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Understanding the morphology and dynamics of the thermosphere is key to understanding the Earth's upper atmosphere as a whole. Thermospheric winds play an important role in this process by transporting momentum and energy and affecting the composition, dynamics and morphology of not only the thermosphere but also of the ionosphere. The general morphology of the winds has been well established over the past decades, but we are only starting to understand its variability. In this process the lower atmosphere plays an important role due to direct penetration of waves from the lower atmosphere into the ionosphere/thermosphere, secondary waves generated on the way, or internal feedback mechanisms in the coupled ionosphere-thermosphere system. Therefore, knowledge about thermospheric variability and its causes is critical for an improved understanding of the global ionosphere-thermosphere system and its coupling to the lower atmosphere. We have used low-to mid-latitude zonal wind observations obtained by the Gravity Field and Steady-State Ocean Explorer (GOCE) satellite near 260 km altitude during geomagnetically quiet times to investigate the interannual and spatial zonal wind variability near dawn and dusk, during December solstice. The temporal and spatial variability is presented as a variation about the zonal mean values and decomposed into its underlying wavenumbers using a Fourier analysis. The obtained wave features are compared between different years and clear interannual changes are observed in the individual wave components, which appear to align with changes in the solar flux but do not correlate with variations in either El Nino Southern Oscillation or the Quasi Biennial Oscillation. The obtained wave features are compared and contrasted with results from the Climatological Tidal Model of the Thermosphere (CTMT) and revealed a very good agreement between CTMT and the 2009 and 2010 December GOCE zonal wind perturbations at dawn. However, during dusk, the CTMT zonal wind perturbations and in particular the zonal wave-1 component show significant differences with those observed by GOCE. [ABSTRACT FROM AUTHOR]

Thermosphere cooling is a known effect of increasing CO2 in the atmosphere. In this study, we explore the changes of thermosphere circulation and tides in the cooled thermosphere via a doubled CO2 numerical experiment using the Ground‐to‐topside Atmosphere Ionosphere model for Aeronomy (GAIA). The results reveal three major features. (1) The thermosphere cools about 10 K more around solstices than equinoxes, more at the summer pole than the winter pole. (2) The meridional circulation shifts downward and strongly accelerates by 5–15 m s−1. (3) The tidal activity experiences dramatic changes, with a 40–60% reduction in the semidiurnal tides (SW2) throughout the thermosphere but an 30–50% enhancement in diurnal tides (DW1) below 200 km altitude. The nonmigrating tide DE3 has only minor changes. These changes in temperature, meridional circulation, and tides are persistent features in all seasons and can profoundly affect the spatial distribution and diurnal cycles of the ionospheric responses to CO2 doubling via atmosphere composition and electrodynamics. Key Points: Thermosphere cooling is stronger around solstice than equinoxesThe meridional circulation is accelerated by 5–15 m/sSemidiurnal tides are strongly reduced [ABSTRACT FROM AUTHOR]

The seeding mechanism of equatorial plasma bubbles (EPBs) in the ionosphere has not been fully understood for several decades. Before investigating the complex seeding process by atmospheric gravity waves, which have been considered as a possible candidate, we investigate the vertical neutral wind effect on the EPB seeding by making the numerical simulation setup as simple as possible. It is presented that the vertical wind over the dip equator with an amplitude of as low as 5 m/s can seed EPBs effectively. The upward wind reduces the eastward current and produces the eastward polarization electric field, so that the bottomside F region pushed by the upward wind is going to form upwellings and penetrate into the topside F region accelerated by the Rayleigh‐Taylor instability. It is suggested that the vertical wind perturbations driven by gravity waves should play an important role in seeding EPBs. Key Points: Plasma bubble can be seeded by small amplitude of vertical neutral windPlasma bubble evolves after short interaction periodAtmospheric gravity waves can be a possible seeding source [ABSTRACT FROM AUTHOR]

The effects of decreasing the intrinsic magnetic field on the upper atmospheric dynamics at low to middle latitudes are investigated using the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). GAIA incorporates a meteorological reanalysis data set at low altitudes (<30 km), which enables us to investigate the atmospheric response to various waves under dynamic and chemical interactions with the ionosphere. In this simulation experiment, we reduced the magnetic field strength to as low as 10% of the current value. The averaged neutral velocity, density, and temperature at low to middle latitudes at 300 km altitude show little change with the magnetic field variation, while the dynamo field, current density, and the ionospheric conductivities are modified significantly. The wind velocity and tidal wave amplitude in the thermosphere remain large owing to the small constraint on plasma motion for a small field. On the other hand, the superrotation feature at the dip equator is weakened by 20% for a 10% magnetic field because the increase in ion drag for the small magnetic field prevents the superrotation. [ABSTRACT FROM AUTHOR]

