We report the first lidar observations of thermospheric Na layers up to 170 km at Lijiang (geomagnetic 21.6°N, 171.8°E), China, in March, April, and December 2012. The Na densities inside the layers are low, ranging from ~1 to ~6 cm−3 at altitudes of 130–170 km, about 3 orders of magnitude lower than the Na peak density in the mesopause region. All of these layers exhibit an apparent downward phase progression with a descending rate of 11–12 km/h or ~3 m/s, consistent with the vertical phase speed of semidiurnal tides around 140 km. We have identified at least 12 events from the total 37 nights of lidar observations with four shown in this report, giving an occurrence frequency of ~33% over Lijiang. These thermospheric layer events correspond to strong to moderate equatorial fountain effects, bolstering our hypothesis that the deposit of metallic ions from the equatorial region to low latitudes via the fountain effect provides the Na+ ions in the thermosphere over Lijiang. Adopting the theory by Chu et al. (2011) and the hypothesis by Tsuda et al. (2015), we further hypothesize that the thermospheric Na layers are formed through the neutralization of the tidal‐wind‐shear‐converged Na+ layers via direct electron‐Na+ recombination Na+ + e− → Na + hν. An envelope calculation using reasonable ion and electron densities shows good consistency with the observations.

The quasi 2 day wave (QTDW) observed during 2007 austral summer period is well reproduced in an reanalysis produced by the data assimilation version of the Whole Atmosphere Community Climate Model (WACCM + Data Assimilation Research Testbed) developed at National Center for Atmospheric Research (NCAR). It is found that the QTDW peaked 3 times from January to February but with different zonal wave numbers. Diagnostic analysis shows that the mean flow instabilities, refractive index, and critical layers of QTDWs are fundamental for their propagation and amplification, and thus, the temporal variations of the background wind are responsible for the different wave number structures at different times. The westward propagating wave number 2 mode (W2) grew and maximized in the first half of January, when the mean flow instabilities related to the summer easterly jet were enclosed by the critical layers of the westward propagating wave number 3 (W3) and wave number 4 (W4) modes. This prevented W3 and W4 from approaching and extracting energy from the unstable region. The W2 decayed rapidly thereafter due to the recession of critical layer and thus the lack of additional amplification by the mean flow instability. The W3 peaked in late January, when the instabilities were still encircled by the critical layer of W4. The attenuation of W3 afterward was also due to the disappearance of critical layer and thus the lack of overreflection. Finally, the W4 peaked in late February when both the instability and critical layer were appropriate.

We present a first analysis of 9 and 6.75 day periodic oscillations observed in the neutral mesospheric density in 2005 and 2006. Mesospheric densities near 90 km are derived using data from the Davis meteor radar (68.5°S, 77.9°E; magnetic latitude, 74.6°S), Antarctica. Spectral analysis indicates that the pronounced periodicities of 9 and 6.75 days observed in the mesosphere densities are associated with variations in solar wind high‐speed streams and recurrent geomagnetic activity. Neutral mesospheric winds and temperatures, simultaneously measured by the Davis meteor radar, also exhibit 9 and 6.75 day periodicities. A Morlet wavelet analysis shows that the time evolution of the 9 and 6.75 day oscillations in the neutral mesosphere densities and winds are similar to those in the solar wind and in planetary magnetic activity index, Kp in 2005 and 2006. These results demonstrate a direct coupling between Sun's corona (upper atmosphere) and the Earth's mesosphere.

We report an analysis of the neutral mesosphere density response to geomagnetic activity from January 2016 to February 2017 over Antarctica. Neutral mesospheric densities from 85 to 95 km are derived using data from the Davis meteor radar (68.5°S, 77.9°E) and the Microwave Limb Sounder on the Aura satellite. Spectral and Morlet wavelet analyses indicate that a prominent oscillation with a periodicity of 13.5 days is observed in the mesospheric density during the declining phase of solar cycle 24 and is associated with variations in solar wind high‐speed streams and recurrent geomagnetic activity. The periodic oscillation in density shows a strong anticorrelation with periodic changes in the auroral electrojet index. These results indicate that a significant decrease in neutral mesospheric density as the geomagnetic activity enhances.

In this study, mesopause temperatures over a low‐latitude station were derived by applying the temperature gradient model technique to data from a meteor radar installation located in Kunming (25.6°N, 103.8°E), China. The estimated temperatures are in good agreement with Sounding of the Atmosphere by Broadband Emission Radiometry (SABER) temperatures and exhibit clear seasonal and interannual variations with dominant spectral peaks at annual, semiannual, quasi 90 day, and terannual oscillations. However, the amplitudes of the temperature fluctuations and the dominant spectral peaks are larger than those from SABER. An improved method that accounts for the temperature sensitivity of the slope estimated from the meteor radar data was developed to calibrate the larger fluctuations obtained using the temperature gradient model technique. The resulting calibrated temperatures are more consistent with SABER observations, and the accuracy of the derived temperatures is significantly improved.