Current Sheet Forms Research Articles

Initiation mechanism of the first on-disk X-class flare of solar cycle 25

In this paper we study the initiation mechanism of the first on-disk X-class eruptive flare in solar cycle 25. Coronal magnetic field reconstructions reveal a magnetic flux rope (MFR) whose configuration is highly consistent with a filament existing for a long period before the flare; the eruption of the whole filament indicates that the MFR erupted during the flare. However, quantitative analysis shows that the pre-flare MFR resides too low to trigger a torus instability (TI). The filament experienced a slow rise before the flare onset, for which we estimated evolution of the filament height using a triangulation method by combining the SDO and STEREO observations, and find it is also much lower than the critical height for triggering TI. On the other hand, the pre-flare evolution of the current density shows progressive thinning of a vertical current layer on top of the flare polarity inversion line (PIL), which suggests that a vertical current sheet forms before the eruption. Meanwhile, there is continuous shearing motion along the PIL under the main branch of the filament, which can drive the coronal field to form such a current sheet. We thus suggest that the event follows a reconnection-based initiation mechanism as recently established using a high-accuracy magnetohydrodynamics simulation, in which an eruption is initiated by reconnection in a current sheet that forms gradually within a continuously sheared magnetic arcade. The eruption should be further driven by TI as the filament quickly rises into the TI domain during the eruption.

Spinning black holes magnetically connected to a Keplerian disk

Context.Accreting black holes (BHs) may be surrounded by a highly magnetized plasma threaded by an organized poloidal magnetic field. Nonthermal flares and power-law spectral components at high energy could originate from a hot, collisionless, and nearly force-free corona. The jets we often observe from these systems are believed to be rotation-powered and magnetically driven.Aims.We study axisymmetric BH magnetospheres, where a fraction of the magnetic field lines anchored in a surrounding disk are connected to the event horizon of a rotating BH. For different BH spins, we identify the conditions and sites of magnetic reconnection within 30 gravitational radii.Methods.With the fully general relativistic particle-in-cell codeGRZeltron, we solve the time-dependent dynamics of the electron–positron pair plasma and of the electromagnetic fields around the BH. The aligned disk is represented by a steady and perfectly conducting plasma in Keplerian rotation, threaded by a dipolar magnetic field.Results.For prograde disks around Kerr BHs, the topology of the magnetosphere is hybrid. Twisted open magnetic field lines crossing the horizon power a Blandford-Znajek jet, while open field lines with their footpoint beyond a critical distance on the disk could launch a magneto-centrifugal wind. In the innermost regions, coupling magnetic field lines ensure the transfer of significant amounts of angular momentum and energy between the BH and the disk. From the Y point at the intersection of these three regions, a current sheet forms where vivid particle acceleration via magnetic reconnection takes place. We compute the synchrotron images of the current sheet emission.Conclusions.Our estimates for jet power and BH–disk exchanges match those derived from purely force-free models. Particles are accelerated at the Y point, which acts as a heat source for the so-called corona. It provides a physically motivated ring-shaped source of hard X-rays above the disk for reflection models. Episodic plasmoid ejection might explain millisecond flares observed in Cygnus X-1 in the high-soft state, but are too fast to account for daily nonthermal flares from Sgr A*. Particles flowing from the Y point down to the disk could produce a hot spot at the footpoint of the outermost closed magnetic field line.

MHD simulations of cold bubble formation from 2/1 tearing mode during massive gas injection in a tokamak

