Numerical Simulations Of Magnetic Reconnection Research Articles

Temperature-reducing shocks in optically thin radiative MHD—Analytical and numerical results

Shocks are often invoked as heating mechanisms in astrophysical systems, with both adiabatic compression and dissipative heating that leading to the increase in temperature. While shocks are reasonably well understood for ideal magnetohydrodynamic (MHD) systems, in many astrophysical plasmas, radiation is an important phenomena, which can allow energy to leave the system. As such, energy becomes non-conservative, which can fundamentally change the behavior of shocks. The energy emitted through optically thin radiation post-shock can exceed the thermal energy increase, resulting in shocks that reduce the temperature of the medium, i.e., cooling shocks that have a net decrease in temperature across the interface. In this paper, semi-analytical solutions for radiative shocks are derived to demonstrate that both cooling (temperature decreasing) and heating (temperature increasing) shock solutions are possible across the whole temperature range in optically thin radiative MHD. Numerical simulations of magnetic reconnection for solar-like temperatures and plasma-β with optically thin radiative losses also yield both heating and cooling shocks in roughly equal abundances. The detected cooling shocks feature a significantly lower pressure jump across the shock than their heating counterparts. The compression at the shock front leads to locally enhanced radiative losses, resulting in significant cooling within a few grid cells in the upstream and downstream directions. The presence of temperature-reducing (cooling) shocks is critical in determining the thermal evolution, and heating or cooling, across a wealth of radiative astrophysical plasmas including magnetic reconnection in the solar corona.

Numerical simulations of the laser-driven Petschek-type magnetic reconnection

This paper describes a numerical study of the magnetic reconnection between two magnetic fields of opposite polarity. The magnetic fields are created by an electric current in a coil connected to two metal disks. One of the disks is irradiated by a strong laser beam, whereby large amounts of free electrons flow toward the other disk, constituting a closed circuit for the electric current flowing through the coil. Two parallel coils are arranged to connect the two disks, and irradiation of the laser beam on one disk results in parallel electric currents in the two coils, inducing two magnetic fields of opposite polarity in the region between them. The magnetic reconnection that occurs in this region is three-dimensional. This three-dimensional magnetic reconnection is investigated via magnetohydrodynamic numerical simulations. The characteristics of the Petschek-type magnetic reconnection are observed for the first time in such numerical simulations of magnetic reconnection. Changes in the shape of the magnetic field lines form the boundary of the dissipation region and the outflow region. Moreover, the thermal plasma generated by reconnection is strongly confined to the region where the reconnecting current sheet and the slow-mode shock are located, and no leaks of thermal plasma are observed. Comparisons with existing laboratory experiment results confirm that our numerical simulations reproduce the experimental outcomes and provide reasonable explanations for the results observed in laboratories.

Numerical Simulation of Magnetic Reconnection Driven by the Laser Helmholtz Capacitor-coil Targets

Experiments of magnetic reconnection driven by the laser in laboratory with the Helmholtz capacitor-coil target have been carried out for years. In the experiment, the reconnecting magnetic fields are induced by the electric current inside two conjugate coils. The free electrons are produced as one of the metal plate is irradiated by the intense laser, and the electric current results from the flow of free electrons between two metal plates connecting to one another via two coils. This kind of experiments of reconnection differs from others that performed by the Biermann battery effect. The three-dimensional numerical experiments in the magnetohydrodynamical framework has been performed for the first time in this work with respect to the laser experiment. Our results indicate clearly that the outflow plasma of magnetic reconnection shows accumulative features around the coils. In our simulation, the reconnecting magnetic field produced by the induced electric current inside the coils could be as strong as 100 T, and the plasma β around the reconnection region is around 10−2. Comparing with the existing results of the laboratory experiments, the numerical experiment duplicates most features within, which helps examine the physics behind the phenomena shown by the laboratory experiments.

