Observations of low-latitude traveling ionospheric disturbances by a 630-nm airglow imager and the CHAMP satellite over Indonesia

We report the first comparison of ground and satellite measurements of low-latitude traveling ionospheric disturbances (TIDs). Three TID events were simultaneously observed by a 630-nm airglow imager and the CHAMP satellite on April 30, 2006 (event 1), September 28, 2006 (event 2), and April 12, 2004 (event 3) at Kototabang, Indonesia (geographic coordinates: 0.2$^\circ$S, 100.3$^\circ$E, geomagnetic latitude: 10.6$^\circ$S). In 630-nm airglow images of all three events, there are clear southward-moving structures. Events 1 and 2 are a single pulse with horizontal scales of $\sim$500--1000 km.  Event 3 shows five wave fronts with a horizontal scale size of 500--1000 km. All three TIDs are medium-scale TIDs. Horizontal wavelengths of both airglow intensity at an average emission altitude of 250 km and CHAMP neutral density variations measured at 400 km are estimated by fitting a sinusoidal function to the observed data. For events 1 and 3, estimated horizontal wavelengths are nearly equal, indicating both instruments are observing the same wave.   For event 1, the CHAMP electron density mapped along the geomagnetic field line onto the airglow altitude does not show wave structure similar
to the airglow variation. For events 2 and 3, the relationship of electron density and airglow intensity is unclear. These results suggest that the cause of the observed TID is not caused by ionospheric plasma instability but by gravity waves in the thermosphere. According to these results, we conclude that the observed TIDs in all three cases are caused by gravity waves in the thermosphere.