Magnetic Reconnection

Our knowledge about the origin and transformation mechanisms of the bright points in the solar network has a significant role in understanding the ejection of materials and the transfer of energy into the solar corona. Outside the active region of the Sun (AR), although it is called the Quiet Sun (QS), various types of small-scale bright phenomena constantly occur within the boundary of the super granular cells above the magnetic network. Knowing the bright points is an effective key in considering the solar spicules. In this research, we study the solar transition region bright points and examine their apparent velocities with the local correlation tracking Fourier (FLCT) method. The results illustrate that these points differ in apparent velocity direction and brightness. Their lifetime and average horizontal velocity were estimated at 100 s and 4 kms-1, respectively. Recently, a new group of solar spicules has been observed, those lifetimes are around 100 s, and show a typical horizontal velocity of 3-4 kms-1. According to the analysis of the two-dimensional, apparent velocity of the bright points on the rosettes of the network, these points can be the disk counterpart of the type II spicules. In addition, the analysis of the two-dimensional field of velocities shows rotations that can cause the excitation of Alfvenic pulses.

The transport of magnetic flux to outside of collapsing molecular clouds is a required step to allow the formation of stars. Although ambipolar diffusion is often regarded as a key mechanism for that, it has been recently argued that it may not be efficient enough. In this review, we discuss the role that MHD turbulence plays in the transport of magnetic flux in star forming flows. In particular, based on recent advances in the theory of fast magnetic reconnection in turbulent flows, we will show results of three-dimensional numerical simulations that indicate that the diffusion of magnetic field induced by turbulent reconnection can be a very efficient mechanism, especially in the early stages of cloud collapse and star formation. To conclude, we will also briefly discuss the turbulence-star formation connection and feedback in different astrophysical environments: from galactic to cluster of galaxy scales.

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We examine Cluster observations of a so-called magnetosphere "crater FTE," employing data from five instruments (FGM, CIS, EDI, EFW, and WHISPER), some at the highest resolution. The aim of doing this is to deepen our understanding of the reconnection nature of these events by applying recent advances in the theory of collisionless reconnection and in detailed observational work. Our data support the hypothesis of a stratified structure with regions which we show to be spatial structures. We support the bulge-like topology of the core region (R3) made up of plasma jetting transverse to reconnected field lines. We document encounters with a magnetic separatrix as a thin layer embedded in the region (R2) just outside the bulge, where the speed of the protons flowing approximately parallel to the field maximizes: (1) short (fraction of a sec) bursts of enhanced electric field strengths (up to ∼30 mV/m) and (2) electrons flowing against the field toward the X line at approximately the same time as the bursts of intense electric fields. R2 also contains a density decrease concomitant with an enhanced magnetic field strength. At its interface with the core region, R3, electric field activity ceases abruptly. The accelerated plasma flow profile has a catenary shape consisting of beams parallel to the field in R2 close to the R2/R3 boundary and slower jets moving across the magnetic field within the bulge region. We detail commonalities our observations of crater FTEs have with reconnection structures in other scenarios. We suggest that in view of these properties and their frequency of occurrence, crater FTEs are ideal places to study processes at the separatrices, key regions in magnetic reconnection. This is a good preparation for the MMS mission.

Asymmetric current sheets are likely to be prevalent in both astrophysical and laboratory plasmas with complex three dimensional (3D) magnetic topologies. This work presents kinematic analytical models for spine and fan reconnection at a radially symmetric 3D null (i.e., a null where the eigenvalues associated with the fan plane are equal) with asymmetric current sheets. Asymmetric fan reconnection is characterized by an asymmetric reconnection of flux past each spine line and a bulk flow of plasma across the null point. In contrast, asymmetric spine reconnection is characterized by the reconnection of an equal quantity of flux across the fan plane in both directions. The higher modes of spine reconnection also include localized wedges of vortical flux transport in each half of the fan. In this situation, two definitions for reconnection rate become appropriate: a local reconnection rate quantifying how much flux is genuinely reconnected across the fan plane and a global rate associ...

The nonthermal particle acceleration during magnetic reconnection remains a fundamental topic in several astrophysical phenomena, such as solar flares, pulsar wind, magnetars, etc, for more than half a century, and one of the unresolved questions is its efficiency. Recently, nonthermal particle acceleration mechanisms during reconnection have been extensively studied by particle-in-cell simulations, yet it is an intriguing enigma as to how the magnetic field energy is divided into thermally heated plasmas and nonthermal particles. Here we study both non-relativistic and relativistic magnetic reconnections using large-scale particle-in-cell simulation for a pair plasma, and indicate that the production of the nonthermal particle becomes efficient with increasing the plasma temperature. In the relativistic hot plasma case, we determine that the heated plasmas by reconnection can be approximated by a kappa distribution function with the kappa index of approximately 3 or less (equivalent to 2 or less for the power-law index), and the nonthermal energy density of reconnection is approximately over 95% of the total internal energy in the downstream exhaust.

