Transient magnetic reconnection and unstable shear layers

We study three-dimensional magnetic reconnection caused by the Kelvin-Helmholtz (KH) instability and differential rotation in subsonic and sub-Alfvenic flows. The flows, which are modeled by the resistive magnetohydrodynamic equations with constant resistivity, are stable in the direction of the magnetic field but unstable perpendicular to the magnetic field. Localized transient reconnection is observed on the KH time scale, and kinetic energy increases with decreasing resistivity. As in flux-transfer events in the Earth's magnetopause boundary layer, bipolar structures in the normal flux and bidirectional jetting away from reconnection zones are observed.     

Time window for magnetic reconnection in plasma configurations with velocity shear

It is shown that the rate of magnetic field line reconnection can be clocked by the evolution of the large-scale processes that are responsible for the formation of the current layers where reconnection can take place. In unsteady plasma configurations, such as those produced by the onset of the Kelvin-Helmholtz instability in a plasma with a velocity shear, qualitatively different magnetic structures are produced depending on how fast the reconnection process develops on the external clock set by the evolving large-scale configuration.     

Model of line preserving field line motions using Euler potentials

We consider behavior of finite magnetic field lines during reconnection processes. We portray field line motions using Euler potentials representation. Here, we propose a new insight into plasma flow fields related with magnetic reconnection. In this approach reconnection is treated as a breakage of magnetic topology, which results in deviation from the line preserving flow regime. We derive constraints and the general equations for these flows. In our approach the flux preserving flows are treated as a special case of line preserving regime. We also derive a constraint on a non-ideal term in Ohm’s Law within diffusion regions, which relates plasma flow with resistivity, and which must hold for non-reconnective diffusion. We also propose a new method of detecting magnetic reconnection.     

Catastrophe model for fast magnetic reconnection onset

A catastrophe model for the onset of fast magnetic reconnection is presented that suggests why plasma systems with magnetic free energy remain apparently stable for long times and then suddenly release their energy. For a given set of plasma parameters there are generally two stable reconnection solutions: a slow (Sweet-Parker) solution and a fast (Alfvénic) Hall reconnection solution. Below a critical resistivity the slow solution disappears and fast reconnection dominates. Scaling arguments predicting the two solutions and the critical resistivity are confirmed with two-fluid simulations.     

Out-of-plane shear flow effects on fast magnetic reconnection in a two-dimensional hybrid simulation model

The effects of out-of-plane shear flows on fast magnetic reconnection are numerically investigated by a twodimensional(2D)hybrid model in an initial Harris sheet equilibrium with flows.The equilibrium and driven shear flows out of the 2D reconnection plane with symmetric and antisymmetric profiles respectively are used in the simulation.It is found that the out-of-plane flows with shears in-plane can change the quadrupolar structure of the out-of-plane magnetic field and,therefore,modify the growth rate of magnetic reconnection.Furthermore,the driven flow varying along the anti-parallel magnetic field can either enhance or reduce the reconnection rate as the direction of flow changes.Secondary islands are also generated in the process with converting the initial X-point into an O-point.     

Magnetic effects on the coalescence of Kelvin-Helmholtz vortices

We simulate the coalescence process of MHD-scale Kelvin-Helmholtz vortices with the electron inertial effects taken into account. Reconnection of highly stretched magnetic field lines within a rolled-up vortex destroys the vortex itself and the coalescence process, which is well known in ordinary fluid dynamics, is seen to be inhibited. When the magnetic field is initially antiparallel across the shear layer, on the other hand, multiple vortices are seen to coalesce continuously because another type of magnetic reconnection prevents the vortex decay. This type of reconnection at the hyperbolic point also changes the field line connectivity and thus leads to large-scale plasma mixing across the shear layer.     

Observation of the temperature dependence of the structure factor in a metallic glass

The x-ray structure factor has been measured as a function of temperature over the range T = 300 K to T = 675 K for the metallic glass Nb₆₀Ni₄₀. In this range of temperature it was found that at the first peak, k_p, in the structure factor, S(k), the magnitude of S(k_p) decreased linearly with increasing temperature. Furthermore this variation was large enough to account quantitatively for the anomalous transport results on this sample, namely, that the resistivity decreases with increasing temperature.     

