Dynamics of cosmic current layers with localized lower-hybrid-drift turbulence

A two-dimensional magnetohydrodynamic model of the dynamics of tail-like current layers caused by anomalous electrical resistivity in a plasma with lower-hybrid-drift (LHD) turbulence is considered. Additionally to the LHD-resistivity, a resistivity pulse in the magnetic neutral sheet is given initiating a magnetic reconnection process. Then the temporal and spatial evolution of the magnetic and electric fields, the plasma convection and the anomalous resistivity are obtained numerically. Taking into account more exact expressions for the LHD-resistivity in the current layer as done in former works, the LHD-turbulence is found to be excited farther from the neutral sheet, and thus, with the time, secondary current sheets are obtained in the plasma-magnetic field system. It is shown that the inductive electric field moving from the magnetic neutral sheet to the current layer periphery during the reconnection process may be considered as indicator of the plasma disturbances.     

Turbulence generation in the magnetopause on account of magnetic islands

The present model is proposed to study the effect of thickness of Harris sheet and strength of guide field on the evolution of magnetic islands and generation of turbulence in magnetic reconnection sites. The governing model equation has been derived using EMHD model in the presence of the equilibrium magnetic field, consisting of guide field and shear field in the Harris sheet. We have carried out a numerical simulation of the dynamical equation for magnetopause region parameters. Simulation results reveal that as the thickness of Harris sheet increases, the intensity of evolution of magnetic islands decreases, but with increasing strength of guide field, intensity gradually increases and at later times irregular structures are formed. These structures give the indication of turbulence in magnetic reconnection site. Further, we have calculated power spectrum, which follows power index \({\sim}\,{-}1.5\) in the inertial range.     

Breakout and Tether-Cutting Eruption Models Are Both Catastrophic (Sometimes)

We present a simplified analytic model of a quadrupolar magnetic field and flux rope to model coronal mass ejections. The model magnetic field is two-dimensional, force-free and has current only on the axis of the flux rope and within two current sheets. It is a generalization of previous models containing a single current sheet anchored to a bipolar flux distribution. Our new model can undergo quasi-static evolution either due to changes at the boundary or due to magnetic reconnection at either current sheet. We find that all three kinds of evolution can lead to a catastrophe, known as loss of equilibrium. Some equilibria can be driven to catastrophic instability either through reconnection at the lower current sheet, known as tether cutting, or through reconnection at the upper current sheet, known as breakout. Other equilibria can be destabilized through only one and not the other. Still others undergo no instability, but they evolve increasingly rapidly in response to slow steady driving (ideal or reconnective). One key feature of every case is a response to reconnection different from that found in simpler systems. In our two-current-sheet model a reconnection electric field in one current sheet causes the current in that sheet to increase rather than decrease. This suggests the possibility for the microscopic reconnection mechanism to run away.     

Current-sheet-associated radio sources and development of the magnetosphere of an active region revealed from 17 GHz and Yohkoh data

We study the evolution of the active region (AR) NOAA 7321 in which appeared a so-called `neutral-line-associated source' (NLS) on the basis of data of the Nobeyama Radioheliograph and Yohkoh/SXT. We provide a physical interpretation of the NLS in terms of a topological magnetic reconnection model in a quadrupole magnetic configuration and discuss its relation to the evolution of the active region. Two kinds of the NLS were observed at 17 GHz. One of them, `rising NLS', was found in the growth stage. The other was `stationary NLS' detected in the maximal stage of the AR. Their presence was associated with substantial expansion of the active region's magnetosphere and accompanied by gradual development of spine-like structures visible in soft X-rays before homologous long-duration arcade flares. We suggest that the rising 17 GHz NLS corresponded to a fragment of a `horizontal' current sheet moving upward. Bright X-ray spines were boundaries of that current sheet. Almost all bursts observed from 26 to 28 October 1992 which accompanied class C and M flares occurred in the rising NLS. Formation of magnetic X-point singularities is believed to be responsible for the low-lying NLS. Reversal of circular polarization due to the effect of radio wave propagation was detected in that NLS on the limb. The initial stage of the microwave burst of the long-duration X9 class flare on 2 November 1992 occurred in this NLS. We also revealed observational manifestations of the presence of `vertical' non-neutral current sheet in the spatial structure of this NLS before the flare.     

