中纬地区日间类扩展F不规则结构的观测研究

基于高频多普勒观测,研究中纬地区日间类扩展F现象及其与其他电离层扰动现象的关联.结果表明:日间类扩展F具有出现时间覆盖面广、持续时间长、与其他电离层扰动相互伴随等特征;形态特征表现为回波弥散（与夜间扩展F相似）,弥散回波的频移多倾向于正向偏移,有时与行进电离层扰动（TIDs）相互伴随.日间类扩展F现象虽然是偶发事件,且发生率极低,但其出现时间及区域的广泛性充分体现了该现象对空间天气及空间环境的影响具有不可忽视的重要性.      

由高频无线电波反射回波参数反演电离层运动和结构的高度剖面

本文从广义电波射线微分方程组出发,导出了时变、不均匀和各向异性电离层介质中高频无线电波反射回波的可测参数与电离层的运动和结构参数之间的积分方程组,并给出了利用现代数字测高仪中的多普勒图、定向图、频高图和测距图,反演电离层的结构和运动随高度的分布剖面的计算方法.根据这种反演方法,单站探测电离层运动和结构剖面时有多种可测参量的组合方式.文中提出了一种最佳的参量组合方案,为探测设备的改进和观测记录的分析提供了理论依据.      

电离层Es对HF和VHF传播信号的影响

Es层是存在于电离层中的电子密度非常高的偶发E层（Sporadic E）,其电子密度可达常规E层的100倍.电离层Es能够反射原本穿透F层的VHF低频段（30~150MHz）无线电波,而且对HF高频段（10~30MHz）无线电波传播具有显著影响.运用垂测和斜测观测数据,研究HF频段Es层电波传播特征,得到了不同类型及不同高度Es层的衰减系数.根据f₀E_s的日变化规律,可得HF频段衰减系数的日变化规律,进而分析并得到Es层对短波传播的影响.不存在电离层Es时,通常无法通过电离层实现VHF超远距离通信.为了对VHF链路通过电离层Es的传播衰减进行定量分析,根据EBU多条链路的观测结果,拟合并建立了电离层Es衰减模型.将该模型、ITU模型和观测数据进行对比,发现本文建立的模型准确度更高.利用建立的模型,对电离层Es不同临频f₀E_s条件下接收信号场强和电压随传播距离的变化进行了计算,结果可为VHF链路设计及建立提供参考.      

电离层Es电波传播模型研究

考虑大气折射的影响,对不同高度Es层条件下大圆路径传播距离与仰角的关系进行计算.基于电离层Es电子密度时空分布特点,建立高阶Es反射模型;运用高阶修正贝塞尔函数表示电离层Es电子密度扰动的自相关函数,综合考虑电离层Es各向异性结构的尺度关系,建立高阶Es散射模型.仿真计算Es层VHF斜入射传播衰减与频率、距离的关系.对于反射模型,Es层厚度越厚,电波衰减越大.对于散射模型,Es层不规则体水平尺度相同,垂直水平漂移的尺度越大,衰减越大;而垂直水平漂移的尺度相同,水平尺度越大,衰减越小.无论反射模型还是散射模型,f₀E_s越高,衰减越小;电波工作频率越高,衰减越大.对比建立的模型与VHF链路的测量结果,明确不同强度的电离层Es反射/散射机制,证明了建立模型的正确性.      

电离层H2O释放扰动效应的数值模拟研究

为充分研究化学物质在电离层释放的扰动效应和后期发展效果,基于化学物质在电离层的扩散模型、化学反应和电离层扩展F的控制模型,通过电离层H₂O的释放,研究电子e,H₂O,O+和H₂O+共4种粒子的分布状态,分析点源、多源和线源释放对电离层的扰动效果,比较不同高度、不同量和不同时间释放的影响结果,模拟夜间释放后期所激发的扩展F发展差异.结果表明,H₂O在电离层释放后,能有效耗散背景电子形成空洞,O+和H₂O+数密度呈椭圆形分布;点源、多源和运动目标线源等不同释放方式对电离层的扰动效果不同,证实了人工影响一定形态和区域电离层的可能性;H₂O释放扰动幅度,低层大于高层,白天强于夜晚,释放量越多扰动越突出;夜间化学释放能激发扩展F,并且释放量越多,激发效果越好.      

