Statistical characteristics of polar cap mesospheric gravity waves observed by an all-sky airglow imager at Resolute Bay, Canada

An airglow imager at Resolute Bay, Canada (74.7 degrees N, 265.1 degrees E) has been used for the observation of gravity waves in the polar cap mesosphere since January 2005. On the basis of the long-term imaging observations obtained over 137 nights during the winter seasons of 2005 and 2006, we extracted 143 events of small-scale (< 100 km) gravity waves from sequential sodium airglow (589.3 nm) images. The observed waves typically had horizontal wavelengths of 20-50 km and phase speeds of 30-60 ms(-1). The propagation directions were predominantly westward. We also identified larger-scale (> 100 km) gravity waves using airglow keograms, which represent a time series of the horizontal cross sections of the airglow images. The horizontal wavelengths and phase speeds of the large-scale waves were typically 100-400 km and 60-100 ms(-1), respectively. The propagation directions of these large-scale waves also showed a westward preference. The wave parameters appeared to have no relation with the geomagnetic activities represented by the K-p index. On the other hand, the propagation directions of the observed waves were consistent with the prevailing eastward winds in the mesosphere and the updraft associated with a low-pressure area that generally appears to the east of Resolute Bay at tropospheric altitudes. These results suggest that the main source of the gravity waves in the polar cap mesosphere is tropospheric dynamics rather than auroral activities.