Spatially resolved measurements of ion heating during impulsive reconnection in the Madison Symmetric Torus

The impurity ion temperature evolution has been measured during three types of impulsive reconnection events in the Madison Symmetric Torus reversed field pinch. During an edge reconnection event, the drop in stored magnetic energy is small and ion heating is observed to be limited to the outer half of the plasma. Conversely, during a global reconnection event the drop in stored magnetic energy is large, and significant heating is observed at all radii. For both kinds of events, the drop in magnetic energy is sufficient to explain the increase in ion thermal energy. However, not all types of reconnection lead to ion heating. During a core reconnection event, both the stored magnetic energy and impurity ion temperature remain constant. The results suggest that a drop in magnetic energy is required for ions to be heated during reconnection, and that when this occurs heating is localized near the reconnection layer.     

Nanoflare heating model for collisionless solar corona

The problem of coronal heating remains one of the greatest unresolved problems in space science. Magnetic reconnection plays a significant role in heating the solar corona. When two oppositely directed magnetic fields come closer to form a current sheet, the current density of the plasma increases due to which magnetic reconnection and conversion of magnetic energy into thermal energy takes place. The present paper deals with a model for reconnection occurring in the solar corona under steady state in collisionless regime. The model predicts that reconnection time in the solar corona varies inversely with the cube of magnetic field and varies directly with the Lindquist number. Our analysis shows that reconnections are occurring within a time interval of 600 s in the solar corona, producing nanoflares in the energy range 10 ²¹–10 ²³ erg /s which matches with Yohkoh X-ray observations.     

Measurement of the Hall dynamo effect during magnetic reconnection in a high-temperature plasma

The fluctuation-induced Hall electromotive force, [deltaJ x deltaB]/nee, is experimentally measured in the high-temperature interior of a reversed-field pinch plasma by a fast Faraday rotation diagnostic. It is found that the Hall dynamo effect is significant, redistributing (flattening) the equilibrium core current near the resonant surface during a reconnection event. These results imply that effects beyond single-fluid MHD are important for the dynamo and magnetic reconnection.     

Cause of sudden magnetic reconnection in a laboratory plasma

The cause for sudden reconnection in reversed field pinch plasmas is determined experimentally for two cases: large reconnection events (the sawtooth crash) and small reconnection events during improved confinement. We measure the term in the MHD equations which represents the driving (or damping) of edge tearing modes due to the axisymmetric magnetic field. The term is negative for large reconnection events (the modes are stable, implying that reconnection may be driven by nonlinear coupling to other modes) and positive for small reconnection events (modes are unstable, reconnection is spontaneous).     

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.     

Numerical Simulation on a Possible Formation Mechanism of Interplanetary Magnetic Cloud Boundaries

The formation med2aniRm of the interplanetary magnetic cloud (MC) boundaries is numerically investigated by simulating the interactions between an MC of some initial momentum and a local interplanetary current sheet.The compressible 2.51:) MHD equations are solved. R~sults show that the magnetic reconnection process is a possible formation mechanism when an MC interacts with a surrounding current sheet. A number of interesting features are found. For instance, the front boundary of the MCs is a magnetic reconnection boundary that could be caused by a driven reconnection ahead of the cloud, and the tail boundary might be caused by the driving of the entrained flow as a result of the Bernoulli principle. Analysis of the magnetic field and plasma data demonstrates that at these two boundaries appear large value of the plasma parameter β, clear increase of plasma temperature and density, distinct decrease of magnetic magnitude, and a transition of maguetic field direction of about 180 degrees. The outcome of the present simulation agrees qualitatively with the observational results on MC boundary inferred from IMP-8, etc.     

Suppression of Hall-term effects by gyroviscous cancellation in steady collisionless magnetic reconnection

The formation of an ion-dissipation region, in which motions of electrons and ions decouple and fast magnetic reconnection occurs, is demonstrated during a steady state of two-dimensional collisionless driven reconnection by means of full-particle simulations. The Hall-term effect is suppressed due to the gyroviscous cancellation at scales between the ion-skin depth and ion-meandering-orbit scale, and thus ions are tied to the magnetic field. The ion frozen-in constraint is strongly broken by nongyrotropic pressure tensor effects due to ion-meandering motion, and thus the ion-dissipation region is formed at scales below the ion-meandering-orbit scale. A similar process is observed in the formation of an electron-dissipation region. These two dissipation regions are clearly observed in an out-of-plane current density profile.     

