using data from GNSS receivers, a continuous Doppler sounding system, ionosondes, and airglow spectrometers. The results demonstrate the capability of our data set to contribute to an extensive effort to provide interpretations to still unexplained ionospheric variations.

Since June 2019, a scanning airglow camera is operated operationally every night at DLR Oberpfaffenhofen (48.09 degrees N, 11.28 degrees E), Germany. It provides nearly all-sky images (diameter 500 km) of the OH* airglow layer (height ca. 85-87 km) with an average spatial resolution of ca. 150 m and a temporal resolution of ca. 2 min.We analyse about three years (941 nights between October 2020 and September 2023) of OH* airglow all-sky images for spatially confined wave structures with horizontal wavelengths of ca. 20 km and less. Such structures are often referred to as ripples and are considered to be instability structures. However, Li et al. (2017) showed that they could also be secondary waves. While ripples move with the background wind, secondary waves do not.To identify small-scale and spatially confined structures, we adapt and train YOLOv7 (You Only Look Once, version 7), a machine learning approach, to determine their position and extent on the sky as well as their horizontal wavelength. Those wavelengths are compared to two-dimensional FFT (Fast Fourier Transform) results. We analyse the seasonal variations in the orientation of the wave fronts, the direction of advection and the horizontal wavelengths of these structures and deduce that instability signatures are observed especially in summer.Finally, we introduce a concept for "operating-on-demand" in order to derive energy dissipation rates from our measurements.

Gravity waves transport momentum and energy vertically and horizontally and play a key role for the circulation in the upper mesosphere and lower thermosphere (UMLT). They can experience convective or dynamic instabilities or undergo nonlinear interactions with the background flow. The UMLT is of particular importance, as gravity waves frequently reach their breaking levels in this region, often referred to as the turbopause.This altitude range is observed using two FAIM cameras measuring the OH-airglow emission centered at approximately 86 km altitude, with a full width at half maximum of about 7–8 km, from different locations. By applying a newly developed tomographic reconstruction technique to coordinated dual-camera OH-airglow observations of the same air volume, the three-dimensional structure of gravity waves in the UMLT can be recovered. The resulting volumetric data provide detailed information about horizontal and vertical gravity-wave features, representing a middle-atmosphere sounding technique complementary to established methods such as lidar or radar observations. To characterize these waves, vertical wavelengths are extracted in a dedicated post-processing step by applying a two-dimensional FFT to selected altitude layers of the tomographically reconstructed volume. This approach provides access to vertical phase progression and vertical wavelength information that is fundamentally unattainable with a single OH airglow imager. By analyzing the phase differences of the wave signals in the FFT spectra between different altitude layers, the vertical propagation angle can be derived. In combination with the horizontal wavelength, this enables the determination of the vertical wavelength and thus a full three-dimensional gravity-wave characterization.First results from this dual-FAIM tomographic approach are presented, demonstrating both the feasibility and the performance of the method. The analysis is based on coordinated OH-airglow observations from FAIM installations at Oberpfaffenhofen (lon = 11.28, lat = 48.09) and Otlica (lon = 13.91, lat = 45.94) over a one-year period. These data are used to assess retrieval quality, identify sensitivity limits for vertical wavelength derivations, and demonstrate the enhanced scientific value of three-dimensional gravity-wave characterization for multi-instrument analyses of middle-atmosphere dynamics.Within the project GIGAWATT, a collaboration of the German Aerospace Center, the University of Augsburg and the University of Bern, we are currently advancing this work by incorporating new measurements and combining complementary observational techniques, including radiometric temperature and wind observations in the stratosphere and lower mesosphere and multi-static OH airglow tomography, to establish a high-resolution gravity-wave observatory for the Alpine region. This work is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under the project number 540878795.

