On five of eight observation nights during the 1996 SEEK (Sporadic E Experiment over Kyushu) campaign, Japan, unusual wave-like structures were imaged in the 630 nm thermospheric nightglow emission. Four of these events were observed to travel towards the southwest, providing new evidence in support of recent theories describing the coupling of medium-scale gravity waves into the mid-latitude F region ionosphere. Available ionosonde data and the visual characteristics of the wave structures indicate no association with the occurrence of mid-latitude spread F or F region upwellings. Instead, the data support the novel concept of feedback from the ionosphere into the gravity wave, via the Perkins instability, to enhance its visibility. ¿ 1998 American Geophysical Union