Nascent Flux Rope Observations at Earth's Dayside Magnetopause

AbstractTwo THEMIS satellites, separated by only 0.38 Earth radii (RE), traversed two consecutive dayside magnetopause (MP) current sheets at 3.5 < ZGSM < 3.8 RE on 15 November 2010. An early‐stage crater‐like flux transfer event (FTE) with weakly enhanced total pressure is sampled at the first, complete outbound MP crossing. A mature FTE flux rope is observed just 70 s later across the second, inbound crossing of the MP current sheet. Two counterstreaming magnetosheath ion beams at the interface of two converging reconnection exhausts earthward of the MP current sheet provide direct evidence of two X‐lines across the early‐stage crater FTE. A D‐shaped ion beam observed along the southward magnetic field in the magnetosheath boundary layer and northward exhausts observed earthward of the MP provide evidence of two X‐lines across the mature FTE flux rope. Tripolar out‐of‐plane magnetic field perturbations (ΔBM) of substantial magnitudes were sampled across the MP current sheet in both stages of the FTE flux rope evolution. These dayside observations suggest an association between a tripolar ΔBM perturbation with two magnetic reconnection X‐lines and FTE flux rope formation. A dedicated particle‐in‐cell numerical simulation of magnetic reconnection reproduces similar ion velocity distributions and out‐of‐plane magnetic field perturbations at an asymmetric current sheet as those observed across a nascent flux rope between two X‐lines at the dayside asymmetric MP.

Numerical Simulation of Magnetic Reconnection around a Black Hole

We performed numerical simulations of general relativistic magnetohydrodynamics with uniform resistivity to investigatethe occurrence of magnetic reconnection in a split-monopole magnetic field around a Schwarzschild black hole. We found that magnetic reconnection happens near the black hole at its equatorial plane. The magnetic reconnection has a point-like reconnection region and slow shock waves, as in the Petschek reconnection model. The magnetic reconnection rate decreases as the resistivity becomes smaller. When the global magnetic Reynolds number is $10^4$ or larger, the magnetic reconnection rate increases linearly with time from $2 \tau_{\rm S}$ to $\sim 10 \tau_{\rm S}$ ($\tau_{\rm S}=r_{\rm S}/c, r_{\rm S}$ is the Schwarzschild radius and $c$ is the speed of light). The linear increase of the reconnection rate agrees with the magnetic reconnection in the Rutherford regime of the tearing mode instability.

Energy transfer and electron energization in collisionless magnetic reconnection for different guide-field intensities

Electron dynamics and energization are one of the key components of magnetic field dissipation in collisionless reconnection. In 2D numerical simulations of magnetic reconnection, the main mechanism that limits the current density and provides an effective dissipation is most probably the electron pressure tensor term, which has been shown to break the frozen-in condition at the x-point. In addition, the electron-meandering-orbit scale controls the width of the electron dissipation region, where the electron temperature has been observed to increase both in recent Magnetospheric Multiple-Scale (MMS) observations and in laboratory experiments, such as the Magnetic Reconnection Experiment (MRX). By means of two-dimensional full-particle simulations in an open system, we investigate how the energy conversion and particle energization depend on the guide field intensity. We study the energy transfer from the magnetic field to the plasma in the vicinity of the x-point and close downstream regions, and E·J and the threshold guide field separating two regimes where either the parallel component, E||J||, or the perpendicular component, E⊥·J⊥, dominate the energy transfer, confirming recent MRX results and also consistent with MMS observations. We calculate the energy partition between fields and kinetic and thermal energies of different species, from electron to ion scales, showing that there is no significant variation for different guide field configurations. Finally, we study possible mechanisms for electron perpendicular heating by examining electron distribution functions and self-consistently evolved particle orbits in high guide field configurations.

Intermittent Reconnection and Plasmoids in UV Bursts in the Low Solar Atmosphere

Abstract Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet, the detailed physical mechanisms driving reconnection are difficult to discern in the remote sensing observations that are used to study the solar atmosphere. In this Letter, we exploit the high-resolution instruments Interface Region Imaging Spectrograph and the new CHROMIS Fabry–Pérot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in so-called UV bursts in the low solar atmosphere. The Si iv 1403 Å UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca ii K 3934 Å line now provide compelling evidence for the presence of plasmoids by revealing highly dynamic and rapidly moving brightenings that are smaller than 0.″2 and that evolve on timescales of the order of seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere.