不可压缩等离子体的2维磁场重联模型

提出了一种2维磁场重联模型。磁场重联过程中的电荷分离在等离子体中产生静电场,等离子体在电场中的漂移运动可以解释阿尔芬速度量级的出流。该磁场重联模型给出如下结论:Sweet-Parker模型描述的重联率强烈地依赖于电子质量与离子质量之比;反常电阻率正比于离子惯性长度和电流片宽度比值的平方; 相对论效应和高温等离子体中电子-正电子对的产生可以提高重联率; 电磁波的激发对于磁能的损耗是必要的。     

Collisionless reconnection in an electron-positron plasma

Electromagnetic particle-in-cell simulations of fast collisionless reconnection in a two-dimensional electron-positron plasma (without an equilibrium guide field) are presented. A generalized Ohm's law in which the Hall current cancels out exactly is given. It is suggested that the key to fast reconnection in this plasma is the localization caused by the off-diagonal components of the pressure tensors, which produce an effect analogous to a spatially localized resistivity.     

垂直磁重联平面的驱动流对磁岛链影响的模拟

近20年来,大量的磁岛链现象从空间、天体物理到磁约束实验室等离子体中被观察到,并且有关磁岛链现象的许多物理特性可以直接被计算机模拟结果所证实.磁岛链理论在磁重联理论中的重要进展为快速磁重联的发生机制提供了更加具有说服力的解释.本文采用二维三分量的磁流体力学模型,数值研究了不同宽度和不同强度的垂直平面驱动流对磁重联中磁岛链不稳定性的影响,并分析了导向场和垂直平面的驱动流对磁岛链的共同作用.研究结果表明:垂直平面驱动流的宽度越宽或强度越强,越容易产生磁岛链结构.电流片中的小磁岛个数及重联率随着垂直平面驱动流宽度及强度的增加而增加.另外,导向场会改变重联平面内磁岛链的对称性.相同导向场情况下,驱动流强度越大,小磁岛的增长速度越快.     

Numerical evidence of undriven, fast reconnection in the solar-wind interaction with earth's magnetosphere: formation of electromagnetic coherent structures

We give evidence for the first time of the onset of undriven fast, collisionless magnetic reconnection during the evolution of an initially homogeneous magnetic field advected in a sheared velocity field. We consider the interaction of the solar wind with the magnetospheric plasma at low latitude and show that reconnection takes place in the layer between adjacent vortices generated by the Kelvin-Helmholtz instability. This process generates coherent magnetic structures with a size comparable to the ion inertial scale, much smaller than the system dimensions but much larger than the electron inertial scale. These magnetic structures are further advected in the plasma in a complex pattern but remain stable over a time interval much longer than their formation time. These results can be crucial for the interpretation of satellite data showing coherent magnetic structures in the Earth's magnetosheath or the magnetotail.     

Applications of a Cole–Hopf transform to the 3D Navier–Stokes equations

The Navier–Stokes equations written in the vector potential can be recast as non-linear Schrödinger equations at imaginary times, i.e. heat equations with a potential term, using the Cole–Hopf transform. On this basis, we study two kinds of Navier–Stokes flows by means of direct numerical simulations. In an experiment on vortex reconnection, it is found that the potential term takes large negative values in regions where intensive reconnection takes place, whereas the signature of the non-linear term is more broadly spread. For decaying turbulence starting from a random initial condition, such a correspondence is also observed in the early stage when the flow is dominated by vorticity layers. At later times, when the flow features several tubular vortices, this correspondence becomes weaker. Finally, a similar set of transformations is presented for the magneto–hydrodynamic equations, which reduces them to a set of heat equations with suitable potential terms, thereby obtaining new criteria for the regularity of their solutions.     

Reconnection rate for the inhomogeneous resistivity petschek model

The reconnection rate for the canonical simplest case of steady-state two-dimensional symmetric reconnection in an incompressible plasma is found by matching of an outer Petschek solution and an internal diffusion region solution. The reconnection rate obtained naturally incorporates both Sweet-Parker and Petschek regimes, while the latter is possible only for a strongly localized resistivity.     

温度各向异性等离子体中的电磁波不稳定性产生的反常电阻率

本文考察了由于温度各向异性分布产生的电磁波不稳定性所联系的反常电阻率问题,讨论了这一现象与地球磁场磁尾区中“磁再联”现象的关系。     

μSR study of the S=1/2 triangular lattice antiferromagnet AgNiO2

AgNiO₂, a model compound of an S=1/2 triangular lattice, was studied by muon spin relaxation in addition to ac, dc susceptibility, electrical resistivity and neutron diffraction. The relaxation rate shows a sharp peak at around T_N=28 K followed by a sudden decrease of initial asymmetry indicating a magnetic ordering. Three internal fields ranging from 0.2 to 0.3 T were obtained from the muon precession period. However, a neutron diffraction experiment failed to detect any magnetic order at low temperatures. From these results, it was concluded that magnetic coherence is confined to small domain compared with the coherence length of neutron diffraction due to spin frustration. This revised version was published online in August 2006 with corrections to the Cover Date.     

