Evolution Of Current Sheet Research Articles

Current Sheet Evolution in a Planar Inductive Pulsed Plasma Thruster

Abstract The evolution of the current sheet is fundamental to the understanding and operation of planar inductive pulsed plasma thrusters. Methods for experimentally determining the time-varying mutual inductance and plasma resistance associated with the current sheet are presented. From these, time-histories of the resistive heating in and electromagnetic acceleration of the sheet are found. Analysis of experimental data obtained from a compact, low discharge energy inductive pulsed plasma thruster shows that current sheet evolution is well described by three phases: ionization, formation, and acceleration. Evidence is provided for the plasma current and resistive heating becoming localized near the upstream edge of the current sheet as the sheet forms. The influence of initial propellant density and pre-ionization on the characteristics of the sheet are examined. Both higher propellant densities and increased pre-ionization are found to produce current sheets which exhibit greater magnetic impermeability. Higher densities cause the sheet to form closer to the coil face, improving coupling, while stronger pre-ionization leads to more rapid resistive heating, causing the sheet to form earlier in time.

Effect of nonlinearity, magnetic islands on turbulence and observation of electron energization, temperature anisotropy at Earth’s magnetopause (magnetosphere)

Space missions’ observations have shown that waves such as lower hybrid waves (LHWs), whistler waves, and kinetic Alfven waves play a vital role in magnetic reconnection, turbulence, and particle acceleration. This paper studies the effect of nonlinearity and the magnetic islands on lower hybrid turbulence and current sheets in Earth’s magnetopause region. The evolution of electromagnetic LHW has been studied with numerical model using pseudo-spectral method for spatial integration and finite difference method with modified predictor-corrector approach for temporal integration. We have considered both ion and electron dynamics and included electromagnetic and warm plasma effects in our model. The study outcomes reveal that both the nonlinear effects and magnetic islands are responsible for the evolution of LHWs and current sheets to a chaotic and turbulent state. We have also used the semi-analytical model to elaborate on the physics behind the localization. Finally, the nonlinear model with field perturbations (magnetic islands) is further used to elaborate on the electron energization and temperature anisotropy near reconnection regions. We have also discussed the relevance of model predictions in the context of the MMS mission observations at Earth’s magnetopause.

Structure of Current Sheets Formed in 2D Magnetic Configurations with X-Type Null Lines in the Presence of the Hall Currents and Inverse Currents

We present experimental results on the formation and evolution of current sheets in two-dimensional magnetic configurations with an X-type null line. Typical features of both the initial magnetic field and the current sheet are their symmetry properties. The experiments were carried out using the CS-3D setup. The formation of a current sheet occurs just after the magneto-sonic wave converges at the null line; then, both the electric current and plasma become compressed in a planar 2D sheet, which accumulates an excess of magnetic energy. The excitation of the Hall currents, which build up the out-of-plane magnetic fields inside the 2D current sheet, brings about the modification of the sheet structure. As a result, the magnetic fields and plasma currents become 3D. The dynamic plasma processes give rise to additional current sheet deformations, which are caused by the excitation of inverse currents at the side edges of the sheet. As a consequence, the out-of-plane magnetic fields are reversed, and strong Ampère’s forces of the opposite directions come into play. These forces slow down the previously accelerated high-speed plasma flows so that the flows become limited in time and space.

Formation and evolution of coherent structures in 3D strongly turbulent magnetized plasmas