The Low Frequency Array (LOFAR) is one of the most advanced radio telescopes in the world. When radio waves from a distant astronomical source traverse the ionosphere, structures in this plasma affect the signal. Results are presented from a statistical study using 2,810 hours of observations of Cassiopeia A from a LOFAR station located in the Netherlands (station CS032, located at 52.9o N; 6.9o E) between 28th June 2014 and 27th November 2016. Ionospheric structures were identified in 469 (~17 %) of these observations. A comparison with proxies for geomagnetic activity (the Kp index) and solar activity (the F10.7 cm solar radio flux) showed that geomagnetic or solar effects were not the primary driver of these ionospheric structures. Ionospheric structures were more common in summer and between ~21 LT – 02 LT. These patterns in season and local time showed similarities to the occurrence of lightning strikes. When ionospheric structures were present, the mean number of lightning strikes in a spatial region close to the LOFAR observations (51.9o – 56.5o N; 3.9o – 9.9o E) two hours prior to the LOFAR observations was (70 ± 25) per hour. This was substantially larger than the mean value of (19 ± 5) per hour when the ionospheric structures were absent. This suggests that quasi-upward propagating Atmospheric Gravity Waves (AGWs) launched by thunderstorm activity could be one of the sources of the ionospheric structures. Collectively, these observations show that LOFAR can be used to infer ionospheric signatures of vertical coupling processes in the mid-latitude atmosphere. Such observations have the potential to be used to develop or validate existing Global Circulation Models (GCMs) or new models of vertical coupling within the terrestrial atmosphere.

The dynamics in the atmosphere, especially the upper mesosphere and mesopause are significantly driven by atmospheric gravity waves. OH airglow offers an unique possibility to observe atmospheric dynamics in this altitude region with a high spatio-temporal resolution simultaneously using imager and spectrometer systems. Especially, characteristics of gravity waves as well as features like wave breaking and wave-wave interaction can be observed. Spectroscopic observations allow observing rotational temperature changes. Thus, both instrument types complement each other very well. Since November 2022 two airglow imagers (FAIM) and one airglow spectrometer (GRIPS) with high temporal resolution (1 image every 2 seconds, 1 spectrum every 15 seconds) started routine observations during each night in cooperation with and at ESO’s Very Large Telescope (VLT) in the Atacama Desert at Cerro Paranal, Chile (24.6°S, 70.4°W). During the night from 31st July to 1st August 2023 we observed an exceptional bright night that is much brighter than any other we observed so far: a single wave front propagates from West to East with an observed phase speed of about 60m/s. After the passing of the wave front the OH intensity decreases by around 50% within only one hour. Pronounced wave activity of small-scale waves is observed especially before the passing of the event. Similar events in literature are often stated as “wall events”, but seem to occur very rarely in the extent observed. We present and interpret the wall event and discuss the observed phenomenon and its causes using data from multiple instruments and data sources.

Below about 2.3 µm, the nighttime emission of the Earth's atmosphere is dominated by non-thermal radiation. Excluding aurorae, the emission is caused by chemical reaction chains that are driven by the daytime photolysis and photoionisation of constituents of the middle and upper atmosphere by hard ultraviolet photons from the Sun. As this airglow can outshine even scattered moonlight in the near-infrared regime, the understanding of the Earth's night-sky brightness requires good knowledge of the complex airglow emission spectrum and its variability. However, airglow modelling is very challenging, as it would require atomic and molecular parameters, rate coefficients for chemical reactions, and knowledge of the complex dynamics at the emission heights with a level of detail that is difficult to achieve. In part, even the chemical reaction pathways remain unclear. Hence, the comprehensive characterisation of airglow emission requires large data sets of empirical data. For fixed locations, this can be best achieved by archived spectra of large astronomical telescopes with wide wavelength coverage, high spectral resolving power, and good temporal sampling. Using 10 years of data from the X-shooter echelle spectrograph in the wavelength range from 0.3 to 2.5 µm and additional data from the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope at Cerro Paranal in Chile, we have succeeded in building a comprehensive spectroscopic airglow model for this low-latitude site with consideration of theoretical data from the HITRAN database for molecules and from different sources for atoms. The Paranal Airglow Line And Continuum Emission (PALACE) model comprises nine chemical species, 26 541 emission lines, and three unresolved continuum components. Moreover, there are climatologies of relative intensity, solar cycle effect, and residual variability with respect to local time and day of year for 23 variability classes. Spectra can be calculated with a stand-alone code for different conditions, including optional atmospheric absorption and scattering. In comparison to the observed X-shooter spectra, PALACE shows convincing agreement and is significantly better than the previous, widely used airglow model for Cerro Paranal.