Abstract: The generation mechanism for the 4-peak longitudinal structure of the neutral density in the upper thermosphere is examined using an atmosphere–ionosphere coupled model. Our result indicates that the wave-4 structure of the neutral density in the upper thermosphere is caused by the upward propagation of the eastward diurnal tide with zonal wavenumber 3 (DE3) and the eastward semidiurnal tide with zonal wavenumber 2 (SE2) from the troposphere. The wave-4 structure of the neutral density in the equatorial region is mainly generated by the DE3, while the SE2 is important for the generation of the wave-4 structure in middle latitudes. Our simulation demonstrates that the wave-4 structure is evident at the height range from 150km to 500km. Furthermore, we examine the day-to-day variation of the wave-4 amplitude of the neutral density at 400km height and its relation with the SE2 and DE3 amplitudes at various heights. [Copyright &y& Elsevier]

The relationship between day-to-day variability of equatorial plasma bubbles (EPBs) and the neutral atmosphere is studied. This study is based on the previous study in which the GPS scintillation index and the tropospheric cloud-top temperature are used as proxies for EPB activity and atmospheric perturbations, respectively, and a correlation was found between their day-to-day variations. In this paper, we maintained the same GPS scintillation data but substituted the atmospheric data via an assimilation run of the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). Cross-correlation between the EPB activity and the atmospheric temperature is similar to the results in Ogawa et al. (Earth Planets Space 61:397-410, 2009). The new findings from our study include (1) an enhanced correlation between the EPB activity and the neutral atmosphere is found in horizontally and vertically large areas, (2) the longitudinal disturbance of atmospheric temperature and wind velocity during the EPB-active days is enhanced, and (3) the enhancement of atmospheric disturbance during the EPB-active days shows a similarity to the characteristics of large-scale wave structures in the ionosphere. These results more clearly support couplings between EPBs and the neutral atmosphere. [ABSTRACT FROM AUTHOR]

The linear growth rates of the Rayleigh-Taylor (R-T) instability in the ionosphere from 2011 to 2013 were obtained with a whole atmosphere-ionosphere coupled model GAIA (ground-to-topside model of atmosphere and ionosphere for aeronomy). The effects of thermospheric dynamics driven by atmospheric waves propagating from below on the R-T growth rate are included in the model by incorporating meteorological reanalysis data in the region below 30 km altitude. The daily maximum R-T growth rates for these periods are compared with the observed occurrence days of the equatorial plasma bubble (EPB) determined by the Equatorial Atmosphere Radar (EAR) and Global Positioning System (GPS) in West Sumatra, Indonesia. We found that a high R-T growth rate tends to correspond to the actual EPB occurrence, suggesting the possibility of predicting EPB occurrences with numerical models. [ABSTRACT FROM AUTHOR]

Using an atmosphere-ionosphere coupled model, the excitation source and temporal (seasonal and interannual) variations in non-migrating tides are investigated in this study. We first focus our attention on temporal variations in eastward moving diurnal tide with zonal wavenumber 3 (DE3), which is the largest of all the non-migrating tides in the mesosphere and lower thermosphere (MLT). Our simulation results indicate that upward propagation of the DE3 excited in the troposphere is sensitive to the zonal mean zonal wind in the stratosphere and mesosphere. The DE3 amplitude is enhanced in the region where the vertical shear of the zonal mean zonal wind is positive (westerly shear). Quasi-2-year variation in the DE3 amplitude in the MLT region is generated by quasi-2-year variation in the zonal mean zonal wind between 40 and 70 km, which is modulated by the stratospheric QBO. The excitation mechanisms of SW3 (westward moving semidiurnal tide with zonal wavenumber 3) and SW1 (westward moving semidiurnal tide with zonal wavenumber 1) are also investigated. During equinoxes, the SW3 and SW1 are excited by tropospheric heating (latent heat release and solar radiative heating) associated with cumulus convection in the tropics, and propagate upward into the MLT region. On the other hand, during solstices, SW3 and SW1 are generated in the winter stratosphere and mesosphere through the nonlinear interaction between the stationary planetary wave and migrating semidiurnal tide, and propagate upward to the lower thermosphere. The excitation sources of other non-migrating tides are also discussed. [ABSTRACT FROM AUTHOR]