Massive gas injection (MGI) experiments have been carried out in many tokamaks to study disruption dynamics and mitigation schemes. Two events often observed in those experiments are the excitation of the m = 2, n = 1 magnetohydrodynamic mode, and the formation of cold bubble structure in the temperature distribution before the thermal quench (TQ). Here m is the poloidal mode number, n the toroidal mode number. The physics mechanisms underlying those phenomena, however, have not been entirely clear. In this work, our recent NIMROD simulations of the MGI process in a tokamak have reproduced the main features of both events, which has allowed us to examine and establish the causal relation between them. In these simulations, the 3/1 and 2/1 islands are found to form successively after the arrival of impurity ion cold front at the corresponding q = 3 and q = 2 rational surfaces. At the interface between impurity and plasma, a local thin current sheet forms due to an enhanced local pressure gradient and moves inward following the gas cold front, this may contribute to the formation of a dominant 2/1 mode. Following the growth of the 2/1 tearing mode, the impurity penetration into the core region inside the q = 2 surface gives rise to the formation of the cold bubble temperature structure and initiates the final TQ. A subdominant 1/1 mode developed earlier near the q = 1 surface alone does not cause such a cold bubble formation, however, the exact manner of the preceding impurity penetration depends on the nature of the 1/1 mode: kink-tearing or quasi-interchange.

A ring of fire in the pulsar magnetosphere

Contopoulos 2019 proposed that a dissipation zone develops in the magnetosphere of young pulsars at the edge of the closed-line region beyond the light cylinder. This is necessary in order to supply the charge carriers that will establish current closure through the equatorial and separatrix current-sheets. In the present work, we propose to investigate in greater detail this region with a simplified model that we would like to call the `ring-of-fire'. According to this simple model, the dissipation zone is a narrow reconnection layer where electrons and positrons are accelerated inwards and outwards respectively along Speiser orbits that are deflected in the azimuthal direction by the pulsar rotation. After they exit the reconnection layer, the accelerated positrons form the positively charged equatorial current-sheet, and the accelerated electrons form the negatively charged separatrix current-sheet along the boundary of the closed-line region. During their acceleration, particles lose only a small part of their energy to radiation. Most of their energy is lost outside the dissipation region, in the equatorial and separatrix current sheets. Our simple model allows us to obtain high-energy spectra and efficiencies. The radiation emitted by the positrons in the equatorial current-sheet forms a very-high energy tail that extends up to the TeV range.

Magnetic vortex growth in the transition layer of a mildly relativistic plasma shock

A two-dimensional particle simulation models the collision of two electron-ion plasma clouds along a quasiparallel magnetic field. The collision speed is 0.9c and the density ratio, 10. A current sheet forms at the front of the dense cloud, in which the electrons and the magnetic field reach energy equipartition with the ions. A structure composed of a solenoidal and a toroidal magnetic field grows in this sheet. It resembles the cross-section of the torus of a spheromak, which may provide the coherent magnetic fields in gamma-ray burst jets needed for their prompt emissions.

FORMATION AND RECONNECTION OF THREE-DIMENSIONAL CURRENT SHEETS IN THE SOLAR CORONA

Current-sheet formation and magnetic reconnection are believed to be the basic physical processes responsible for much of the activity observed in astrophysical plasmas, such as the Sun s corona. We investigate these processes for a magnetic configuration consisting of a uniform background field and an embedded line dipole, a topology that is expected to be ubiquitous in the corona. This magnetic system is driven by a uniform horizontal flow applied at the line-tied photosphere. Although both the initial field and the driver are translationally symmetric, the resulting evolution is calculated using a fully three-dimensional magnetohydrodynamic (3D MHD) simulation with adaptive mesh refinement that resolves the current sheet and reconnection dynamics in detail. The advantage of our approach is that it allows us to apply directly the vast body of knowledge gained from the many studies of 2D reconnection to the fully 3D case. We find that a current sheet forms in close analogy to the classic Syrovatskii 2D mechanism, but the resulting evolution is different than expected. The current sheet is globally stable, showing no evidence for a disruption or a secondary instability even for aspect ratios as high as 80:1. The global evolution generally follows the standard Sweet- Parker 2D reconnection model except for an accelerated reconnection rate at a very thin current sheet, due to the tearing instability and the formation of magnetic islands. An interesting conclusion is that despite the formation of fully 3D structures at small scales, the system remains close to 2D at global scales. We discuss the implications of our results for observations of the solar corona. Subject Headings: Sun: corona Sun: magnetic fields Sun: reconnection

