Abstract
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.
Highlights
Gravity waves, tides and planetary waves with periods of a few minutes to days are most dominant dynamical entities responsible for modification of the ambient dynamics of the mesosphere and lower thermosphere (MLT)
It is interesting to note that all the plots show clear four-crest and fourtrough structure over 24 h span indicating 6-h oscillation to be very regular and most dominant wave feature among the waves with periods less than 7 h in the MLT and it confirms the existence of the quarterdiurnal tide (QDT) at the present sites
Summary and conclusions The features of the QDT are investigated from three lowlatitude Southern hemispheric stations using meteor wind observations
Summary
Tides and planetary waves with periods of a few minutes to days are most dominant dynamical entities responsible for modification of the ambient dynamics of the mesosphere and lower thermosphere (MLT). The tidal periods are sub-harmonic of the solar day, i.e., 24, 12, 8, 6 h and so on, showing strongest amplitude for the diurnal (24 h) and semidiurnal (12 h) components. Among these major tidal components, the quarterdiurnal tide (6 h) (QDT) is least explored by observational and model studies due to its weak manifestation of significantly smaller amplitude as compared to other three dominant harmonics. It should be mentioned that the QDT could be a major contributor to the MLT dynamics at times depending on ambient dynamical conditions as pointed out by observational and modeling investigations (Smith et al 2004; Xu et al 2014; Jacobi et al 2017)
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