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.     

Dissipation in turbulent plasma due to reconnection in thin current sheets

We present in situ measurements in a space plasma showing that thin current sheets the size of an ion inertial length exist and are abundant in strong and intermittent plasma turbulence. Many of these current sheets exhibit the microphysical signatures of reconnection. The spatial scale where intermittency occurs corresponds to the observed structures. The reconnecting current sheets represent a type of dissipation mechanism, with observed dissipation rates comparable to or even dominating over collisionless damping rates of waves at ion inertial length scales (x100), and can have far reaching implications for small-scale dissipation in all turbulent plasmas.     

Structure of the magnetic reconnection diffusion region from four-spacecraft observations

Magnetic reconnection leads to energy conversion in large volumes in space but is initiated in small diffusion regions. Because of the small sizes of the diffusion regions, their crossings by spacecraft are rare. We report four-spacecraft observations of a diffusion region encounter at the Earth's magnetopause that allow us to reliably distinguish spatial from temporal features. We find that the diffusion region is stable on ion time and length scales in agreement with numerical simulations. The electric field normal to the current sheet is balanced by the Hall term in the generalized Ohm's law, E(n) approximately jxB/ne.n, thus establishing that Hall physics is dominating inside the diffusion region. The reconnection rate is fast, approximately 0.1. We show that strong parallel currents flow along the separatrices; they are correlated with observations of high-frequency Langmuir/upper hybrid waves.     

Measurement of the current sheet during magnetic reconnection in a toroidal plasma

The current and magnetic-field fluctuations associated with magnetic-field-line reconnection have been measured in the reversed field pinch plasma configuration. The current sheet resulting from this reconnection has been measured. The current layer is radially broad, comparable to a magnetic-island width, as may be expected from current transport along magnetic-field lines. It is much larger than that predicted by resistive MHD for linear tearing modes and larger than prediction from two-fluid linear theory.     

Hall magnetic shocks in plasma current layers

We present new analytical and numerical results of the dynamics of reversed field current layers in the Hall limit (i.e., characteristic length scales smaller than the ion inertial length). A rapid, localized thinning of the current layer leads to the generation of a nonlinear, shocklike structure that propagates in the B x inverted Delta(n) direction. This magnetic structure is self-supportive and can lead to a nonlocal thinning of the current layer and the release of magnetic energy.     

Three-dimensional evolution of a relativistic current sheet: triggering of magnetic reconnection by the guide field

The linear and nonlinear evolution of a relativistic current sheet of pair (e(+/-)) plasmas is investigated by three-dimensional particle-in-cell simulations. In a Harris configuration, it is obtained that the magnetic energy is fast dissipated by the relativistic drift kink instability (RDKI). However, when a current-aligned magnetic field (the so-called "guide field") is introduced, the RDKI is stabilized by the magnetic tension force and it separates into two obliquely propagating modes, which we call the relativistic drift-kink-tearing instability. These two waves deform the current sheet so that they trigger relativistic magnetic reconnection at a crossover thinning point. Since relativistic reconnection produces a lot of nonthermal particles, the guide field is of critical importance to study the energetics of a relativistic current sheet.     

Onset of fast magnetic reconnection

We demonstrate the existence of a new steady-state magnetic reconnection configuration which lies at the boundary of the basins of attraction between the Sweet-Parker and Hall reconnection configurations. The solution is linearly unstable to small perturbations and its identification required a novel iterative numerical technique. The eigenmodes of the unstable solution are localized near the X line, suggesting that the onset of fast reconnection in a weakly collisional plasma is initiated locally at the X line as opposed to remotely at the boundaries.     

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.     