Formation and Reconnection of Three-dimensional Current Sheets with a Guide Field in the Solar Corona

Abstract We analyze a series of three-dimensional magnetohydrodynamic numerical simulations of magnetic reconnection in a model solar corona to study the effect of the guide-field component on quasi-steady-state interchange reconnection in a pseudostreamer arcade configuration. This work extends the analysis of Edmondson et al. by quantifying the mass density enhancement coherency scale in the current sheet associated with magnetic island formation during the nonlinear phase of plasmoid-unstable reconnection. We compare the results of four simulations of a zero, weak, moderate, and a strong guide field, , to quantify the plasmoid density enhancement’s longitudinal and transverse coherency scales as a function of the guide-field strength. We derive these coherency scales from autocorrelation and wavelet analyses, and demonstrate how these scales may be used to interpret the density enhancement fluctuation’s Fourier power spectra in terms of a structure formation range, an energy continuation range, and an inertial range—each population with a distinct spectral slope. We discuss the simulation results in the context of solar and heliospheric observations of pseudostreamer solar wind outflow and possible signatures of reconnection-generated structure.

Quantifying gyrotropy in magnetic reconnection

AbstractA new scalar measure of the gyrotropy of a pressure tensor is defined. Previously suggested measures are shown to be incomplete by means of examples for which they give unphysical results. To demonstrate its usefulness as an indicator of magnetic topology, the new measure is calculated for electron data taken from numerical simulations of magnetic reconnection, shown to peak at separatrices and X points, and compared to the other measures. The new diagnostic has potential uses in analyzing spacecraft observations, and so a method for calculating it from measurements performed in an arbitrary coordinate system is derived.

Numerical Simulations of Magnetic Reconnection in an Asymmetric Current Sheet

Previous particle-in-cell simulations have shown that electron phase-space holes (electron holes), where the associated parallel electric field has a bipolar structure, exist near the four separatrices in anti-parallel magnetic reconnection. By performing two-dimensional (2-D) particle-in-cell (PIC) simulations, here we investigate magnetic reconnection in an asymmetric current sheet, with emphasis on the parallel electric field near the separatrices. Compared with magnetic reconnection in a symmetric current sheet, it is found that the parallel electric field with a bipolar structure only exists around the separatrices in the upper region with a lower density (upper separatrices). Such a bipolar structure of the parallel electric field is considered to be associated with electron holes resulting from the nonlinear evolution of the electron beam instability excited by the high-speed electron flow formed after their acceleration around the X line. The disappearance of the parallel electric field around the separatrices in the lower region with a higher density (lower separatrices) may be due to the transverse instability, which is unstable in a weak magnetized plasma.

Continuous transitions between discontinuous magnetohydrodynamic flows of plasma and its heating

The possibility that the type of discontinuous flow changes as the conditions gradually (continuously) change is investigated in connection with the problems arising when the results of numerical simulations of magnetic reconnection in plasma are interpreted. The conservation laws at a discontinuity surface in magnetohydrodynamics admit such transitions, but the so-called transition solutions for the boundary conditions that simultaneously satisfy two types of discontinuities should exist in this case. The specific form of such solutions has been found, and a generalized scheme of permitted transitions has been constructed on their basis. An expression for the jump in internal energy at discontinuity is derived. The dependence of the plasma heating efficiency on the type of discontinuity is considered.

Formation of a transient front structure near reconnection point in 3‐D PIC simulations

Massively parallel numerical simulations of magnetic reconnection are presented in this study. Electromagnetic full‐particle implicit code iPIC3D is used to study the dynamics and 3‐D evolution of reconnection outflows. Such features as Hall magnetic field, inflow and outflow, and diffusion region formation are very similar to 2‐D particle‐in‐cell (PIC) simulations. In addition, it is well known that instabilities develop in the current flow direction or oblique directions. These modes could provide for anomalous resistivity and diffusive drag and can serve as additional proxies for magnetic reconnection. In our work, the unstable evolution of reconnection transient front structures is studied. Reconnection configuration in the absence of guide field is considered, and it is initialized with a localized perturbation aligned in the cross‐tail direction. Our study suggests that the instabilities lead to the development of finger‐like density structures on ion‐electron hybrid scales. These structures are characterized by a rapid increase of the magnetic field, normal to the current sheet (Bz). A small decrease in the magnetic field component parallel to the reconnection X line and the component perpendicular to the current sheet is observed in the region ahead of the front. The instabilities form due to fact that the density gradient inside the front region is opposite to the direction of the acceleration Lorentz force. Such density structures may possibly further develop into larger‐scale earthward flux transfer events during magnetotail reconnection. In addition, oscillations mainly in the magnetic and electric fields and the electron density are observed shortly before the arrival of the main front structure which is consistent with recent THEMIS observations.