We review the current literature on the formation of coherent structures (CoSs) in strongly turbulent 3D magnetized plasmas. CoSs [Current Sheets (CS), magnetic filaments, large amplitude magnetic disturbances, vortices, and shocklets] appear intermittently inside a turbulent plasma and are collectively the locus of magnetic energy transfer (dissipation) into particle kinetic energy, leading to heating and/or acceleration of the latter. CoSs and especially CSs are also evolving and fragmenting, becoming locally the source of new clusters of CoSs. Strong turbulence can be generated by the nonlinear coupling of large amplitude unstable plasma modes, by the explosive reorganization of large-scale magnetic fields, or by the fragmentation of CoSs. A small fraction of CSs inside a strongly turbulent plasma will end up reconnecting. Magnetic Reconnection (MR) is one of the potential forms of energy dissipation of a turbulent plasma. Analyzing the evolution of CSs and MR in isolation from the surrounding CoSs and plasma flows may be convenient for 2D numerical studies, but it is far from a realistic modeling of 3D astrophysical, space, and laboratory environments, where strong turbulence can be exited, such as in the solar wind, the solar atmosphere, solar flares and Coronal Mass Ejections, large-scale space and astrophysical shocks, the magnetosheath, the magnetotail, astrophysical jets, and Edge Localized Modes in confined laboratory plasmas (tokamaks).

Distinctive features of the structure of current sheets formed in plasma in three-dimensional magnetic configurations with an X line (a review)

A review is presented on experimental results related to investigation of distinctive features of the structure and evolution of plasma current sheets formed in three
 dimensional (3D) magnetic configurations with an X line, in the presence of a longitudinal magnetic field component (guide field) directed along the X line. It is shown that formation of a plasma current sheet results in enhancement of the guide field within the sheet. The excessive guide field is maintained by plasma currents that flow in the transverse plane relative to the main current in the sheet. As a result, the structure of the currents becomes three-dimensional. 
 Increasing the initial value of the guide field brings about a decrease of compression into the sheet of both the electric current and plasma. This effect is caused by changing the pres-
 sure balance in the sheet when an excessive guide field appears in it. Deformation of plasma current sheets in 3D magnetic configurations, namely, an appearance of asymmetric 
 and tilted sheets, results from excitation of the Hall currents and their interaction with the guide field. It is shown that the formation of current sheets in 3D magnetic configurations 
 with an X line is possible in a relatively wide, but limited range of initial conditions

Reconnection and particle acceleration in three-dimensional current sheet evolution in moderately magnetized astrophysical pair plasma

Magnetic reconnection, a plasma process converting magnetic energy to particle kinetic energy, is often invoked to explain magnetic energy releases powering high-energy flares in astrophysical sources including pulsar wind nebulae and black hole jets. Reconnection is usually seen as the (essentially two-dimensional) nonlinear evolution of the tearing instability disrupting a thin current sheet. To test how this process operates in three dimensions, we conduct a comprehensive particle-in-cell simulation study comparing two- and three-dimensional evolution of long, thin current sheets in moderately magnetized, collisionless, relativistically hot electron–positron plasma, and find dramatic differences. We first systematically characterize this process in two dimensions, where classic, hierarchical plasmoid-chain reconnection determines energy release, and explore a wide range of initial configurations, guide magnetic field strengths and system sizes. We then show that three-dimensional (3-D) simulations of similar configurations exhibit a diversity of behaviours, including some where energy release is determined by the nonlinear relativistic drift-kink instability. Thus, 3-D current sheet evolution is not always fundamentally classical reconnection with perturbing 3-D effects but, rather, a complex interplay of multiple linear and nonlinear instabilities whose relative importance depends sensitively on the ambient plasma, minor configuration details and even stochastic events. It often yields slower but longer-lasting and ultimately greater magnetic energy release than in two dimensions. Intriguingly, non-thermal particle acceleration is astonishingly robust, depending on the upstream magnetization and guide field, but otherwise yielding similar particle energy spectra in two and three dimensions. Although the variety of underlying current sheet behaviours is interesting, the similarities in overall energy release and particle spectra may be more remarkable.

