A 3rd Order WENO GLM-MHD Scheme for Magnetic Reconnection

A new numerical scheme of 3rd order Weighted Essentially Non-Oscillatory (WENO) type for 2.5D mixed GLM-MHD in Cartesian coordinates is proposed. The MHD equations are modified by combining the arguments as by Dellar and Dedner et al to couple the divergence constraint with the evolution equations using a Generalized Lagrange Multiplier (GLM). Moreover, the magnetohydrodynamic part of the GLM-MHD system is still in conservation form. Meanwhile, this method is very easy to add to an existing code since the underlying MHD solver does not have to be modified. To show the validation and capacity of its application to MHD problem modelling, interaction between a magnetosonic shock and a denser cloud and magnetic reconnection problems are used to verify this new MHD code. The numerical tests for 2D Orszag and Tang's MHD vortex, interaction between a magnetosonic shock and a denser cloud and magnetic reconnection problems show that the third order WENO MHD solvers are robust and yield reliable results by the new mixed GLM or the mixed EGLM correction here even if it can not be shown that how the divergence errors are transported as well as damped as done for one dimensional ideal MHD by Dedner et al.

Magnetosphere response to the IMF turning from north to south

In this paper, the Space Weather Modeling Framework(SWMF) is used to simulate the real-time response of the magnetosphere to a solar wind event on June 5, 1998, in which the interplanetary magnetic field shifted its direction from north to south.Since most current models do not take into account convective effects of the inner magnetosphere, we first study the importance of Rice Convection Model(RCM) in the global model.We then focus on the following four aspects of the magnetosphere's response: the magnetosphere's density distribution, the structure of its magnetic field lines, the area of the polar cap boundary, and the corresponding ionospheric current change.We find that(1) when the IMF changes from north to south in this event, high magnetosheath density is observed to flow downstream along the magnetopause with the solar wind; low-latitude reconnection at dayside occurs under the southward IMF, while the magnetic field lines in the tail lobe caudal, caused by the nightside high latitude reconnection, extend into the interplanetary space.Open magnetic field lines exist simultaneously at both high and low latitudes at the magnetopause;(2) the area of the polar cap is obviously increased if the IMF turns from the north to the south; this observation is highly consistent with empirical observations;(3) the ionospheric field align current in the northern hemisphere is stronger than in the southern hemisphere and also increases as the IMF changes from north to south.SWMF with the Rice Convection effect provides reliable modeling of the magnetospheric and ionospheric response to this solar wind variation.

Simulation of Plasma Flow Injectionwith Multi-Hierarchy Model Aiming Magnetic Reconnection Studies

A multi-hierarchy simulation model aiming magnetic reconnection studies is developed and improved in which macroscopic and microscopic physics are computed consistently and simultaneously.Macroscopic physics is solved by magnetohydrodynamics(MHD)algorithm,while microscopic dynamics is expressed by particle-in-cell(PIC)algorithm.The multi-hierarchy model relies on the domain decomposition method,and macro-and micro-hierarchies are interlocked smoothly by hand-shake scheme.As examination,plasma flow injection is simulated in the multihierarchy model.It is observed that plasmas flow from a macro-hierarchy to a microhierarchy across the magnetic field smoothly and continuously.

Roles of initial current carrier in the distribution of field-aligned current in 3-D Hall MHD simulations

A three-dimensional (3-D) Hall MHD simulation is carried out to study the roles of initial current carrier in the topology of magnetic field, the generation and distribu- tion of field aligned currents (FACs), and the appearance of Alfvén waves. Consid- ering the contribution of ions to the initial current, the topology of the obtained magnetic field turns to be more complex. In some cases, it is found that not only the traditional By quadrupole structure but also a reversal By quadrupole structure appears in the simulation box. This can explain the observational features near the diffusion region, which are inconsistent with the Hall MHD theory with the total ini- tial current carried by electrons. Several other interesting features are also emerged. First, motions of electrons and ions are decoupled from each other in the small plasma region (Hall effect region) with a scale less than or comparable with the ion inertial length or ion skin depth di=c/ωp. In the non-Hall effect region, the global magnetic structure is shifted in +y direction under the influence of ions with initial y directional motion. However, in the Hall effect region, magnetic field lines are bent in ?y direction, mainly controlled by the motion of electrons, then By is generated. Second, FACs emerge as a result of the appearance of By. Compared with the prior Hall MHD simulation results, the generated FACs shift in +y direction, and hence the dawn-dusk symmetry is broken. Third, the Walén relation in our simulations is consistent with the Walén relation in Hall plasma, thus the presence of Alfvén wave is confirmed.

不考虑磁层顶磁重联的全球三维MHD模型

本文介绍了一个新的磁层全球三维MHD模型 ,该模型可通过把IMF和地球磁场分开处理的方法“关闭”磁层顶的磁重联 ,从而可直观地显示不同IMF条件下磁层顶的IMF与地磁场的反平行区域 ,即磁重联最可能发生的地方 .结果表明 ,IMFBx 分量对磁层顶磁重联有重要影响 .由于可关闭磁重联 ,该模型还可有效地研究有无磁重联时 ,太阳风对磁层位形 (如晨 昏不对称性 )、粒子输运等重要问题的影响 ,有助于揭示磁层物理现象的基本特性 .

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.