A Possible Interpretation of Burst-Like Characteristics of Explosive Events

Explosive events have been observed to occur consecutively in bursts at intermittent locations along the boundary near the opposite polarity. The aim of the present paper is to explore a possible mechanism to interpret this burst-like characteristic of explosive events. The 2D magnetohydrodynamic (MHD) numerical simulations with resistivity have been carried out to reproduce the intermittent spatial-temporal magnetic reconnection events taking place along the long, compressible current sheet. The observed density enhancements in previously published results have been verified to be associated to magnetic reconnection sites. Late observational evidences, which support present attempts, have also been found, at least in morphological evolution of the consecutive explosive events.     

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。     

Observations of slow electron holes at a magnetic reconnection site

We report in situ observations of high-frequency electrostatic waves in the vicinity of a reconnection site in the Earth's magnetotail. Two different types of waves are observed inside an ion-scale magnetic flux rope embedded in a reconnecting current sheet. Electron holes (weak double layers) produced by the Buneman instability are observed in the density minimum in the center of the flux rope. Higher frequency broadband electrostatic waves with frequencies extending up to f(pe) are driven by the electron beam and are observed in the denser part of the rope. Our observations demonstrate multiscale coupling during the reconnection: Electron-scale physics is induced by the dynamics of an ion-scale flux rope embedded in a yet larger-scale magnetic reconnection process.     

Single-particle dynamics in collisionless magnetic reconnection

The role of single-particle dynamics in driven magnetic reconnection in collisionless plasmas is investigated experimentally and analytically. The trapping of particle orbits in the magnetic cusp is observed to allow fast reconnection in the absence of a macroscopic current layer, at a rate identical to that of vacuum. The development of an electrostatic potential structure around the magnetic X line during reconnection is predicted theoretically and observed experimentally.     

Ion acceleration during reconnection in MAST

Spontaneous acceleration of ions to suprathermal energies is observed during magnetic reconnection in the Mega-Ampere Spherical Tokamak (MAST). A high-energy tail is observed in the ion-distribution function following each internal reconnection event in Ohmic discharges. This phenomenon is explained in terms of runaway ion acceleration in the electric field induced by the reconnection.     

Three-dimensional dynamics of collisionless magnetic reconnection in large-scale pair plasmas

Using the largest three-dimensional particle-in-cell simulations to date, collisionless magnetic reconnection in large-scale electron-positron plasmas without a guide field is shown to involve complex interaction of tearing and kink modes. The reconnection onset is patchy and occurs at multiple sites which self-organize to form a single, large diffusion region. The diffusion region tends to elongate in the outflow direction and become unstable to secondary kinking and formation of "plasmoid-rope" structures with finite extent in the current direction. The secondary kink folds the reconnection current layer, while plasmoid ropes at times follow the folding of the current layer. The interplay between these secondary instabilities plays a key role in controlling the time-dependent reconnection rate in large-scale systems.     

Numerical Simulation on a Possible Formation Mechanism of Interplanetary Magnetic Cloud Boundaries

The formation mechanism of the interplanetary magnetic cloud (MC) boundaries is numerically investigatedby simulating the interactions between an MC of some initial momentum and a local interplanetary current sheet.The compressible 2.5D MHD equations are solved. Results show that the magnetic reconnection process is a possibleformation mechanism when an MC interacts with a surrounding current sheet. A number of interesting features arefound. For instance, the front boundary of the MCs is a magnetic reconnection boundary that could be caused by adriven reconnection ahead of the cloud, and the tail boundary might be caused by the driving of the entrained flowas a result of the Bernoulli principle. Analysis of the magnetic field and plasma data demonstrates that at these twoboundaries appear large value of the plasma parameterβ, clear increase of plasma temperature and density, distinctdecrease of magnetic magnitude, and a transition of magnetic field direction of about 180 degrees. The outcome of thepresent simulation agrees qualitatively with the observational results on MC boundary inferred from IMP-8, etc.     

