典型甚低频电磁波对辐射带高能电子的散射损失效应

辐射带中高能电子与空间甚低频电磁波由于波粒共振相互作用发生投掷角散射, 进而沉降入稠密大气而损失. 为研究甚低频电磁波对辐射带中高能电子的散射作用机制, 本文基于准线性扩散理论, 利用库仑作用和波粒共振相互作用扩散系数的物理模型, 得到了两组典型甚低频电磁波与高能电子波粒共振相互作用的赤道投掷角弹跳周期平均扩散系数, 并分析了甚低频电磁波共振散射作用与大气库仑散射作用对不同磁壳及不同能量的辐射带电子扩散损失的影响规律. 以磁壳参数L=2.2, 能量E=0.5 MeV的辐射带电子作为算例, 采用有限差分方法数值求解扩散方程, 计算分析了电子单向通量和全向通量随时间的沉降损失演化规律. 研究结果表明: 当电子能量大于0.5 MeV, 磁壳参数大于1.6时, 甚低频电磁波的共振散射作用显著; 随着磁壳参数或电子能量的增大, 斜传播甚低频电磁波引起的高阶共振相互作用越来越大; 电子全向通量近似随时间呈指数函数形式衰减.     

场向传播的内磁层哨声波对辐射带高能电子的共振扩散

基于高斯分布的哨声波谱密度分布、偶极子背景磁场模型以及建立在卫星观测数据基础上的半经验电子密度纬度分布模型,对于等离子体层顶以外区域(4≤L≤7),计算了准线性当地及弹跳平均电子共振扩散系数,并估算了与磁层哨声波回旋共振导致的辐射带电子损失及加速时间尺度.结果表明,波粒共振相互作用区域取决于电子能量、波谱分布、电子赤道抛射角以及当地电子密度及背景磁场.哨声波共振频率除了与以上5个参量有关外,还与地磁纬度有关.赤道哨声波主要影响较低能量辐射带电子的加速,中高纬度哨声波主要作用于较高能量辐射带电 关键词： 共振波粒相互作用 地球辐射带 哨声波 回旋共振加速及散射沉降     

基于AKEBONO哨声波参数的内辐射带高能电子扩散模拟

为能准确地模拟内辐射带中哨声波对高能电子扩散损失的影响,基于内辐射带AKEBONO哨声波参数统计模型,及随纬度分布的背景冷等离子体密度模型,对引起电子扩散损失的大气分子,空间等离子体嘶声、闪电激发的哨声、人工激发的甚低频三类哨声波,利用准线性扩散理论,计算1.4≤ L≤2.0区域的不同能量电子,受到库仑碰撞和波粒回旋共振相互作用的弹跳周期平均赤道投掷角扩散系数,分析不同作用机制、不同类哨声波、不同能量、不同磁壳参数等对辐射带高能电子扩散损失的影响.结果表明：在赤道面损失锥角附近,高能电子主要受到库仑碰撞作用而扩散;在赤道投掷角接近90°附近区域,等离子体嘶声和闪电激发的哨声是引起扩散的主要因素;内辐射带电子主要受到甚低频电磁波波粒回旋共振扩散影响;扩散系数对高能电子能量及其所处磁壳参数比较敏感,通常,高能电子的能量或所处磁壳参数越大,扩散系数越大.     

低纬电离层人工调制所激发的ELF波射线追踪

通过大功率极低频(ELF)/甚低频(VLF)高频调幅波能有效地扰动低电离层电流, 形成等效的ELF/VLF电离层虚拟天线,辐射ELF/VLF波, 所辐射出的低频信号能够传播进入到磁层,对其传播特性的研究对于理解辐射带高能电子沉降具有重要意义.本文基于磁层射线追踪理论,通过数值模拟得到在低纬地区所激发出的ELF波在磁层中的射线路径,并对其特征进行分析.数值模拟结果表明, 从低纬激发的ELF波在南北半球来回弹跳,并逐渐传播到更远处,对于不同频率的ELF波, 频率越高,传播距离越近,频率越低,传播距离越远,在传播过程中, ELF波会逐渐倾向于在一个固定的磁层区域附近来回反射,在此过程中波法向角也逐渐变为90°,射线方向倾向于沿着背景磁场方向传播.     

地基高频加热激励ELF/VLF波对辐射带高能电子的准线性散射

地球内、外辐射带电子通量的变化对于空间飞行器,尤其是中低轨卫星的防护有着非常重要的影响.基于回旋共振波粒相互作用的准线性理论,使用地基高频发射器发射电波调制低电离层背景电流可以人工激励ELF/VLF波,这些波能使辐射带相对论电子发生抛射角散射沉降进入大气层从而降低其生存期.为了定量地分析人工激励ELF/VLF波散射辐射带高能粒子的可行性,针对内、外辐射带,本文选取了两个典型区域：L=4.6和L=1.5.数值计算结果表明,在内、外辐射带由于ELF/VLF波的人工注入而造成的高能电子损失时间尺度很大程度上取决于冷等离子体参量α^*(∝B²/N₀,这里B是背景磁场,N₀是电子数密度)、电波频谱特性和功率,以及与波发生回旋共振的电子能量.一般来讲,在外辐射带人工ELF/VLF哨声波散射相对论电子使之沉降到大气层要容易得多;低能量的高能电子(200keV)要比高能量的相对论电子(500keV)更有效地通过抛射角散射进入大气层.考虑到高频电波加热电离层激励的ELF/VLF波可能会被捕获在磁层空腔中,来回反射从而得到增强,因此在适当的条件下,地基高频加热装置发射足够的电波功率进入电离层诱导大幅度ELF/VLF波注入到内磁层,能够在1至3天的时间尺度内快速散射外辐射带相对论电子使之沉降,也能够在10天量级的时间尺度里散射生存周期一般为100天甚至更长的内辐射带相对论电子. 关键词： 地基高频加热电离层 ELF/VLF波激励 高能电子散射和沉降 共振波粒相互作用     