Onset of Plasmoid Reconnection during Magnetorotational Instability

Abstract The evolution of current sheets in accretion flows undergoing magnetorotational instability (MRI) is examined through two- and three-dimensional numerical modeling of the resistive MHD equations in global cylindrical geometry. With an initial uniform magnetic field aligned in the vertical (z) direction, MRI produces radially extended toroidal (azimuthal) current sheets. In both 2D and 3D when axisymmetric modes dominate, these current sheets attract each other and merge in the poloidal (rz) plane, driving magnetic reconnection when the Lundquist number S > 3 × 102, making it a possible source of plasmoids (closed magnetic loops) in accretion disks. At high Lundquist numbers in the 2D regime, starting at S = 5 × 103, self-consistent MRI-generated current sheets become thin and subject to plasmoid instability, and therefore spontaneous magnetic reconnection. When nonaxisymmetric 3D modes dominate, turbulence makes the azimuthal current sheets more unstable and stretch vertically. Toroidally extended vertical current sheets in the inner region, as well as larger 3D magnetic islands in the outer regions of the disks are also formed. These findings have strong ramifications for astrophysical disks as potential sources of plasmoids that could cause local heating, particle acceleration, and high energy EM radiation.

Magnetic island merging: Two-dimensional MHD simulation and test-particle modeling

In an idealized system where four current channels interact in a two-dimensional periodic setting, we follow the detailed evolution of current sheets (CSs) forming in between the channels, as a result of a large-scale merging. A central X-point collapses and a gradually extending CS marks the site of continuous magnetic reconnection. Using grid-adaptive, non-relativistic, resistive magnetohydrodynamic (MHD) simulations, we establish that slow, near-steady Sweet-Parker reconnection transits to a chaotic, multi-plasmoid fragmented state, when the Lundquist number exceeds about ten to the fourth power, well in the range of previous studies on plasmoid instability. The extreme resolution employed in the MHD study shows significant magnetic island substructures. With relativistic test-particle simulations, we explore how charged particles can be accelerated in the vicinity of an O-point, either at embedded tiny-islands within larger "monster"-islands or near the centers of monster-islands. While the planar MHD setting artificially causes strong acceleration in the ignored third direction, it also allows for the full analytic study of all aspects leading to the acceleration and the in-plane-projected trapping of particles in the vicinities of O-points. Our analytic approach uses a decomposition of the particle velocity in slow- and fast-changing components, akin to the Reynolds decomposition in turbulence studies. Our analytic description is validated with several representative test-particle simulations. We find that after an initial non-relativistic motion throughout a monster island, particles can experience acceleration in the vicinity of an O-point beyond 0.7c, at which speed the acceleration is at its highest efficiency

Chaos-induced resistivity in collisionless reconnection region with magnetic island-chain structure

The mechanism of anomalous resistivity is still an open and unsolved problem in collisionless magnetic reconnection. Studies show that magnetic reconnection regions often have magnetic island-chain structures. By using the test particle simulation, the chaos-induced resistivity is calculated in the regions around X points and O points, respectively. The results show that the chaos-induced resistivity in the X-type region is higher than that in the O-type region by an order of magnitude for the case with a strong guide field or a high magnetic island parameter. For the case of a weak guide field, however, the chaos-induced resistivity in the X-type region sensitively depends on the guide field and may be close to or less than that in the O-type region in the case of a low magnetic island parameter. This work will be helpful for us to understand the magnetic energy dissipation of occurring in collisionless reconnection regions as well as the dynamical evolution of reconnection current sheets.

On the Generation of Opposite Currents in Current Sheets

A consistent interpretation of phenomena at late stages of the evolution of current sheets has been given for the first time in terms of the generation of inductive electric fields at the motion of plasma flows in a magnetic field. It has been shown that inductive fields are inhomogeneous within the width of the sheet and maximal at its edges, where opposite currents appear. The time of appearance of opposite currents depends on the time of plasma acceleration and increases for the plasma with heavy ions. A decrease in the current density and the broadening of the current sheet far from the center are due to the generation of opposite currents, which are manifested here indirectly. In regions with opposite currents, Hall currents in the directions opposite to those at early stages of the evolution of the sheet have been detected for the first time.

Determination of the Structure of Current Sheets and Detection of Sheet Disruption Based on Measurements by External Magnetic Probes

A simple macroscopic method for detection of current sheet disruptions and determination of the width of metastable sheets is proposed based on the measurements of magnetic fields at the external surface of the vacuum chamber. This contactless diagnostic method does not affect the plasma processes in the current sheet and allows one to increase the signal-to-noise ratio. Such changes will allow us to relatively quickly determine the conditions of current sheet development under which sheet disruptions are most probable.

