磁化等离子体中斜传播的离子孤波

近几年卫星空间电场测量经常证认出局地非线性离子静电波,它们可能与极光粒子加速有直接关系。这些静电波被认为或者是离子声波模的演化,或者是静电离子逥旋波模的演化结果。本文研究了磁场中斜传播小振幅离子非线性波的演化,得到非线性Schrodinger方程。结果表明离子声孤波和离子迴旋孤波都是可能的。计算结果与卫星S(3-3)电场测量比较,可以很好说明部分测量结果。      

离子温度对磁化等离子体中非线性静电波的影响

本文讨论了无碰撞磁化低β等离子体中离子温度对非线性静电波的影响。结果表明,在参量α≡T_i/T_e≠0条件下,存在着三种非线性静电波(T_i和T_e分别为离子和电子的热能):在波速ν_p>(1+α)(1/2)c_s情况下存在着非线性离子回旋周期波;在(1+α)(1/2)c_scosθp<(1+α)(1/2)c_s情况下存在着离子声孤立波;在v_p<(1+α)(1/2)c_scosθ情况下存在着非线性离子声周期波。当参量α增加时,孤立波的波幅(最大电位)减小,而另外两种非线性周期波的电位幅度都几乎保持不变。      

太阳风中低频离子静电孤波

本文给出在太阳风超声速流动条件下,离子静电孤波的传播特性,结果与Helios1,2卫星观测的静电离子噪声做了比较。离子声波扰动的非线性发展使太阳风等离子体呈规则的小尺度起伏,离子声波在马赫锥外传播,因此理论预言密度起伏不沿着太阳风速度方向,而是在横向方向.      

离子迴旋波的参量激发

本文分析了极光区上空以低混杂波作为泵波激发静电离子迴旋波的可能性,并计算了增长率。结果表明,当泵波较强时(峰值100mV/m)增长率可达0.5ω_ci,大于电流驱动不稳定性及离子束流不稳定性等线性理论所可能期待的增长率。从而在某些情况下,离子迥旋波的这种参量激发可能是主要的。      

损失锥电子速度分布激发的哨声波

利用一维粒子模拟方法研究了损失锥电子速度分布激发的哨声波非线性演化过程, 并与双麦克斯韦电子分布激发哨声波的情况进行了比较. 结果表明, 这两种情况下, 在线性增长阶段哨声波的主导频率 (能量最集中的一个频率) 是一样的, 波动的激发使得平行于背景磁场方向的电子温度有所提高. 比较而言,在损失锥分布情况下, 哨声波具有主导频率的波模可较早地被激发, 而在线性增长阶段, 哨声波的能量更倾向于集中在高频(短波长)波段. 部分粒子被散射到损失锥中, 而使损失锥分布得以补充. 本文还研究了不同各向异性分布和磁场强度条件下的激发过程.      

月尘静电迁移研究进展

月尘的静电悬浮和迁移是Apollo时期留下的最有争议的问题之一. 其既是研 究月球表面物质演化历史的重要线索, 也是探月工程必须考虑的重要因素. 月尘在月表环境下易因电子附着、光电效应、二次电子发射等过程带电, 并 在月球全球性静电场作用下发生迁移运动. 但目前对月尘静电迁移过程的认 识还不全面, 其主要原因在于对月尘静电特性的了解不够准确, 对静电迁移 过程的地面模拟不够充分以及对月球尘埃环境的探测较为缺乏. 未来需进一 步开展模拟月尘的研制, 月尘静电特性的分析, 静电迁移过程模拟以及尘埃 环境的探测等工作.      

基于一维模型的高频电波加热电离层F层数值研究

基于电离层一维仿真加热模型,详细介绍了模型中电子的动量方程、连续性方程、能量方程和各类参量表达式,利用对角矩阵追赶法数值求解电离层F层加热过程,分析了不同时次电子数密度和电子温度的变化,讨论了不同频率和不同功率电波加热的情形.结果表明:当高频电波加热高电离层时,电子温度迅速增加,并很快趋近于饱和状态;电子密度的变化较为迟缓,但在加热过程中其变化幅度却越来越大;电子密度变化量在沿磁场方向上形成空洞和上下稠团两峰一谷构型;频率越大、功率越高的电波加热时,电子密度的变化也越大,但存在一适值频率的电波对电子温度的影响最小.      

镜模波识别方法研究及其在火星磁鞘中的应用

镜模波是温度各向异性等离子体中的一种波动结构，根据磁场和离子分布及波动特性可以进行识别.本文对比了只使用磁场数据与同时使用磁场及离子数据两种识别方法，分析了两类方法的特点.只使用磁场数据的方法基于磁场强度变化大、方向沿背景磁场的特征，通常使用磁场强度的波动幅度ΔB/|B|以及磁场变化方向与背景磁场的夹角θ_min，θ_max作为参数；同时使用磁场及粒子数据的方法利用的是磁场纵波特性、总压平衡和波动在等离子体坐标系下静止的特征.使用两种方法对MAVEN卫星在火星磁鞘内的数据进行识别，结果表明在某些情况下，只使用磁场数据会导致对镜模波的误判.通过研究改变上述参数阈值时识别结果的变化，发现当θ_min> 40°，θ_max < 40°，ΔB/|B|> 80%时，只用磁场数据可取得较好的识别效果.      

