Study of wave signatures observed in thermospheric airglow imaging over the dip equatorial region

The observational data on OI 630 nm thermospheric nightglow from the Indian dip equatorial station, Tirunelveli (8.7°N, 77.8°E geographic, 1.6°N dip latitude) obtained between January 2013 and January 2015 using an All-Sky Airglow Imager (ASAI) are utilized in this work to examine the presence of gravity waves in the thermosphere. Two types of wave signatures were observed: (1) quasi-periodic waves consisting of alternating crests and troughs and (2) single bands of enhanced intensity (SBEI). The phase speed, wavelength and time period of the quasi-periodic waves were in the range of 70-160 m/s, 130-575 km and 25-75 min, respectively, while the phase speed and scale size of the SBEI features were found to be in the range of 150-250 m/s and 230-470 km, respectively. During equinoxes, quasi-periodic waves propagated towards north-northwest direction, while during winter they were observed to propagate towards northwest, south-southwest and southeast directions.