空间等离子体与垂直无碰撞激波相互作用的数值实验

应用混合模拟方法数值研究了高Ａｌｆｖｅｎ－Ｍａｃｈ数垂直无碰撞激波与等离子体间的相互作用。结果表明，激波上游的磁场非常稳定，粒子分布近似为Ｍａｘｗｅｌｌ分布。激波下游磁场存在不规则的湍动，质子分布有一个高能尾，且有些质子被激波反射。跟踪少量高速质子的计算结果表明，在ｔ＝４０Ωｉ＾－１时约有４０％的质子被激波反射，而７％质子可被加速，最大速度值可达到２０ＶＡ激波区的电场分布对质子的减速或加速起了主要     

无碰撞激波的理想MHD模拟

采用了WENO格式数值求解一维理想MHD方程组，模拟了行星际无碰撞激波，研究了垂直无碰撞激波与行星际反向磁场结构和高密度等离子体团的相互作用过程，并与粒子模拟的结果进行比对，两者的结果非常类似．模拟结果表明，对涉及无碰撞激波的大部分现象中理想MHD模拟是准确且可行的，同时相对于粒子模拟又有很好的计算效率，便于扩展至二维或三维的情形．     

行星际结构与准平行无碰撞激波的相互作用

应用一维混合模拟方法数值研究了高密度等离子体团和行星际激波与准平行无碰撞激波的相互作用．结果表明，由于推平行无碰撞激波上游的大振幅低频波动的散射，除了在通过激波过渡区时稍有压缩外，等离子体团从激波的上游开始就一直是不断弥散的．行星际激波在向准平行无碰撞激波靠近的过程中，会在其上游产生大振幅的低频波动，同时行星际激波的强度不断增加，最后和准平行无碰撞激波会并成一个新的激波，在新激波前继续有大振幅的低频波动产生     

准平行无碰撞激波的混合模拟

本文应用一维混合模拟方法数值研究了准平行无碰撞激波的结构.结果表明,激波上游的质子和准平行无碰撞激波相互作用后,有部分质子被激波反射,并向激波上游运动很长一段距离,从而激发起束流不稳定性,引起大振幅的共振右旋偏振的低频波动.这些波动在太阳风的带动下向激波下游运动,靠近激波后与激波合井,同时在激波的上游不断有新的波动产生.此过程能不断重复地进行.在平行无碰撞激波的情况下,在激波的下游还有大振幅的非共振右旋偏振的低频波动.激波上游的低频波动在向下游运动的过程中强度不断加强,最后超过原来激波的强度,形成新的激波.      

磁云边界层中的重联慢激波观测分析

在Petschek模型中，排空区边界处的一对慢激波是能量耗散的重要机制.已有大量行星际空间的Petschek型磁场重联排空区观测事件被报道，但是只有少量的排空区边界处观测到了慢激波.针对一例位于磁云边界层中的Petschek型磁场重联排空区观测事件，在排空区靠近磁云一侧边界处证认了一例慢激波.激波跃变层两侧的磁场和等离子体参数满足Rankine-Hugoniot关系，且激波上下游的中间马赫数均小于1，上游的慢马赫数为2.94（>1），下游的慢马赫数为0.65（<1），符合慢激波的观测特征.磁云内部的等离子体β值很低，局地阿尔芬速度高，同时磁云边界层中可能发生丰富的磁场重联活动，这可能是磁云前边界处慢激波形成的原因.      

无碰撞激波上游低频波的激发及其对激波耗散机制的影响

由于很大一部分来自激波上游的粒子被激波面所反射,因此在准平行无碰撞激波的上游存在着等离子体束流．通过一维混合模拟方法,计算了束流密度较小(nb／n0=0．02)和较大(nb／n0=0．2)两种不同情形下的等离子体束流不稳定性．结果表明,在束流密度较小时,束流激发的主要是平行于背景磁场方向传播的右旋波,此波动只能对束流粒子产生影响,包括减速和加热．在束流密度较大时,束流可同时激发平行和反平行于背景磁场方向传播的右旋波,除能对束流粒子产生影响外,还可通过非共振作用加速和加热背景粒子．文中对准平行无碰撞激波耗散机制的影响也进行了讨论。     