We study the formation of slow-mode shocks in collisionless magnetic reconnection by using one-and two-dimensional collisionless MHD codes based on the double adiabatic approximation and the Landau closure model. We bridge the gap between the Petschek-type MHD reconnection model accompanied by a pair of slow shocks and the observational evidence of the rare occasion of in-situ slow shock observations. Our results showed that once magnetic reconnection takes place, a firehose-sense (p || > p ⊥) pressure anisotropy arises in the downstream region, and the generated slow shocks are quite weak comparing with those in an isotropic MHD. In spite of the weakness of the shocks, however, the resultant reconnection rate is 10 − 30% higher than that in an isotropic case. This result implies that the slow shock does not necessarily play an important role in the energy conversion in the reconnection system, and is consistent with the satellite observation in the Earth's magnetosphere.

The plasmoid observed in the Earth's magnetotail shows a wide variety of the complicated plasma structures that are not simply described by the standard Petschek reconnection model. The interaction of the plasmoid propagating tailward with the surrounding plasmas of the plasma sheet at rest may be important to understand the plasma sheet structure and the plasma heating observed in the magnetotail. The nonlinear time evolution of the plasmoid is studied by using a large-scale, high-resolution, two dimensional MHD simulation code. Several discontinuities/shocks are formed in association with magnetic reconnection: 1) a pair of the standard Petscheck-type slow shock waves emanating from the X-type neutral point, 2) the tangential discontinuity inside the plasmoid that separates the accelerated plasmas from the original plasma sheet plasmas, 3) the slow shock with a "crab-hand" structure surrounding the front-side of the plasmoid, 4) the intermediate shocks in the edge of the plasma sheet inside the plasmoid, and 5) the contact discontinuity inside the plasma sheet that separates the shock-heated plasmas from the Joule heated plasmas by the magnetic diffusion at the X-type neutral point. We also discuss how those plasma discontinuities/shocks structures are affected by the lobe/mantle plasma condition.

The debate surrounding fast magnetic energy dissipation by magnetic reconnection has remained a fundamental topic in the plasma universe, not only in the Earth's magnetosphere but in astrophysical objects such as pulsar magnetospheres and magnetars, for more than half a century. Recently, nonthermal particle acceleration and plasma heating during reconnection have been extensively studied, and it has been argued that rapid energy dissipation can occur for a collisionless "thin" current sheet, the thickness of which is of the order of the particle gyro-radius. However, it is an intriguing enigma as to how the fast energy dissipation can occur for a "thick" current sheet with thickness larger than the particle gyro-radius. Here we demonstrate, using a high-resolution particle-in-cell simulation for a pair plasma, that an explosive reconnection can emerge with the enhancement of the inertia resistivity due to the magnetization of the meandering particles by the reconnecting magnetic field and the shrinkage of the current sheet. In addition, regardless of the initial thickness of the current sheet, the time scale of the nonlinear explosive reconnection is tens of the Alfvén transit time.

An evolution of a magnetic reconnection in a collisionless accretion disk is investigated using a 2.5 dimensional hybrid code simulation. In astrophysical disks magnetorotational instability (MRI) is considered to play an important role by generating turbulence in the disk and contributes to an effective angular momentum transport through a turbulent viscosity. Magnetic reconnection, on the other hand also plays an important role on the evolution of the disk through a dissipation of a magnetic field enhanced by a dynamo effect of MRI. In this study, we developed a hybrid code to calculate an evolution of a differentially rotating system. With this code, we first confirmed a linear growth of MRI. We also investigated a behavior of a particular structure of a current sheet which would exist in the turbulence in the disk. From the calculation of the magnetic reconnection, we found an asymmetric structure in the out-of-plane magnetic field during the evolution of reconnection which can be understood by a coupling of the Hall effect and the differential rotation. We also found a migration of X-point whose direction is determined only by an initial sign of J 0 ×Ω 0 where J 0 is the initial current density in the neutral sheet and Ω 0 is the rotational vector of the background Keplerian rotation. Associated with the migration of X-point we also found a significant enhancement of the perpendicular magnetic field compared to an ordinary MRI. MRI-Magnetic reconnection coupling and the resulting magnetic field enhancement can be an effective process to sustain a strong turbulence in the accretion disk and to a transport of angular momentum.