Magnetic reconnection at 3D null points: effect of magnetic field asymmetry

The aim of this paper is to investigate the properties of magnetic reconnection at a 3D null point, with respect to their dependence on the symmetry of the magnetic field around the null. In particular we examine the rate of flux transport across the null point with symmetric/asymmetric diffusion regions, as well as how the current sheet forms in time, and its properties. Mathematical modelling and finite difference resistive MHD simulations are used. It is found that the basic structure of the mode of magnetic reconnection considered is unaffected by varying the magnetic field symmetry, that is, the plasma flow is found cross both the spine and fan of the null. However, the peak intensity and dimensions of the current sheet are dependent on the symmetry/ asymmetry of the field lines. As a result, the reconnection rate is also found to be strongly dependent on the field asymmetry.

Relative timing of substorm processes as derived from multifluid/multiscale simulations: Internally driven substorms

The role of tail reconnection in initiating substorm onset remains highly controversial. Multifluid/multiscale simulations are used to examine this problem with the unique capability of being able to resolve ion skin depth and ion cyclotron processes within a very thin tail current sheet within a global magnetosphere. It is shown that, consistent with the near‐Earth neutral line model, a thin current sheet forms during the growth phase with the eventual formation of an X line and the ejection of a plasmoid. However, these processes do not produce substorm onset but are precursors to onset. The modeling indicates that in the wake of the plasmoid, a Y line forms within which there is the intermittent generation of flux ropes that can propagate either earthward or tailward with equal probability. The intensity and size of the flux ropes are seen to increase in time up to substorm onset. Depending on the size of the earthward moving flux rope, pseudobreakup or substorm onset occurs when the energy within the earthward moving flux rope is dissipated against the inner edge of the plasma sheet. This interaction leads to rapid (1 min) dipolarization, magnetospheric injection of energetic particles, and intensification of the nightside auroral currents that expand in local time and to higher latitudes. These processes are consistent with the current disruption model where onset occurs as a direct consequence of processes occurring at the inner edge of the plasma sheet. Ionospheric outflows and their energization with the thin current sheet play important roles in the buildup of energy in the plasma sheet that eventually produces substorm onset and drives breakup.

Heating the corona by nanoflares: simulations of energy release triggered by a kink instability

Context. The heating of solar coronal plasma to millions of degrees is likely to be due to the superposition of many small energy-releasing events, known as nanoflares. Nanoflares dissipate magnetic energy through magnetic reconnection. Aims. A model has been recently proposed in which nanoflare-like heating naturally arises, with a sequence of dissipation events of various magnitudes. It is proposed that heating is triggered by the onset of ideal instability, with energy release occurring in the nonlinear phase due to fast magnetic reconnection. The aim is to use numerical simulations to investigate this heating process. Methods. Three-dimensional magnetohydrodynamic numerical simulations of energy release are presented for a cylindrical coronal loop model. Initial equilibrium magnetic-field profiles are chosen to be linearly unstable, with a two-layer parameterisation of the current profile. The results are compared with calculations of linear instability, with line-tying, which are extended to account for a potential field layer surrounding the loop. The energy release is also compared with predictions that the field relaxes to a state of minimum magnetic energy with conserved magnetic helicity (a constant a force-free field). Results. The loop initially develops a helical kink, whose structure and growth rate are generally in accordance with linear stability theory, and subsequently a current sheet forms. During this phase, there is a burst of kinetic energy while the magnetic energy decays. A new relaxed equilibrium is established that corresponds quite closely to a constant a field. The fraction of stored magnetic energy released depends strongly on the initial current profile, which agrees with the predictions of relaxation theory. Conclusions. Energy dissipation events in a coronal loop are triggered by the onset of ideal kink instability. Magnetic energy is dissipated, leading to large or small heating events according to the initial current profile.