Mesoscale Phenomena during a Macroscopic Solar Eruption

Abstract Our previous magnetohydrodynamic simulation of a macroscopic solar eruption discussed in Zhao et al. (2019) showed that the current sheet (CS) evolution during eruption went through four stages: the CS growth stage, the dynamic growth stage, the hot CS stage, and the dynamic hot CS stage. We now focus on various mesoscale phenomena associated with the ongoing reconnection. In the dynamic growth stage, the remnant chromospheric matter in the CS is quasi-periodically pushed into the prominence, inducing fast shocks propagating at a speed of 210 km s−1. In the hot CS phase, various shock features relevant for particle acceleration are identified throughout the flare loop. Finally, during both dynamic stages, we quantify the properties of magnetic islands. A typical island is accelerated to Alfvénic speed by the Lorentz force and cools down by radiative cooling and thermal conduction. It also tends to expand in size before colliding with another island, with the FR or with the flare arcade. Islands in the dynamic growth stage have a higher density and lower temperature, and vice versa in the dynamic hot CS stage. Islands tend to move upward in the dynamic growth stage, while almost equal fractions of downward-moving and upward-moving islands in the dynamic hot CS stage. Translating the island trajectories to phase space, we find that the function fits the trajectory well, and its two fixed points represent the creation and the annihilation of the island.

Plasmoid formation in global GRMHD simulations and AGN flares

ABSTRACT One of the main dissipation processes acting on all scales in relativistic jets is thought to be governed by magnetic reconnection. Such dissipation processes have been studied in idealized environments, such as reconnection layers, which evolve in merging islands and lead to the production of ‘plasmoids’, ultimately resulting in efficient particle acceleration. In accretion flows on to black holes, reconnection layers can be developed and destroyed rapidly during the turbulent evolution of the flow. We present a series of two-dimensional general-relativistic magnetohydrodynamic simulations of tori accreting on to rotating black holes focusing our attention on the formation and evolution of current sheets. Initially, the tori are endowed with a poloidal magnetic field having a multiloop structure along the radial direction and with an alternating polarity. During reconnection processes, plasmoids and plasmoid chains are developed leading to a flaring activity and hence to a variable electromagnetic luminosity. We describe the methods developed to track automatically the plasmoids that are generated and ejected during the simulation, contrasting the behaviour of multiloop initial data with that encountered in typical simulations of accreting black holes having initial dipolar field composed of one loop only. Finally, we discuss the implications that our results have on the variability to be expected in accreting supermassive black holes.

Effect of Ionization and Recombination on the Evolution of the Harris-type Current Sheet in Partially Ionized Plasmas

Abstract Two-dimensional magnetohydrodynamics (MHD) simulations, treating plasma and neutral populations (hereafter, neutrals) as two separate components of the magneto-fluid, are performed in order to investigate the effect of ionization and recombination (or I/R) on the time evolution of the Harris-type current sheet in partially ionized plasmas. Our MHD simulations, including the effect of ambipolar diffusion (arising due to ion-neutral interactions) along with the I/R, show that the current sheet thinning occurs due to the diffusion of neutral particles from the current sheet. In addition to ambipolar diffusion, frictional heating also appears and affects the evolution of the current sheet. In a current sheet that is formed in a partially ionized plasma, the neutral population tries to spread outward and the plasma population tries to converge toward the center of the current sheet, and the overall process is influenced by the I/R. One of the important feature that is captured in our 2D simulations is that the escape of neutrals from the current sheet is sometimes suppressed due to the increase in ionization rate at the center of the current sheet, for the case of collisional I/R. As long as the ionization degree is kept low inside the current sheet, the current sheet thinning and elongation takes place and the current sheet becomes unstable due to the tearing-mode and plasmoid formation. The ion-neutral interactions coupled with I/R and the dynamics of the magnetic reconnection play an important role in plasmoid-mediated reconnection, therefore, the present study on the current sheet thinning and plasmoid formation could serve as a key for understanding bursty and intermittent plasma ejections observed in the solar chromosphere.