Three-Dimensional Simulations of Magnetic Reconnection with or Without Velocity Shears

The properties of spontaneous reconnection of a current sheet analyzed via direct three-dimensional simulations are presented. In particular the non-linear dynamics of resistive instabilities has been studied in absence or in presence of velocity shears. It is shown that full three-dimensional simulations allow the inclusion of a rich variety of (ideal) secondary instabilities which, depending on the initial equilibrium magnetic field configuration, determine the final fate of the system in the fully non linear regime. In particular in presence of a guide-field the dynamic is similar to what observed in two-dimensional simulations with energy driven toward both smaller and larger scales and energy spectra anisotropy. For different magnetic field configurations, the final state is characterized by the disruption of the coalesced structure created during the resistive phase and the system is characterized by a more chaotic state. A discussion on the importance of high-order numerical techniques in numerical simulations of magnetic reconnection is also present.

Numerical simulations of magnetic reconnection in the lower solar atmosphere

Observations indicate that Ellerman bombs (EBs) and chromospheric microflares both occur in the lower solar atmosphere, and share many common features, such as temperature enhancements, accompanying jet-like mass motions, short lifetime, and so on. These strongly suggest that EBs and chromospheric microflares could both probably be induced by magnetic reconnection in the lower solar atmosphere. With gravity, ionization and radiation considered, we perform two-dimensional numerical simulations of magnetic reconnection in the lower solar atmosphere. The influence of different parameters, such as intensity of the magnetic field and anomalous resistivity, on the results are investigated. Our result demonstrates that the temperature increases are mainly due to the joule dissipation caused by magnetic reconnection. The spectral profiles of EBs and chromospheric microflares are calculated with the non-LTE radiative transfer theory and compared with observations. It is found that the typical features of the two phenomena can be qualitatively reproduced.

Transition-Region Explosive Events: Reconnection Modulated by p-Mode Waves

Transition-region explosive events (TREEs) have long been proposed as a consequence of magnetic reconnection. However, several critical issues have not been well addressed, such as the location of the reconnection site, their unusually short lifetime (about one minute), and the recently discovered repetitive behaviour with a period of three to five minutes. In this paper, we perform MHD numerical simulations of magnetic reconnection, where the effect of five-minute solar p-mode oscillations is examined. UV emission lines are synthesised on the basis of numerical results in order to compare with observations directly. It is found that several typical and puzzling features of the TREEs with impulsive bursty behaviour can only be explained if there exist p-mode oscillations and the reconnection site is located in the upper chromosphere at a height range of around 1900 km < h < 2150 km above the solar surface. Furthermore, the lack of proper motions of the high-velocity ejection may be due to a rapid change of temperature along the reconnection ejecta.

Two-dimensional Magnetohydrodynamic Simulations of Relativistic Magnetic Reconnection

It has been recognized that the magnetic reconnection process is of great importance in high-energy astrophysics. We develop a new two-dimensional relativistic resistive magnetohydrodynamic (R2MHD) code and carry out numerical simulations of magnetic reconnection. We find that the outflow velocity reaches the Alfven velocity in the inflow region and that a higher Alfven velocity provides a higher reconnection rate. We also find that Lorentz contraction plays an important role in enhancement of the reconnection rate.

