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 (26.9 • 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.

We describe performance of a new two-channel Fabry-Perot interferometer (FPI) with two thermoelectric-cooled CCD detectors to measure neutral winds for both the 558-nm (mesopause region) and 630-nm (thermosphere) airglow emissions simultaneously at Shigaraki (34.8 • N, 136.1 • E), Japan. The employment of the thermoelectric cooling system enables us continuous automatic operation of the FPI since October 2000. The random errors of wind measurement are estimated to be ∼5-8 m/s and ∼10-50 m/s for 558 nm and 630 nm, respectively. The wind velocities obtained by the FPI (558 and 630 nm) fit well with those obtained by the collocated Middle and Upper (MU) atmosphere radar.

[1] Using a comprehensive data set from optical and radio instruments, we investigate a midnight brightness wave that appeared in 630-nm airglow images over Japan on the night of 9 September 1999. This may be the first such observation of the brightness wave with an all-sky imager in the East-Asian longitudinal sector. The imager at Shigaraki (35.6°N, 136.1°E) tracked a north-northeastward propagation of the wave with an apparent velocity of 500 m s À1 after midnight. Ionosonde observations at five stations in Japan showed that rapid descent of the F 2 layer propagated northward beyond 35°N with decreasing amplitude. Incoherent scatter observations with the MU radar at Shigaraki also revealed that the F 2 peak altitude decreased from 360 to 280 km during the event. During the F 2 layer descent the altitude profile of the electron density became sharp, enhancing the F 2 peak electron density. After the F 2 layer altitude reached 280 km, electron density in the F 2 layer rapidly decreased because of increased neutral density at low altitude. A FabryPerot interferometer (FPI) at Shigaraki observed northward neutral winds of 10-70 m s À1 during the event. A model calculation demonstrates that the meridional winds estimated from the MU radar electron density profiles are fairly consistent with those observed with the FPI. From these results we conclude that the observed northward wind enhancements, probably caused by the midnight temperature maximum, pushed down the plasma in the F 2 layer to lower altitudes along the geomagnetic field to cause the 630-nm airglow intensity enhancement.

We have developed a three-channel imaging Fabry-Perot interferometer with which to measure atmospheric wind and temperature in the mesosphere and thermosphere through nocturnal airglow emissions. The interferometer measures two-dimensional wind and temperature for wavelengths of 630.0 nm (OI, altitude, 200-300 km), 557.7 nm (OI, 96 km), and 839.9 nm (OH, 86 km) simultaneously with a time resolution of 20 min, using three cooled CCD detectors with liquid-N(2) Dewars. Because we found that the CCD sensor moves as a result of changes in the level of liquid N(2) in the Dewars, the cooling system has been replaced by thermoelectric coolers. The fringe drift that is due to changes in temperature of the etalon is monitored with a frequency-stabilized He-Ne laser. We also describe a data-reduction scheme for calculating wind and temperature from the observed fringes. The system is fully automated and has been in operation since June 1999 at the Shigaraki Observatory (34.8N, 136.1E), Shiga, Japan.