Parallel flow in hele-shaw cells with ferrofluids

Parallel flow in a Hele-Shaw cell occurs when two immiscible liquids flow with relative velocity parallel to the interface between them. The interface is unstable due to a Kelvin-Helmholtz type of instability in which fluid flow couples with inertial effects to cause an initial small perturbation to grow. Large amplitude disturbances form stable solitons. We consider the effects of applied magnetic fields when one of the two fluids is a ferrofluid. The dispersion relation governing mode growth is modified so that the magnetic field can destabilize the interface even in the absence of inertial effects. However, the magnetic field does not affect the speed of wave propogation for a given wave number. We note that the magnetic field creates an effective interaction between the solitons.     

超高速激光纹影技术测量脉冲功率驱动的磁重联现象

用超高速激光纹影技术测量了Z箍缩等离子体磁重联现象。实验采用超高速光电分幅相机,配合激光纹影技术,测量了XP-1装置上两根金属丝产生的等离子体分布,论证了超高速激光纹影技术研究Z箍缩磁重联现象的可行性。双钨丝实验结果表明,电流加载约10ns后金属丝已有明显膨胀,线性拟合得到平均膨胀速度约8km/s,金属丝内外两侧出现了规则的极有可能是垂直磁场的电热不稳定性扰动,并沿角向高度关联。铝丝负载的实验结果表明,早期的不稳定性波长为0.4mm,电流峰值之后金属丝初始位置仍有大量等离子体,后期的不稳定性波长约1.5mm。这些现象揭示了不稳定性发展的一个主要特征:短波模式受抑制,长波模式将占主导。     

Vortex dynamics and entrainment mechanisms in low Reynolds orifice jets

Classical planar 2D-PIV measurements and time-resolved visualizations enriched by low-level processing are used for the reconstruction of the Kelvin-Helmholtz vortex passing in the near field of a circular and a 6-lobed orifice jet flow. In the circular jet, the entrainment is produced in the braid region, being interrupted in the presence of the Kelvin-Helmholtz ring. The latter compresses the streamwise vortices and alters their self-induction role. Conversely, the 6-lobed orifice geometry allows the cutting of the Kelvin-Helmholtz structures into discontinuous ring segments. Consequently, into these discontinuity regions streamwise large scale structures are developing. These streamwise structures are permanent thus controlling and enhancing the jet entrainment which is not altered by the Kelvin-Helmholtz structures passing.     

Hall magnetic reconnection rate

Two-dimensional Hall magnetohydrodynamic simulations are used to determine the magnetic reconnection rate in the Hall limit. The simulations are run until a steady state is achieved for four initial current sheet thicknesses: L=1,5,10, and 20c/omega(pi), where c/omega(pi) is the ion inertial length. It is found that the asymptotic (i.e., time independent) state of the system is nearly independent of the initial current sheet width. Specifically, the Hall reconnection rate is weakly dependent on the initial current layer width and is partial differential Phi/ partial differential t less, similar 0.1V(A0)B0, where Phi the reconnected flux, and V(A0) and B0 are the Alfvén velocity and magnetic field strength in the upstream region. Moreover, this rate appears to be independent of the scale length on which the electron "frozen-in" condition is broken (as long as it is 相似文献   

二维可压缩流体Kelvin-Helmholtz不稳定性

利用高精度数值格式,研究了二维可压缩流体中的Kelvin-Helmholtz不稳定性,主要研究了可压缩性对Kelvin-Helmholtz稳定性增长率的影响.模拟定量的给出低Mach和高Mach数两种情况下,初始静压和对流Mach数以及Kelvin-Helmholtz不稳定性线性增长率的关系.模拟结果和自由剪切层以及混合层的实验结果以及理论分析一致.模拟表明,对流Mach数是描述流体可压缩性的合适参数,对流Mach数越小流体越不可压,Kelvin-Helmholtz不稳定性的线性增长率随对流Mach数的增加而减小. 关键词： Kelvin-Helmholtz不稳定性 可压缩流体 Mach数 超音速流体