磁场重联中的电子加速机制的数值模拟研究

在应用2.5维混合模拟方法研究Petschek模型磁场重联的基础上,考察了试验电子被加速的特征. 模拟结果表明,稳态的低频重联场能将少量试验电子加速到高能,电子的能谱为幂律谱,但总体分布函数未发生显著变化. 电子在整个加速过程中被束缚在低磁场的加速区内,由重联产生的感应电场Ey分量对其直接加速,根据加速时间和加速区域可以将这些电子分为两种情况：初始位于加速区和漂移到加速区被加速.     

带电粒子在磁场中的运动区及其模型实验

本文讨论了带电粒子在电磁场中的运动区域.在计算过程中假定磁场是有势的,并且势函数分别考虑为轴对称与平面函数.计算结果表明:当磁场足够强时,粒子运动区的边界与磁力线是相似的.这个结果表明,我们可以根据磁力线图形确定带电粒子的运动区.此外,还将上述计算结果与模型实验做了对此,在磁场中的辉光放电实验里,放电中的辉光区部分相当于粒子的运动区,暗区部分相当于粒子禁区.对比结果表明,辉光区的边界与磁力线相似,其中有以下几个结果:（1）当磁场是一个偶极磁场,并且其磁短的大小大于其临界值时,则放电辉光将被捕获于偶极磁场中,其边界近似于磁力线;（2）当偶极磁场受到扰动时,设扰动场分别考虑为与赤道面上的偶极磁场方向相反的均匀磁场,以及位于捕获区之外在赤道面上的电流环磁场时,则偶极磁场磁力线将向外伸长,实验观测到被捕获于偶极磁场中的辉光将同时伸长,并且辉光区边界的变化与磁力线的变化是相似的;（3）根据中性线磁场中的辉光放电实验表明,辉光区的边界仍然与中性楱磁场的磁力线相似.由于气体放电中的现象比较复杂,因此要进行定量的计算是比较困难的,为此我们引进了一些简化假定,定量计算了辉光区在上进各种磁场中的边界,计算出来的辉光边界的大小和形状与实验结果是相同的.     

引力场中磁场重联的数值研究（Ⅱ）磁力线足点的剪切运动

本文基于二维三分量可压缩磁流体动力学模拟，数值研究由于磁力线足点在光球层的剪切运动引起日冕电流片中的磁场重联过程。结果表明，磁力线足点的剪切运动作为引起强迫磁场重联的一种触发机制，将加速磁场重联的发展和磁岛的合并过程。结合不稳定性导致等离子体急剧加速，在β_∞＝０．１的情况下其加速度达到０．３４ν_Ａ∞／τＡ，等离子体的最大下落速度可达１．９０ν_Ａ∞，大于纯电阻撕裂模情况。还讨论了β_∞值对这种磁场重联过程的影响。β_∞值越小，磁场重联和磁岛合并过程发展得越快。     

非均匀动压冲击和停止后产生的瞬时重联过程

在非均匀动压冲击期间和冲击突然停止,可引起等离子体边界层的局部瞬时重联过程．本文用二维可压缩ＭＨＤ数值模拟方法研究了这两个过程．结果表明：当大尺度的均匀横向流从一侧边冲击边界层时,磁力线不弯曲,也不发生磁场重联,只是边界层被推着向下游运动;当局部的非均匀动压（特别是横向剪切流）冲击边界层时,被冲击的同向磁场区磁力线逐渐弯曲,在弯曲的反磁场区,出现磁岛,然后在电流片区发生磁场重联,且逐渐形成准稳态的“反Ｋ型”重联结构;当横向剪切流冲击停止后,边界层区变为非常不稳定的系统,产生多种流体涡旋和流型,并相应地产生多种类型的磁场重联结构,直到涡旋消失变为湍动状态时,磁场拓扑才逐渐恢复到未扰动状态。我们提出,外力作用的突然停止,可能是驱动重联的一种新机制,并对这种重联过程在磁层物理中可能的应用进行了讨论．     