Spontaneous magnetic reconnection

The present review concerns the relevance of collisionless reconnection in the astrophysical context. Emphasis is put on recent developments in theory obtained from collisionless numerical simulations in two and three dimensions. It is stressed that magnetic reconnection is a universal process of particular importance under collisionless conditions, when both collisional and anomalous dissipation are irrelevant. While collisional (resistive) reconnection is a slow, diffusive process, collisionless reconnection is spontaneous. On any astrophysical time scale, it is explosive. It sets on when electric current widths become comparable to the leptonic inertial length in the so-called lepton (electron/positron) “diffusion region”, where leptons de-magnetise. Here, the magnetic field contacts its oppositely directed partner and annihilates. Spontaneous reconnection breaks the original magnetic symmetry, violently releases the stored free energy of the electric current, and causes plasma heating and particle acceleration. Ultimately, the released energy is provided by mechanical motion of either the two colliding magnetised plasmas that generate the current sheet or the internal turbulence cascading down to lepton-scale current filaments. Spontaneous reconnection in such extended current sheets that separate two colliding plasmas results in the generation of many reconnection sites (tearing modes) distributed over the current surface, each consisting of lepton exhausts and jets which are separated by plasmoids. Volume-filling factors of reconnection sites are estimated to be as large as \({<}10^{-5}\) per current sheet. Lepton currents inside exhausts may be strong enough to excite Buneman and, for large thermal pressure anisotropy, also Weibel instabilities. They bifurcate and break off into many small-scale current filaments and magnetic flux ropes exhibiting turbulent magnetic power spectra of very flat power-law shape \(W_b\propto k^{-\alpha }\) in wavenumber k with power becoming as low as \(\alpha \approx 2\). Spontaneous reconnection generates small-scale turbulence. Imposed external turbulence tends to temporarily increase the reconnection rate. Reconnecting ultra-relativistic current sheets decay into large numbers of magnetic flux ropes composed of chains of plasmoids and lepton exhausts. They form highly structured current surfaces, “current carpets”. By including synchrotron radiation losses, one favours tearing-mode reconnection over the drift-kink deformation of the current sheet. Lepton acceleration occurs in the reconnection-electric field in multiple encounters with the exhausts and plasmoids. This is a Fermi-like process. It results in power-law tails on the lepton energy distribution. This effect becomes pronounced in ultra-relativistic reconnection where it yields extremely hard lepton power-law energy spectra approaching \(F(\gamma )\propto \gamma ^{-1}\), with \(\gamma \) the lepton energy. The synchrotron radiation limit becomes substantially exceeded. Relativistic reconnection is a probable generator of current and magnetic turbulence, and a mechanism that produces high-energy radiation. It is also identified as the ultimate dissipation mechanism of the mechanical energy in collisionless magnetohydrodynamic turbulent cascades via lepton-inertial-scale turbulent current filaments. In this case, the volume-filling factor is large. Magnetic turbulence causes strong plasma heating of the entire turbulent volume and violent acceleration via spontaneous lepton-scale reconnection. This may lead to high-energy particle populations filling the whole volume. In this case, it causes non-thermal radiation spectra that span the entire interval from radio waves to gamma rays.     

新磁重联理论

本文研究了磁流体力学与高频等离子体波（ 包括纵横模式） 之间的精巧的相互作用。研究表明,这些等离激元会在电流片内诱发一种阻抗不稳定,并最终导至磁重联,出现爆发性不稳定。在高涨的离声湍动情况下,高温电流片模型必须采用反常电导率,而非库仑电导率。理论估算的结果与观测相一致。因此这种计及等离激元有质动力作用的新磁重联理论,基本上能解释耀斑现象。     

Magnetic reconnection model for X-ray flare loop interaction

The subtle interactions between the magnetohydrodynamics (MHD) and transverse plasmons are investigated. It is shown that there is a resistive instability by the plasmon's soliton in a current sheet, which eventually turns into an eruptive instability at the magnetic field reconnection. In the case of ion-acoustic turbulence, the high temperature current sheet model must adopt the aromalous conductivity instead of the Coulomb conductivity. The numerical results are consistent with the observations obtained by Hanaoka (1994). Thus the flare caused by X-ray loop coalescence can be basically interpreted by this model of magnetic field reconnection driven by ponderomotive force.     