东亚地区Es特性研究

Es是发生在电离层E层高度上的偶发不规则结构,其电子密度最高可达E层背景电子密度的100倍以上. 远东地区是全球Es的高发区,其相关研究具有独特性和典型性,该地区Es形态学特性的深入研究对于探索Es成因及揭示其本质具有重要意义. 本文利用中国电离层垂测网50余年的垂测数据,结合日本垂测网数据对东亚地区Es的强度特性、空间分布特性、日变化、季节变化以及长期变化趋势进行了深入分析,得到一些有意义的结论.      

高频返回散射回波频谱测量

本文采用脉冲多普勒雷达技术, 测量了电离层高频返回散射回波的时延-功率-多普勒频率的三维频谱图.对典型的频谱图作了分析, 并由谱图推算了电离层等效反射面的垂直运动速度, 它与垂直探测资料推算的结果相符.      

高精度多普勒频高图的获取和分析

电离层高频反射回波的多普勒频移是研究电离层扰动的一个重要参数。在数字测高仪探测中，高分辩率的多普勒频移一般在漂移测量中获得，但这种测量方式探测的频点少，不能获得扰动的高度剖面；而在频高图模式下，虽然工作的频点多，能够获得电子浓度的高度剖面的信息，但探测的多普勒频移的分辩率低，无法用来精确检测电离层扰动。实际的电离层高频回波一般为窄带信号，由此本文中提出了一种新的分析方法，利用DGS-256数字测高仪频高图模式下记录的16个多普勒通道的数据，通过反傅里叶变换还原成时域信号，采用最小二乘法估计相邻时间点的相位差，获得高精度多普勒频高图。作为实例，利用该方法分析武汉电离层观象台数字测高仪观测站的观测数据，得到了多频点的多普勒频移曲线。结果表明：在DGS-256数字测高仪频高图模式下能够得到高精度多普勒频高图，这在电离层扰动探测和研究中很在意义。     

乌鲁木齐地区电离层环境长期变化特性研究

电离层环境变化对电子信息系统具有重要影响,其可能引起短波通信质量下降甚至中断,导致导航信号闪烁和误差增加等. 研究电离层环境特性对于评估空间环境对通信、雷达及导航装备的影响具有重要意义. 本文根据乌鲁木齐电波环境观测站50余年数据,研究了乌鲁木齐地区电离层F层临界频率、F层底高、Es层临界频率的日变化、季节变化和长期变化情况,同时根据HWM07高空风场模型研究了乌鲁木齐地区高空风场的昼夜和季节变化特性.      

中性气体释放人工产生气辉

电离层中分子性的离子与电子的复合要比氧离子与电子的辐射性复合快得多,因此火箭发动机产生的尾气和空间等离子体主动实验中主动释放的中性气体会对电离层有很大的影响,这么大的电离层扰动现象在过去的实验中经常可以观测到.根据中性气体在热层背景中的扩散方程,考虑电离层F区主要的离子化学反应,研究了H2,H2O和CO2气体在电离层高度上的扩散过程和电离层对所释放气体的响应,计算了气辉的体发射系数和发射强度.结果表明,中性气体在电离层高度上扩散非常迅速,在F区的一些高度上,主要正离子成分由O+转变为其他分子离子,且在释放过程中伴随气辉发射,发射气辉的波长和特征与释放物质的种类有关.      

