Small-scale gravity waves near the mesopause observed by four all-sky airglow imagers

Nocturnal airglow images obtained at the Shigaraki middle and upper atmosph ere (MU) observatory (34.9 degreesN, 136.1 degreesE), Japan, were analyzed to study the three-dimensional structure of small-scale gravity waves in th e mesopause region. Airglow images from the near-infrared OH (layer height of similar to 86 km) and O-2(0,1) (similar to 94 kin) bands and the visible O I (similar to 96 km) and Na (similar to 90 kin) lines were obtained simu ltaneously by using four all-sky cooled charge-coupled device imagers with a high time resolution of 0.5-2.0 min. A clear wave packet (horizontal wave length of 60-70 km) propagating downward was observed for 2000-2030 LT on J anuary 28, 1998. The downward motion of the wave packet is identified from a comparison of the edge location of the packet at the four airglow layers. The observed waves became faint first in 0 1 (the highest altitude) and la st in Off (the lower altitude), suggesting also that the wave packet passed through each of the airglow layers downward. From the simultaneous wind ob servation by the MU radar, we conclude that the observed downward propagati ng wave packet was generated by wave reflection at higher altitudes in the background wind field that was opposite to the horizontal k vector of the w aves. For the same event, we estimate the vertical wavelength of the waves to be similar to5 kin or 20-30 km by comparing the. wave phases observed in the four airglow layers. The vertical wavelengths estimated from the dispe rsion relation are consistent with the latter value.