双等离子体团相互作用的磁流体力学模拟

利用商用磁流体力学模拟程序USIM对双等离子体团相互作用过程进行了数值模拟,分别考察和比较了双对流等离子体团在外加磁场和无外加磁场情况下,相互作用的物理过程.发现在外加磁场情况下等离子体团相互作用会伴随着磁重联(反向磁场)、磁排斥(同向磁场)以及一些不稳定过程.针对激光产生等离子体团错位相互作用实验,进行了标度模拟,发现外加磁场起着重要作用,进一步表明激光等离子体的磁化特征.研究结果为下一步在神光Ⅱ激光装置进行强磁环境下等离子体实验提供理论指导.     

Generation of Electric Field and Net Charge in Hall Reconnection

Generation of Hall electric field and net charge associated initial conditions of plasma density and magnetic field. with magnetic reconnection is studied under different With inclusion of the Hall effects, decoupling of the electron and ion motions leads to the formation of a narrow layer with strong electric field and large net charge density along the separatrix. The asymmetry of the plasma density or magnetic field or both across the current sheet will largely increase the magnitude of the electric field and net charge. The results indicate that the asymmetry of the magnetic field is more effective in producing larger electric field and charge density. The electric field and net charge are always much larger in the low density or/and high magnetic field side than those in the high density or/and low magnetic field side. Both the electric field and net charge density are linearly dependent on the ratios of the plasma density or the square of the magnetic field across the current sheet. For the case with both initial asymmetries of the magnetic field and density, rather large Hall electric field and charge density are generated.     

Angular momentum transport in thin accretion disks and intermittent accretion

The plasma modes, transporting angular momentum in accretion disks, under minimally restrictive conditions when the magnetic energy density is significant relative to the thermal energy density, are shown to be singular if the ideal MHD approximation is adopted. A similarity with the modes producing magnetic reconnection in current carrying plasmas is established. The combined effects of finite plasma temperature, of plasma compressibility, of the gradient of the rotation frequency, and of appropriate transport processes (outside ideal MHD) are involved in the onset of these nonaxisymmetric and locally corotating modes.     

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.     

Observation and analysis of whistler-mode wave and electrostatic solitary waves within density depletion near magnetic reconnection X-line

The PWI/WFC data onboard Geotail during one burst time interval when Geotail is skimming a magnetic reconnection diffusion region in the near-Earth magnetotail is carefully analyzed.Both the whistler-mode wave and the electrostatic solitary wave are found within the region with density depletion on the boundary layer near the magnetic reconnection X-line.The whistler-mode wave is electromagnetic whistler wave propagating quasi-parallel to the ambient field with a small angle between the wave vector and the ambient magnetic field.The whistler-mode wave associated with ESWs suggests that enhanced electromagnetic whistler-mode fluctuations can also be generated after the decay of the ESWs,which is different from the 2-D PIC simulation results.     

Magnetic field annihilation and reconnection driven by femtosecond lasers in inhomogeneous plasma

The process of fast magnetic reconnection driven by intense ultra-short laser pulses in underdense plasma is investigated by particle-in-cell simulations. In the wakefield of such laser pulses, quasi-static magnetic fields at a few mega-Gauss are generated due to nonvanishing cross product ▽(n/) × p. Excited in an inhomogeneous plasma of decreasing density, the quasi-static magnetic field structure is shown to drift quickly both in lateral and longitudinal directions. When two parallel-propagating laser pulses with close focal spot separation are used, such field drifts can develop into magnetic reconnection(annihilation) in their overlapping region, resulting in the conversion of magnetic energy to kinetic energy of particles. The reconnection rate is found to be much higher than the value obtained in the Hall magnetic reconnection model. Our work proposes a potential way to study magnetic reconnection-related physics with short-pulse lasers of terawatt peak power only.