等离子体对GPS信号载噪比的影响

飞行器再入过程产生等离子体,等离子体中电子密度的变化对电磁波信号造成幅度衰减和相位偏移,严重时导致通信中断。理论分析了等离子体对GPS信号载噪比的影响;利用实验室环境下的等离子体模拟装置,设计了GPS二键相控(BPSK)调制信号透过等离子体的地面模拟方案,进行了等离子体对GPS接收机性能影响的实验研究,获得了不同电子密度下的衰减和导航信号载噪比的数据。实验结果与理论分析一致,不仅证明了等离子体下衰减与导航信号载噪比的变化是一致的结论,而且验证了实验方法的有效性。     

Effects of adiabaticity and non-adiabaticity of dust charge fluctuation on the propagation of the dust ion acoustic waves in inhomogeneous dusty plasma

A theoretical investigation has been made for adiabatic positive and negative dust charge fluctuations on the propagation of dust-ion acoustic waves (DIAWs) in a weakly inhomogeneous, collisionless, unmagnetized dusty plasmas consisting of cold positive ions, stationary positively and negatively charged dust particles and isothermal electrons. The reductive perturbation method is employed to reduce the basic set of fluid equations to the variable coefficients Korteweg–de Vries (KdV) equation. Either compressive or rarefactive solitons are shown to exist depending on the critical value of the ion density, which in turn, depends on the inhomogeneous distribution of the ion. The dissipative effects of non-adiabatic dust charge variation has been studied which cause generation of dust ion acoustic shock waves governed by KdV-Burger (KdVB) equation. The results of the present investigation may be applicable to some dusty plasma environments, such as dusty plasma existing in polar mesosphere region.     

Cassini Plasma Spectrometer Investigation

The Cassini Plasma Spectrometer (CAPS) will make comprehensive three-dimensional mass-resolved measurements of the full variety of plasma phenomena found in Saturn’s magnetosphere. Our fundamental scientific goals are to understand the nature of saturnian plasmas primarily their sources of ionization, and the means by which they are accelerated, transported, and lost. In so doing the CAPS investigation will contribute to understanding Saturn’s magnetosphere and its complex interactions with Titan, the icy satellites and rings, Saturn’s ionosphere and aurora, and the solar wind. Our design approach meets these goals by emphasizing two complementary types of measurements: high-time resolution velocity distributions of electrons and all major ion species; and lower-time resolution, high-mass resolution spectra of all ion species. The CAPS instrument is made up of three sensors: the Electron Spectrometer (ELS), the Ion Beam Spectrometer (IBS), and the Ion Mass Spectrometer (IMS). The ELS measures the velocity distribution of electrons from 0.6 eV to 28,250 keV, a range that permits coverage of thermal electrons found at Titan and near the ring plane as well as more energetic trapped electrons and auroral particles. The IBS measures ion velocity distributions with very high angular and energy resolution from 1 eV to 49,800 keV. It is specially designed to measure sharply defined ion beams expected in the solar wind at 9.5 AU, highly directional rammed ion fluxes encountered in Titan’s ionosphere, and anticipated field-aligned auroral fluxes. The IMS is designed to measure the composition of hot, diffuse magnetospheric plasmas and low-concentration ion species 1 eV to 50,280 eV with an atomic resolution M/ΔM ∼70 and, for certain molecules, (such asN ₂ ⁺ and CO⁺), effective resolution as high as ∼2500. The three sensors are mounted on a motor-driven actuator that rotates the entire instrument over approximately one-half of the sky every 3 min.This revised version was published online in July 2005 with a corrected cover date.     

Thin Current Sheets in the Magnetotail Observed by Cluster

The dynamics of the current sheet is one of the most essential elements in magnetotail physics. Particularly, thin current sheets, which we define here as those with a thickness of less than several ion inertia lengths, are known to play an important role in the energy conversion process in the magnetotail. With its capability of multi-point observation, Cluster succeeded to obtain the current density continuously and therefore identify structures of thin current sheets. We discuss characteristics of the thin current sheets by showing their temporal evolution and the spatial structures based on several Cluster observations.     