Petschek‐like Reconnection with Current‐driven Anomalous Resistivity and Its Application to Solar Flares

Recent numerical simulations of magnetic reconnection in two dimensions have shown that, when the resistivity is strongly localized, the reconnection region develops a Petschek-like structure, with the width of the inner diffusion region being of the order of the resistivity localization scale. In this paper, we combine this fact with a realistic model for locally-enhanced anomalous resistivity generated by current-driven microturbulence. The result is a qualitative model of the reconnection layer where the size of Petschek's diffusion region and, therefore, the final reconnection rate are determined self-consistently in terms of the main parameters of the functional dependence of anomalous resistivity on the electric current density. We then consider anomalous resistivity due to ion-acoustic turbulence as a particular case. This enables us to express the reconnection region's parameters directly in terms of the basic parameters of the plasma. Finally, we apply this reconnection model to solar flares and obtain specific predictions for typical reconnection times, which are very consistent with observations.

Two‐dimensional Magnetohydrodynamic Numerical Simulations of Magnetic Reconnection Triggered by a Supernova Shock in the Interstellar Medium: Generation of X‐Ray Gas in the Galaxy

We examine magnetic reconnection triggered by a supernova (or a point explosion) in the interstellar medium by performing two-dimensional resistive magnetohydrodynamic (MHD) numerical simulations with high spatial resolution. We find that magnetic reconnection starts long after a supernova shock (fast-mode MHD shock) passes a current sheet. A current sheet evolves as follows: (1) Tearing-mode is excited by the supernova shock, and in its nonlinear stage the current sheet becomes thin. (2) The current-sheet thinning is saturated when the current-sheet thickness becomes comparable to that of the Sweet-Parker current sheet. After that, Sweet-Parker reconnection starts, and the current-sheet length increases. (3) Secondary tearing-mode instability occurs in the thin Sweet-Parker current sheet. (4) As a result, further current-sheet thinning occurs and anomalous resistivity sets in, because gas density decreases in the current sheet. Petschek reconnection starts and heats the interstellar gas. Magnetic energy is released quickly as magnetic islands move in the current sheet during Petschek reconnection. The released magnetic energy is determined by the interstellar magnetic field strength, not the energy of the initial explosion or the distance to the explosion. We suggest that magnetic reconnection is a possible mechanism to generate X-ray gas in the Galaxy.

Ellerman Bombs, Type II White-light Flares and Magnetic Reconnection in the Solar Lower Atmosphere

Ellerman bombs and Type II white-light flares share many common features despite the large energy gap between them. Both are considered to result from local heating in the solar lower atmosphere. This paper presents numerical simulations of magnetic reconnection occurring in such a deep atmosphere, with the aim to account for the common features of the two phenomena. Our numerical results manifest the following two typical characteristics of the assumed reconnection process: (1) magnetic reconnection saturates in ~ 600--900 s, which is just the lifetime of the two phenomena; (2) ionization in the upper chromosphere consumes quite a large part of the energy released through reconnection, making the heating effect most significant in the lower chromosphere. The application of the reconnection model to the two phenomena is discussed in detail.

Flaring Loop Motion and a Unified Model for Solar Flares

We performed 2.5-dimensional numerical simulations of magnetic reconnection for several models, some with the reconnection point at a high altitude (the X-type point in magnetic reconnection), and one with the reconnection point at a low altitude. In the high-altitude cases, the bright loop appears to rise for a long time, with its two footpoints separating and the —eld lines below the bright loop shrinking, which are all typical features of two-ribbon —ares. The rise speed of the loop and the separation speed of its footpoints depend strongly on the magnetic —eld to a medium extent on the density and B 0 , o 0 , weakly on the temperature the resistivity g, and the length scale by which the size of current T 0 , L 0 , sheet and the height of the X-point are both scaled. The strong dependence means that the Lorentz B 0 force is the dominant factor; the inertia of the plasma may account for the moderate dependence; o 0 and the weak g dependence may imply that ii fast reconnection ˇˇ occurs; the weak dependence implies L 0 that the —aring loop motion has geometrical self-similarity. In the low-altitude case, the bright loops cease rising only a short time after the impulsive phase of the reconnection and then become rather stable, which shows a distinct similarity to the compact —ares. The results imply that the two types of solar —ares, i.e., the two-ribbon —ares and the compact ones, might be uni—ed into the same magnetic reconnection model, where the height of the reconnection point leads to the bifurcation. Subject headings: MHDmethods: numericalSun: —aresSun: magnetic —elds