2 1/2-Dimensional Reconnection Model and Energy Release in Solar Flares

We employ a 2 1/2-dimensional reconnection model to analyse different aspects of the energy release in two-ribbon flares. In particular, we investigate in which way the systematic change of inflow region variables, associated with the vertical elongation of current sheet, affects the flare evolution. It is assumed that as the transversal magnetic field decreases, the ambient plasma-to-magnetic pressure ratio increases, and the reconnection rate diminishes. As the transversal field decreases due to the arcade stretching, the energy release enhances and the temperature rises. Furthermore, the magnetosonic Mach number of the reconnection outflow increases, providing the formation of fast mode standing shocks above the flare loops and below the erupting flux rope. Eventually, in the limit of a very small transversal field the reconnection becomes turbulent due to a highly non-linear response of the system to small fluctuations of the transversal field. The turbulence results in the energy release fragmentation which increases the release efficiency, and is likely to be responsible for the impulsive phase of the flare. On the other hand, as the current sheet stretches to larger heights, the ambient plasma-to-magnetic pressure ratio increases which causes a gradual decrease of the reconnection rate, energy release rate, and temperature in the late phase of flare. The described magnetohydrodynamical changes affect also the electron distribution function in space and time. At large reconnection rates (impulsive phase of the flare) the ratio of the inflow-to-outflow magnetic field strength is much smaller than at lower reconnection rates (late phase of the flare), i.e., the corresponding loss-cone angle becomes narrower. Consequently, in the impulsive phase a larger fraction of energized electrons can escape from the current sheet downwards to the chromosphere and upwards into the corona – the dominant flare features are the foot-point hard X-ray sources and type III radio bursts. On the other hand, at low reconnection rates, more particles stay trapped in the outflow region, and the thermal conduction flux becomes strongly reduced. As a result, a superhot loop-top, and above-the-loop plasma appears, as sometimes observed, to be a dominant feature of the gradual phase.     

Evolution of morphological features of CMEs deduced from catastrophe model of solar eruptions

We describe the evolution of morphological features of the magnetic configuration of CME according to the catastrophe model developed previously. For the parameters chosen for the present work, roughly half of the total mass is nominally contained in the initial flux rope, while the remaining plasma is brought by magnetic reconnection from the corona into the current sheet and from there into the CME bubble. The physical attributes of the difference in the observable features between CME bubble and flare loop system were studied. We tentatively identified distinguishable evolutionary features like the outer shell, the expanding bubble and the flux rope with the leading edge, void and core of the 3-component CME structure. The role of magnetic reconnection is discussed as a possible mechanism for the heating of the prominence material during eruptions. Several aspects of this explanation that need improvement are outlined.     

Numerical Study for the Bursty Nature of Spontaneous Fast Reconnection

In this paper, spontaneous fast reconnection in a neutral current sheet, which is initially perturbed by a localized resistivity, is studied by the newly developed Space-Time Conservation Element and Solution Element (CESE) method. After the initial perturbation is switched off, an anomalous resistivity is allowed to occur if a threshold of the local electron-ion drift velocity is exceeded. For a given threshold value, the amount of the reconnected magnetic flux introduced by the initial perturbation is very crucial for the onset of the anomalous resistivity. The numerical results indicate that fast reconnection can develop self-consistently with slow shocks extending between the diffusion region and a large-scale plasmoid-like structure, which is pushed forward by the reconnection outflow. A Petschek-like configuration is then built up, but it can not be sustained as a quasi-steady state. In fact, during the reconnection evolution, the diffusion region undergoes an elongation process so that after the dynamic process is nonlinearly saturated secondary tearing is subject to occur at the center of the system. This leads to enhanced and time-dependent reconnection. The reconnection evolution is further studied in various physical situations, also confirming the bursty nature of the spontaneous fast reconnection mechanism.     

Magnetic reconnection and energy release in the solar corona by Taylor relaxation

The heating of the solar corona by resistive turbulence of coronal magnetic fields is considered. The theory of this process, based on the Taylor-Heyvaerts-Priest hypothesis and a magnetic relaxation equation, is developed. Such an approach allows one to obtain the successive magnetic reconnection configurations and energy balance of the coronal magnetic field in response to prescribed motions of the photospheric footpoints. Two specific models of the coronal magnetic configuration are investigated, namely an array of closely packed flux tubes and a two-dimensional magnetic arcade.     