Space Weather in the Equatorial Ionosphere

The ‘scintillations’ observed on signals received in the equatorial region from GPS satellites are due to plasma instabilities in the F region of the ionosphere, also detected as spread F. These instabilities give rise to depletions of ionisation or ‘bubbles’. The occurrence of these events and their relation to the equatorial electrojet are reviewed. Possibilities of short-term forecasting are examined with particular attention to problems encountered in modelling the equatorial electrojet. This revised version was published online in August 2006 with corrections to the Cover Date.     

再入弹头非平衡尾迹的回波特性分析

本文利用文[1]方法计算了纯空气非平衡再入弹头尾迹流场,分析了尾迹等离子体场对雷达波的频响效应,并进一步估算了几种高程条件下的雷达散射截面,说明影响截面大小和分布的几个重要因素。分析和计算表明,以物体底部直径和环境参数表征的雷诺数达到转捩值以后,在完全湍流尾迹中可能出现散射截面的突增现象。     

毛细管放电型脉冲等离子体推力器输出特性分析

为了研究毛细管放电型脉冲等离子体推力器输出特性,借助电学诊断手段展开实验研究,获得了推力器典型放电波形,系统研究了不同毛细管内径和施加电压对等离子体等效阻抗、沉积能量效率的影响规律。利用微冲量测量台架,测试了不同参数下毛细管推力器输出元冲量,并通过计算获得了推力器比冲、总体效率的变化规律。实验结果表明,当毛细管内径不断增大时,能量沉积效率不断下降,元冲量下降,比冲降低。主电容电压增大时,放电能量不断增大,能量沉积效率降低,元冲量和单次等效烧蚀质量不断增大,但推力器比冲和总体效率均先增加并趋于稳定。当毛细管腔体长度为16mm,内径3mm,主电容2.5μF,充电电压为2kV时,输出元冲量350.79±7.50μN?s,比冲531s,总体效率可达18.3%。     

面向立方星变轨任务的机动控制策略

为了实现立方星在轨飞行与变轨,基于模块化推进器系统提出混合控制策略实现微小卫星轨道持续变化任务需求。首先,针对多单元立方星单一主推进器的结构部署,基于高斯变分方程采用连续低推进力实现轨道机动变化。为了实现对立方星主推进器的指向调整,基于姿态动力学模型利用PD连续姿态控制求得所需扭矩,实现对立方星的指向角和指向角速度调节。针对配置的微脉冲等离子推进器(μ-PPT)不连续的特点,通过搜寻μ PPT最优脉冲序列组合获得实际扭矩,满足对外部干扰的持续补偿以及立方星的姿态稳定和指向调整操作需求。此外,引入姿态误差敏感度阈值,使姿态控制器在能够提高系统鲁棒性的同时减少μ-PPT消耗。最后,通过对3U立方星在轨飞行与变轨的具体案例分析,表明所提出的基于微推进器系统的混合控制策略能够实现立方星轨道机动变化需求。     

Superthermal electron’s effects on lump solitons structures in magnetized auroral plasma

Based on the considerations of the acknowledgements of the numerous space observations by satellites in the auroral plasma, astrophysics plasmas, spacecraft observations, various plasma models have been framed and revealed different interesting features, like electrostatic structures, solitary waves, double layers, supersolitons, etc. Soliton theory is a very efficient and competent way to describe nonlinear features. Using Viking satellite data in the auroral plasma, we have derived lump soliton solutions of the Kadomstev-Petviashvili (KP) equation by employing Hirota bilinear method. Due to its wide range of applications, the study of lump soliton is very attractive and important too. It has been shown that the lump solitons structures as well as in the one-dimensional form of lump soliton are varied with associated parameters in the auroral magnetized plasma. During the analysis of the features of the lump solitons, it is found that the system parameters play a pivotal role on the lump solitons structures.     

微波等离子推力器喷管流动数值模拟

为了准确把握微波等离子推力器喷管流动的机理与特点,采用冻结流和非平衡流两种模型对其进行了对比数值模拟分析。非平衡流模型考虑了流动过程中的分解反应、电离反应和复合反应,化学动力学模型为4组分、4反应的有限速率化学反应模型,采用二阶精度NND格式数值求解耦合化学反应源项的N S方程组。数值模拟的结果揭示了喷管内的流场结构,反映了喷管内的离解电离状况,得到了推力器的推力和比冲。分析表明,数值方法有效,计算结果合理,具有工程应用价值,能为相关研究提供参考。