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This paper reports the first attempt to observe the equatorward limit of medium-scale traveling ionospheric dis- turbances (TIDs) in the middle latitudes. The TIDs usually propagate southwestward in the northern hemisphere. An all-sky cooled-CCD imager measured 630-nm airglow at a southern island of Japan, Okinawa (26.9◦N, 128.3◦E, geomagnetic latitude (MLAT) = 17.0◦), during the FRONT-2 campaign of August 4-15, 1999. The TIDs were detected at the mainland of Japan (∼21◦-36◦ MLAT) by the total electron content (TEC) observations of more than 1000 GPS receivers. In the August 4 event, the TIDs moving southwestward was seen only in the northern sky of Okinawa as a depletion band in the 630-nm airglow images. In the August 6 event, the TIDs were not seen in the 630-nm images at Okinawa, although weak TID activity was observed by the GPS network at the mainland of Japan. The TEC data also showed weakening of the TID activity below 18◦ MLAT. Based on these observations, we suggest that there is a possible limit of medium-scale TID propagation around ∼18◦ MLAT.

Imaging observations of the equatorward limit of midlatitude traveling ionospheric disturbances