【作者】 刘宇；

【导师】 曹金祥；

【作者基本信息】 中国科学技术大学 ， 等离子体物理， 2015， 博士

【摘要】 电离层是地球大气的一个电离区域,其存在对大气电学和地球磁层的形成起着重要作用。电离层活动异常会对空间及地面技术系统造成严重危害,尤其是随着人类活动范围的进一步扩大,无线电通讯、卫星导航、航天器飞行、空间天气预报等活动都会受到电离层变化的影响。因此开展人工影响电离层空间天气的研究具有重要意义,而化学物质释放扰动电离层形成电离层空洞就是一种可行的人工影响空间天气的方法。电离层空洞可以影响无线电波的传播、高功率微波加热电离层的效率等一系列空间活动；同时在基础研究方面也提出了许多亟待解决的问题,例如电离层空洞边界层存在着从离子频率到低混杂频率的等离子体不稳定性,而这些不稳定性的产生与增长是电离层空洞的演化过程必须考虑的因素。因此,化学物质释放形成电离层空洞的研究是一个兼具国防安全和基础等离子体物理前沿的研究课题。世界各国科学家在过去几十年进行了大量化学物质主动释放制造电离层空洞空间实验和数值模拟研究。由于空间主动实验的主要手段依赖于相干散射雷达、非相干散射雷达、全天光谱仪以及一些地基、箭载和星载设备,这些对设备磁流体大尺度的物理问题研究很有优势。但是设备自身时间空间分辨率的限制,对电子离子混杂不稳定性这样的动理学尺度的不稳定性过程目前并没有很好的研究结果。我们在实验室等离子体中研究了化学物质主动释放形成电离层空洞边界层的非线性演化,首先验证了实验室环境下研究电离层空洞的可行性。由于空气辉光放电的组分和化学过程都接近于真实电离层环境,本论文采用空气辉光放电等离子体来模拟电离层环境。在背景等离子体形成之后,我们释放SF6、CC12F2和C02进入等离子体来形成模拟电离层空洞。实验采用微波干涉法和光谱法来研究模拟电离层空洞的演化,观察到了密度梯度随着等离子体气压和释放比例的演化,发现不同化学物质释放造成的密度梯度与空间主动实验的观测结果相一致。同时,我们还发现负离子中间产物(NI)物质比正离子中间产物(PI)物质降低电子密度效果明显；并且NI物质形成电离层空洞需要的时间较PI物质更短。这些结果与相应的空间主动实验观测结果一致。同时,我们还观察到了SF6释放造成777.4m的气辉增强和C02释放造成的630nm的气辉增强,这与全天光谱仪观测的结果一致。证明实验室等离子发生着和主动空间实验相同的过程,从而验证了我们研究方法的可行性。在此基础上我们发展了一种在实验室环境下研究电离层空洞产生与演化的方法,即通过无量纲参量定标的方法在实验室中产生ωpe/ωce、β值等一系列无量纲参量与真实空间环境相同或接近的等离子体；基于此我们可以在实验室实现和空间等离子体相同的物理过程。由于实验室环境下,可通过控制实验参数实现对电离层空洞边界层的物理问题进行详细研究。同时实验室等离子体诊断的时空分辨率相比空间观测都有较大优势,因此实验室环境通过定标方法可以精确研究电离层空洞边界层演化等微观物理问题。我们通过该方法在实验室研究了电离层空洞边界层的非线性演化过程,得到了边界层等离子体电子密度和等离子体电势的演化过程。我们观测到边界层上存在巨大的密度梯度Vn。和等离子体电势中f上升,发现在边界层电子密度梯度Vn。为等离子体电位涨落提供自由能。由于等离子体悬浮电位中f的变化会导致非均匀电场E(r)的产生,进一步在边界层激发剪切E×B流的产生。剪切流会驱动一系列从离子频率到混杂频率的等离子体不稳定性。通过对电子密度和等离子体电位的涨落做数字信号谱分析,我们发现悬浮电位涨落中存在一个低混杂频率范围内波结构。通过对该结构进行互功率谱分析和双谱分析,我们证实了该结构为剪切流驱动的电子离子混杂不稳定性(Electron-Ion Hybrid Instability)形成的的涡状相干结构。边界层电子离子混杂不稳定性的增长以及涡状相干结构的形成对电离层空洞的非线性演化起着重要作用。例如,边界层密度不规则体的形成就与电子离子混杂不稳定性相关。我们在实验室发现了该结构的存在对解释诸多空间主动实验的观测结果具有重要意义。我们还研究了电离层空洞边界层出现的电磁涨落。磁探针的信号显示,边界层上存在较强的电磁涨落。该涨落具有BT、BZ和Bθ分量,并且θ的分量远远大于其他分量。数字信号谱分析显示磁场涨落中存在一个低混杂频率的结构。经过对信号进一步进行互相关分析,我们发现这是右旋极化的哨声模式。并且通过静电电子离子混杂模式和电磁模式的频率对比,我们判断该哨声模式来源于静电电子离子混杂模式的非线性散射,这是首次给出从静电频率向电磁频率转化的实验证据。总之,我们在实验室环境下研究了化学主动释放形成的电离层空洞边界层的演化过程。通过采用无量纲参量定标的方法,我们研究了该区域的静电涨落和电磁涨落；发现了静电电子离子混杂模式以及其电磁波段哨声模式存在的证据,同时间接证明了边界层静电电子离子混杂模式可以经过非线性散射转化为电磁哨声模式。这些静电以及电磁涨落对边界层的演化过程动力学行为起着重要作用。由于实验室研究能够对许多空间观测不到的微观物理能进行详细研究,因此它能与目前电离层空洞的主要两种研究方法(空间主动实验和数值模拟)形成了一个很好互补效果,为国家即将开展的主动空间实验研究积累经验。更多还原