We study linear and nonlinear development of relativistic and ultrarelativistic current sheets of pair (e ±) plasmas with antiparallel magnetic fields. Two types of two-dimensional problems are investigated by particle-in-cell simulations. First, we present the development of relativistic magnetic reconnection, whose outflow

In this paper, we present a comprehensive study of the energetic ion acceleration during magnetic reconnection in the Earth's magnetosphere using the Geotail data. A clear example of the energetic ion acceleration up to 1 MeV around an X-type neutral line is shown. We find that the energetic ions are localized at far downstream of reconnection outflow. The time variation of energetic ion and electron is almost the same. We observe ∼100 keV ions over the entire observation period. We study ten events in which the Geotail satellite observed in the vicinity of diffusion region in order to understand the reconnection characteristics that determine the energetic ion acceleration efficiency. We find that the reconnection electric field, total amount of reduced magnetic energy, reconnection rate, satellite location in the Earth's magnetosphere (both X GSM and Y GSM) show high correlation with energetic ion acceleration efficiency. Also, ion temperature, electron temperature, ion/electron temperature ratio, current sheet thickness, and electric field normal to the neutral sheet show low correlation. We do not find any correlation with absolute value of outflow velocity and current density parallel to magnetic field. The energetic ion acceleration efficiency is well correlated with large-scale parameters (e.g., total amount of reduced magnetic energy and satellite location), whereas the energetic electron acceleration efficiency is correlated with small-scale parameters (e.g., current sheet thickness and electric field normal to the neutral sheet). We conclude that the spatial size of magnetic reconnection is important for energetic ion acceleration in the Earth's magnetotail.

We study the role of the guide field in relativistic magnetic reconnection in a Harris current sheet of pair (e ±) plasmas, using linear theories and particlein-cell (PIC) simulations. Two-dimensional PIC simulations exhibit the guide field dependence to the linear instabilities; the tearing or reconnection modes are relatively insensitive, while the relativistic drift-kink instability (RDKI), the fastest mode in a relativistic current sheet, is stabilized by the guide field. Particle acceleration in the nonlinear stage is also investigated. A three-dimensional PIC simulation demonstrates that the current sheet is unstable to the RDKI, although a small reconnection occurs in the deformed current sheet. Another three-dimensional PIC simulation with a guide field demonstrates a completely different scenario. Secondary magnetic reconnection is triggered by nonlinear coupling of oblique instabilities, which we call the relativistic drift-sausage tearing instability. Therefore, particle acceleration by relativistic guide field reconnection occurs in three-dimensional configuration. Based on the plasma theories, we discuss an important role of the guide field: to enable non-thermal particle acceleration by magnetic reconnection.

Magnetic reconnections play essential roles in space, astrophysical, and laboratory plasmas, where the anti-parallel magnetic field components reconnect and the magnetic energy is converted to the plasma energy as Alfvénic out flows. Although the electron dynamics is considered to be essential, it is highly challenging to observe electron scale reconnections. Here we show the experimental results on an electron scale reconnection driven by the electron dynamics in laser-produced plasmas. We apply a weak-external magnetic field in the direction perpendicular to the plasma propagation, where the magnetic field is directly coupled with only the electrons but not for the ions. Since the kinetic pressure of plasma is much larger than the magnetic pressure, the magnetic field is distorted and locally antiparallel. We observe plasma collimations, cusp and plasmoid like features with optical diagnostics. The plasmoid propagates at the electron Alfvén velocity, indicating a reconnection driven by the electron dynamics.

In a recent paper 1 about electron heating at the reconnection separatrix, two figures depicting the contributions to the electron energy balance and the contribution to the total, quasi-viscous heating are incorrectly displayed. The correct figures are as follows: FIG. 11. Integration of the various terms of the energy equation over a volume bounded by flux tubes at X i t ¼ 29.94. The figure shows that the quasi-viscous contribution is the main energy source. It becomes important as soon as the integration volume extends past the separatrix field line.

We report simulation results for turbulent magnetic reconnection obtained using a newly developed Reynolds-averaged magnetohydrodynamics model. We find that the initial Harris current sheet develops in three ways, depending on the strength of turbulence: laminar reconnection, turbulent reconnection, and turbulent diffusion. The turbulent reconnection explosively converts the magnetic field energy into both kinetic and thermal energy of plasmas, and generates open fast reconnection jets. This fast turbulent reconnection is achieved by the localization of turbulent diffusion. Additionally, localized structure forms through the interaction of the mean field and turbulence.

