Quasiperiodic southward moving waves in 630-nm airglow images in the equatorial thermosphere

We report quasiperiodic southward moving waves, which are commonly observed in the OI 630-nm airglow images (emission altitudes of 200-300 km) near the equator, in 2-year airglow observations at Kototabang, Indonesia (0.2 degrees S, 100.3 degrees E, geomagnetic latitude of -10.4 degrees). The waves have predominantly east-west phase fronts and repeatedly propagate southward with a velocity of 310 +/- 110 m/s and a period of 40 +/- 15 min. They are frequently observed in May-July with an occurrence rate of 53% and are also observed in other seasons with occurrences of similar to 20%. The waves are observed in and to the south (geomagnetically poleward) of the equatorial ionospheric anomaly, which is identified as an airglow enhancement region moving gradually to lower geomagnetic latitudes at the premidnight local times. We suggest that gravity waves in the lower thermosphere below similar to 300 km are a plausible cause of the observed quasiperiodic waves in the airglow images.