通量传输事件的可压缩MHD模拟及与不可压缩模型的比较

用二维可压缩ＭＨＤ方程组模拟研究了地球向阳面磁层顶区通量传输事件（ＦＴＥｓ）的基本特性，并与不可压缩情况下的结果进行了比较，指出了二者的适用范围。研究结果表明，可压缩模型与不可压缩模型所描述的通量管的基本性质是相同的，可压缩模型还可描述一些不可压缩情况下不能描述的新特性。

Pseudo-reconnection in MHD numerical simulation

A class of pseudo-reconnections caused by a shifted mesh in magnetohydrodynamics(MHD) simulations is reported. In terms of this mesh system, some non-physical results may be obtained in certain circumstances, e.g. magnetic reconnection occurs without resistivity. After comparison, another kind of mesh is strongly recommended.

太阳大气中自发磁重联的数值模拟(Ⅱ)重力、热传导下二维三分量的计算结果

在无力场中引入反常电阻，数值模拟电阻撕裂模不稳定性引起的均匀重力场中的二维三分量磁重联过程．首先研究了Vc（反常电阻模型中的临界漂移速度）和β0（气压磁压比）对磁重联的影响，结果表明：（1）在Vc大的情况下，重联率开始很小，但最终出现爆发现象．另外，温度的增加不如Vc小的情况明显。（2）Vc相同而β0不同的情况具有相似的演化特征，但β0越小，动力学过程越剧烈。（3）Vc大巨β0也大的情况不能产生爆发现象．还研究了各向同性热传导对磁重联过程及扩散区结构的影响．结果表明热传导加快磁重联速率．在无热传导的情况中，开始时温度增加迅速，重联率较低，但最终出现磁重联率迅速上升的现象．对上述结果及其在解释耀斑、日冕物质抛射及冕流等太阳大气现象方面进行了讨论．

X箍缩三维数值模拟

利用FOI-PERFECT程序对X箍缩进行了3D数值模拟研究,给出了X箍缩的物理图像和动力学过程,探讨了Z箍缩中出现磁重联的可能性,并指出如果双丝Z箍缩中能够出现磁重联,那么X箍缩是更有利于磁重联出现的位形,并且,X箍缩中出现多重X射线暴的一个可能原因是z轴上多个位置出现磁重联。

行星际磁场北向时磁层顶区磁场重联的全球模式

在对背阳面磁层顶区局域磁场重联模拟的基础上提出了一个行星际磁场北向时磁层顶磁场重联的全球模式．行星际磁场北向时碰层顶磁场重联导致近地尾瓣的能量被输送到远磁尾，太阳风能量不在磁尾储存，向阳面磁层顶变厚，磁层受到一系列扰动．

K-H不稳定性在多电流片系统磁场重联中的效应

等离子体系统中存在两个或多个电流片时，电流片中发生的不稳定性可能会相互作用．行星际磁场北向时，背阳面碰层顶电流片与磁尾等离子体片之间可能发生相互作用，高纬边界层强烈的流场剪切可能促进磁场重联，产生磁层亚暴．本文运用二维可压缩磁流体模拟研究具有强流场剪切的多个电流片系统中磁场重联的演化．结果表明，Kelvin－Helmholtz不稳定性使多电流片系统的磁场重联过程明显加快；相邻电流片之间的距离越近，两者相互作用越强，重联增长率越大；在三电流片系统中，超Alfven速度强流场导致外侧两个电流片中出现强烈的磁场重联，并引发中心电流片的磁场重联．行星际磁场北向时，也可能发生磁层亚暴．

太阳表面水平运动驱动的磁圈与开放磁场重联的模拟研究

太阳大气的诸多观测事件(如耀斑、喷流等)均被归因于磁重联产生的能量转换.近年来,关于太阳风起源,有研究提出了磁重联使闭合磁圈开放为太阳风供应物质的新模式.在该模式中,闭合磁圈被光球超米粒组织对流携带,向超米粒边界运动,与位于边界的开放磁场相碰撞进而发生磁重联.该模式中磁重联的驱动及其效应是本文的研究目标.磁流体力学(MHD)数值模拟是研究太阳大气磁重联物理过程的重要途径.本文建立了一个二维MHD数值模型,结合太阳大气温度和密度的分层分布,在超米粒组织尺度上模拟了水平流动驱动的闭合磁圈与开放磁场的重联过程.通过对模拟结果的定量分析,认为磁重联确实能够将闭合磁圈的物质释放,进而供应给新的开放磁结构并产生向上流动.该结果为进一步模拟研究太阳风初始外流奠定了基础.

Simulation of the dawn-dusk magnetosheath asymmetry under quasi-steady states

The dawn-dusk asymmetry of the magne- tosheath under quasi-steady states has been studied by using a newly developed 3D MHD magnetosphere simulation model. The results show that the dawn-dusk asymmetry is substantial because of the Parker spiral IMF. It is found that the dawn-dusk magnetosheath thickness asymmetry is the effect of different shock conditions. The plasma density and flux asymmetry are mainly caused by the different thickness of the dawn-dusk magnetosheath, and the magnetic recon- nection on the magnetopause has no significant effects. It is also showed that the Plasma Depletion Layer in front of the dayside magnetopause can cause duskward plasma flow, and the total plasma flux on the dusk side will be higher.

太阳耀斑演化的数值模拟与可视化

太阳耀斑是太阳大气中的剧烈活动之一,它常伴随日冕物质抛射、射电爆发及高能粒子辐射等,并可能影响日地空间环境。因此,太阳耀斑研究不仅在天体物理中有重要的意义,同时也有利于空间物理、航天技术等学科的发展。现在已普遍认为太阳耀斑是磁场重联产生的爆发活动,对其动力学演化的研究主要有2种途径:一种是研究磁重联释放能量后在磁流管中的响应过程,被称为耀斑动力学模型研究。它可以和光谱诊断相结合。