Thermospheric gravity wave modes over low and equatorial latitudes during daytime

[1] The small-scale intensity fluctuations obtained from 56 days of observations of OI 630.0 nm dayglow emission intensities from Mount Abu (24.6degreesN, 73.7degreesE, dip latitude 19.09degreesN), a station under the crest of equatorial ionization anomaly (EIA), have been subjected to harmonic analysis to unravel different modes of atmospheric gravity waves (AGW). Similar analysis is carried out for 26 days of dayglow intensity data recorded from Waltair (17.7degreesN, 83.3degreesE, dip latitude 10.09degreesN), a low-latitude station, and for 8 days of available data from Thumba (8.68degreesN, 77.0degreesE, dip latitude 0.45degreesN), an equatorial station with the same objective. The analysis reveals the relative dominance ("preferred mode'') of 0.5 - 1.0 hour periodic component over Mount Abu, 2.0 - 2.5 hour periodic component over Waltair, and 1.0 - 1.5 hour component over Thumba during geomagnetically quiet periods. Qualitative arguments have been propounded to explain the differences in the preferred modes over these places, and it is suggested that the source responsible for the preferred modes may lie in the lower atmosphere. Further, the analysis also indicates the presence of additional preferred modes over low latitudes during geomagnetically disturbed periods.