We studied the correlations between the migrating and non-migrating tides and solar cycle in the mesosphere and lower thermosphere (MLT) regions between 60° S and 60° N, which are in LAT-LON Earth coordinates, by analyzing the simulation datasets from the thermosphere and ionosphere extension of the Whole Atmosphere Community Climate Model (WACCM-X). A least squares fitting method was utilized to obtain the daily mean migrating tides and non-migrating tides. The Pearson linear correlation coefficient was used to analyze the correlations between tides and solar activity. Our analysis shows that the negative correlations between tides and solar activity are mostly impacted by the first symmetrical structure of the tidal modes for both migrating and non-migrating components. The coefficient of molecular thermal conductivity for the first symmetrical structure is small at low solar flux, so the tides dissipate more slowly when the F10.7 cm radio flux level is low. Thus, the amplitudes of tidal variations under a solar minimum condition are larger than those under a solar maximum condition. The correlation between tides and solar activity could also be influenced by some other factors, such as geomagnetic activity and the density of carbon dioxide  CO2 on Earth. The tidal variations can be influenced by westward background wind, which grows stronger as geomagnetic activity rises. Further, dissipation of the tides decreases because the heat conduction and molecular viscosity are weakened in the cooling thermosphere caused by increasing CO2, which results in larger tidal amplitudes under the solar maximum condition. It is found that the correlations between tides and solar cycle vary at different altitudes and latitudes. The negative correlations are most possibly influenced by the first symmetrical structure of tidal variations and may also be impacted by geomagnetic activity. The positive correlations are impacted by the density of CO2.

We studied the responses of the mesosphere and ionosphere to the quasi‐biennial oscillation (QBO) in the stratosphere using meteor radar zonal wind observations over monitoring station Kototabang (KB, 0.2°S, 100.3°E), global zonal wind data from the thermosphere and ionosphere extension of the Whole Atmosphere Community Climate Model (WACCM‐X), equatorial electrojet (EEJ) data over the monitoring station Jicamarca (12°S, 77°W) and global total electron content (TEC) maps. The Fourier fitting method was applied to obtain the monthly mean amplitudes of zonal wind, TEC and EEJ. The least squares fitting method was utilized to perform a spectral analysis. Our data indicate that the QBOs of the stratosphere and mesosphere are present in the zonal winds between 20 and 40 km and 82 and 96 km at KB. The QBO is also present in the global TEC at 40°S to 40°N latitude and is characterized by the equatorial ionization anomaly (EIA). We propose that the QBOs in the mesosphere are linked to the QBOs in the ionosphere through the modulation of the E region dynamo by neutral winds and the equatorial ionospheric fountain effect. This model is verified by the QBO signals in the EEJ and diurnal tides in the E region of the ionosphere. The absence of QBO signals in the zonal wind at 250 km confirms that QBO signals in the TEC are not modulated by the zonal wind in the F region ionosphere, which further substantiates the model.

We studied the characteristics of quasi-two-day wave (QTDW) using the meridional wind in the mesosphere and lower thermosphere (MLT) obtained from a meteor radar over Kototabang (KB, 0.2S, 100.3E) from 2003 to 2012. Atmospheric oscillations have a crucial impact on atmospheric dynamics, which contributes to more accurate space weather forecasting, thus providing a more secure space environment for human space exploration activities such as remote sensing and satellite navigation. QTDWs are typical atmospheric oscillations in the upper stratosphere, mesosphere and lower thermosphere. The occurrence time, amplitudes, periods and vertical wavelengths of QTDW events are analyzed statistically. Data obtained from the TIMED Doppler Interferometer (TIDI), which can measure wind and temperature and is onboard the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite, are used to analyze the global distribution and spatial structure of QTDWs with different zonal wavenumbers. The characteristics of the QTDWs over KB are compared with the QTDWs at the middle latitudes using the meridional wind data from a meteor radar over Wuhan (114.4E, 30.6N), Beijing (116.5E, 39.9N) and Mohe (121.1E, 50.1N). The amplitudes of the QTDW and spectral analysis are calculated by the least squares fitting method. Our results demonstrate that QTDWs are present almost all year around over KB. The occurrence time, amplitudes, periods and vertical wavelengths of QTDW events with different zonal wavenumbers are determined in this study. We also find that the statistical characteristics of the QTDWs in KB are different from those at middle latitudes. The westward zonal wavenumber −4 (W4) events gradually increase with increasing latitude, whereas westward zonal wavenumbers −1, −2, and −3 (W1, W2 and W3, respectively) events all decrease with increasing latitude.

Based on the Modern‐Era Retrospective analysis for Research and Applications version 2 reanalysis and Aura Microwave Limb Sounder geopotential height observations, the anomalously strong secondary planetary waves (PWs) with westward zonal wavenumber 1 and periods of 10–16 days are unexpectedly captured in the Southern (summer) Hemisphere (SH) during the Arctic sudden stratospheric final warmings (SSFWs) in 2015 and 2022, while no distinct secondary PW signal occurs in the SH during the 2005, 2014, and 2016 Arctic SSFWs. The Eliassen‐Palm (EP) flux diagnostics show that the secondary PWs during the 2015 and 2022 SSFWs have strong trans‐equatorial components at ∼35–60 km, but the southward EP flux from the Northern Hemisphere during the other three SSFWs decays dramatically around the equator. Further diagnostic results on the background conditions indicate that the phase of the quasi‐biennial oscillation (QBO) in the middle stratosphere plays a crucial role in the SH response to the secondary PW during SSFW. The strong equatorial westward winds at ∼32–42 km result in the critical layers and negative waveguides for the secondary W1 PW during the 2005, 2014, and 2016 SSFWs, which significantly block the trans‐equatorial propagation of PW. Such PW suppression is not observed during the 2015 and 2022 SSFWs since the middle stratospheric QBO is in the eastward phase. Moreover, the strong secondary PW in the SH during the 2015 and 2022 SSFWs can additionally induce the poleward residual circulation to facilitate the mesospheric total residual circulation transition from summer pattern to winter pattern in the SH.