Through the enhancement of transport, turbulence is expected to contribute to the fast reconnection. However the effects of turbulence are not so straightforward. In addition to the enhancement of transport, turbulence under some environment shows effects that suppress the transport. In the presence of turbulent cross helicity, such a dynamic balance between the transport enhancement and suppression occurs. As this result of dynamic balance, the region of effective enhanced magnetic diffusivity is confined to a narrow region, leading to the fast reconnection. In order to confirm this idea, a self-consistent turbulence model for the magnetic reconnection is proposed. With the aid of numerical simulations where turbulence effects are incorporated in a consistent manner through the turbulence model, the dynamic balance in the turbulence magnetic reconnection is confirmed.

Roles of turbulence in the context of magnetic reconnection are investigated with special emphasis on the mutual interaction between flow (large-scale inhomogeneous structure) and turbulence. In order to evaluate the effective transport due to turbulence, in addition to the intensity information of turbulence represented by the turbulent energy, the structure information represented by pseudoscalar statistical quantities (helicities) is important. On the basis of the evolution equation, mechanisms that provide turbulence with cross helicity are presented. Magnetic-flux freezing in highly turbulent media is considered with special emphasis on the spatial distribution of the turbulent cross helicity. The cross-helicity effects in the context of magnetic reconnection are also investigated. It is shown that the large-scale flow and magnetic-field configurations favorable for the cross-helicity generation is compatible with the fast reconnection. In this sense, turbulence and large-scale structures promote magnetic reconnection mediated by the turbulent cross helicity.

Extraction of knowledge from massive and complex data sets generated from peta-scale simulations poses a major obstacle to scientific progress. We propose a new approach to solving this problem by utilizing an innovative feature extraction technique in combination with specialized data mining algorithms which can be incorporated as part of the scientific visualization pipeline. In this paper we show how data from simulations as well as many other real life examples can be represented in a form of multivariate time series. Accordingly, we have adapted a multivariate time series analysis data mining technique to handle simulation data. The technique extracts global features and metafeatures in the 2D simulation dataset in order to capture the necessary time-lapse information. The features are then used to create a static, intermediate data set that is suitable for analysis using the standard supervised data mining techniques. The viability of the new algorithm called MineTool-M is dem...

Jets are ubiquitous in astronomy. It has been conjectured that the existence of jets is intimately connected with the spin of the central object and the viscous angular momentum transport of the inner disk. Bipolar jet-like structures propelled by the viscous torque on a spinning central object are also known in a completely different context, namely the flow in the laboratory of a viscoelastic fluid. On the basis of an analogy of the tangled magnetic field lines of magnetohydrodynamic (MHD) turbulence to the tangled polymers of viscoelastic polymer solutions, we propose a viscoelastic description of the dynamics of highly turbulent conductive fluid. We argue that the same mechanism that forms jets in viscoelastic fluids in the laboratory may be responsible for collimating and powering astrophysical jets by the angular momentum of the central object.

Rising tone bursts whose frequencies are drifting in decametric wavelength range (20-40 MHz) are sometimes observed associated with strong flares; the studies on these bursts have been made for the case of flare event on February 6, 1986 by analyzing dynamic spectra obtained at the Tsukidate observatory, in relation to the configuration of coronal magnetic field. The present analyses for the frequency drift of decametric radiation suggest the motion of energetic particles along the special form of the magnetic loop and its origin and evolution. That is, a large scale isolatedly closed magnetic loop is formed by magnetic reconnection and is pushed out with a speed of 400-500 km/s toward outer coronal region.

The results of many investigations have shown that equilibrium current structures are frequently formed in the presence of developed turbulence. Therefore internal current instabilities cannot be the cause of observed reconnection phenomena. However external factors may be responsible for the observed changes in the magnetic field topology (reconnection). Some possible sources of such a kind, which could result in the reconnection on the Earth's magnetopause and in the near tail regions, are analyzed here. It is shown that an effective eastward ring current can lead to many of the observed reconnection phenomena.

We present multi-wavelength observations of an M-class flare (M3.9) that occurred on 2014 June 11. Our observations were conducted with the Dunn Solar Telescope (DST), adaptive optics, the multi-camera system ROSA (Rapid Oscillations in Solar Atmosphere) and new HARDcam (Hydrogen-Alpha Rapid Dynamics) camera in various wavelengths, such as Ca II K, Mg I b 2 (at 5172.7Å), and Hα narrow-band, and G-band continuum filters.

