Abstract
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.
Highlights
The first report about the Na D line being present in the night sky spectrum was by Slipher et al [1].From on, large studies of the sodium layer have been conducted using airglow imager and lidar data
We report the extensive analyses on over 11,600 h of broadband Na lidar observations made near Beijing to look for thermospheric Na layers above 105 km
We found that three hours before this low thermospheric sodium layer (LTSL), an extremely low temperature was observed by SABER
Summary
The first report about the Na D line being present in the night sky spectrum was by Slipher et al [1]. Researchers found that the density of the sodium atoms can increase and decrease suddenly over a narrow height range This phenomenon was called the sporadic sodium layer (SSL). Compared the peak densities, peak altitudes, FWHMs, and peak times of the “thermospheric enhanced sodium layers” observed at Beijing (40.2◦ N, 116.2◦ E), Hefei (31.8◦ N, 117.3◦ E), Wuhan Gao et al reported “thermospheric sodium layers at up to 170 km” in March, April, and December 2012 over Lijiang (26.7◦ N, 100.0◦ E), China [24]. Yuan et al suggested that at a tidal wind shear node, increased atmospheric pressure and density components contribute to the formation of Es and “high-altitude sporadic sodium layers” [30].
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