Theoretical Predictions of X‐Ray and Extreme‐UV Flare Emissions Using a Loss‐of‐Equilibrium Model of Solar Eruptions

In this paper, we present numerical simulations of solar flares that couple a loss-of-equilibrium solar eruption model with a one-dimensional hydrodynamic model. In these calculations, the eruption is initiated by footpoint motions that disrupt the balance of forces acting on a flux rope. After the eruption begins, a current sheet forms and an arcade of flare loops is created by reconnecting magnetic fields. Thermal energy input into the flare loops is found by assuming the complete thermalization of the Poynting flux swept into the current sheet. This thermal energy is input into a one-dimensional hydrodynamic code for each loop formed in the multithreaded flare arcade. We find that a density enhancement occurs at the loop top when the two evaporating plasma fronts in each leg of the loop collide there. Simulated flare images show that these loop-top density enhancements produce bars of bright emission similar to those observed in the Transition Region and Coronal Explorer (TRACE) 195 A bandpass and loop-top knots of bright emission seen in flare observations by the Soft X-Ray Telescope (SXT) on Yohkoh. We also simulate flare spectra from the Bragg Crystal Spectrometer (BCS) on Yohkoh. We find that during the early stages of flare initiation, there are significant blueshifts in the Ca XIX line, but the intensities are too faint to be observed with BCS. In general, the results of this model simulate observed flare emissions quite well, indicating that the reconnection model of solar flares is energetically consistent with observations.

The Relationship between Flux Rope Acceleration and Thermal Energy Release in a Model of Eruptive Solar Phenomena

In this paper, we examine the relationship between the thermal energy release rate and the flux rope acceleration in a loss-of-equilibrium CME model. The model consists of a flux rope in equilibrium in the corona and two point sources at the chromosphere. As the point sources are brought together quasi-statically, the equilibrium state of the flux rope is disturbed, and an eruption occurs. After the eruption begins, a current sheet forms under the flux rope. The thermal energy release rate is assumed to be equal to the Poynting flux swept into the current sheet as the flux rope accelerates. We find that the flux rope acceleration and the thermal energy release rate are well correlated when the background magnetic field is high and the reconnection rate is fast. They are not well correlated for events with low magnetic fields and slow reconnection rates.

Catastrophic Behavior of Multiple Coronal Flux Rope System

A major solar active event called Bastille Day Event occurred in AR 9077 on July 14, 2000. Simultaneous occurrence of a filament eruption, a flare and a coronal mass ejection was observed in this event. Previous analyses of this event show that before the event, there existed an activation and eruption of a huge trans-equatorial filament, which might play a crucial role in triggering the Bastille Day event. This implies that independent flux systems are closely related to and affect each other, which has encouraged us to investigate the catastrophic behavior of a multiple coronal flux rope system with the use of a 2.5-D time-dependent MHD model. A force-free field that contains three separate coronal flux ropes is taken to be the initial state. Starting from this state, we increase either the annular or the axial flux of a certain flux rope to examine the catastrophic behavior of the system in two regimes, the ideal MHD regime and the resistive MHD regime. It is found that a catastrophe occurs if the flux exceeds a certain critical value, or the magnetic energy of the system exceeds a certain threshold: the rope of interest breaks away from the base and escapes to infinity, leaving a current sheet below. Moreover, the destiny of the remainder flux ropes relies on whether reconnection takes place across the current sheet. In the ideal MHD regime, i.e., in the absence of reconnection, these ropes remain to be attached to the base in equilibrium, whereas in the resistive MHD regime they abruptly erupt upward during reconnection and escape to infinity. Reconnection causes the field lines to close back to the base and thus changes the background field outside the attached flux ropes in such a way that the constraint on these ropes is substantially relaxed and the corresponding catastrophic energy threshold is reduced accordingly, leading to a catastrophic eruption of these ropes. Since magnetic reconnection is generally inevitable when a current sheet forms and develops through an eruption of one flux rope, the eruption of this flux rope must lead to an eruption of the others. This provides an example to demonstrate the interaction between several independent magnetic flux systems in different regions, as implied by the Bastille Day event, and may serve as a possible mechanism for sympathetic events occurring on the Sun.