在静电离子迴旋波中电子分布的演化及不稳定性分析

静电离子迴旋波可以导致异常电阻及平行电场的出现.本文分析了电子分布函数在这种波场及平行电场中的演化.结果表明, 电子分布可分成捕获和逃逸两部分.双流不稳定性的发展使逃逸部分变平, 形成具有长而平的尾巴的分布函数.后者在一定速度空间范围内对异常迴旋共振是不稳定的, 结果导致电子的热化和投掷角扩散, 使电子损失掉平行方向的能量, 形成尾部隆起的分布特征, 导致新的不稳定性的出现, 使隆起的部分基本拉平.此后虽然有少部分电子仍可能被加速, 但要慢得多, 且边加速边被热化.      

回旋波共振加热加速的多重离子流体模型

在回旋波共振准线性理论基础上建立了自恰描述质子和氧离子各向异性温度和整体速度的双离子流体模型,求得数值解,并与UVCS观测相比较.发现在波谱斜率固定的假设下,模型的数值结果虽然与观测有相同趋势,但不能在误差范围之内解释全部观测现象.进一步的研究需要考虑波谱形状的自洽演化.      

Furthering our understanding of electrostatic solitary waves through Cluster multispacecraft observations and theory

Nonlinear isolated electrostatic solitary waves (ESWs) are observed routinely at many of Earth’s major boundaries by the Wideband Data (WBD) plasma wave receivers that are mounted on the four Cluster satellites. The current study discusses two aspects of ESWs: their characteristics in the magnetosheath, and their propagation in the magnetosheath and in the auroral acceleration (upward current) region. The characteristics (amplitude and time duration) of ESWs detected in the magnetosheath are presented for one case in which special mutual impedance tests were conducted allowing for the determination of the density and temperature of the hot and cold electrons. These electron parameters, together with those from the ion experiment, were used as inputs to an electron acoustic soliton model as a consideration for the generation of the observed ESWs. The results from this model showed that negative potential ESWs of a few Debye lengths (10–50 m) could be generated in this plasma. Other models of ESW generation are discussed, including beam instabilities and spontaneous generation out of turbulence. The results of two types of ESW propagation (in situ and remote sensing) studies are also presented. The first involves the propagation of bipolar type ESWs from one Cluster spacecraft to another in the magnetosheath, thus obtaining the velocity and size of the solitary structures. The structures were found to be very flat, with large scale perpendicular to the magnetic field (>40 km) and small scale parallel to the field (<1 km). These results were then discussed in terms of various models which predict such flat structures to be generated. The second type of propagation study uses striated Auroral Kilometric Radiation (SAKR) bursts, observed on multiple Cluster satellites, as tracers of ion solitary waves in the upward current region. The results of all studies discussed here (pulse characteristics and ESW velocity, lifetime, and size) are compared to in situ measurements previously made on one spacecraft and to theoretical predictions for these quantities, where available. The primary conclusion drawn from the propagation studies is that the multiple spacecraft technique allows us to better assess the stability (lifetime) of ESWs, which can be as large as a few seconds, than can be achieved with single satellites.     

Small amplitude ion acoustic solitons in a weakly magnetized plasma with anisotropic ion pressure and kappa distributed electrons

The Zakharov–Kuznetzov (ZK) equation is derived for nonlinear electrostatic waves in a weakly magnetized plasma in the presence of anisotropic ion pressure and superthermal electrons. The anisotropic ion pressure is defined using Chew–Goldberger–Low (CGL) while a generalized Lorentzian (kappa) distribution is assumed for the non-thermal electrons. The standard reductive perturbation method (RPM) is employed to derive the two dimensional ZK equation for the dynamics of obliquely propagating low frequency ion acoustic wave. The influence of spectral index (kappa) of non-thermal electron on the soliton is discussed in the presence of anisotropic ion pressure in plasmas. It is found that ion pressure anisotropy and superthermality of electrons affect both the width and amplitude of the solitary waves. On the other hand the magnetic field is found to alter the dispersive property of the plasma only, and hence the width of the solitons is affected while the amplitude of the solitary waves is independent of external magnetic field. The numerical results are also presented for illustrations.     