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。     

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。     

Structure, properties and applications of thin Fe--Al layers

Fe-Al alloys show interesting physical properties and offer some special industrial applications. There are phase transitions combined with changes in the magnetic behaviour. Another interesting fact is the excellent oxidation and corrosion resistance of iron aluminides even at high temperatures. Thin Fe-Al layers can be produced in different ways. Ion beam methods are able to produce surface layers on bulk material modifying the initial properties completely. The properties depend strongly on the phase structure induced by the preparation process. 57-Fe Mössbauer spectroscopy, and in the case of surface layers conversion electron Mössbauer spectroscopy, is very suitable to identify this phase structure. In the present work, it is described for Al implanted Fe layers. Depending on implantation dose and energy both magnetic and non-magnetic phases can be produced. Due to the inhomogeneous distribution of Al in the Fe target a layer structure of different phases can be created. Moreover, due to double implantation an Fe-Si-Al alloy can be prepared.     

Magnetohydrodynamic theories of magnetic field reconnection

This article is concerned with a review of the prominent magnetohydrodynamic theories proposed to date to explain magnetic field reconnection. These theories fall into three categories: (i) resistive tearing-mode instability, (ii) steady externally driven processes, (iii) nonsteady externally driven processes. The purpose of this article is to give on the analytical side - (i) a detailed discussion including a critical appraisal of the existing pr ominent theories of magnetic reconnection, (ii) a further elaboration and more correct versions and extensions of some of the existing theories of magnetic reconnection, and a review of the laboratory and computational work on the problem. The controversies that surround the application of these theories to problems involving explosive releases of magnetic energy are discussed.     

Electron self-reinforcing process of magnetic reconnection

The growth of collisionless magnetic reconnection is discovered to be a nonlinear electron self-reinforcing process. Accelerated by the reconnection electric field, the small portion of energetic electrons in the vicinity of the X point are found to be the cause of the fast reconnection rate. This new mechanism explains that recent simulation results of different reconnection evolutions (i.e., steady state, quasisteady state, or nonsteady state) are essentially determined by the availability of feeding plasma inflows. Simulations are carried out with open boundary conditions.     

Laboratory observations of spontaneous magnetic reconnection

Detailed measurements of spontaneous magnetic reconnection are presented. The experimental data, which were obtained in the new closed Versatile Toroidal Facility magnetic configuration, document the profile evolution of the plasma density, magnetic flux function, reconnection rate, and the current density during a spontaneous reconnection event in the presence of a strong guide magnetic field. The reconnection process is at first slow, which allows magnetic stress to build in the system while the current channel becomes increasingly narrow and intense. The onset of a fast reconnection event occurs as the width of the current channel approaches the ion-sound-Larmor radius rho s. During the reconnection event magnetically stored energy is channeled into energetic ion outflows and a rapid increase in the electron temperature.     

Surface morphology and local magnetic properties of electrodeposited thin iron layers

Thin iron layers with different thickness were prepared by electrodeposition on the polycrystalline substrate. The surface morphology of the layers, their structure and local magnetic properties were studied using scanning tunneling microscopy (STM), X-ray diffraction (XRD) and conversion electron Mössbauer spectroscopy (CEMS). STM studies revealed the granular structure of the surface of the electrodeposited iron layers with the roughness up to 10 nm. XRD analysis proved that these layers were highly strained. The CEMS spectra showed an in-plane magnetic anisotropy in the iron layers. Isomer shift of the electrodeposited iron was different than that of the -Fe. This difference was attributed to the internal stresses existing in the electrodeposited layers.     

Exact energy principle in magnetic reconnection for current-sheet models

On the basis of an exact nonlinear energy principle, it is shown that the change in magnetic topology (i.e., reconnection) in a finite-domain system leads to the conversion of magnetic field energy to particle energy. However, it is also shown that the conversion efficiency gradually disappears as the system size increases. This principle is demonstrated with model current-sheet equilibria including Harris and Fadeev solutions, as well as a current-sheet equilibrium which contains a singular current layer. The finding that energy conversion in reconnection is highly dependent on the system size may have an important implication for numerical simulations performed under finite geometry.