地球磁尾和磁层顶磁场变化的随机特性

本文讨论了平板模型下电阻不稳定性使磁力线由对称平衡状态进入随机状态的可能性;给出该模型下以（x,z）为正则坐标的哈密顿函数表达式;在Harris平衡磁场分布下分析了撕裂模不稳定性,驱动重联和涡旋诱发重联引起的磁力线随机特性;估计了两个磁岛相互作用时磁力线进入随机态的临界值.本文结果可用于讨论地球磁层顶和磁尾处磁场的性质.     

带电粒子在中性线磁场中的运动

本文利用了小扰动方法、轨道法以及粒子的运动区域,比较系统地研究了带电粒子在中性线磁场中的运动。其结果是: 1.带电粒子的运动轨道可分为漂移轨道、波动轨道与8字形轨道三种形式。小扰动方法不能使用于中性线的区域内,在这个区域出现一个与小扰动漂移运动相反方向的运动。 2.在沿中性线方向的电场的作用下,中性线周围的部分粒子可以聚集到中性线附近。当粒子进入非小扰动区时,它们将被中性线磁场反射,并被电场加速。 3.计算出磁尾中性片的厚度为在这个区域内,大部分带正电的粒子的平均运动是沿晨昏方向,带负电的粒子的运动则相反。磁尾区出现一个等离子体中性片电流。     

模拟IMF北向且By分量占主导时磁层顶重联

本文基于自己开发的全球三维磁层模型, 模拟研究了IMF(Interplanetary Magnetic Field)北向并且B_y分量较大(时钟角为60°)时磁层顶三维结构及其重联图像. 结果发现, IMF B_y为正时, 在北极隙区附近尾-昏侧存在IMF与地磁场之间稳定持续的重联现象;参与重联的地球磁场既有闭合磁力线也有开放磁力线;IMF在北极隙区与地球闭合磁力重联后一端与南磁极相连的磁力线在尾向运动时还可能与北尾瓣的开放磁力线重联而重新闭合, 这种重联与磁力线循环过程不同于同一条IMF磁力线分别在南北半球与地磁场重联的模型. 南极隙区的重联发生在尾-晨侧, 其动力学过程与北极隙区情形类似. 我们的模拟结果表明, IMF B_y较大时不可能发生IMF同一条磁力线分别在南北极隙区重联的情形, 也不会因此而减少尾瓣的开放磁力线.     

磁场重联耗散区内X线附近的强低混杂波的Geotail卫星观测研究

Geotail卫星于2003年5月15日在近地磁尾观测到磁场重联并穿越重联耗散区.卫星从尾向—南尾瓣一侧穿越磁场重联耗散区到地向—北尾瓣一侧的过程中,随着等离子体流反向,霍尔磁场(B_y) 被观测到.本文研究了该磁场重联耗散区内的低混杂波.观测结果显示,在磁场重联耗散区核心及其附近区域观测到在低混杂频率附近存在强烈的等离子体波动的增强,其传播方向主要垂直于背景磁场,该等离子体波动为低混杂波.前人的模拟结果认为,当磁场重联得到充分发展之前,低混杂波将消失.本文的观测结果充分说明,当磁场重联充分发展之后,在核心区域仍然存在增强的低混杂波,说明低混杂波贯穿磁场重联的整个发展过程.这种观测结果与计算机等离子体模拟的结果有所不同.本文的观测对低混杂波在磁场重联中的具体表现提供了新的观测证据并有可能修正前人的理论.     

