Mechanisms For The Formation Of Thermospheric Sodium Layer And Sporadic Sodium Layer

MLT(Mesopause and Lower Thermosphere)region is a transitional area in middle/upper atmosphere.Due to the influence of solar radiation,there exist short-term variation(diurnal and semidiurnal variation)and long-term variation(annual,semiannual and solar cycle variation).The composition of the MLT region is also very complex,except molecular component(such as O2 and N2),there is a large number of O which is related to strong photolysis.When meteors flow into this region,they will be ablated and then inject many metal atoms and ions(such as Na,Fe,K and so on),creating a metal layer in this area.The atomic density of metal layer will be influenced by both dynamical and chemical process.Using sodium resonance fluorescence wind/temperature lidar,we can measure sodium density,wind and temperature in the MLT region.At some particular height within the main sodium layer,sodium density sometimes suddenly increases for several times or even ten times and last for several minutes to few hours,this phenomenon known as the sporadic sodium layer.In recently years,thermospheric metal atom layers(density is very low comparing to main layer)are frequently observed between 110?160 km by]idar.In this thesis,we use 2 sodium resonance fluorescence wind/temperature lidars,located in Hefei,China and Andes mountain,Chile,to study the formation mechanism of sporadic sodium layer and thermospheric sodium layer.Based on datasets observed by the USTC sodium lidar,Wuhan meteor radar and satellite,we study the sporadic sodium layer appeared right after a gravity wave breaking event.The observation indicates this gravity wave breaking event influenced a large region and had a strong effect on wind and temperature.45 minutes after gravity wave breaking,sporadic sodium layer first appeared at lidar west channel signal,2 minutes later appeared at east channel signal.This 2 minutes delay lasted for several minutes which indicate that the sporadic sodium layer event may be formed by a high-density sodium atom cluster propagation.According to lidar and meteor radar wind,the scale,transport speed and direction of this sporadic sodium layers were analyzed.ERAU(Embry-Riddle Aeronautical University)lidar at Chile station,in April 2015,observed thermospheric sodium layers(TSL)and neutral wind between 110 and 140 km.Wind comparison between observation and Thermosphere-Ionosphere Electrodynamics Global Circulation Model(TIEGCM)simulation shows very good agreement,we therefore conclude that we can safely use TIEGCM wind in the higher altitude region.Then using TIEGCM wind,we find that the TSL appeared around the maximum ion divergence region,therefore its formation cannot be explained by neutralization of metallic ions converge induced by horizontal-wind-shear-induced vertical drift.We further analyze more thermospheric metal layers observations at different locations and sites,and find out that most of these layers also appeared around maximum ion divergence region.Above 120 km,because air density is very low,meteor sputtering process can directly inject sodium atoms into the background.And atmospheric molecule ions(such as NO+ and O2+)can ionize sodium atoms via chemical reactions.Based on upon point,we proposed a possible mechanism for the formation of the TSL.First,meteor sputtering provides the source of Na atoms in the thermosphere.Then strong semidiurnal tide can induce wind shear which in turn causes divergence of NO+ and O2+ and reduce their density to a very low level.And the neutral Na would have a much longer lifetime and then reach a higher density and detectable by the lidar.And a simple numerical simulation is able to reproduce key features of the observed TSL.Finally,we analyze the propagation characteristics of the TSL at ALO.