Several phenomena connected with the magnetic field of the Sunthe cool sunspots, the hot corona, solar flares, the solar windare collectively known as solar activity. This paper discusses how one uses the MHD equations to understand how the magnetic field of the Sun is produced by the dynamo process and then gives rise to these diverse activities, making the Sun the best laboratory for plasma physics in the limit of high magnetic Reynolds number (defined at the end of the Introduction).

Several phenomena connected with the magnetic field of the Sun (the cool sunspots, the hot corona, solar flares, the solar wind) are collectively known as solar activity. This paper discusses how one uses the MHD equations to understand how the magnetic field of the Sun is produced by the dynamo process and then gives rise to these diverse activities, making the Sun the best laboratory for plasma physics in the limit of high magnetic Reynolds number (defined at the end of the Introduction).

We summarize recent 2D MHD simulations of line-driven stellar winds from rotating hot-stars with a dipole magnetic field aligned to the star's rotation axis. For moderate to strong fields, much wind outflow is initially along closed magnetic loops that nearly corotate as a solid body with the underlying star, thus providing a torque that results in an effective angular momentum

With the recent detection of large scale magnetic fields on several O and B stars, the anomalous X-ray emission detected from a subset of hot stars, and advances in numerical modeling, there is increasing interest in the role magnetic fields play in hot stars. We present synthesized diagnostics of magnetic hot stars, derived from state-of-the-art numerical simulations of both slowly rotating O stars with strong winds (e.g. theta1 Ori C) and rapidly rotating B stars with weaker winds and stronger fields (e.g. sigma Ori E). The numerical simulations include 2-D ZEUS MHD simulations, as well as rigidly-rotating magnetosphere (RRM) simulations of the more magnetically dominated winds of early B stars. We present results on rotationally modulated H-alpha emission, as well as x-ray emission arising from magnetically channeled wind shocks and centrifugally driven magnetospheric breakout with the associated magnetic reconnection.

This talk summarizes results from recent MHD simulations of the role of a dipole magnetic field in inducing large-scale structure in the line-driven stellar winds of hot, luminous stars. Unlike previous fixed-field analyses, the MHD simulations here take full account of the dynamical competition between the field and the flow. A key result is that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless 'wind magnetic confinement parameter', η * (= B 2 eq R 2 * /Ṁ v ∞), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind. For weak confinement, η * ≤ 1, the field is fully opened by wind outflow, but nonetheless, for confinement as small as η * = 1/10 it can have significant back-influence in enhancing the density and reducing the flow speed near the magnetic equator. For stronger confinement, η * > 1, the magnetic field remains closed over limited range of latitude and height above the equatorial surface, but eventually is opened into nearly radial configuration at large radii. Within the closed loops, the flow is channeled toward loop tops into shock collisions that are strong enough to produce hard X-rays. Within the open field region, the equatorial channeling leads to oblique shocks that are again strong enough to produce X-rays and also lead to a thin, dense, slowly outflowing "disk" at the magnetic equator.

Magnetic reconnection phenomena are investigated taking into account all three vector components of the magnetic field in a laboratory experiment. Two toroidal magnetized plasmas carrying identical toroidal currents and poloidai field configurations are made to collide, thereby inducing magnetic reconnection. The directions of the toroidal field play an important role in the merging process. It is found that plasmas of anti-parallel heliciry merge much faster than those of parallel helicity. It is also found that the reconnection rate is proportional to the initial relative velocity of the two plasma tori ; suggesting that magnetic reconnecrion, in the present experiment, is a forced pheonomenon. DISCLAIMER This report was P^red as » « '-^^J^^^-fS Government. Neither the United States ^^"^^l^^jiiiy or responsiemp.oyees. »*. any-^ »'j£of"r^^fe.u, product or bilhy for the accuracy. cocnpWcKSS.« u »'»™=?. ». rivalely owmd rights. Referprocess .Closed, or represent that .tt use *ould not^««™&J ^ name> tfademarl , £» terei-to «, 1*** »*"«™ al^^S u " *""?,*its endowment, xccommanufacturer, or o.herwUe does «?,"?""*££££ °'.£Agency thereof. The views mendation. or favoring by the United S a, « ^'™"^y J aW " reneet those of the and opinions of authors expressed here* do not necessarily state United States Government or any agency thereof. MASTER^