Sudden Sodium Layers: Their Appearance and Disappearance

Abstract

Temperature variation has been proposed to play an important role in the formation of the sporadic sodium layers (SSLs or NaS) in subtropic area, based on the observed significant correlation between SSLs and high temperatures. The icy-dust particle, which could form in the extremely cold conditions and act as absorbers of sodium species, was proposed to be a possible candidate for the sodium reservoir of the SSLs. In this study, the University of Science and Technology of China temperature/wind lidar and the sodium fluorescence lidar at a subtropic station Hefei (31°N, 117°E), China, were used to observe sodium density, temperature, and wind profiles simultaneously throughout the SSL events. Based on the observations of two SSLs occurring on 12 and 13 May 2013, the possibility of an icy-dust layer existing and acting as the sodium reservoir is tested for the first time in details. Both events experienced an extremely cold temperature (<150 K) several hours before the onset of SSLs, followed by a subsequently fast production of sodium atoms during a large temperature enhancement (>40 K). An empirical model including two main steps is then proposed: first, sodium species are collected by an icy-dust reservoir and stored during the extremely cold phase; second, free sodium atoms could be released from the reservoir by a possible trigger. As a result, this kind of SSLs could possibly be regarded as a quasi-continuous phenomenon caused and modulated by temperature variations with an icy-dust model that can exhibit intermittent time variations related to the water vapor concentration.