Regional Characteristics And Possible Mechanisms Of The Sporadic Sodium Layers

Sporadic sodium layer (SSLs or Nas) is mesospheric phenomenon that the sodium layer increases very fast (e.g. within five minute) in a narrow altitude region to more than2times of the average sodium layer density of the background. The SSLs usually last for several tens of minute, and some time can last over several hours. The full width of the half maximum is often1-2km.SSLs were first observed by Clemesha et al.(1978), and after that many possible mechanisms were proposed to explain the formation progresses of the SSLs, such as the direct injection of meteors, the energetic auroral particles bamdment on the dust surface to release free sodium atoms, the recombination of sodium ions from sporadic E layers and the mechanism depending on the temperature variations. However, up to now none of these theories could explain all the observations of SSLs, and more and more results implied that SSLs in different latitudes and altitudes might have distinct mechanisms.In this study, we analyzed the relationship between low temperature (<150K) and SSL occurrence in Hefei (31.8°N,117.3°E), China, a subtropical area, and detected an extremely statistically significant link that the low temperature (<150K) occurred in three days before an SSL with an occurrence rate of93.4%(57/61). Based upon this link and a number of other evidences, we proposed a four-step model for the formation of SSLs in the studied subtropical area. First, the icy dust forms in the extremely cold region where the surrounding temperature falls below150K. Second, the icy dust will absorb sodium atoms and form a solid sodium metal film on the ice surface. Third, the ice surface sublimates rapidly when meeting with warm air (e.g.,150K<T<190K, which is the normal condition for the mesopause region above subtropical area), and leave behind the metal atom film and the solid dust particle containing the sodium species. Fourth, the remanent sodium metal film releases vapor sodium atoms by some means through high temperature (e.g.,>190K and sometimes even>230K). Although not fully conclusive, this model not only provides a good explanation for the observed characteristics of SSLs; it is also in good agreement with many observations, such as the simultaneous sporadic sodium and iron layers, the behavior of SSLs on small time scale, the deviation of the sodium density profile of SSLs from the normal one, and the sharply decreased scale height above the peak of the sodium layer. These results suggest that the icy dust is a viable option of sodium reservoir for the formation of SSLs.Regional characteristics and possible mechanisms for the SSLs in different latitudes and altitudes are discussed in section3. SSLs in high latitude have a maximum frequency near mid night. There were significant seasonal differences of height and intensity between high latitude SSLs in summer and in winter:the averaged height of summer SSLs was much lower than that of winter SSLs; meanwhile, the averaged factor of summer SSLs was much larger than that of winter SSLs. Most simultaneous observations of the sporadic sodium layers and sporadic E layers in high latitude support the Es mechanism that Es could act as a direct source for the SSLs. However, the temperature mechanism seemed to be unsuitable for the high latitude SSLs, since SSLs were often observed in the temperature minimumThe observations of mid and low latitude show that the sporadic sodium layers in the sub tropical area from20°N to35°N may have a similar occurrence and mechanism. The onset of SSLs in Hefei keeps high occurrence between20-03LT. The peak height distribution is quite different from that in high latitude:the winter sporadic sodium events seldom locate above96km, but in summer the high SSLs above96km increase apparently. The SSLs observed in Hefei have a good correlation with the high temperatures, but a very weak correlation with Es layers. So the most probably mechanism for the sub tropical SSLs is the temperature theory.The distributions of the onset of the SSLs observed in a low latitude station at Haikou also have no frequency at mid night. The seasonal distribution of the peak altitudes of the SSLs in Haikou roughly looks like that for the sub tropical SSLs, with a lower location for the winter SSLs and higher location for the summer SSLs except that the occurrence of SSLs above96km in Haikou is nearly the same in summer and in winter. All the results from the low latitudes showe both the temperature involved mechanism and Es theory may work in this region and generate distinct types of sporadic sodium layers respectively.Sporadic sodium layers very rarely occurred in the mid latitude area from40°N to50°N. The observations near40°N show that the mid latitude SSLs are also probably controlled by high temperature and the double sodium layers, a similar phenomenon to SSLs, could have a good relationship with Es layers.