The ionosphere as a part of Earth’s atmosphere supports a wide range of oscillations, of which acoustic–gravity waves (AGWs) form an important part. AGWs distribute energy and momentum from the source region over large distances. A significant portion of AGWs originates in the lower atmosphere and propagates through the atmosphere up to the ionospheric heights where, due to the coupling between neutral and ionized particles, it could be detected as wavelike disturbances of the plasma. Primarily, the ionospheric behavior is driven by solar and geomagnetic activity, while the influence from neutral and below-laying regions of the atmosphere most of the time forms a substantially smaller part of the observed variability. However, it could significantly alter ionospheric behavior. Our study is limited to a time span of rather low solar and geomagnetic activity in order to highlight neutral atmosphere influence. In this study, we focus on two tropospheric situations above Europe that may lead to AGW generation, which propagate up to the F-layer where they potentially induce variability that we observe within ionospheric plasma parameters.

In this poster contribution, we present a scheduling system for automated remote operation of instruments at high-altitude research facilities and similar remote sites. Via web-based interfaces, the system allows instrument owners as well as authorized third-party scientists to schedule and execute measurements and observations.The system has been developed as a thesis project in the context of the AlpEnDAC-II ("Alpine Environmental Data Analysis Centre", www.alpendac.eu) collaboration (funded by the Bavarian State Ministry of the Environment and Consumer Protection). Consequently, the scheduler and interfaces have been integrated with the AlpEnDAC Operating-on-Demand functionalities. A first use case for the framework has been the operation of an airglow imager (FAIM) in Oberpfaffenhofen (DE).We describe the design and implementation of our system for scheduling and execution of multi-user observations on instruments, including scheduling-data transfers and data retrieval. Our core implementation uses an optimization-based scheduler (Google's OR-Tools) to ensure maximum instrument use and to minimize idle times. Results show that the scheduler is reliable, fast, and is consistently able to provide optimal observation plans. The extensibility of the system is guaranteed by the usage of modern software in the core of the system, including well-defined and specified communication through REST APIs. Thus, it can easily be adapted to other settings and instruments, which is also facilitated by a modern deployment strategy using Docker and Kubernetes.

We demonstrate how machine learning can be easily applied to support the analysis of large quantities of excited hydroxyl (OH*) airglow imager data. We use a TCN (temporal convolutional network) classification algorithm to automatically pre-sort images into the three categories “dynamic” (images where small-scale motions like turbulence are likely to be found), “calm” (clear-sky images with weak airglow variations) and “cloudy” (cloudy images where no airglow analyses can be performed). The proposed approach is demonstrated using image data of FAIM 3 (Fast Airglow IMager), acquired at Oberpfaffenhofen, Germany, between 11 June 2019 and 25 February 2020, achieving a mean average precision of 0.82 in image classification. The attached video sequence demonstrates the classification abilities of the learned TCN. Within the dynamic category, we find a subset of 13 episodes of image series showing turbulence. As FAIM 3 exhibits a high spatial (23 m per pixel) and temporal (2.8 s per image) resolution, turbulence parameters can be derived to estimate the energy diffusion rate. Similarly to the results the authors found for another FAIM station (Sedlak et al., 2021), the values of the energy dissipation rate range from 0.03 to 3.18 W kg−1.

Atmospheric gravity waves transport energy and momentum through the atmosphere and can travel large horizontal and vertical distances from the troposphere to the mesosphere and higher. They contribute to atmospheric dynamics and among others drive the meridional pole-to-pole circulation in the mesosphere. Thus, knowing about gravity waves, their spatio-temporal characteristics, their interaction with other waves and the atmospheric background is attracting more and more attention in order to further improve climate and even meteorological models.In the upper mesosphere / lower thermosphere (UMLT) region around an altitude of 80km to 100km, OH airglow can be utilized for passive remote sensing and continuous nightly observations of atmospheric dynamics, especially of gravity waves. The OH airglow layer is a chemiluminescent layer with a strong emission in the short wave infrared spectral range (at about 1500nm) and is located at an altitude of about 86-87km with a layer halfwidth of about 4km. The OH airglow intensity is modulated by traversing atmospheric gravity waves which lead amongst others to a vertical transport of atomic oxygen. Observing the OH airglow with short-wave infrared imagers allows characterizing gravity waves. From these observations the horizontal wave parameters (horizontal wavelength, horizontal direction of propagation, etc.) can be derived.In this study we present measurements of two ground-based FAIM (Fast Airglow IMager) systems, which are cameras sensitive in the short-wave infrared region observing the OH airglow layer with a high temporal resolution. The cameras are located at Oberpfaffenhofen, Germany and Otlica, Slovenia, about 300km apart from each other and are pointing to the same volume at about 87km located in the Alpine Region above Northern Italy. We developed a novel tomographic algorithm to allow for a three-dimensional reconstruction of the airglow layer by combining images from the two viewing angles. In order to solve the highly underdetermined equation system, prior knowledge of the OH airglow layer vertical profile is needed e.g. from multi-year observations of SABER on the TIMED satellite on a statistical basis, or Gaussian and Chapman basis functions. This allows us, among others, to derive the vertical wavelength of the waves, their three-dimensional propagation direction, and their three-dimensional structure. From that knowledge, further wave parameters but also the horizontal wind along the wave propagation can be estimated via the wave’s dispersion relation.We will explain the tomographic reconstruction method, its capabilities and limits and will present a detailed case study showing a 3D-reconstructed gravity wave and the derivation of its parameters.This work received funding from the Bavarian State Ministry of the Environment and Consumer Protection.