Dynamic Evolution of Current Sheets, Ideal Tearing, Plasmoid Formation and Generalized Fractal Reconnection Scaling Relations

Abstract Magnetic reconnection may be the fundamental process allowing energy stored in magnetic fields to be released abruptly, with solar flares and coronal mass ejection being archetypal natural plasma examples. Magnetic reconnection is much too slow of a process to be efficient on the large scales, but accelerates once small enough scales are formed in the system. For this reason, the fractal reconnection scenario was introduced to explain explosive events in the solar atmosphere; it was based on the recursive triggering and collapse via tearing instability of a current sheet originally thinned during the rise of a filament in the solar corona. Here we compare the different fractal reconnection scenarios that have been proposed, and derive generalized scaling relations for the recursive triggering of fast, “ideal” —i.e., Lundquist number independent—tearing in collapsing current sheet configurations with arbitrary current profile shapes. An important result is that the Sweet–Parker scaling with Lundquist number, if interpreted as the aspect ratio of the singular layer in an ideally unstable sheet, is universal and does not depend on the details of the current profile in the sheet. Such a scaling, however, must not be interpreted in terms of stationary reconnection, rather it defines a step in the accelerating sequence of events of the ideal tearing mediated fractal cascade. We calculate scalings for the expected number of plasmoids for such generic profiles and realistic Lundquist numbers, showing that in ideal tearing scenarios a smaller number of plasmoids, by orders of magnitude, is generated compared to the original fractal model.

Localized Structures and Turbulent Spectra in the Magnetopause

AbstractKinetic Alfvén wave (KAW) and whistler wave play a key role in the framework of turbulence and reconnection. There are lot of observations of these wave in the magnetopause region. The present paper deals with the nonlinear evolution of KAW and a weak whistler wave through the preexisting magnetic reconnection site. For this study the dynamical evolution equations are derived by taking into account the ponderomotive force driven density modification and magnetic field fluctuations due to shear field modeled by the Harris sheet. Furthermore, these equations have been solved numerically as well as semianalytically. For numerical integrations we have used the pseudospectral method and finite difference method and for semianalytically Runge Kutta method. Simulated results have shown the evolution of coherent structures or current sheets, which are capable to energy transfer efficiently. These structures have scale size around ion gyroradius as well as electron inertial length as calculated analytically. At a later time the chaotic structures arise in this reconnection site. This gives the signature of turbulence generation. Therefore, the magnetic power spectrum with scaling is also presented in this manuscript and their relevance with the observed spectrum (calculated from the Cluster data (Chaston, 2008, https://doi.org/10.1029/2008GL033601)) is also pointed out.

Forward Modeling of SDO/AIA and X-Ray Emission from a Simulated Flux Rope Ejection

Abstract We conduct forward-modeling analysis based on our 2.5 dimensional magnetohydrodynamics (MHD) simulation of magnetic flux rope (MFR) formation and eruption driven by photospheric converging motion. The current sheet (CS) evolution during the MFR formation and eruption process in our MHD simulation can be divided into four stages. The first stage shows the CS forming and gradually lengthening. Resistive instabilities that disrupt the CS mark the beginning of the second stage. Magnetic islands disappear in the third stage and reappear in the fourth stage. Synthetic images and light curves of the seven Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) channels, i.e., 94 Å, 131 Å, 171 Å, 193 Å, 211 Å, 304 Å, and 335 Å, and the 3–25 keV thermal X-ray are obtained with forward-modeling analysis. The loop-top source and the coronal sources of the soft X-ray are reproduced in forward modeling. The light curves of the seven SDO/AIA channels start to rise once resistive instabilities develop. The light curve of the 3–25 keV thermal X-ray starts to go up when the reconnection rate reaches one of its peaks. Quasiperiodic pulsations (QPPs) appear twice in the SDO/AIA 171 Å, 211 Å, and 304 Å channels, corresponding to the period of chaotic (re)appearance and CS-guided displacements of the magnetic islands. QPPs appear once in the SDO/AIA 94 Å and 335 Å channels after the disruption of the CS by resistive instabilities and in the 193 Å channel when the chaotic motion of the magnetic islands reappears.

