| The air in the middle and upper atmosphere(above 20 km)is very thin,where the physics,chymical and dynamic processes are very complex.It is a key area in the study of solar-terrestrial physics.The deposition of materials with extraterrestrial origin into the upper atmosphere results in layers of free metal atoms or ions,mainly in the mesosphere and lower thermosphere(MLT),which ranges from 80 to 105 km.Among many metel atoms,sodium atoms have a relatively high concentration and longevity,and their backscattering crosssection is large.As a result,sodium atoms are good tracers for atmospheric activities in the upper and middle atmosphere.Middle and upper atmosphere observational instruments with high temporal and high spatial resolution,high signal-to-noise and high stability are necessary to better understand the complex dynamics and chemical processes occurring in the stratosphere,mesosphere and lower thermosphere regions.There are some special phenomena in the sodium layer,such as sporadic sodium layer and thermospheric enhanced metal layer,collectively referred to as the enhanced sodium layer.Both of them are defined as a sudden increase in metal density that exceeds the magnitude of the background,but for different altitudes.Since Clemesha et al.(1978)first observed sporadic sodium layer,its formation mechanism has always been the focus of scientists.The main mechanisms include:direct meteor injuection,ion neutralization,temperature enhancement,and electron deposition,while the formation mechanism of the thermospheric enhanced metal layer is even more complicated.Gravity wave(GW)is an important medium for the exchange and redistribution of energy,momentum,and composition between the upper and lower atmospheres,and the processes of saturation,decay and dissipation of GWs are important dynamic processes in the large-scale atmospheric circulation.In this thesis,we report a Rayleigh and sodium lidar system which can detect from stratosphere to MLT region.In addition,we use lidar systems located in the northern and southern hemispheres(China and Chile)to study the formation mechanism of the large-scale horizontally enhanced sodium layer and the coupling process of gravity waves and spordic sodium layers.Lidar can obtain the atmospheric temperature and density of the stratosphere and the middle layer by detecting the Rayleigh scattering signal,while the sodium atom density of the sodium layer needs to invert the sodium resonance fluorescence scattering signal.Generally speaking,simultaneous observation of atmospheric data and sodium layer data requires multiple sets of lidar systems to work together to complete the observation.The Lidar system introduced in this thesis uses the time-division and wavelength-division multiplexing methods to measure atmosphere data from 30 to 105 km by only one piece of photomultiplier tube in the optical receiver,which makes the system compact and robust.The system was upgraded in Hefei,China(31.5°N,117°E)in September 2016,and then obtained a large amount of observational data.The lidar system features high temporal and vertical resolutions,a high signal-to-noise ratio and mobility.The analysis of seasonal variations in atmospheric parameters and the observations of meteor trails and stratospheric aerosols prove that the observational data from this system can promote a deeper understanding of atmospheric dynamics and chemical processes.The existing Lidar reports on the enhanced sodium layer generally focus on a single site.However,the characteristics of the large-scale horizontally enhanced sodium layers and its relationship with the background conditions are still to be studied.Therefore,it is necessary to combine multiple sites and observational data of multiple instrument for more in-depth research.We use multiple lidar systems along the Meridian Chain to systematically study 19 large-scale horizontally enhanced sodium events observed in Hefei and Wuhan(30.5°N,114.4°E)from 2011 to 2019,and discuss their main formation mechanism by combining sodium atom,wind,temperature and ionospheric E-region observations.The conclusion includes:(1)The SSL and TeSL occurrence rates in summer were considerably higher than those in other seasons.(2)In general,the correlation coefficients between Hefei and Wuhan for long-duration cases(more than two hours)were high due to some large-scale mechanism,and the short-duration cases(less than two hours)had poor correlation due to different local characteristics.(3)Seven TeSLs and Four out of the nine SSLs were accompanied by ionospheric sporadic E(Es),suggesting that an"Es-SSL" chain formed via the wave-induced wind shear mechanism.(4)Three SSLs were modulated by waves and the two other cases were related with gravity wave overturning,which are related to dynamic process.(5)Three cases were observed with an apparent time delay over Hefei and Wuhan,which might indicate the possible existence of long-distance transport processes.This thesis also studied the ducted gravity wave-related spordic sodium layer event observed by the Andes Lidar Observatory(ALO,30.3°S,70.7°W)on 2 May,2019.Observations of wind field and temperature also showed obvious medium and high frequency GWs.This study provides a new method for calculating the parameters of gravity waves from lidar observation data,using the ratio of relative temperature to the amplitude of the wind field in various directions and the non-dissipative polarization relationship to calculate the important parameters of GWs such as intrinsic frequency,horizontal wavelength and wave vector direction.These parameters can be used to calculate the critical layer and inversion layer formed by gravity wave propagation during the spordic sodium layer event,which can be used to explain the actual observed gravity wave dissipation and ducted phenomena.This case study can be combined with numerical simulations and comparative analysis with various observational data to better understand the dynamic processes related to gravity waves in the sodium layer. |
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