低频Alfvén波对等离子体非共振加热与随机加热的粒子模拟

采用粒子模拟方法,研究沿背景磁场方向传播的低频Alfvén波对磁化等离子体加热的物理过程.模拟结果表明:离子在垂直和平行于背景磁场的方向都得到明显的加热,在非共振加热阶段,垂直方向比平行方向的加热效果更加显著,形成温度各向异性;在随机加热阶段,垂直和平行方向的温度最终达到饱和且趋于一致.加热过程中,离子所获得动力学温度的最大值由外加磁场能量密度与等离子体密度的比值决定,与Alfvén波的频率及振幅无关;离子在平行于背景磁场方向上被加速,并最终获得相当于Alfvén波相速度大小的流速.     

甚低频台站信号对地球内辐射带和槽区能量电子的散射效应分析

人工地面甚低频台站发射的10?30 kHz信号主要在地球?低电离层波导传播,部分能量会泄露进入内磁层,进而会影响近地空间中高能电子的动态变化过程.本文详细研究了NWC,NAA和DHO38三个人工甚低频台站信号对内辐射带和槽区高能电子的散射作用.基于准线性理论,分别计算了三个甚低频台站信号单独和共同作用时对高能电子的弹跳...     

Test particle simulations of resonant interactions between energetic electrons and discrete,multi-frequency artificial whistler waves in the plasmasphere

Modulated high frequency(HF) heating of the ionosphere provides a feasible means of artificially generating extremely low frequency(ELF)/very low frequency(VLF) whistler waves, which can leak into the inner magnetosphere and contribute to resonant interactions with high energy electrons. Combining the ray tracing method and test particle simulations, we evaluate the effects of energetic electron resonant scattering driven by the discrete, multi-frequency artificially generated ELF/VLF waves. The simulation results indicate a stochastic behavior of electrons and a linear profile of pitch angle and kinetic energy variations averaged over all test electrons. These features are similar to those associated with single-frequency waves. The computed local diffusion coefficients show that, although the momentum diffusion of relativistic electrons due to artificial ELF/VLF whistlers with a nominal amplitude of ～ 1 pT is minor, the pitch angle scattering can be notably efficient at low pitch angles near the loss cone, which supports the feasibility of artificial triggering of multi-frequency ELF/VLF whistler waves for the removal of high energy electrons from the magnetosphere. We also investigate the dependences of diffusion coefficients on the frequency interval(△f) of the discrete, multi-frequency waves.We find that there is a threshold value of △f for which the net diffusion coefficient of multi-frequency whistlers is inversely proportional to △f(proportional to the frequency components Nw) when △f is below the threshold value but it remains unchanged with increasing △f when △f is larger than the threshold value. This is explained as being due to the fact that the resonant scattering effect of broadband waves is the sum of the effects of each frequency in the ‘effective frequency band’. Our results suggest that the modulation frequency of HF heating of the ionosphere can be appropriately selected with reasonable frequency intervals so that better performance of controlled precipitation of high energy electrons in the plasmasphere by artificial ELF/VLF whistler waves can be achieved.     

Determine the physical mechanism and source region of beat wave modulation by changing the frequency of high-frequency waves

This paper introduces a new approach for the determination of the source region of beat wave(BW)modulation.This type of modulation is achieved by transmitting high-frequency(HF)continuous waves with a frequency difference f,where f is the frequency of modulated ELF/VLF(extremely low frequency/very low frequency)waves from two sub-arrays of a high power HF transmitter.Despite the advantages of BW modulation in terms of generating more stable ELF/VLF signal and high modulation efficiency,there exists a controversy on the physical mechanism of BW and its source region.In this paper,the two controversial theories,i.e.,BW based on D-E region thermal nonlinearity and BW based on F region ponderomotive nonlinearity are examined for cases where each of these two theories exists exclusively or both of them exist simultaneously.According to the analysis and simulation results presented in this paper,it is found that the generated VLF signal amplitude exhibits significant variation as a function of HF frequency in different source regions.Therefore,this characteristic can be utilized as a potential new approach to determine the physical mechanism and source location of BW.     

A diagnostic tool for the study of stochastic properties of density striations excited by powerful electromagnetic waves in the ionospheric plasma

Powerful electromagnetic waves illuminating the ionospheric plasma generate small-scale density irregularities elongated in the direction of the geomagnetic field. Stochastic motion of these irregularities results in fluctuations of HF radio signals backscattered by the illuminated region. Observations of the full wave form of the radio signals make it possible to reconstruct the distribution function of the irregularities over their velocities. An experiment with such a reconstruction is proposed.     

Theoretical analysis of interaction between a particle and an oscillating bubble driven by ultrasound waves in liquid

A theoretical model is developed to describe the interaction of a particle and an oscillating bubble at arbitrary separation between them. The derivation of the model is based on the multipole expansion of the particle and bubble velocity potentials and the use of Lagrangian mechanics. The model consists of three coupled ordinary differential equations. One of them accounts for the pulsation of the bubble and the other two describe the translation of the bubble and particle in an infinite, incompressible liquid. The model here is accurate to order 1/d~(10), where d is the distance between the centers of the particle and bubble. The effects of the size and density of the particle are investigated, namely, the interaction between the particle and bubble changes from repulsion to attraction with the increment of the particle density, and the increment of the particle size makes the interaction between the particle and bubble stronger. It is demonstrated that the driving frequency and acoustic pressure amplitude can affect the interaction of the particle and bubble. It is shown that the correct modeling of the translational dynamics of the bubble and particle at small separation distances requires terms accurate up to the tenth order.