On the Nature of the X-Ray Bright Core in a Stable Filament Channel

In a search for the cause of the intense heating revealed by X-ray emission in filament channels, we have simulated the evolution of a twisted toroidal flux rope emerging quasi-statically into the corona. Initially, the simulated flux rope remains confined in equilibrium as the stored magnetic energy increases. With enough twist buildup, there is a sudden catastrophic loss of equilibrium and total expulsion of the flux rope. We focused on the quasi-static phase in which a current sheet forms within the flux rope cavity, along the so-called bald-patch separatrix surface (BPSS). This comprises an envelope of field lines that graze the anchoring lower boundary, enclosing the detached helical field that supports the prominence. Significant magnetic energy dissipation and heating are expected to center around such current sheets. The heating that should result provides a plausible explanation for the hot X-ray sources, although they appear to be colocated with cool material. If our physical picture is correct, then the development of X-ray bright cores or sigmoids in a filament channel suggests the presence of a BPSS separating the helical field of a twisted flux rope in stable confinement from the surrounding untwisted fields.

Slow wind and magnetic topology in the solar minimum corona in 1996–1997

This study examines the physical conditions of the outer solar corona in order to identify the regions where the slow solar wind is accelerated and to investigate the latitudinal transition from slow to fast wind during the minimum of the solar cycle. The analysis is based on observations of six streamers obtained during the years of solar minimum, 1996 and 1997, with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard the Solar and Heliospheric Observatory (SOHO). The outflow velocity of the oxygen ions and the electron density of the coronal plasma are determined in altitude ranging from 1.5 to 3.5 solar radii (). The adopted diagnostic method, based on spectroscopic analysis of the O VI 1032 and 1038 Å lines, fully accounts for the large expansion factor of the magnetic field lines expected in the regions surrounding the streamers. The analysis leads to the conclusion that the slow coronal wind is observed (i) in the region external to and running along the streamer boundary; and (ii) in the region above the streamer core beyond 2.7 , where the transition between closed and open magnetic field lines takes place and the heliospheric current sheet forms. Regions in the immediate vicinity of the streamer boundary can be identified with the edges of the large polar coronal holes that characterize solar minimum. Results point to gradual variations of the properties of a coronal hole from the streamer boundary to its polar core, most likely related to the topology of the coronal magnetic field.

Numerical study of nonlinear forced magnetic reconnection

Two-dimensional numerical simulations are used to investigate nonlinear aspects of forced magnetic reconnection in a zero β highly conducting plasma. This is representative of the solar corona, where reconnection may be induced by external perturbations, e.g., at the photospheric boundary of the corona. The aim is to investigate the energy dissipation by the reconnection, which may provide a mechanism for heating the coronal plasma. The field is taken to be initially a sheared force-free equilibrium in a slab, and the effects of applying a slow deformation to the boundaries are investigated. Previous analytical studies assuming small departures from the initial equilibrium have found that a current sheet forms during an initial ideal phase of evolution, which subsequently relaxes to a reconnected equilibrium, releasing some magnetic energy. The linear theory predicts that the energy release has a singularity when the field is marginally stable to the tearing mode. The nonlinear evolution of the field is calculated numerically, focusing on the energy release. In particular, the strongly nonlinear behavior is studied in the parameter regime in which the linear theory breaks down. It is found that nonlinearities become strong close to the marginal stability point, and for such highly sheared fields, the energy released by reconnection is large even for weak boundary deformations.

On the ion-scale structure of thin current sheets in the magnetotail

A numerical and analytical investigation of the formation and structure of thin current sheets which form in a magnetotail-like plasma model is presented. Current sheet formation is driven by the application of a boundary electric field similar to what would be expected from solar wind driving. As a result, a thin current sheet forms which involves strong Hall-type electric fields, which, by means of electric field drifts, strongly increase the electron contribution, and reduce the ion contribution to the cross-tail current density. This result is also supported by analytical investigations of the total plasma momentum. The thin current sheet forms on strong gradients in the ion density and the magnetic field. The simulation leads to a well defined equilibrium state.