IMF北向时磁层顶重联的模拟研究

本文基于自己开发的全球三维磁层模型,模拟研究了IMF(Interplanetary Magnetic Field)北向时磁层顶重联及磁尾结构.结果发现磁层顶附近存在两种典型的重联过程:一是高纬极尖区IMF与地球磁场的重联,这与空间观测证据和前人的模拟结果是一致的;二是重联后一端在太阳风中另一端与地球相连的磁力线在向磁尾运动中,会发生弯曲、拖曳,在磁尾晨昏侧低纬区域可与尾瓣开放磁力线满足重联条件而再次发生重联.我们认为前一重联会使磁尾等离子片产生与IMF时钟角方向相反的旋转;而后者可重新形成闭合磁力线,可能是LLBL(Low Latitude Boundary Layer)形成的重要原因.     

行星际磁场By分量对地球磁层顶场向电流调制

采用三维可压缩MHD数值模拟研究了行星际磁场By分量的变化对磁层顶重联区场向电流大小和分布的影响. 行星际磁场通过模拟区x=-Lx处左边界条件By来影响重联过程，从而改变重联区的场向电流. 研究结果表明边界条件By的突然改变，能使重联区场向电流迅速增加，甚至达到增大一个量级的水平.By本身的存在(即不为零)也会使场向电流维持在一个较高的水平. 由于行星际磁场By分量不为零，而形成模拟区磁场By不对称分布，这种不对称分布是场向电流不对称分布产生的主要原因. 这些结果是与Orsted卫星最新观测结果和地 面观测结果相符合的，它表明行星际磁场By分量对地球空间场向电流有较大的调制作用.     

Kinematic reconnection at a magnetic null point: spine-aligned current

Magnetic reconnection at a three-dimensional null point is the natural extension of the familiar two-dimensional X-point reconnection. A model is set up here for reconnection at a spiral null point, by solving the kinematic, steady, resistive magnetohydrodynamic equations in its vicinity. A steady magnetic field is assumed, as well as the existence of a localised diffusion region surrounding the null point. Outside the diffusion region the plasma and magnetic field move ideally. Particular attention is focussed on the way that the magnetic flux changes its connections as a result of the reconnection. The resultant plasma flows are found to be rotational in nature, as is the change in connections of the magnetic field lines.     

Reconnection driven lobe convection: Interball tail probe observations and global simulations

We present Interball Tail Probe observations from the high latitude mid-tail magnetopause which provide evidence of reconnection between the interplanetary magnetic field (IMF) and lobe field lines during a 6 h interval of stable northward and dawnward IMF on October 19, 1995. Results from a global magnetohydrodynamic simulation for this interval compare well with the Interball observations. With the simulations, we provide an extended global view of this event which gives us insight into the reconnection and convection dynamics of the magnetosphere. We find that reconnection occurs in a region of limited spatial extent near the terminator and where the IMF and the lobe field are anti-parallel. Reconnected IMF field lines drape over the dayside magnetosphere, convect along the flanks into the nightside, and enter the magnetotail through a small entry window that is located in the flank opposite to the reconnection site. Ionospheric convection is consistent with previous observations under similar IMF conditions and exhibits a two cell pattern with a dominant lobe cell over the pole. The magnetic mapping between the ionosphere and the lobe boundary is characterized by two singularities: the narrow entry window in the tail maps to a 6 h wide section of the ionospheric lobe cell. A singular mapping line cuts the lobe cell open and maps to almost the entire tail magnetopause. By this singularity the magnetosphere avoids having a stagnation point, i.e., the lobe cell center maps to a tailward convecting field line. The existence of singularities in the magnetic mapping between the ionosphere and the tail has important implications for the study of tail–ionosphere coupling via empirical magnetic field models. Because the lobe–IMF reconnection cuts away old lobe flux and replaces it with flux tubes of magnetosheath origin, solar wind plasma enters the lobes in a process that is similar to the one that operates during southward IMF.     

Solar flare model: Comparison of the results of numerical simulations and observations

The electrodynamic flare model is based on numerical 3D simulations with the real magnetic field of an active region. An energy of ∼10³² erg necessary for a solar flare is shown to accumulate in the magnetic field of a coronal current sheet. The thermal X-ray source in the corona results from plasma heating in the current sheet upon reconnection. The hard X-ray sources are located on the solar surface at the loop foot-points. They are produced by the precipitation of electron beams accelerated in field-aligned currents. Solar cosmic rays appear upon acceleration in the electric field along a singular magnetic X-type line. The generation mechanism of the delayed cosmic-ray component is also discussed.     

