The characteristic features of the quarterdiurnal tide (QDT) in the mesosphere and lower thermosphere (MLT) are investigated using meteor radar observations from three low-latitude southern hemispheric stations, Sao Joao do Cariri (7.4°S, 36.5°W), Cachoeira Paulista (22.7°S, 45°W) and Santa Maria (29.7°S, 53.7°W) in 2005. The QDT shows consistent appearance with a weak enhancement during late summer and fall interval. The QDT amplitudes are found to be much smaller (< 3 m/s) than the longer period tidal components most of the times of the year consistent with the previous investigations carried out from various locations over the globe. The vertical wavelength is found to reach up to 80 km in the MLT maximizing in winter although most of the times it remains within 20 km. Therefore, based on the observed characteristics of the QDT, it is surmised that both the migrating and nonmigrating components are involved in manifestation of the QDT at present locations. Further detailed investigations are sought in connection with the important issues pertained to the actual excitation mechanisms and existing dominant QDT modes in the low-latitude MLT.
An observational evidence of medium‐scale traveling ionospheric disturbances (MSTIDs) reaching to magnetic latitude as low as ~3.5° over the Indian sector is provided for the first time based on OI 630‐nm airglow imaging observation from a low‐latitude station, Gadanki (13.5°N, 79.2°E; 6.6° magnetic latitude), on 12 January 2016. The horizontal wavelength, horizontal phase velocity, and period of the MSTID are found to be 160 ± 6 km, 138 ± 14 m/s, and 19.5 ± 3 min, respectively. These phase fronts are observed to move toward southwest with a propagation angle of ~235° ± 1° with respect to north. In addition to the MSTID, a strong quasiperiodic southward moving wave (QPSMW) from the evening to midnight interval and a small‐scale southward moving wave structure with wavelength and periodicity different from the QPSMW are also detected on the same night. Horizontal wavelength, horizontal phase velocity, and period of the QPSMW are estimated to be 367 ± 14 km, 131 ± 18 m/s, and 46.7 ± 13 min, respectively, and those of the small‐scale southward moving wave are found to be 157 ± 4 km, 121 ± 17.8 m/s, and 21.7 ± 3.4 min, respectively. Global Positioning System‐total electron content maps suggest that the weak and asymmetric equatorial ionization anomaly helped deep ingression of the MSTID on this night. The descent of the F layer seems to have caused the dissipation of the MSTID and QPSMW closer to the dip equator on this night. Therefore, the present investigation shows that the midlatitude MSTIDs can influence the F region plasma processes even over very low latitudes under favorable background conditions.
Abstract. Simultaneous measurements of mesospheric OH (6-2) Meinel and O 2 (0-1) Atmospheric band emissions from a low-latitude station, Maui, Hawaii (20.8 • N, 156.2 • W) are utilized to study the wave characteristics and associated processes. Deduced temperatures show large variability in both OH and O 2 data. The seasonal variability in the temperature shows a well-defined, semiannual type of oscillation, which are comparable to the ground-based rocket sounding data. The "Wave Growth Factor", a ratio of normalized perturbation amplitude in O 2 to the OH temperature variability, is estimated for principal as well as residual smaller period components of the nocturnal variability. It is noticed that smaller period waves (less than 12 h) occasionally have large growth factors of about 3-4 during equinox transitions, an indication of wave amplitude amplification within the 87-94 km altitudes while a strong wavedissipation occurs throughout the year.Keywords. Ionosphere (Equatorial ionosphere) -Atmospheric composition and structure (Airglow and aurora) -Meteorology and atmospheric dynamics (Middle atmosphere dynamics)
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