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Equipped with a 1-meter Cassegrain telescope with 6.2 meter focal length and an electronically gated Intensified Charge-Coupled Device (ICCD), a multilayer Na imager is designed and developed at Wuhan in China. This novel instrument has successfully achieved the first preliminary 3-D image of the mesospheric Sodium (Na) layer when running alongside a Na lidar. The vertical Na layer profile is measured by the lidar, while the horizontal structure of the layer at different altitudes is measured by the ICCD imaging with a horizontal resolution of ~3.7 urad. In this experiment, controlled by the delay and width of the ICCD gating signal, the images of the layer are taken with three-second temporal resolution for every 5 km. The results show highly variable structures in both the vertical and horizontal directions within the Na layer. Horizontal images of the Na layer at different altitudes near both the permanent layer (80–100 km) and a sporadic Na layer at 117.5 km are obtained simultaneously for the first time. The Na number density profiles measured by the lidar and those derived from this imaging technique show excellent agreement, demonstrating the success of this observational technique and the first 3-D imaging of the mesospheric Na layer. [ABSTRACT FROM AUTHOR]

The low thermospheric sodium layer (LTSL) is the separate sodium atom layer above 105 km. Based on 11,607 h of lidar observations from Yanqing (40.5° N, 116.0° E) from 2010 to 2016, we found 38 LTSLs wherein the peak densities were more than five percent above those of the main sodium layers. This work presents the peak altitudes, peak local times and peak densities of the LTSLs as well as the long-term characteristics of the seasonal and inter-annual variations of LTSLs. We analyzed the correlation between the LTSL and sporadic E layer (Es). The seasonal variation trends of the occurrences of LTSL and Es are similar, and the results showed that 95% of the LTSLs were accompanied by Es. We also found that 69% of the LTSL cases exhibited apparent downward phase progressions, while the descending rates of the LTSLs are consistent with the phase speeds of the tide. [ABSTRACT FROM AUTHOR]

Based on application of the atomic filter technology in a signal detection system of lidar, the diurnal observation of sodium lidar were obtained using the system at the National Space Science Center of the Chinese Academy of Sciences at Beijing Yanqing station (40.5° N, 116° E) in April 2014. During the lidar observation period, among the 103 cases of continuous daytime observations, the longest time was 181 h. In the case of a continuous observation period of 5 days (13–18 October 2014), the signal-to-noise ratio reached to 19:1 at 12:00–13:00 Local Time of the daytime, when the spatial and time resolutions were respectively set to 96 m of 167 s. The improvements resulted in the highest detection level of any existing diurnal lidars in China. Some interesting phenomena such as the sporadic sodium layer have also been observed during the daytime. The daytime capability extended the observing time range of the earlier systems that were limited to only nighttime observations. This innovation provides a useful method for the studies of diurnal tides, photochemistry, gravity waves, and correlative modeling studies. [ABSTRACT FROM AUTHOR]

Sporadic sodium layers (SSLs) have been observed all over the globe and always has been of great concern. However, most researches on SSLs are based on only nocturnal observations due to strong daytime solar background noise, thus there is still insufficient understanding of the diurnal variation of SSLs. In this paper, a statistical analysis of SSLs observed by a diurnal sodium (Na) lidar at Beijing (40.5 ° N, 116 ° E) from April 2014 to December 2016 has been reported. A total of 54 and 87 events that peaked during daytime and nighttime, respectively, are identified. The average occurrence interval for daytime observations is ~36 h per SSL, which is slightly longer than the value of ~29 h per SSL for nighttime observations. Both daytime and nighttime SSLs show evident seasonal variations with summer SSLs occurring much more frequently and tending to appear at higher altitude with lower peak density while larger strength factor. The seasonal dependence of SSL occurrence is consistent with that of sporadic E (Es) occurrence, however, their diurnal variations are found to be totally different. The SSLs are observed more frequently around 03–04 LT during nighttime and 16–17 LT during daytime when Es layers appear at lower altitudes (below 110 km), while have a lowest occurrence rate around midday when Es layers occurrence rate peak. The average peak altitude of SSLs during the second half of the night is lower compared to that during the first half of the night. Around midday, the average peak width of SSLs is narrower and peak density tends to be smaller when compared to those at other times. The possible explanations for the diurnal and seasonal variations of SSLs characteristics are discussed, and the key role of the evolution of Es layer height modulated by semidiurnal tide in the local time variation of SSL is highlighted. • The occurrence rate of daytime SSLs has a similar seasonal dependence to those of nighttime SSLs as well as Es. • SSLs occurred more frequently around 03–04 LT during nighttime and 16–17 LT during daytime, while rarely around midday. • Both daytime and nighttime observations SSLs tend to have higher peak altitude, larger strength factor, lower peak density in summer. • Diurnal variation of SSL occurrence is related to tidal-modulated Es height and photochemistry and ionization reactions. [ABSTRACT FROM AUTHOR]