Statistical study of short-period gravity waves in OH and OI nightglow images at two separated sites

[1] Using airglow images of the near-infrared OH band (720 - 910 nm) and OI (557.7 nm) line, we investigated seasonal, latitudinal, and local time variations of short-period gravity waves. The images were obtained at two locations in Japan that are similar to 1200 km apart, Rikubetsu (43.5 degreesN, 143.8 degreesE) and Shigaraki (34.9 degreesN, 136.1 degreesE), between October 1998 and October 1999. Our analysis has focused on small-scale gravity waves with wavelengths less than 40 km and dominant phase speeds of similar to 20 - 50 m/s. Wave occurrences for both OH and OI at Rikubetsu and Shigaraki are significantly higher than 60%, with a slightly larger value in summer. The occurrences increase from evening to midnight. There are no obvious local time dependencies in horizontal wavelength, propagation direction, and phase speed. The propagation directions in summer are either northward or northeastward at both locations. However, in winter the propagation directions at Rikubetsu are generally westward ( NW, W, and SW), whereas those at Shigaraki are only southwestward. From simultaneous wind observation by the MF radars at Wakkanai (45.4 degreesN, 141.7 degreesE) and Yamagawa (31.2 degreesN, 130.6 degreesE), we discuss possible influences of Doppler and thermal ducting, wind filtering, and source distribution of gravity waves propagating from the lower atmosphere to the airglow heights in the mesopause region.