On the maximum energy release in flux-rope models of Eruptive Flares

We determine the photospheric boundary conditions which maximize the magnetic energy released by a loss of ideal-MHD equilibrium in two-dimensional flux-rope models. In these models a loss of equilibrium causes a transition of the flux rope to a lower magnetic energy state at a higher altitude. During the transition a vertical current sheet forms below the flux rope, and reconnection in this current sheet releases additional energy. Here we compute how much energy is released by the loss of equilibrium relative to the total energy release. When the flux-rope radius is small compared to its height, it is possible to obtain general solutions of the Grad-Shafranov equation for a wide range of boundary conditions. Variational principles can then be used to find the particular boundary condition which maximizes the magnetic energy released for a given class of conditions. We apply this procedure to a class of models known as cusp-type catastrophes, and we find that the maximum energy released by the loss of equilibrium is 20.8% of the total energy release for any model in this class. If the additional restriction is imposed that the photospheric magnetic field forms a simple arcade in the absence of coronal currents, then the maximum energy release reduces to 8.6%.     

Quick triggering of magnetic reconnection in magnetotail

Recently, quick triggering of magnetic reconnection (QMRT) even in an ion-scale current sheet is found to be possible with the help of the nonlinear evolution of the lower hybrid drift instability (LHDI). The details of the QMRT mechanism are reviewed mostly based on three-dimensional full-particle simulation results of our group. QMRT is mediated by LHDI and its time scale is comparable to the saturation time scale of LHDI. Depending on the initial current sheet thickness, two types of QMRT, so-called Type-I and Type-II QMRT, are demonstrated.     

Generalized analytical models of Syrovatskii’s current sheet

Two-dimensional stationary magnetic reconnection models that include a thin Syrovatskii-type current sheet and four discontinuous magnetohydrodynamic flows of finite length attached to its endpoints are considered. The flow pattern is not specified but is determined from a self-consistent solution of the problem in the approximation of a strong magnetic field. Generalized analytical solutions that take into account the possibility of a current sheet discontinuity in the region of anomalous plasma resistivity have been found. The global structure of the magnetic field in the reconnection region and its local properties near the current sheet and attached discontinuities are studied. In the reconnection regime in which reverse currents are present in the current sheet, the attached discontinuities are trans-Alfvénic shock waves near the current sheet endpoints. Two types of transitions from nonevolutionary shocks to evolutionary ones along discontinuous flows are shown to be possible, depending on the geometrical model parameters. The relationship between the results obtained and numerical magnetic reconnection experiments is discussed.     

太阳爆发过程中的大尺度磁重联电流片:理论和观测

从理论和观测两个方面来介绍和讨论出现在太阳爆发过程中的磁重联电流片及其物理本质和动力学特征。首先介绍在理论研究和理论模型中,磁重联电流片是如何在爆发磁结构当中形成并发展的,对观测研究有什么指导意义。然后介绍观测工作是从哪几个方面对理论模型预测的电流片进行证认和研究的。第三,将介绍观测研究给出了哪些过去所没有能够预期的结果,这些结果对深入研究耀斑一CME电流片以及其中的磁重联过程的理论工作有什么重要的、挑战性的意义。第四,讨论最新的与此有关的理论研究和数值实验。最后,对未来的研究方向和重要课题进行综述和展望。     

Flux-Rope Twist in Eruptive Flares and CMEs: Due to Zipper and Main-Phase Reconnection

The nature of three-dimensional reconnection when a twisted flux tube erupts during an eruptive flare or coronal mass ejection is considered. The reconnection has two phases: first of all, 3D “zipper reconnection” propagates along the initial coronal arcade, parallel to the polarity inversion line (PIL); then subsequent quasi-2D “main-phase reconnection” in the low corona around a flux rope during its eruption produces coronal loops and chromospheric ribbons that propagate away from the PIL in a direction normal to it. One scenario starts with a sheared arcade: the zipper reconnection creates a twisted flux rope of roughly one turn (\(2\pi \) radians of twist), and then main-phase reconnection builds up the bulk of the erupting flux rope with a relatively uniform twist of a few turns. A second scenario starts with a pre-existing flux rope under the arcade. Here the zipper phase can create a core with many turns that depend on the ratio of the magnetic fluxes in the newly formed flare ribbons and the new flux rope. Main phase reconnection then adds a layer of roughly uniform twist to the twisted central core. Both phases and scenarios are modeled in a simple way that assumes the initial magnetic flux is fragmented along the PIL. The model uses conservation of magnetic helicity and flux, together with equipartition of magnetic helicity, to deduce the twist of the erupting flux rope in terms the geometry of the initial configuration. Interplanetary observations show some flux ropes have a fairly uniform twist, which could be produced when the zipper phase and any pre-existing flux rope possess small or moderate twist (up to one or two turns). Other interplanetary flux ropes have highly twisted cores (up to five turns), which could be produced when there is a pre-existing flux rope and an active zipper phase that creates substantial extra twist.     