【Abstract】 The Earth’s ionosphere, a ionized region of upper atomsphere, plays an important role in atmospheric electricity and formation of the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propaga-tion to distant places on the Earth, especially in the space era. Therefore, it is crucial to progress the study of the artificial modification of the ionosphere. Ionospheric de-pletion, produced by artificial release of attachment chemicals, was widely investigated and taken as a potential technique for the artificial modification of space weather in the past decades. Active release experiment and numerical simulation are the primary men-thods to study the ionospheric depletion until now. However, limited to the resolution of the dignostic tools, some micro-processes, especially the onset and evolution of the electron-ion hybrid(EIH) instability in the the boundary layer, have been few observed and analysed until recently. The EIH mode is believed to play important roles in the evolution of the ionospheric depletion, such as the anomalous trasport and the origin of irregularities.In our work, a new approach for investigating ionosphere chemical depletion in the laboratory is introduced. Air glow discharge plasma closely resembling the ionosphere in both composition and chemical reactions is used as the artificially created ionosphere. The ionospheric depletion experiment is accomplished by releasing chemicals such as SF6, CCl2F2, and CO2into the model discharge. The evolution of the electron density is investigated by varying the plasma pressure and input power. It is found that the neg-ative ion (SF6-,CCl2F2-) intermediary species provide larger reduction of the electron density than the positive ionC(CO2+) intermediary species. The negative ion intermedi-ary species are also more efficient in producing ionospheric holes because of their fast reaction rates. Airglow enhancement attributed to SF6and CO2releases agrees well with the published data. Compared to the traditional methods, the new scheme is sim-pler to use, both in the release of chemicals and in the electron density measurements. It is therefore more efficient for investigating the release of chemicals in the ionosphere.Based upon the previous study, we have experimentally investigated the boundary layer processes of artificially-created ionospheric depletions. Those ionospheric deple-tions were modeled via releasing attachment chemicals, such as SF6, CF4, and CO2, into the ambient plasmas. Boundary layer of width of electric scale length emerged and separated those plasmas into two regions, the ambient plasmas and the negative ions plasmas. In the localized boundary layer, those fluctuations of the electron den-sity and the floating potential were investigated varying with the plasma pressure and the partial pressure of released chemicals. The electron density decreased sharply that yielded steep density gradients▽ne, and the floating potential increased which gen-erated sheared electron flows. It is found that the magnitude of fluctuating floating potential is proportional to that of the▽ne. Those fluctuations were analyzed in detail using digital spectra analysis techniques. Vortex-like coherent structures were observed in the fluctuations of electrostatic potentials. These coherent frequencies are sensitive to the mass of the negative ions, and all lie in the lower hybrid(LH) range. By compar-ing the experimental results with theoretical predictions, the modes have been identified as the coherent structures resulting from the electron-ion hybrid instability. Our results are important to study the early phase nonlinear evolution of the ionospheric depletion, and also may be applied to the plasma sheet boundary layer in where often encounters the narrow electron density gradients and sheared electron flows.In addition, we also studied the spontaneously generated electromagnetic fluctua-tion in the boundary layer of laboratory-created ionospheric depletion. These depletions were modelled via releasing attachment chemicals into the ambient plasmas. Electron density gradients and sheared flows appeared in the boundary layer of nagative ions and positive ion plasmas. The electromagnetic fluctuation lies in the LH range, and it was indentied as the right-hand polarized whistler wave branch. Besides, we also con-firmed that thees whistler modes were tranformed from the electrostatic EIH modes in the boundary layer. The research can be applicated to the geospace enviroment such as the early phase nonlinear evolution of the ionospheric depletion, and also may be ap-plied to the plasma sheet boundary layer in where often encounters the narrow electron density gradients and sheared electron flows.In summary, we have studied the nonlinear evolution of the bounday layer of lab-oratory. Electrostatic and electromagnetic fluctuations were successfully observed in the region, and the EIH mode and whistler mode also were studied in detail. Besides, we found that the EIH mode can transform to the whistler mode via nonlinear scatter-ing processes. Therefore,the project provide a good supplement for the active release experiment and numerical simulation, and will certainly gain experiences for the future active releasing experiments of China.更多还原