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

Five years of photometric data of the lambda 630 nm ox In auroral/airglow e mission (similar to 240-km altitude) over Davis, Antarctica (Lambda=-74.6(0 )) are searched for evidence of periodic oscillations due. to atmospheric g ravity waves. Data were selected for intervals when the auroral oval was eq uatorward 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 s ubjected to a full spectral analysis. Wave-like features were seen oil 45 n ights (42%), with some nights showing evidence for two intervals of waves: A total of 53 waves were identified, Periods range from similar to 13 to 40 min. We identify the oscillations as the signatures of gravity waves. Deri ved wave properties include horizontal wavelengths from 220 to 1600 km and phase speeds ranging from similar to 200 to similar to 850 ms(-1). Around h alf of the waves are directed within similar to +/- 40(0) of magnetic south , with most of the remainder being directed either approximately magnetic e ast or magnetic west. We interpret this as indicating the wave source regio ns are of significant linear extent and are aligned either parallel or perp endicular to the auroral oval. Source locations are estimated using the kno wn dependence on frequency of the gravity wave angle of ascent, assuming th e 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 l atitude of the poleward edge of the auroral oval. Phase speeds and travel d irections are similar to the speeds and direction of substorm surges, which is suggestive of the source of these waves.