MAGNETIC RECONNECTION IN MULTIPLE HELIOSPHERIC CURRENT SHEETS

Satellite observations of the heliospheric current sheet indicate that the internal structure of sector boundaries is a very complex structure with many directional discontinuities in the magnetic field. This implies that the heliospheric current sheet is not a single surface but a constantly changing layer with a varying number of current sheets. In this paper, we investigate magnetic reconnection caused by the resistive tearing mode instability in non-periodic multiple current sheets by using two-dimensional magnetohydrodynamic simulation. The results show that it is complex unsteady magnetic reconnection. Accompanying the nonlinear development of the tearing mode, the width of each magnetic island in multiple current sheets increases with time, and this leads to new magnetic reconnection. At the same time, the width of each current sheet increases, and the current intensity decreases gradually. Finally, the reverse current disappears, and a big magnetic island is formed in the central region. This process is faster when the separation between the current sheets is smaller. We suggest that the occurrence of multiple directional discontinuities observed at sector boundary crossings in the heliosphere may be associated with the magnetic islands and plasmoids caused by magnetic reconnection in multiple current sheets.     

磁拱脚点缓慢运动引起的磁场演化和日冕加热

Heyvaerts和Priest最近提出了一个线性无力场演化的简化模型来定量计算磁拱脚点做缓慢的剪切运动所引起的日冕加热。由于他们在能量的计算中漏掉了一些二阶项,并且保留了导致位移无界的磁场的线性演化项,本文对他们的工作进行了修正,同时还对脚点运动引起的磁拱无力场演化进行了进一步探讨。本文主要结果如下:(1)得到的加热效率(即耗散能量在光球供给能量中所占比例)比Heyvaerts和Priest所得结果大。(2)磁拱无力场的无耗散线性演化是不可能的。(3)由磁场位形具体说明了非线性无力场发生磁力线重联的可能性,并指出最容易发生磁力线重联的高度大约为一个磁拱宽度。     

Particle Acceleration Due to Coronal Non-null Magnetic Reconnection

Various topological features, for example magnetic null points and separators, have been inferred as likely sites of magnetic reconnection and particle acceleration in the solar atmosphere. In fact, magnetic reconnection is not constrained to solely take place at or near such topological features and may also take place in the absence of such features. Studies of particle acceleration using non-topological reconnection experiments embedded in the solar atmosphere are uncommon. We aim to investigate and characterise particle behaviour in a model of magnetic reconnection which causes an arcade of solar coronal magnetic field to twist and form an erupting flux rope, crucially in the absence of any common topological features where reconnection is often thought to occur. We use a numerical scheme that evolves the gyro-averaged orbit equations of single electrons and protons in time and space, and simulate the gyromotion of particles in a fully analytical global field model. We observe and discuss how the magnetic and electric fields of the model and the initial conditions of each orbit may lead to acceleration of protons and electrons up to 2 MeV in energy (depending on model parameters). We describe the morphology of time-dependent acceleration and impact sites for each particle species and compare our findings to those recovered by topologically based studies of three-dimensional (3D) reconnection and particle acceleration. We also broadly compare aspects of our findings to general observational features typically seen during two-ribbon flare events.     

Detailed analysis of fan-shaped jets in three dimensional numerical simulation

We performed three dimensional resistive magnetohydrodynamic simulations to study the magnetic reconnection using an initially shearing magnetic field configuration(force free field with a current sheet in the middle of the computational box).It is shown that there are two types of reconnection jets:the ordinary reconnection jets and fan-shaped jets,which are formed along the guide magnetic field.The fan-shaped jets are significantly different from the ordinary reconnection jets which are ejected by magneti...