Forced three‐dimensional magnetic reconnection due to linkage of magnetic flux tubes

During periods of southward interplanetary magnetic field (IMF) orientation the magnetic field geometry at the dayside magnetopause is susceptible to magnetic reconnection. It has been suggested that reconnection may occur in a localized manner at several patches on the magnetopause. A major problem with this picture is the interaction of magnetic flux ropes which are generated by different reconnection processes. An individual flux rope is bent elbowlike where it intersects the magnetopause and the magnetic field changes from magnetospheric to interplanetary magnetic field orientation. Multiple patches of reconnection can lead to the formation of interlinked magnetic flux tubes. Although the corresponding flux is connected to the IMF the northward and southward connected branches are hooked into each other and cannot develop independently. We have studied this problem in the framework of three‐dimensional magnetohydrodynamic simulations. The results indicate that a singular current sheet forms at the interface of two interlinked flux tubes if no resistivity is present in the simulation. This current sheet is strongly tilted compared to the original current sheet. In the presence of resistivity the interaction of the two flux tubes forces a fast reconnection process which generates helically twisted closed magnetospheric flux. This linkage induced reconnection generates a boundary layer with layers of open and closed magnetospheric flux and may account for the brightening of auroral arcs poleward of the boundary between open and closed magnetic flux.

Magnetotail dynamics under isobaric constraints

Using linear theory and nonlinear MHD simulations, we investigate the resistive and ideal MHD stability of two‐dimensional plasma configurations under the isobaric constraint dP/dt = 0, which in ideal MHD is equivalent to conserving the pressure function P = P(A), where A denotes the magnetic flux. This constraint is satisfied for incompressible modes, such as Alfvén waves, and for systems undergoing energy losses. The linear stability analysis leads to a Schrödinger equation, which can be investigated by standard quantum mechanics procedures. We present an application to a typical stretched magnetotail configuration. For a one‐dimensional sheet equilibrium characteristic properties of tearing instability are rediscovered. However, the maximum growth rate scales with the 1/7 power of the resistivity, which implies much faster growth than for the standard tearing mode (assuming that the resistivity is small). The same basic eigenmode is found also for weakly two‐dimensional equilibria, even in the ideal MHD limit. In this case the growth rate scales with the 1/4 power of the normal magnetic field. The results of the linear stability analysis are confirmed qualitatively by nonlinear dynamic MHD simulations. These results suggest the interesting possibility that substorm onset, or the thinning in the late growth phase, is caused by the release of a thermodynamic constraint without the (immediate) necessity of releasing the ideal MHD constraint. In the nonlinear regime the resistive and ideal developments differ in that the ideal mode does not lead to neutral line formation without the further release of the ideal MHD constraint; instead a thin current sheet forms. The isobaric constraint is critically discussed. Under perhaps more realistic adiabatic conditions the ideal mode appears to be stable but could be driven by external perturbations and thus generate the thin current sheet in the late growth phase, before a nonideal instability sets in.

Post-flare loops: formation and velocity

Post-flare loops are generally observed between two ribbon flares. The formation of post flare loops and active region or plage filaments has been explained in a model based on magnetic reconnection and chromospheric ablation /1,2/. This model uses a magnetic topology which is relevant to solar flares /3/ : a large flare or an instability opens the magnetic lines of a coronal arcade (or arch) and a vertical current sheet forms. Then the reconnection of the magnetic field follows according to the scenario of Kopp and Pneuman /4/. In this magnetic configuration, we show that the formation of condensations or dense loops is induced by reconnection shocks. We give in this communication a new method of diagnostic to derive from Hα profiles physical parameters, i.e. source function, optical depth and velocity. This method called “differential cloud method” is very promising for future observations.

Double current sheet and coronal transients

A double current sheet forms when an opposite magnetic flux emerges into a background magnetic field which has a zero field-line in the corona. It consists of an upper sheet, resulting from the squeezing of field lines near the original zero field region and a lower sheet formed in the region between the new and old fields. We use a pair of linear dipoles to model the background and a pair of line charges to model the emerging field and discuss the formation and evolution of the double current sheet. Matter will condense onto the sheets during their formation. The matter in the lower sheet comes mainly from the transition region and the photosphere; it is further cooled by radiation, giving rise to a low-temperature prominence loop. The matter in the upper sheet comes from the corona and forms a high-temperature coronal loop.This scenario seems to be realized in the coronal transient of 1984 April 14.