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 per 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; Hannawald et al., 2019), 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 during winter may be due to instability features from breaking secondary gravity waves that were created 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. We present multiple observations of turbulence episodes captured by our high-resolution airglow imager and estimated the energy dissipation rate in the UMLT from image sequences in 25 cases. Values range around 0.08 and 9.03 W kg−1 and are on average higher than those in recent literature. The values found here would lead to an approximated localized maximum heating of 0.03–3.02 K per turbulence event. These are in the same range as the daily chemical heating rates for the entire atmosphere reported by Marsh (2011), which apparently stresses the importance of dynamical energy conversion in the UMLT.

Atmospheric dynamics is strongly influenced by waves on different scales. Airflow over mountains can lead to all kinds of atmospheric waves, planetary and gravity waves as well as infrasound. Under certain circumstances these waves can propagate through the atmosphere and lead to a re-distribution of energy. In the case of gravity waves, a stably stratified atmosphere is a mandatory requirement for their generation and vertical propagation. Additionally, the vertical propagation depends on the horizontal wind field. In the Alpine and pre-Alpine region, we currently operate five OH-airglow imaging systems, which allow the investigation of orographic gravity waves. Depending on tropo-, strato- and mesospheric wind and temperature, it is checked which wavelengths can propagate into the fields of view of our instruments. This is done for a whole year in order to take into account annual and semi-annual cycles in wind and temperature. Concerning the generation of gravity waves, we put our focus on our OH-airglow imager (FAIM) deployed at Otlica (45.9°N, 13.9°E), Slovenia. Here, we also have additional measurements of an OH-airglow spectrometer (GRIPS). In case studies, we investigate whether strong wind events (Bora) lead to strong gravity waves activity or enhanced potential energy density. This work received funding from the Bavarian State Ministry of the Environment and Consumer Protection.

Atmospheric gravity waves transport energy and momentum trough the different atmospheric layers from the troposphere up to the mesosphere and above. On the one hand this transport has influence on atmospheric circulation patterns and drives for example the meridional circulation in the mesosphere. On the other hand the prevailing wind field selectively influences the vertical propagation conditions of gravity waves of different phase speed and horizontal propagation direction. The OH-airglow layer at ca. 86 km altitude (upper mesosphere / lower thermosphere, UMLT) is well-suited for the investigation of atmospheric dynamics, allowing continuous observations of the night-sky throughout the year. Especially, atmospheric gravity waves are prominent features in the data of airglow imaging systems. Furthermore, this altitude region is known to be a region where wave breaking occurs quite often making it particular interesting for quantifying the amount of energy and momentum released due to gravity waves. Five years of airglow observations with three FAIM (Fast Airglow Imager) systems in and around the Alpine region are analysed regarding high-frequency gravity waves. Prevailing wind fields and tides from meteor radar wind data and ERA5 data are compared with the propagation direction of these waves and show patterns with high correlation. On seasonal timescales, the gravity waves clearly propagate predominantly to the East in summer and to the West in winter regarding the zonal direction. The meridional direction varies between the different years. On diurnal timescales, we find that atmospheric tides significantly impact the main propagation directions of the gravity waves. We further present a case study of a stereoscopic reconstruction using two synchronized airglow-imagers with overlapping field-of-views. This allows deriving the wave amplitude and a 3D visualization of gravity wave patterns within the airglow layer. This work received funding from the Bavarian State Ministry of the Environment and Consumer Protection.

