Charscteristic Analysis And Exprimental Study Of Radar Echoes In High-latitude Mesosphere

Polar Mesospheric Summer Echoes(PMSE)are strong radar echoes that have been observed typically from the MF to UHF band in the summer polar mesosphere.These polar mesospheric summer echoes are fundamentally related to humanity's greenhouse gas emissions and global atmospheric circulation.At the same time,it is also an important scientific topic which is widely concerned by scientists in the field of dust plasma physics and space physics.PMSE is associated to the ice grains in mesospheric ice clouds,and are thus of interest for both fundamental dusty plasma research and as a tool for monitoring the mesospheric ice clouds.The theoretical and experimental study of PMSE can improve the understanding of the ionosphere,and it has important scientific significance and application value to the study of space physics.This dissertation mainly focuses on the characteristics of PMSE,the experimental characteristics of layered PMSE and its correlation with solar and geomagnetic activities,the frequency dependence of PMSE,the aspect sensitivity of PMSE,the characteristics of Polar mesosphere summer echoes(PMSE-Es)and the correlation between Polar Mesosphere Clouds(PMC)and PMSE.The main contents of this dissertation are given as follows:First,a brief introduction to the basic characteristics of the ionosphere at high latitudes and a survey of domestic and foreign research on the Polar Mesosphere Echoes(PME).The basic requirements of PMSE observation experiment,data processing methods and typical characteristics of PMSE are introduced,which pave the way for detailed analysis and discussion in the following content.Second,study on the characteristics of layered PMSE.The phenomenon of layered PMSE and the criteria for mono-,double-and triple-layer PMSE are introduced.The calculation method of mono-,double-and triple-layer PMSE occurrence rate is given,and the occurrence rate of layered PMSE is calculated.On this basis,the seasonal and annual changes of the occurrence rate of layered PMSE and the characteristics of layered PMSE under different thresholds are studied.Then the correlation between the layered PMSE and solar and geomagnetic activities is analyzed.It is found that the correlation between the occurrence rate of mono-,double-and triple-layer PMSE and geomagnetic activity is positively,while the occurrence rate of tripe-layer PMSE is negatively correlated with solar activity.Third,study on the frequency dependence of PMSE.The calculation formula of volume reflectivity is given,and the literature results are statistically analyzed.It was found that volume reflectivity is inversely proportional to the fourth power of frequency.The frequency dependence of PMSE under the influence of high-energy particle deposition was studied by using the data of the EISCAT VHF radar observation experiment.It is consistent with the results of the literature.A theoretical model is established using layered media theory,and then substituted into experimental data to verify the conclusion that the volume reflectivity is inversely proportional to the fourth power of frequency.Fourth,study on the aspect sensitivity of PMSE.The aspect sensitivity parameters of PMSE are introduced.The observation experiments under different elevation angles conducted on 24-28 June,2013 and 1 July,2013 are used to analyze the aspect sensitivity of PMSE in detail.It is found that the scattering structure of PMSE is not purely isotropic when radar echoes occur at the off-vertical elevation angle.The aspect sensitivity of the upper PMSE layer is weaker than that of the lower PMSE layer,when double-layer PMSE occurs.It indicates that the anisotropic scattering of the upper PMSE layer is stronger than that of the upper PMSE layer.And possible explanations are given.Fifth,study on the characteristics of PMSE-Es.The criterion of PMSE-Es is given,using the data of PMSE-Es observed by the digital ionosonde.The daily and seasonal variation characteristics of PMSE-Es are analyzed.Then,the daily,seasonal and annual changes of PMSE occurrence rate were compared with that of the PMSE-Es occurrence rate,and it was found that they had similar trends in daily,seasonal and annual changes,but there were still some differences.And possible explanations are given.Sixth,study on the correlation between PMSE and PMC.The PMC observation equipment is introduced briefly.The correlation between PMSE observed by EISCAT VHF radar and PMC observed by CIPS are discussed.It was found that there is a positive correlation between double-layer PMSE and PMC.This study provides a new idea for the research of PMSE formation mechanism.