A review of latitudinal characteristics of sporadic sodium layers, including new results from the Chinese Meridian Project

Abstract

Characteristics of the sporadic sodium layers (SSLs or NaS) in different latitudes and altitudes are summarized in this paper and the possible mechanisms are discussed. The SSLs in high latitude have a maximum frequency near midnight with significant seasonal differences of height and intensity being reported in previous studies. The average height of summer SSLs was much lower than that of winter SSLs and the average density enhancement factor for summer SSLs was much larger than that for winter SSLs. The simultaneous observations of the SSLs and sporadic E layers (ES) suggested that the ES probably contribute significantly to the formation of high latitude SSLs. In contrast, the temperature mechanism seemed to contribute little to the high latitude SSLs, especially since plenty or frequent high latitude SSLs were observed under minimum temperature conditions. The SSLs in the subtropical area from 20˚N to 35˚N show similar characteristics between different stations. The onset of SSLs in Hefei (31.8˚N, 117.3˚E) maintains (?) high occurrence from 20 to 03 LT and the peak height distribution is distinctly different between summer and winter. The winter sporadic sodium events are largely confined to altitudes below 96 km, while summer SSLs occurred frequently above 96 km. The simultaneous observations of SSLs, ES and high temperature suggested that the SSLs in subtropical area have a closer link with high temperature conditions than with the formation of ES. The high temperature are most likely caused by gravity wave breaking, together with a newly proposed icy dust sodium reservoir, both processes probably contribute importantly for the formation of subtropical SSLs. In contrast, the ES are not essential for most SSLs in the subtropical area, and they seem to influence only a few SSLs above 96 km, especially in the summer months from May to August. The observations from the Haikou station (19.5˚N, 109.1˚E) suggested that the SSLs in low latitude have some similar characteristics with that of the subtropical area. SSLs in Haikou also have stable occurrence frequency between 20-03 LT and the average altitude of the winter SSLs is lower than that of the summer SSLs. However, the occurrence frequency of SSLs above 96 km does not show any obvious seasonal difference for this station. The abundance of data and observations from low-latitude stations indicate that both the temperature control mechanism and ES theory are likely responsible for the formation of SSLs in the low latitudes and each mechanism can generate distinct types of SSLs. Temperature control mechanism is more likely to account for the large SSLs below 96 km, while ES theory probably contributes more directly to some SSLs above 96 km. Our results highlighted both the single-mechanism and combined actions of the temperature mechanism and ES theory on the formation of the SSLs in different latitudes and altitudes. The high latitude SSLs have a closer link with the ES theory and the subtropical SSLs correlate more strongly to high temperature, while both the...

Sudden Sodium Layers: Their Appearance and Disappearance

“…We propose that the supposed sodium reservoir should contain some specific properties: it needs to have efficient adsorption ability to store enough sodium sources, it has a fast release mechanism, and lastly, the release process is related to a warming temperature enhancement; it might have no correlation to E S or auroral activities. Recently, Qiu et al () and Qiu et al () observed the following empirical scenario for typical subtropical SSLs: extremely low temperatures below 150 K have always occurred several hours before the SSLs and most of these SSLs were formed or generated with a temperature maximum over 190 K. These statistical results showed that the occurrence of low temperature and the corresponding subsequent temperature increase is a specific character of these SSLs. According to these results, formation of icy‐dust particles, which could form in the extremely cold weather condition and then adsorbs and accumulates sodium species, and could also sublimate in the hot mesospheric weather to release free sodium atoms, was proposed as a candidate for the sodium reservoir (Qiu et al, , ).…”

“…In general, the occurrence of SSLs observed in subtropical latitudes, from 20°N to 35°N, has a weak correlation to E S activities (Dou et al, ; Gong et al, ; Miyagawa et al, ; Qiu et al, , ). These SSLs, usually lasting for a long period and locating near the centroid height of normal sodium layer at 92 km, are often detected accompanied by large temperature enhancements (Gardner et al, ; Li et al, ; Qian et al, ; Qiu et al, , ). Thus, the temperature‐controlled theory, instead of the E S theory, seems to be more appropriate for explaining the subtropical SSLs (Qiu et al, , ).…”

“…These SSLs, usually lasting for a long period and locating near the centroid height of normal sodium layer at 92 km, are often detected accompanied by large temperature enhancements (Gardner et al, ; Li et al, ; Qian et al, ; Qiu et al, , ). Thus, the temperature‐controlled theory, instead of the E S theory, seems to be more appropriate for explaining the subtropical SSLs (Qiu et al, , ).…”

Relationship Analysis of PM<sub>2.5</sub> and BLH using Aerosol and Turbulence Detection Lidar

“…In recent decades, lidars are widely used in lower atmosphere via Mie scattering, Raman scattering and differential absorption (Godin et al, 1989;Campbell et al, 2002;Renaut et al, 1980;Murray and van der Laan, 1978;Xia et al, 2007;Reitebuch, 2012), in middle atmosphere via Rayleigh scattering (Xia et al, 2012;Xia et al, 2014;Dou et al, 2009;Lux et al, 2018), in mesosphere and lower thermosphere via fluorescence backscatter (She et al, 1992;Gao et al, 2015;Li et al, 2018;Qiu et al, 2016). Aerosol, trace gas concentration, atmospheric density and temperature, wind, and neutral metal atom can be detected by these lidars.…”

On the Long Lasting “C‐Type” Structures in the Sodium Lidargram: The Lifetime of Kelvin‐Helmholtz Billows in the Mesosphere and Lower Thermosphere Region

“…However, if there is a significant reduction in the magnitude of a wind shear, the further evolution and deformation are ceased and the remnant of the previously generated KH billows can get nearly frozen‐in the background. Qiu et al () also support the frozen‐in concept. They observed the same C‐type structure in Na lidargram from different lidar locations in China.…”