| Summary: | Small-scale structure of the thermosphere is studied at high-latitudes for its important role in ion-neutral coupling. Four Fabry-Perot Interferometers (FPIs) in Scandinavia are primarily used. These are supplemented by a range of other instruments, including the Spectrograph Imaging Facility, radars, magnetometers, all-sky cameras, and satellite data. The FPIs measure the atomic oxygen emission line at 6300 A, from 240 km altitude. Emission intensities, thermospheric line of sight wind speeds, and neutral temperatures are obtained. Comparisons of electron densities from tomography data and EISCAT (European Incoherent SCATter) radar with FPI intensities allow the investigation whether dissociative recombination is the dominant production mechanisms of the nighttime 6300 A oxygen line. Cross correlations indicate that the thermosphere varies on short temporal scales. Altitude variations have less effect due to the broad (-50 km) emission height band. Atmospheric gravity waves in the thermosphere have been detected for the first time in ground-based FPI data using Lomb-Scargle analysis. Joule heating from electrojet currents, and particle precipitation in the auroral oval, have been identified as the primary source mechanisms using two case study nights. High time resolution data shows a limit to the variability of the thermosphere to be approximately 1-minute. Statistical studies of the gravity waves from 567 nights of FPI data show that the length of the night and time resolution are the most important influences on the number and periods of waves detected. Greater numbers of short period waves are detected in the rapidly responding intensities than in the winds and temperatures. Little variation with geomagnetic activity or solar cycle is observed. Periods at particular harmonics of the length of the night are preferred between October and February. Comparisons of mainland and Svalbard data show that the shorter period waves that are formed equatorward in the auroral oval mostly dissipate before reaching Svalbard.
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