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

Characteristics of the sporadic sodium layers (SSLs or Na-s) 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 (E-s) suggested that the E-s 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 degrees N to 35 degrees N show similar characteristics between different stations. The onset of SSLs in Hefei (31.8 degrees N, 117.3 degrees 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 E-s. 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 degrees N, 109.1 degrees 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 and 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 E-s 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 E-s 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 temperature mechanism and Es theory probably contribute equally to the SSLs in the low-latitude Haikou station. In fact, we show that the observations in Haikou yield the example that two SSL events could be controlled by different mechanisms even though they occurred at the same location within the same night. (C) 2016 Published by Elsevier B.V.