Abstract:
This study presents the quasi-two-day wave (Q2DW) characteristics of the mesosphere and lower thermosphere (MLT) region
obtained by taking hourly mean values of horizontal wind velocities for 4 years (August 2013–July 2017) through continuous measurements
using a medium-frequency (MF) radar (operating frequency – 1.98 MHz) located at the low-latitude Indian station Kolhapur
(16.8 N; 74.2 E). The MF radar located at Kolhapur was upgraded in 2013, and these results of Q2DW have been reported for the first
time after upgrading. The present study investigated variability in seasonal, annual, interannual, and solar indices of Q2DWs traveling in
zonal (EW) and meridional (NS) components in the MLT region. The Q2DW activity is observed to be stronger during austral summer
(January–February) (EW = 5 m/s and NS = 8–10 m/s) than during boreal summer (June–July) (EW = 5 m/s and NS = 6–8 m/s).
The Q2DW amplitudes are larger in the meridional component than in the zonal one. A strong semiannual oscillation (SAO) has been
observed in Q2DWs, with peak during January–February and June–July. In addition, small enhancement is seen in meridional Q2DW in
October ( 5–6 m/s). It is observed that the entire spectrum (40–60 h) measured between 86 and 94 km contributes to the SAO amplitudes
during January–February and June–July, whereas the waves measured between 42 h and 52 h contribute to enhancement in October similar
to that reported elsewhere. In general, the Q2DW amplitude shows large interannual variability. The easterlies developed in the global
circulation model in Northern hemisphere during May intensify up to around summer solstice. Q2DW activity peaks during westerly
shear zone and intensifies with time at a lower thermospheric altitude (above 90 km). Small positive correlations (r = 0.2 for sunspot
number and r = 0.1 for 10.7 cm solar flux) have been observed between Q2DW amplitudes and solar activity.