Solitary and periodic waves in magneto–rotating plasmas: Effect of the Kappa–Cairns distributed electrons

The nonlinear propagation of ion–acoustic (IA) waves in a magneto–rotating plasma is studied by considering the Kappa-Cairns electron distribution. Employing the perturbation scheme, Korteweg–de Vries equation is derived. It is seen that both positive and negative potential solitons can be supported in the present plasma model. The numerical results reveal that the Kappa-Cairns distributed electrons modify features of the electrostatic waves significantly. The effects of non–thermal parameters, plasma rotation frequency, ion temperature, and the wave propagation angle on electrostatic solitary wave structures are also discussed here. It is found that the plasma parameters considerably influence the propagation of IA waves in rotating plasmas. Furthermore, using the bifurcation theory of planar dynamical systems to the K-dV equation, we have presented the existence of solitary and periodic traveling waves. Our study may be helpful to understand the behavior of ion–acoustic wave in the rotating plasma.     

Theoretical properties of offset bipolar electric field solitary structures in space plasmas

The bipolar electric field solitary (EFS) structures have been frequently observed in the near Earth plasma regions, such as auroral zone, magnetopause, cusp regions, and magneto-tail. Sometimes these structures are observed as offset bipolar structures. In this paper, the properties of the offset bipolar EFS structures parallel to the magnetic field are studied with an ion fluid model in a cylindrical symmetry by considering electrostatic condition. The model results show that the offset bipolar EFS structures can develop from both ion-acoustic waves and ion cyclotron waves, and propagate along the magnetic field line in the space plasmas if plasma satisfies some conditions. The offset bipolar EFS structures can have both polarities. It will be first negative pulse and then positive pulse if the initial electric field E₀ < 0 or reverse in order if E₀ > 0. The amplitude of the offset bipolar EFS structures first decreases and then increases with the wave propagation velocity. Some results from our model are consistent with the observations.     

准平行无碰撞激波上游能量粒子的观测研究

通过Cluster卫星在2005年3月16日观测到的一个准平行激波观测事例,研究了准平行激波上游低频等离子体波动与能量离子之间的关系.卫星观测结果表明,在准平行激波上游,离子微分能通量受到了非线性波动的调制.在磁场强度较小区域,离子微分能通量较高.产生这种现象的可能原因是准平行激波上游的非线性波动可以捕获离子,被捕获的离子在波动中来回弹跳并被电场加速,从而导致磁场强度较小区域离子微分能通量较高.这一观测结果与已有的混合模拟结果相吻合.      

Generation mechanism of the whistler-mode waves in the plasma sheet prior to magnetic reconnection

The whistler-mode waves and electron temperature anisotropy play a key role prior to and during magnetic reconnection. On August 21, 2002, the Cluster spacecrafts encountered a quasi-collisionless magnetic reconnection event when they crossed the plasma sheet. Prior to the southward turning of magnetospheric magnetic field and high speed ion flow, the whistler-mode waves and positive electron temperature anisotropy are simultaneously observed. Theoretic analysis shows that the electrons with positive temperature anisotropy can excite the whistler-mode waves via cyclotron resonances. Using the data of particles and magnetic field, we estimated the whistler-mode wave growth rate and the ratio of whistler-mode growth rate to wave frequency. They are 0.0016f_ce (Electron cyclotron frequency) and 0.0086f_ce, respectively. Therefore the whistler-mode waves can grow quickly in the current sheet. The combined observations of energetic electron beams and waves show that after the southward turning of magnetic field, energetic electrons in the reconnection process are accelerated by the whistler-mode waves.     

Electron acoustic solitary waves in the Earth’s magnetotail region

Satellite observations have revealed solitary potential structures in the Earth’s magnetotail region. These structures have both positive (compressive) and negative (rarefactive) electrostatic potentials. In this paper we study the electron-acoustic solitary waves (EASWs) in an unmagnetized plasma consisting of cold plasma electrons and isothermal ions with two different temperatures. Using the reductive perturbation method, the nonlinear evolution of such structures is studied. The numerical computations are performed to study the role of two temperature ions in the generation of EASWs. In this case, the model supports the existence of both positive and negative electrostatic potentials with bipolar pulses. The electric field associated with these positive and negative solitary structures are numerically computed. The present study could be useful to construe the compressive and rarefactive electric field bipolar pulses associated with the BEN type emissions in the magnetospheric regions where the electron beams are not present.     

Ion- and electron-acoustic solitons and double layers in multi-component space plasmas

A general model for the ion- and electron-acoustic solitons and double layers in a multi-component unmagnetized plasma consisting of background electrons, counter-streaming electron beams and ions is discussed. The model is based on the multi-fluid equations and the Poisson equation, and uses the Sagdeev pseudo-potential techniques. For identical counter-streaming electron beams and depending upon the plasma parameters, three types of solutions, namely, ion-acoustic, slow and fast electron-acoustic soliton/double layer, are possible. Generally, the ion acoustic solitons have positive potentials, slow-electron acoustic solitons have negative potentials and fast electron-acoustic solitons and double layers can have either positive or negative potentials depending on the core electron density. As beam speed is increased, first ion-acoustic and then slow electron-acoustic solitons disappear. At large beam speed, only fast electron-acoustic solitons/double layers survive. The results may be relevant to the observations of the electrostatic solitary waves (ESWs) observed in the Earth’s magnetosphere.