Daytime VHF amplitude scintillations recorded at an Indian low-latitude station,Waltair (17.7°N, 83.3°E) during 1997–2003

In this research, it is presented the daytime amplitude scintillations recorded at VHF frequency (244 MHz) at an Indian low-latitude station, Waltair (17.7°N, 83.3°E) during seven continuous years (1997–2003). Contrary to the nighttime scintillation seasonal trends, the occurrence of daytime scintillations maximizes during summer followed by winter and the equinox seasons. The fade depths, scintillation indices and the patch durations of daytime scintillations are meager when compared with their nighttime counterparts. A co-located digital high frequency (HF) ionosonde radar confirms the presence of sporadic (Es) layers when daytime scintillations are observed. The presence of daytime scintillations is evident when the critical frequency of the Es-layer (foEs) is ≥4 MHz and Es-layers are characterized by a highly diffuse range spread Es echoes as can be seen on ionograms. It is surmised that the gradient drift instability (GDI) seems to be the possible mechanism for the generation of these daytime scintillations. It is quite likely that the spread Es-F-layer coupling is done through polarization electric fields (Ep) that develop inside the destabilized patches of sporadic E layers, which are mapped up to the F region along the field lines as to initiate the daytime scintillations through the GDI mechanism. Further, the presence of additional stratification of ionosphere F-layer, popularly known as the F3-layer, is observed on ionograms once the Es-layers and daytime scintillations are ceased.     

基于混沌振子阵列的扩频信号捕获方法

在低信噪比下提出了结合混沌振子阵列及相干相关实现对扩频信号的捕获方法．针对混沌振子的数学模型,用混沌系统的相轨迹特性来分析对扩频信号的捕获过程,以Lyapunov指数（LE）作为实现捕获及提取载波多普勒频率的判断依据,再由跟踪环路完成对扩频码相位的跟踪．对DSSS／BPSK仿真结果表明,本方法可以实现扩频信号快速捕获,同时提高了捕获概率及检测灵敏度。     

NOx羽烟引起的局部电离层电子密度扰动

通过空间飞行器抛出羽烟的球状近似，研究白天ＮＯ和ＮＯ２羽烟在Ｅ层的扩散，光化学过程及它们对电子密度的影响。由于光致电离和郭解复合反应，抛出的ＮＯ羽烟在Ｅ层造成电子密度最大７．５倍的瞬时增加，而ＮＯ２羽烟可以使Ｅ层下部电子密度瞬时升高达２４倍。     

低纬(海南)地区电离层偶发E层的潮汐类周期研究

基于低纬(海南)台站(19.5°N,109.1°E)电离层测高仪在2004年观测到的偶发E层(Es)临界频率(f₀E_s)和虚高(h'E_s)数据,利用谱分析方法,给出了中国低纬地区Es层在全年和不同季节内的短周期变化特性.对全年数据谱分析表明,Es层除具有非常强的24h周期外,还具有显著的12h和8h周期.对不同季节内两个参数的谱分析进一步表明,8h周期主要出现在f₀E_s参数的春秋分季和夏末及在h'E_s参数的几乎所有季节;6h周期主要出现在f₀E_s参数的11-12月.海南低纬Es层中具有显著的6h和8h周期特性,特别是这两个周期还能同时出现在冬季,这是以往研究中不曾发现的新结果.初步分析认为,Es层的6h和8h周期极可能是分别由1/4日和1/3日潮汐所造成的.      

Energetic ions upstream of the Earth''s bow shock: INTERBALL-1 survey

A statistical survey of energetic ions (> 20 keV) observed by the DOK2 experiment on Interball-1 during foreshock intervals from 1996 to 1998 is presented. Flux levels depend on the connection geometry, with higher values at quasi-parallel shocks, particularly at lower energies. The decrease of flux in diffusive events with distance from the bow shock is consistent with results from earlier surveys performed closer to the bow shock. The energy spectra are softer for quasi-parallel than quasi-perpendicular connections. The reflected ions have musch softer spectra. The cumulative frequencies for flux level occurrence patterns exhibit differences for quasi-parallel and quasi-perpendicular geometries up to approximately 200 keV.     

