Imaging observations of the equatorward limit of midlatitude traveling ionospheric disturbances

This paper reports the first attempt to observe the equatorward limit of medium-scale traveling ionospheric disturbances (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 (26.9degreesN, 128.3degreesE, geomagnetic latitude (MLAT) = 17.0degrees), during the FRONT-2 campaign of August 4-15, 1999. The TIDs were detected at the mainland of Japan (similar to21degrees-36degrees 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 18degrees MLAT. Based on these observations, we suggest that there is a possible limit of medium-scale TID propagation around similar to18degrees MLAT.