Temperature controlled icy dust reservoir of sodium: A possible mechanism for the formation of sporadic sodium layers

Abstract

Using seven years, from 2006 to 2013, sodium lidar observations over Hefei, China (31.80°N, 117.3°E), we attempt to propose a possible mechanism for the formation of sporadic sodium layers (SSLs or NaS). We analyze the relationship between low temperature (<150K) and SSL occurrence and detect a statistically significant link that the low temperature (<150K) occur in three days before an SSL with an occurrence rate of 93.4% (57/61). The sharp decrease of water vapor concentration nearby before an SSL and the recover after the SSL are also detected frequently. Based upon these evidences and some case studies, we propose an icy dust reservoir in the formation of an SSL. The icy dust could form in the extremely cold mesopause region where the temperature falls below 150K and it will absorb sodium atoms to form a solid sodium metal film as a sodium reservoir. The icy dust will then sublimate rapidly when meeting with warm air (e.g., 150K<T<190K) and leave behind the solid metal atom film. The remanent sodium film might release vapor sodium atoms finally by some means through high temperature (e.g., >190K and sometimes even >230K) and form a sporadic sodium layer. Although not conclusive and highly uncertain, the icy dust reservoir model not only provides a good explanation for the observed characteristics of SSLs; it is also in good agreement with many other 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 further suggest that the icy dust might be a viable option of sodium reservoir for the formation of SSLs.