太阳风粒子速度分布不对称性的产生机制

1引言太阳风中由于磁场的作用,在垂直磁场平面内质子做回旋运动．通常在此平面的质子速度分布函数是旋转对称的．实际观测中,虽然大部分由测量值内插得到的速度分布是旋转对称的,但少数是不对称的．产生这种不对称性的原因还不清楚．文献川提出o粒子与间断面相互作用可产生非回族对称分布．由于这种非旋转对称的速度分布应是不稳定的,因而会很快弛豫到了旋转对称的分布．很难解释非对称的分布可经常被观测到的事实．我们认为这种非对称分布的观测结果并不一定是真实的非回旋对称的分布函数产生的而可能是由于太阳风等离子体小尺度的不均…     

磁场重联的二维粒子模拟研究

利用二维全粒子模拟方法研究了无碰撞等离子体中的磁场重联过程，得到了不同区域的离子和电子速度分布．计算结果表明，电子和离子在扩散区中的不同动力学特性产生的Hall电流使磁场的y分量By呈现四极形分布．离子和电子的速度分布偏离了初态时的Maxwell分布，呈现非局域的多重分布．同时由于磁场重联而产生的电场使电子在X点附近得到加速和加热，因而在电子的能谱分布中形成-高能尾。     

垂直激波条件下能量粒子加速机制的模拟研究

在具有湍动的磁场和垂直激波条件下对大量测试粒子的轨迹进行了数值计算,研究了激波强度和粒子初始能量对于粒子穿越激波的平均能量变化的影响,分析了漂移加速(SDA)在不同条件下对粒子加速的贡献,并给出了一个与数值结果相符合的漂移加速理论公式△E=amvivup(1-1/s).结果表明,加入磁场湍流后,垂直激波条件下粒子仍主要受到漂移加速作用,而基于粒子引导中心的耗散漂移加速理论在此条件下失效.      

行星际起伏向磁层顶的输运

时间尺度为分钟数量级的太阳风速度和行星际磁场大幅度扰动实际上始终存在于行星际空间的。这些扰动一直传输到紧贴磁层边界面外侧的区域。它们在磁鞘等离子体和磁层顶的相互作用过程中可能起很重要的作用。行星际起伏中的磁场分量在通过地球弓激波时首先经历一次跳跃,然后一部分扰动被带到磁层边界面处。在边界面附近磁场扰动幅度被大大地放大了。弓激波上游的太阳风条件控制了放大因子。本文所作的数值模拟研究结果表明,如果上游有大幅度的扰动,在边界面附近就有大幅度的Alfven起伏的磁场分量。当上游磁场接近垂直于日地联线时,放大因子变得相当大,而且放大因子随上游的等离子体β值和/或Alfven马赫数的增加而增加。上游各向异性对放大因子的影响不大。在磁层边界附近存在大幅度起伏表明这里不存在稳定的片流。      

膨胀磁场和太阳风粒子相互作用三维数值模拟

采用三维模型,使用混合网格质点法HPIC(Hybrid Particle-in-Cell)对膨胀的磁场和太阳风相互作用过程进行数值模拟.研究了线圈产生的偶极子磁场在注入等离子体后和太阳风粒子的相互作用过程,并对以不同速度入射的等离子体引起的太阳风粒子的变化和磁场变化进行了比较.研究结果表明,偶极子磁场和太阳风作用时会产生弓形激波,此时磁压等于太阳风粒子的动压,当向线圈产生的偶极子磁场中注入高能等离子体时引起磁场膨胀,膨胀的磁场将会排斥太阳风粒子向外运动,从而引起弓形激波的变化,增大与太阳风相互作用的面积,并且粒子入射速度越大,磁场膨胀越明显,与太阳风相互作用愈强.      

The plasma regime at comet Giacobini-Zinner

This review of the plasma regime sampled by the encounter of the International Cometary Explorer spacecraft (ICE) with the comet Giacobini-Zinner, discusses the shock, or bow wave, ion pickup, ionization mechanisms, and the cometary plasma tail.

The observations are consistent with the existence of a weak shock, which may be pulsating, but do not exclude the suggestion by Wallis and Dryer that the shock, though present around the sub-solar point, is in process of decaying to a wave on the flanks.

Pickup of cometary ions provokes, by means of several mechanisms, ion cyclotron, mirror, beam and electrostatic instabilities which cause strong turbulence in the inner coma, as indicated in the power spectra of the magnetic field in the coma and the surrounding volume. Heavy mass loading and consequent slowing down of the solar wind is observed. Acceleration of ions by a stochastic mechanism is indicated.

Ionization of cometary neutrals occurs principally by photoionization and charge exchange. Alfvens critical velocity mechanism, likely operates only in the inner coma not visited by ICE. A steep increase of nearly two orders of magnitude in electron density occurs in the tail, where electron velocity distributions show evidence of entry of electrons from the solar wind. The turbulence there is damped by the high ion density and low temperature.