Variations in the polar cap area during intervals of substorm activity on 20–21 March 1990 deduced from AMIE convection patterns

The dynamic behaviour of the northern polar cap area is studied employing Northern Hemisphere electric potential patterns derived by the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure. The rate of change in area of the polar cap, which can be defined as the region of magnetospheric field lines open to the interplanetary magnetic field (IMF), has been calculated during two intervals when the IMF had an approximately constant southward component (1100- 2200 UT, 20 March 1990 and 1300–2100 UT, 21 March 1990). The estimates of the polar cap area are based on the approximation of the polar cap boundary by the flow reversal boundary. The change in the polar cap area is then compared to the predicted expansion rate based on a simple application of Faraday’s Law. Furthermore, timings of magnetospheric substorms are also related to changes in the polar cap area. Once the convection electric field reconfigures following a southward turning of the IMF, the growth rate of the observed polar cap boundary is consistent with that predicted by Faraday’s Law. A delay of typically 20 min to 50 min is observed between a substorm expansion phase onset and a reduction in the polar cap area. Such a delay is consistent with a synthesis between the near Earth neutral line and current disruption models of magnetospheric substorms in which the dipolarisation in the magnetotail may act as a trigger for reconnection. These delays may represent a propagation time between near geosynchronous orbit dipolarisation and subsequent reconnection further down tail. We estimate, from these delays, that the neutral X line occurs between \sim35R_E and \sim75R_E downstream in the tail.     

引力场中磁场重联的数值研究──（Ⅰ）磁岛的形成和合并

数值研究了引力场中电阻撕裂模不稳定性所引起的磁场重联，结果表明，在电流片长度Ｌ＝１Ｈ、半宽度δ＝Ｈ／２４的情况下，电流片两端附近将出现两个Ｘ线的磁场重联，并形成磁岛和高温高密度的等离子体团．磁岛宽度随着时间而增长，在ｔ≈５５τ_Ａ时达到饱和，最大饱和岛宽约为４δ．同时从ｔ≈３７τ_Ａ开始，磁岛中心位置逐渐下降；在ｔ＝５７τ_Ａ时发生结合不稳定性，磁岛与底部附近的闭合磁场区合并，导致磁场湮灭和磁能的快速释放．另一方面，由于引力场和等离子体非均匀性的影响，顶部附近随时间而增长的等离子体外流速度达１．１４Ｖ_Ａ∞；磁岛中等离子体向下运动的最大速度达１．４１Ｖ_Ａ∞，且大于局地声速；在磁岛前方可形成快激波．这些结果可用于解释双带耀斑中后随耀斑环的形成、磁场湮灭和磁能释放、以及Ｄｏｐｐｌｅｒ速度图上观测到的红移现象．     

引力场中磁场重联的数值研究（Ⅲ）等离子体团的形成和运动

应用解析和数值相结合的方法求得了含有闭场区、中性片和开场区的二维局域磁静力平衡日冕基态，进而在此基态下数值研究了由电阻撕裂模不稳定性引起的磁场重联过程．结果表明，在电流片长度远大于其半宽度的情况下，仍将发生具有两个Ｘ线的磁场重联，形成磁岛和高温高密度的等离子体团．等离子体团运动的方向取决于闭合区底部等离子体压力和磁压的比值β_０．当β_０较大时，等离子体团向下运动，引起结合不稳定性和磁岛的合并，等离子体团中的等离子体不断落入闭合磁场区两侧．当β_０较小时，等离子体团向上运动，导致电流片中等离子体的喷发．结果还表明，磁张力的分布是决定等离子体团运动方向的主要因子．