Optical and radio measurements of a 630‐nm airglow enhancement over Japan on 9 September 1999

<jats:p>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<jats:sup>−1</jats:sup> after midnight. Ionosonde observations at five stations in Japan showed that rapid descent of the <jats:italic>F</jats:italic><jats:sub>2</jats:sub> layer propagated northward beyond 35°N with decreasing amplitude. Incoherent scatter observations with the MU radar at Shigaraki also revealed that the <jats:italic>F</jats:italic><jats:sub>2</jats:sub> peak altitude decreased from 360 to 280 km during the event. During the <jats:italic>F</jats:italic><jats:sub>2</jats:sub> layer descent the altitude profile of the electron density became sharp, enhancing the <jats:italic>F</jats:italic><jats:sub>2</jats:sub> peak electron density. After the <jats:italic>F</jats:italic><jats:sub>2</jats:sub> layer altitude reached 280 km, electron density in the <jats:italic>F</jats:italic><jats:sub>2</jats:sub> layer rapidly decreased because of increased neutral density at low altitude. A Fabry‐Perot interferometer (FPI) at Shigaraki observed northward neutral winds of 10–70 m s<jats:sup>−1</jats:sup> 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 <jats:italic>F</jats:italic><jats:sub>2</jats:sub> layer to lower altitudes along the geomagnetic field to cause the 630‐nm airglow intensity enhancement.</jats:p>