In general, the vicinity of the comet is filled with plasma phenomena and a rich variety of corresponding atomic and molecular processes can be studied there. Comparison between the ICE, Giotto, and Vega observations forms a most valuable future study.     

Solar wind interaction with lunar crustal magnetic anomalies

Using Lunar Prospector data, we review the magnetic field and electron signatures of solar wind interaction with lunar crustal magnetic sources. Magnetic field amplifications, too large to represent direct measurements of crustal fields, appear in the solar wind over strong crustal sources, with the chance of observing these amplifications depending on upstream solar wind parameters. We often observe increases in low-energy (?100 eV) electron energy fluxes simultaneously with large magnetic field amplifications, consistent with an increase in plasma density across a shock surface. We also often observe low frequency wave activity in the magnetic field data (both broadband turbulence and monochromatic waves), often associated with electron energization, sometimes up to keV energies. Electron energization appears to be correlated more closely with wave activity than with magnetic amplifications. Detailed studies of the interaction region will be necessary in order to understand the physics of the Moon–solar wind interaction. At present, the Moon represents the only natural laboratory available to us to study solar wind interaction with small-scale crustal magnetic fields, though simulation results and theoretical work can also help us understand the physical processes at work.     

The Compton–Getting effect of energetic particles with an anisotropic pitch-angle distribution

This paper presents a simulation of anisotropy measurements by the low-energy charged particle (LECP) experiment on Voyager 1 for cases when the particle pitch-angle distribution function in the solar wind plasma reference frame is not isotropic. The model includes both the Compton–Getting anisotropy and perpendicular diffusion anisotropy that possibly exists in the upstream region of the termination shock. The results show that the Voyager 1 data cannot rule out either the model with zero solar wind speed or the one with a finite speed on qualitative basis. The determination of solar wind speed using the Compton–Getting effect is affected by the assumption of the magnetic field direction and perpendicular diffusion anisotropy. Because the pitch-angle distribution anisotropy is so large, a small uncertainty in the magnetic field direction can produce very different solar wind speeds ranging from ∼0 to >400 km/s. In fact, if the magnetic field is chosen to be in the Parker spiral direction, which is consistent with the magnetometer measurement on Voyager 1, the derived solar wind speed is still close to the supersonic value. Only the two lowest-energy channels of the LECP instrument may give a definitive answer to the solar wind speed. However, because these channels contain a very high level of cosmic ray background, an uncertainty of just a few percent in the background can entirely hamper the estimate of solar wind speed.     

Fluctuations of cosmic rays and IMF in the vicinity of interplanetary shocks

Fluctuations of cosmic rays and interplanetary magnetic field upstream of interplanetary shocks are studied using data of ground-based polar neutron monitors as well as measurements of energetic particles and solar wind plasma parameters aboard the ACE spacecraft. It is shown that coherent cosmic ray fluctuations in the energy range from 10 keV to 1 GeV are often observed at the Earth’s orbit before the arrival of interplanetary shocks. This corresponds to an increase of solar wind turbulence level by more than the order of magnitude upstream of the shock. We suggest a scenario where the cosmic ray fluctuation spectrum is modulated by fast magnetosonic waves generated by flux of low-energy cosmic rays which are reflected and/or accelerated by an interplanetary shock.     

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.      

微阴极电弧推力器羽流探针诊断研究

微阴极电弧推力器（micro-cathode arc thruster,μCAT）具备功率低和结构简单的特点,能够满足微纳卫星的任务需求,具有良好的发展前景。μCAT羽流的诊断可以揭示推力器的加速机理,对提高其性能具有重要意义。利用朗缪尔三探针对μCAT羽流进行诊断,得到了μCAT羽流不同位置的电子温度、电子密度和离子速度等羽流特性,研究了外加磁场、充电时间和阴极材料对羽流特性的影响。研究结果表明,μCAT放电初期产生的等离子体电子温度较高,密度较大;随着等离子体向下游运动,电子温度和电子密度降低,离子速度增大;外加磁场的磁感应强度越强,电子温度和离子速度越高,电子密度有所降低;磁场位置适当向推力器下游平移,能够有效提高推力器中轴线的电子密度;μCAT充电时间越长,电子温度、电子密度和离子速度越大;相比于CuW和AgW阴极,Ti阴极羽流的电子温度更高,电子密度更低。