【作者】 李海龙；

【导师】 黄际英； 吴健；

【作者基本信息】 西安电子科技大学 ， 无线电物理， 2009， 博士

【摘要】 极区中层夏季回波(PMSE,Polar Mesosphere Summer Echoes)是通过雷达在高纬当地夏季中层顶附近探测到的异常强大雷达回波,是与尘埃等离子体密切相关的自然现象。自从1979年被Ecklund和Balsley发现以来已经引起了广泛的关注。本文以尘埃等离子体电波传播理论为出发点,主要围绕极区中层顶PMSE的产生机制,PMSE层对电磁波的散射与反射,加热对PMSE的影响,测高仪对PMSE的观测开展相关研究。本文主要开展了如下工作:1.首先介绍有关极区中层夏季回波的基本知识;然后利用在仅发生PMSE现象时ECT-02实验数据分析得出极区中层顶区域属于尘埃等离子体,最后利用电子密度不规则运动解释PMSE。2.介绍了不同雷达对PMSE体反射率的测量以及雷达的测量结果,统计得出雷达体反射率与工作频率的依赖关系,然后分别应用尘埃等离子体的尘埃粒子散射统计平均结果和波恩近似分别得到散射截面与工作频率的内在关系,最后利用小尺度扰动理论得到雷达体反射率与频率的关系,发现它们均与频率的四次方成反比,与统计的实验结果趋于一致。这表明带电尘埃粒子在PMSE形成机制的研究中是不可忽略的因素,尘埃等离子体理论是研究PMSE的一个重要工具。3.极区中层顶区域存在大量的带电尘埃粒子,出现明显的分层结构,PMSE回波强度和电子数密度也存在明显的分层结构。采用分层介质波传播理论,分析在频率53.5MHz情况下雷达回波的信噪比。结果表明:利用分层介质理论得到的信噪比与实验结果较吻合。另外分别计算在其它频率的反射系数,发现随频率的增加,反射系数明显减弱。上面结果基本表明PMSE主要是反射现象。可见利用分层介质理论是解释PMSE产生机制的又一重要手段。最后结合分层介质,利用全球定位系统(GPS, Global Positioning System)卫星L2载波频率分析PMSE尘埃层对电磁波的衰减情况,发现GPS信号穿过PMSE层后,信号受到严重衰减,PMSE层能对GPS信号产生重大影响。4.极区中层夏季回波具有明显的分层现象,加热能引起过冲现象。我国从2006年开始开展PMSE加热实验,基于三年的PMSE加热实验,发现PMSE回波强度具有明显的分层现象,并且层数之间变化迅速。另外也得到PMSE加热特征曲线OCC(Overshoot Characteristic Curves)图,回波功率随高度变化的剖面图,回波强度随时间的变化图,得到明显的过冲现象。这为我国开展PMSE研究打下了一定的基础。5.利用电离层测高仪在MF和HF频段开展对PMSE的研究。应用电离层测高仪在中国南极中山站测得的数据,结合北极Troms?站测高仪测量的频高图,分析发现:在南北半球高纬地区,用MF和HF频段的电离层测高仪能够观测到类似PMSE现象,文中称为PMSE-Es现象。并且还发现南极PMSE-Es具有季节性变化、日变化和半日变化等特征,与北半球的PMSE现象较吻合。最后我们也对其中的差异进行了对比分析。另外本文对比1999-2003年PMSE出现率和PMSE-Es出现率的季节性变化,发现它们出现率的相对大小与相对应的年份吻合的比较好,特别是在2002年,PMSE和PMSE-Es的出现率均明显偏低,这从另外一个侧面说明PMSE与PMSE-Es具有一定的相关性。结果表明可以在MF和HF频段通过研究PMSE-Es来研究PMSE现象。更多还原