磁暴期间甚低纬哨声的导管传播

对磁暴期间甚低纬哨声导管传播的机理进行了初步的讨论，根据暴时低纬哨声的特点，认为暴时出现的扩展哨、多跳回波是沿东向扰动电场导致的场列电离增强路径传播的。对在东向电场影响下低纬Ｆ层的数值模拟表明，东向电场导致的场列电离增强结构有利于哨声的捕获和传播。     

An attempt to study long-term variation of sporadic E layers using neural networks

Strong positive correlation between sporadic E layers and the solar activity and the long-term declining trend of Es were found in this paper. Then the feed-forward back propagation neural networks (NNs) were used to simulate the long-term variation of Es at four stations and predict foEs yearly average values. The inputs used for NNs are the yearly mean values of foEs in the daytime of the past ten years and the yearly averaged data of solar 10.7 cm radio flux (F107) of the present year, and the output is the present yearly mean value of daytime foEs. The outputs of trained NNs have high correlation with the desired values and the foEs yearly mean values predicted by NNs have good agreement with the observed data. The results indicate that NNs can make full use of the observed data to simulate the long variation rule of Es. Also, the results confirm the effect of solar activity on Es.     

Acceleration at the termination shock: Anisotropies and spectra

Low-energy termination shock particle populations observed by the Voyagers upstream of the shock exhibited strong field-aligned beaming with anisotropies of the order of unity. The Parker transport equation is valid only for nearly isotropic phase space distributions and is inapplicable to these highly beamed populations. The usual approach is to revert to the more general focused transport equation retaining pitch-angle information. We developed a complimentary technique employing a three-moment expansion of the Skilling equation using Legendre polynomials. We investigate the effects of adiabatic focusing and reflection on the diffusive acceleration process at oblique shock waves. It is shown that low-energy particle intensities are discontinuous and sharply peaked at the shock, consistent with the observations. Particle spectra are not only harder than the power laws predicted from diffusive transport theory, but also exhibit spectral gaps near the low-energy acceleration threshold due to more efficient acceleration by scattering and mirroring. Our model also predicts upstream anisotropies as high as 100% for highly oblique shocks whereas downstream distributions are nearly isotropic.     

Effect of the seed perturbation amplitude on the equatorial spread F initiation during solar minimum

The contribution of gravity wave (GW) to the initiation/development of spread F during a solar minimum year was investigated through the comparison of the observed precursory parameters and characteristics of the corresponding equatorial spread F (ESF) events. The ionospheric parameters were recorded at the magnetic equatorial station Sao Luis (2.3°S, 44°W, dip latitude 2°S) during March and October 2010. These data were used to estimate the influence of the relative gravity wave amplitude and the ambient ionospheric condition on the diurnal variation of the spread F initiation. The vertical velocity drift indicated a clear control and defines the threshold for the seasonal variability of the ESF occurrence. However, it was insufficient to solely determine or predict the day to day variation of ESF occurrence. Thus, few days with contrasting ambient ionospheric condition and magnitude of GW amplitude were analysed in order to investigate the role of the different precursory factors in the observed diurnal variation of the plasma irregularity development. The density scale length and gravity wave amplitude were shown to immensely contribute to the linear instability growth rate, especially during the days with a low post-sunset rise. Thus, the experimental observations have demonstrated the strong inter-dependence between the precursory factors and they have also highlighted the probable control of the ESF morphology.     

The characterization of atmospheric spread functions affecting satellite remote sensing of the earth's surface

A Monte Carlo simulation program has been used to compute the signal contribution of surface elements outside the field of view of a satellite borne sensor in both the visible and infrared spectra for realistic atmospheric models. For azimuthally symmetric reflection the effect of an arbitrary surface albedo pattern can be represented by two spread functions, namely, 1) the ground-satellite spread function, and 2) the ground-ground spread function. Examples of computed spread functions are shown together with their Fourier correspondents, the modulation transfer functions. It has been shown that the central portions of the line spread functions behave as the inverse distance while the point spread functions always possess a region behaving as the inverse square of the radial distance. The median distance is shown to relate to the weighted molecular and aerosol scale height, and for large distances the spread functions become independent of the aerosol loading.