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

Abstract

Nocturnal airglow images obtained at the Shigaraki middle and upper atmosphere (MU) observatory (34.9°N, 136.1°E), Japan, were analyzed to study the three‐dimensional structure of small‐scale gravity waves in the mesopause region. Airglow images from the near‐infrared OH (layer height of ∼86 km) and O2(0, 1) (∼94 km) bands and the visible O I (∼96 km) and Na (∼90 km) lines were obtained simultaneously 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 wavelength of 60–70 km) propagating downward was observed for 2000–2030 LT on January 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 O I (the highest altitude) and last in OH (the lower altitude), suggesting also that the wave packet passed through each of the airglow layers downward. From the simultaneous wind observation by the MU radar, we conclude that the observed downward propagating 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 waves. For the same event, we estimate the vertical wavelength of the waves to be ∼5 km or 20–30 km by comparing the wave phases observed in the four airglow layers. The vertical wavelengths estimated from the dispersion relation are consistent with the latter value.