POSSIBLE EVIDENCE OF GRAVITY-WAVE COUPLING INTO THE MIDLATITUDE F-REGION IONOSPHERE DURING THE SEEK CAMPAIGN

On five of eight observation nights during the 1996 SEEK (Sporadic E E xperiment over Kyushu) campaign, Japan, unusual ''wave-like'' structur es 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 s tructures indicate no association with the occurrence of mid-latitude spread F or F region upwellings. Instead, the data support the novel c oncept of feedback from the ionosphere into the gravity wave, via the Perkins instability, to enhance its visibility.