Horizontal winds in the mesosphere have been measured over Ascension Island (8° S, 14° W) in the tropical mid-Atlantic region throughout the years 2002–2011. The observations were made by a very high frequency (VHF) meteor radar. The observations reveal the presence of atmospheric tides of large amplitude. The observations are analysed to characterise the seasonal and interannual variability of the diurnal and semidiurnal tides. Monthly-mean diurnal tidal amplitudes are found to reach values as large as 48 m s⁻¹ in the meridional component and 41 m s⁻¹ in the zonal. A semiannual seasonal variation is found in diurnal tidal amplitudes with amplitude maxima at the equinoxes and amplitude minima at the solstices. Diurnal tidal meridional vertical wavelengths are generally in the range 24–30 km. The diurnal zonal vertical wavelengths are similar to the meridional, except for the winter months when the zonal vertical wavelengths are much longer, occasionally exceeding 100 km. Semidiurnal amplitudes are observed to be significantly smaller than diurnal amplitudes. Semidiurnal vertical wavelengths range from 20 to more than 100 km. Our observations of tidal amplitudes and phases are compared with the predictions of the extended Canadian Middle Atmosphere Model (eCMAM) and the Whole Atmosphere Community Climate Model (WACCM). Both eCMAM and WACCM reproduce the trend for greater diurnal amplitudes in the meridional component than the zonal. However, eCMAM tends to overestimate meridional amplitudes, while WACCM underestimates both zonal and meridional amplitudes. Vertical wavelength predictions are generally good for both models; however, eCMAM predicts shorter diurnal zonal vertical wavelengths than are observed in winter, while WACCM predicts longer zonal vertical wavelengths than observed for the semidiurnal tide for most months. Semidiurnal amplitude predictions are generally good for both models. It is found that larger-than-average diurnal and semidiurnal tidal amplitudes occur when the stratospheric quasi-biennial oscillation (QBO) at 10 hPa is eastwards, and smaller-than-average amplitudes occur when it is westwards. Correlations between the amplitude perturbations and the El Niño Southern Oscillation are also found. The precise mechanism for these correlations remains unclear.

Comparisons between tidal wind signatures diagnosed from satellite and ground‐based observations and a general circulations model for two (September–October 2005, March–April 2007) of the four Climate and Weather of the Sun‐Earth System (CAWSES) Global Tidal Campaign observation periods are presented (CAWSES is an international program sponsored by Scientific Committee on Solar‐Terrestrial Physics). Specific comparisons are made between model (extended Canadian Middle Atmosphere Model), satellite (Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED)), meteor, MF and incoherent scatter radar (ISR), and lidar tidal signatures in the mesosphere and lower thermosphere. The satellite and ground‐based signatures are in good agreement and demonstrate for the first time that the tidal wind fields observed by both types of observations are consistent with each other. This is the first time that such agreement has been reported and effectively resolves the long‐standing issue between ground‐based radar and satellite optical measurements of winds. This level of agreement, which has proved elusive in the past, was accomplished by superposing the significant tidal components from the satellite analyses to reconstruct the fields observed by the ground stations. Particularly striking in these comparisons is the extent to which the superposed fields show strong geographic variability. This variability is also seen in the component superpositions generated from the extended Canadian Middle Atmosphere Model (eCMAM), although differences in the geographic patterns are evident.

While knowledge of the energy inputs from the Sun (as it is the primary energy source) is important for understanding the solar-terrestrial system, of equal importance is the manner in which the terrestrial part of the system organizes itself in a quasi-equilibrium state to accommodate and re-emit this energy. The ROSMIC project (2014–2018 inclusive) was the component of SCOSTEP’s Variability of the Sun and Its Terrestrial Impact (VarSITI) program which supported research into the terrestrial component of this system. The four themes supported under ROSMIC are solar influence on climate, coupling by dynamics, trends in the mesosphere lower thermosphere, and trends and solar influence in the thermosphere. Over the course of the VarSITI program, scientific advances were made in all four themes. This included improvements in understanding (1) the transport of photochemically produced species from the thermosphere into the lower atmosphere; (2) the manner in which waves produced in the lower atmosphere propagate upward and influence the winds, dynamical variability, and transport of constituents in the mesosphere, ionosphere, and thermosphere; (3) the character of the long-term trends in the mesosphere and lower thermosphere; and (4) the trends and structural changes taking place in the thermosphere. This paper reviews the progress made in these four areas over the past 5 years and summarizes the anticipated research directions in these areas in the future. It also provides a physical context of the elements which maintain the structure of the terrestrial component of this system. The effects that changes to the atmosphere (such as those currently occurring as a result of anthropogenic influences) as well as plausible variations in solar activity may have on the solar terrestrial system need to be understood to support and guide future human activities on Earth.

The advanced E-Region Wind Interferometer (ERWIN II) combines the imaging capabilities of a CCD detector with the wide field associated with field-widened Michelson interferometry. This instrument is capable of simultaneous multi-directional wind observations for three different airglow emissions (oxygen green line (O(¹S)) at a height of ~97 km, the ^PQ(7) and ^P(7) emission lines in the O₂(0–1) atmospheric band at ~93 km and P₁(3) emission line in the (6, 2) hydroxyl Meinel band at ~87 km) on a three minute cadence. In each direction, for 45 s measurements for typical airglow volume emission rates, the instrument is capable of line-of-sight wind precisions of ~1 m s⁻¹ for hydroxyl and O(¹S) and ~4 m s⁻¹ for O₂. This precision is achieved using a new data analysis algorithm which takes advantage of the imaging capabilities of the CCD detector along with knowledge of the instrument phase variation as a function of pixel location across the detector. This instrument is currently located in Eureka, Nunavut as part of the Polar Environment Atmospheric Research Laboratory (PEARL) (80°N, 86° W). The details of the physical configuration, the data analysis algorithm, the measurement calibration and validation of the observations from December 2008 and January 2009 are described. Field measurements which demonstrate the capabilities of this instrument are presented. To our knowledge, the wind determinations with this instrument are the most accurate and have the highest observational cadence for airglow wind observations of this region of the atmosphere and match the capabilities of other wind-measuring techniques.

An international joint research project, entitled Interhemispheric Coupling Study by Observations and Modelling (ICSOM), is ongoing. In the late 2000s, an interesting form of interhemispheric coupling (IHC) was discovered: when warming occurs in the winter polar stratosphere, the upper mesosphere in the summer hemisphere also becomes warmer with a time lag of days. This IHC phenomenon is considered to be a coupling through processes in the middle atmosphere (i.e., stratosphere, mesosphere, and lower thermosphere). Several plausible mechanisms have been proposed so far, but they are still controversial. This is mainly because of the difficulty in observing and simulating gravity waves (GWs) at small scales, despite the important role they are known to play in middle atmosphere dynamics. In this project, by networking sparsely but globally distributed radars, mesospheric GWs have been simultaneously observed in seven boreal winters since 2015/16. We have succeeded in capturing five stratospheric sudden warming events and two polar vortex intensification events. This project also includes the development of a new data assimilation system to generate long‐term reanalysis data for the whole middle atmosphere, and simulations by a state‐of‐the‐art GW‐permitting general circulation model using the reanalysis data as initial values. By analyzing data from these observations, data assimilation, and model simulation, comprehensive studies to investigate the mechanism of IHC are planned. This paper provides an overview of ICSOM, but even initial results suggest that not only GWs but also large‐scale waves are important for the mechanism of the IHC.