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Abstract The plasma and neutral density variations, interactions and coupling processes within ±30° latitudes are examined concurrently by the DMSP‐F18 and Swarm‐C satellite during geomagnetically quiet years in 2020–2021. The wavenumber (WN) patterns are computed in the form of neutral and electron density for two altitudes and their latitudinal profiles are analyzed. We observe that the WN1 structure of the electron density has a significant seasonal dependence in the topside ionosphere and dominates all other structures but WN2 neutral density amplitude dominates all other structures in the middle thermosphere (∼440 km). Additionally, we analyze vertical‐temporal‐latitudinal tidal structures from the Climatological Tidal Model of the Thermosphere (CTMT) to find evidence for the modulation of the large‐scale waves (LSWs) neutral density structures. Through the examination of the in situ observational and modeling approaches, we show that the tidal contributors of WN structures obtained from CTMT can capture the influence of terrestrial sources on the WN structures of plasma‐neutral density and imprint the corresponding vertical coupling in the IT system. Correlation analysis reveals that the amplitudes of the WN1 and WN3 structures of electron density in topside ionosphere and those of neutral density in the middle thermosphere show intermittent but significant correlations with each other, unlike the WN2 and WN4 structures. This study provides new insights into the topside ionospheric response to wave driving in the lower atmosphere, which ultimately improves our capability to understand the interaction and vertical coupling of large‐scale structures, thereby advancing our predictive capabilities of space weather critical for satellite operations. 

Abstract We provide observational evidence that the stability of the stratospheric Polar vortex (PV) is a significant driver of sub‐seasonal variability in the thermosphere during geomagnetically quiet times when the PV is anomalously strong or weak. We find strong positive correlations between the Northern Annular Mode (NAM) index and subseasonal (10–90 days) Global Observations of the Limb and Disk (GOLD) O/N₂perturbations at low to mid‐northern latitudes, with a largest value of +0.55 at ∼30.0°N when anomalously strong or weak (NAM >2.5 or < −2.1) vortex times are considered. Strong agreement for O/N₂variability and O/N₂‐NAM correlations is found between GOLD observations and the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) simulations, which is then used to delineate the global distribution of O/N₂‐NAM correlations. We find negative correlations between subseasonal variability in WACCM‐X O/N₂and NAM at high northern and southern latitudes (as large as −0.54 at ∼60.0°S during anomalous vortex times). These correlations suggest that PV driven upwelling at low latitudes is accompanied by corresponding downwelling at high latitudes in the lower thermosphere (∼80–120 km), which is confirmed using calculations of residual mean meridional circulation from WACCM‐X. 

Abstract A statistical study of 18 years of diurnal temperature tides observed by the SABER instrument on board the TIMED satellite reveals a substantial response of the tides in the upper atmosphere (>60 km) to the Madden‐Julian Oscillation (MJO) in the tropical troposphere. Nonmigrating tidal amplitudes are modulated at the intraseasonal MJO periods up to ~25% relative to the seasonal mean, twice as much as for the migrating tides (~10%). We fully characterize the tidal response for active MJO days as a function of season and MJO location as prescribed by the MJO index. The MJO modulation of the tides was predicted by models but could not be unequivocally observed before. Our results further point to an important role of background winds that partly cause a different response for equatorial and nonequatorial tidal modes in different seasons, which has implications for the MJO imprint on the ionospheric dynamo region. 

Abstract The Madden‐Julian Oscillation (MJO), an eastward‐moving disturbance near the equator (±30°) that typically recurs every ∼30–90 days in tropical winds and clouds, is the dominant mode of intraseasonal variability in tropical convection and circulation and has been extensively studied due to its importance for medium‐range weather forecasting. A previous statistical diagnostic of SABER/TIMED observations and the MJO index showed that the migrating diurnal (DW1) and the important nonmigrating diurnal (DE3) tide modulates on MJO‐timescale in the mesosphere/lower thermosphere (MLT) by about 20%–30%, depending on the MJO phase. In this study, we address the physics of the underlying coupling mechanisms using SABER, MERRA‐2 reanalysis, and SD‐WACCMX. Our emphasis was on the 2008–2010 time period when several strong MJO events occurred. SD‐WACCMX and SABER tides show characteristically similar MJO‐signal in the MLT region. The tides largely respond to the MJO in the tropospheric tidal forcing and less so to the MJO in tropospheric/stratospheric background winds. We further quantify the MJO response in the MLT region in the SD‐WACCMX zonal and meridional momentum forcing by separating the relative contributions of classical (Coriolis force and pressure gradient) and nonclassical forcing (advection and gravity wave drag [GWD]) which transport the MJO‐signal into the upper atmosphere. Interestingly, the tidal MJO‐response is larger in summer due to larger momentum forcing in the MLT region despite the MJO being most active in winter. We find that tidal advection and GWD forcing in MLT can work together or against each other depending on their phase relationship to the MJO‐phases. 

Abstract Analyzing Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations from 2003 to 2018, the interannual variability of 2–5d eastward propagating planetary waves is found to correlate positively with zonal‐mean zonal winds averaged over 67.5°±10°S but negatively with the quasi‐biennial oscillation (QBO) index in austral winter. The composite‐mean wave amplitudes are ~20% larger in QBOe than in QBOw. On statistical average, the poleward flank strengthening and the equatorward flank weakening of polar night jet (PNJ) during QBOe form a dipole‐cell pattern. In contrast, only a single negative cell is seen in the Northern Hemisphere zonal‐mean zonal winds (January) previously explained by the Holton‐Tan theory. Such difference implies an interhemispheric asymmetry and other processes needed to explain the additional positive cell in Antarctica. Mechanistic modeling illustrates that the stronger PNJ generates eastward propagating planetary waves with larger growth rates (stronger waves) in QBOe than QBOw, explaining the QBO‐like signal in the Antarctic planetary waves.