Particle acceleration in explosive relativistic reconnection events and Crab Nebula gamma-ray flares

We develop a model of gamma-ray flares of the Crab Nebula resulting from the magnetic reconnection events in a highly magnetised relativistic plasma. We first discuss physical parameters of the Crab Nebula and review the theory of pulsar winds and termination shocks. We also review the principle points of particle acceleration in explosive reconnection events [Lyutikov et al., J. Plasma Phys., vol. 83(6), p. 635830601 (2017a); J. Plasma Phys., vol. 83(6), p. 635830602 (2017b)]. It is required that particles producing flares are accelerated in highly magnetised regions of the nebula. Flares originate from the poleward regions at the base of the Crab’s polar outflow, where both the magnetisation and the magnetic field strength are sufficiently high. The post-termination shock flow develops macroscopic (not related to the plasma properties on the skin-depth scale) kink-type instabilities. The resulting large-scale magnetic stresses drive explosive reconnection events on the light-crossing time of the reconnection region. Flares are produced at the initial stage of the current sheet development, during the X-point collapse. The model has all the ingredients needed for Crab flares: natural formation of highly magnetised regions, explosive dynamics on the light travel time, development of high electric fields on macroscopic scales and acceleration of particles to energies well exceeding the average magnetic energy per particle.

Evolution of three-dimensional relativistic current sheets and development of self-generated turbulence

In this paper, the temporal evolution of 3-dimensional relativistic current sheets in Poynting-dominated plasma is studied for the first time. Over the past few decades, a lot of efforts have been conducted on studying the evolution of current sheets in 2-dimensional space, and concluded that sufficiently long current sheets always evolves into the so-called "plasmoid-chain", which provides fast reconnection rate independent of its resistivity. However, it is suspected that plasmoid-chain can exist only in the case of 2-dimensional approximation, and would show transition to turbulence in 3-dimensional space. We performed 3-dimensional numerical simulation of relativistic current sheet using resistive relativistic magnetohydrodynamic approximation. The results showed that the 3-dimensional current sheet evolve not into plasmoid-chain but turbulence. The resulting reconnection rate is $0.004$ which is much smaller than that of plasmoid-chain. The energy conversion from magnetic field to kinetic energy of turbulence is just 0.01\% which is much smaller than typical non-relativistic cases. Using the energy principle, we also showed that the plasmoid is always unstable for a displacement in opposite direction to its acceleration, probably interchange-type instability, and this always results in seeds of turbulence behind the plasmoids. Finally, the temperature distribution along the sheet is discussed, and it is found that the sheet is less active than plasmoid-chain. Our finding can be applied for many high energy astrophysical phenomena, and can provide a basic model of the general current sheet in Poynting-dominated plasma.

Spectroscopic Observations of Current Sheet Formation and Evolution

Abstract We report on the structure and evolution of a current sheet that formed in the wake of an eruptive X8.3 flare observed at the west limb of the Sun on 2017 September 10. Using observations from the EUV Imaging Spectrometer (EIS) on Hinode and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory, we find that plasma in the current sheet reaches temperatures of about 20 MK and that the range of temperatures is relatively narrow. The highest temperatures occur at the base of the current sheet, in the region near the top of the post-flare loop arcade. The broadest high temperature line profiles, in contrast, occur at the largest observed heights. Furthermore, line broadening is strong very early in the flare and diminishes over time. The current sheet can be observed in the AIA 211 and 171 channels, which have a considerable contribution from thermal bremsstrahlung at flare temperatures. Comparisons of the emission measure in these channels with other EIS wavelengths and AIA channels dominated by Fe line emission indicate a coronal composition and suggest that the current sheet is formed by the heating of plasma already in the corona. Taken together, these observations suggest that some flare heating occurs in the current sheet, while additional energy is released as newly reconnected field lines relax and become more dipolar.