Thermospheric gravity waves in the southern polar cap from 5 years of photometric observations at Davis, Antarctica

Abstract

Five years of photometric data of the λ630 nm oxygen auroral/airglow emission (∼240‐km altitude) over Davis, Antarctica (Λ=−74.6°), are searched for evidence of periodic oscillations due to atmospheric gravity waves. Data were selected for intervals when the auroral oval was equatorward of Davis and hence are from the thermospheric polar cap. A total of 106 cloud‐free nights, each with a minimum of 2 hours in the cap, were subjected to a full spectral analysis. Wave‐like features were seen on 45 nights (42%), with some nights showing evidence for two intervals of waves: A total of 53 waves were identified. Periods range from ∼13 to 40 min. We identify the oscillations as the signatures of gravity waves. Derived wave properties include horizontal wavelengths from 220 to 1600 km and phase speeds ranging from ∼200 to ∼850 ms−1. Around half of the waves are directed within ∼ ±40° of magnetic south, with most of the remainder being directed either approximately magnetic east or magnetic west. We interpret this as indicating the wave source regions are of significant linear extent and are aligned either parallel or perpendicular to the auroral oval. Source locations are estimated using the known dependence on frequency of the gravity wave angle of ascent, assuming the source is near the electrojet height (120 km) and ignoring the effects of background winds. The source locations are found to be near the magnetic latitude of the poleward edge of the auroral oval. Phase speeds and travel directions are similar to the speeds and direction of substorm surges, which is suggestive of the source of these waves.