【Abstract】 Recently, considerable attention has been paid to the anomalously high power radar echoes from the mesosphere at high latitude in summer: the polar mesosphere summer echoes (PMSE). PMSE considerable relate to the charged dust particles in polar mesopause. Much research on PMSE has been done since it was first found by Ecklund and Balsley as a fascinating phenomenon in 1979. On the basis of theoretical research on electromagnetic wave propagation in dusty plasma, the generation mechanism is studied with layered media theory. The effect of heating on PMSE is discussed and the ionosonde is used to analyze PMSE. The main topics and results of the study are as follows:Firstly, the basic knowledge of PMSE is introduced; the data of ECT-02 only occurring PMSE are given. The data provide a good base to study various features of PMSE. By analyzing the data, one can find that the region in polar summer mesopause where charged dust particles exist should be studies with dusty plasma theory. The variable disturbance of electron density is used to explain PMSE.Secondly, by statistically analyzing the volume reflectivities of PMSE at different time, stations and frequencies, the relation between the volume reflectivity and the work frequency is obtained. The differential scattering cross section obtained with Born approximation in dusty plasma is also derived, the same result is obtained. Others, the relation between the volume reflectivity and frequency occurring PMSE is studies using the small scale structure of electron density caused by charged dust particles. A theoretical expression for the radar volume reflectivity is derived and shows that the statistical result agrees with the theoretical results. When studying the generation mechanism of PMSE, the charged dust particles must be considered. The theory of dusty plasma may be a useful tool to investigate the generation mechanism of PMSE.Thirdly, the data obtained from sounding rocket campaigns indicate that radar echo power, electron density and dust charge density of polar mesosphere in summer are obvious layered structures. Based on the theory of wave propagation in layered media, the reflection coefficient and radar signal to noise ratio (SNR) at 53.5 MHz at each layer in polar mesosphere is studied. The agreement of the SNR calculated with the theoretical results and the measured results are good. Others, the theory of wave propagation in layered media are also used to study the reflection coefficient at other frequencies. The intensity of the radar echoes reflected by layered structure decreases greatly with the frequency increasing. So the phenomenon of PMSE may mainly be caused by the reflection of the layered structure. Combining dusty plasma theory, the attenuation of L2 carrier frequency of Global Positioning System (GPS) traverses in PMSE layer is studied with the layered media theory. One find that the signals attenuate greatly in PMSE layer, so the PMSE layer can affect communication.Fourthly, PMSE have obvious layered structures, the overshoot can be caused by artificial electron heating experiments during PMSE conditions. Since 2006, we have carried out the heating campaign for three years. Based on the data from 2006 to 2008, the different layered structures are observed, the number of layer change very quickly. The overshoot characteristic curve (OCC) is obtained in our heating campaign too. Others, the echo power changing with height and time are also got. The heating campaign may be the found of studying PMSE in the future.Finally, the ionosonde is used to study PMSE at MF and HF bands. Using the ionograms detected by ionosonde at Troms? Station, Arctic and Zhongshan Station, Antarctica, PMSE are studied with data observed by a Digisonde Portable Spunder-4 ionosonde. By analyzing the irregular characteristics of E and Es layers in ionograms, the PMSE-Es may be observed at MF and HF bands. The phenomena of irregular E and Es layer are called PMSE-Es (Polar Mesosphere Summer Echoes-Es). The occurrence rate variations of PMSE-Es exhibit similarity to those of PMSE, such as: seasonal variation, diurnal variation, and semidiurnal variation. Others, in order to compare the seasonal variation of the Arctic PMSE’occurrence rate from 1999 to 2003, a new statically method is used to obtain the PMSE-Es’seasonal variation. Both occurrence rates of PMSE in Arctic and that of PMSE-Es in Antarctica occurrence rates are relatively lower in 2002. At the same time, the occurrence rate of PMSE-Es in the other four years is nearly consistent with that of PMSE. The results show that PMSE-Es should have some relations with PMSE. The ionosonde in Arctic and Antarctica may be used to detect PMSE with the phenomenon of PMSE-Es at MF and HF bands.更多还原