Multi-year temperature time series from OH-airglow infrared (IR) spectrometers deployed at different sites in Europe as part of the Network for the Detection of Mesospheric Change (NDMC) are used to estimate the gravity wave activity in the upper mesosphere / lower thermosphere (UMLT) region. The seasonal course of gravity wave activity is found to be strongly dependent on the wave period. While there is almost no clear variability of gravity wave activity for periods lower than about 60 minutes, we find strong evidence for an increasing variation throughout the year for periods longer than ca. 60 min. A dominant semi-annual structure with maxima at the solstices is found up to a periodicity of about 200 minutes, where a gradual transition to an annual cycle with maximum activity during winter and minimum activity during summer is observed. The energy and momentum carried by gravity waves is dissipated in terms of turbulent wave breaking. Using observations of airglow imagers with high spatial and temporal resolution which were operated at the same time as the abovementioned IR-spectrometers we performed an investigation of turbulent gravity wave dynamics. The estimations of the turbulent eddy diffusion coefficient and the energy dissipation rate from the image series of a turbulent wave front agree quite well with the few available values in literature. A machine learning approach for the systematic extraction of turbulent episodes from the very large data set is presented. This work received funding from the Bavarian State Ministry of the Environment and Consumer Protection.

In January and February 2016, the OH airglow camera system FAIM (Fast Airglow Imager) measured during six flights on board the research aircraft FALCON in northern Scandinavia. Flight 1 (14 January 2016) covering the same ground track in several flight legs and flight 5 (28 January 2016) along the shoreline of Norway are discussed in detail in this study. The images of the OH airglow intensity are analysed with a two-dimensional FFT regarding horizontal periodic structures between 3 and 26 km horizontal wavelength and their direction of propagation. Two ground-based spectrometers (GRIPS, Ground-based Infrared P-branch Spectrometer) provided OH airglow temperatures. One was placed at ALOMAR, Northern Norway (Arctic Lidar Observatory for Middle Atmosphere Research; 69.28∘ N, 16.01∘ E) and the other one at Kiruna, northern Sweden (67.86∘ N, 20.24∘ E). Especially during the last third of January 2016, the weather conditions at Kiruna were good enough for the computation of nightly means of gravity wave potential energy density. Coincident TIMED-SABER (Thermosphere Ionosphere Mesosphere Energetics Dynamics–Sounding of the Atmosphere using Broadband Emission Radiometry) measurements complete the data set. They allow for the derivation of information about the Brunt–Väisälä frequency and about the height of the OH airglow layer as well as its thickness. The data are analysed with respect to the temporal and spatial evolution of mesopause gravity wave activity just before a minor stratospheric warming at the end of January 2016. Wave events with periods longer (shorter) than 60 min might mainly be generated in the troposphere (at or above the height of the stratospheric jet). Special emphasis is placed on small-scale signatures, i.e. on ripples, which may be signatures of local instability and which may be related to a step in a wave-breaking process. The most mountainous regions are characterized by the highest occurrence rate of wave-like structures in both flights.

Between December 2013 and August 2017 the instrument FAIM (Fast Airglow IMager) observed the OH airglow emission at two Alpine stations. A year of measurements was performed at Oberpfaffenhofen, Germany (48.09∘ N, 11.28∘ E) and 2 years at Sonnblick, Austria (47.05∘ N, 12.96∘ E). Both stations are part of the network for the detection of mesospheric change (NDMC). The temporal resolution is two frames per second and the field-of-view is 55 km × 60 km and 75 km × 90 km at the OH layer altitude of 87 km with a spatial resolution of 200 and 280 m per pixel, respectively. This resulted in two dense data sets allowing precise derivation of horizontal gravity wave parameters. The analysis is based on a two-dimensional fast Fourier transform with fully automatic peak extraction. By combining the information of consecutive images, time-dependent parameters such as the horizontal phase speed are extracted. The instrument is mainly sensitive to high-frequency small- and medium-scale gravity waves. A clear seasonal dependency concerning the meridional propagation direction is found for these waves in summer in the direction to the summer pole. The zonal direction of propagation is eastwards in summer and westwards in winter. Investigations of the data set revealed an intra-diurnal variability, which may be related to tides. The observed horizontal phase speed and the number of wave events per observation hour are higher in summer than in winter.