Evidence of guide field magnetic reconnection in flapping current sheets

Based on current sheet flapping motion on 27 August 2018 in the dusk flank magnetotail,as recorded by instruments aboard Magnetospheric Multiscale(MMS)spacecraft,we present the first study of guide field reconnection observed in the flux rope embedded in kink-like flapping current sheets near the dusk-side flank of the magnetotail.Unlike more common magnetotail reconnections,which are symmetric,these asymmetric small-scale(λ_(i)~650 km)reconnections were found in the highly twisted current sheet when the direction normal to the sheet changes from the Z direction into the Y direction.The unique feature of this unusual reconnection is that the reconnection jets are along the Z direction-different from outflow in the X direction,which is the more usual situation.This vertical reconnection jet is parallel or antiparallel to the up-and-down motion of the tail’s current sheet.The normalized reconnection rate R is estimated to be~0.1.Our results indicate that such asymmetric reconnections can significantly enlarge current sheet flapping,with large oscillation amplitudes.This letter presents direct evidence of guide field reconnection in a highly twisted current sheet,characterized by enlarged current sheet flapping as a consequence of the reconnection outflow.

Probing the peripheral self-generated magnetic field distribution in laser-plasma magnetic reconnection with Martin-Puplett interferometer polarimeter

Magnetic reconnection of the self-generated magnetic fields in laser-plasma interaction is an important laboratory method for modeling high-energy density astronomical and astrophysical phenomena.We use the Martin-Puplett interferometer(MPI)polarimeter to probe the peripheral magnetic fields generated in the common magnetic reconnection configuration,two separated coplanar plane targets,in laser-target interaction.We introduce a new method that can obtain polarization information from the interference pattern instead of the sinusoidal function fitting of the intensity.A bidirectional magnetic field is observed from the side view,which is consistent with the magneto-hydro-dynamical(MHD)simulation results of self-generated magnetic field reconnection.We find that the cancellation of reverse magnetic fields after averaging and integration along the observing direction could reduce the magnetic field strength by one to two orders of magnitude.It indicates that imaging resolution can significantly affect the accuracy of measured magnetic field strength.

The influence of boundary conditions on the distribution of energetic electrons during collisionless magnetic reconnection

We conducted 2-D particle-in-cell simulations to investigate the impact of boundary conditions on the evolution of magnetic reconnection. The results demonstrate that the boundary conditions are crucial to this evolution. Specifically, in the cases of traditional periodic boundary(PB) and fully-opened boundary(OB) conditions, the evolutions are quite similar before the system achieves the fastest reconnection rate. However, differences emerge between the two cases afterward. In the PB case, the reconnection electric field experiences a rapid decline and even becomes negative, indicating a reversal of the reconnection process. In contrast, the system maintains a fast reconnection stage in the OB case. Suprathermal electrons are generated near the separatrix and in the exhaust region of both simulation cases. In the electron density depletion layer and the dipolarization front region, a larger proportion of suprathermal electrons are produced in the OB case. Medium-energy electrons are mainly located in the vicinity of the X-line and downstream of the reconnection site in both cases. However, in the OB case, they can also be generated in the electron holes along the separatrix. Before the reverse reconnection stage, no high-energy electrons are present in the PB case. In contrast, about 20% of the electrons in the thin and elongated electron current layer are high-energy in the OB case.

Three-dimensional magnetic reconnection in complex multiple X-point configurations in an ancient solar-lunar terrestrial system

Magnetic reconnection processes in three-dimensional(3D)complex field configurations have been investigated in different magneto-plasma systems in space,laboratory,and astrophysical systems.Two-dimensional(2D)features of magnetic reconnection have been well developed and applied successfully to systems with symmetrical property,such as toroidal fusion plasmas and laboratory experiments with an axial symmetry.But in asymmetric systems,the 3D features are inevitably different from those in the 2D case.Magnetic reconnection structures in multiple celestial body systems,particularly star-planet-Moon systems,bring fresh insights to the understanding of the 3D geometry of reconnection.Thus,we take magnetic reconnection in an ancient solar-lunar terrestrial magneto-plasma system as an example by using its crucial parameters approximately estimated already and also some specific applications in pathways for energy and matter transports among Earth,ancient Moon,and the interplanetary magnetic field(IMF).Then,magnetic reconnection of the ancient lunar-terrestrial magnetospheres with the IMF is investigated numerically in this work.In a 3D simulation for the Earth-Moon-IMF system,topological features of complex magnetic reconnection configurations and dynamical characteristics of magnetic reconnection processes are studied.It is found that a coupled lunar-terrestrial magnetosphere is formed,and under various IMF orientations,multiple X-points emerge at distinct locations,showing three typical magnetic reconnection structures in such a geometry,i.e.,the X-line,the triple current sheets,and the A-B null pairs.The results can conduce to further understanding of reconnection physics in 3D for plasmas in complex magnetic configurations,and also a possible mechanism for energy and matters transport in evolutions of similar astrophysical systems.

Ion and electron motions in the outer electron diffusion region of collisionless magnetic reconnection

Two-dimensional particle-in-cell simulations are performed to study the coupling between ion and electron motions in collisionless magnetic reconnection.The electron diffusion region(EDR),where the electron motions are demagnetized,is found to have a two-layer structure:an inner EDR near the reconnection site and an outer EDR that is elongated to nearly 10 ion inertial lengths in the outflow direction.In the inner EDR,the speed of the electron outflow increases when the electrons move away from the X line.In the outer EDR,the speed of the electron outflow first increases and then decreases until the electrons reach the boundary of the outer EDR.In the boundary of the outer EDR,the magnetic field piles up and forms a depolarization front.From the perspective of the fluid,a force analysis on the formation of electron and ion outflows has also been investigated.Around the X line,the electrons are accelerated by the reconnection electric field in the out-of-plane direction.When the electrons move away from the X line,we find that the Lorentz force converts the direction of the accelerated electrons to the x direction,forming an electron outflow.Both electric field forces and electron gradient forces tend to drag the electron outflow.Ion acceleration along the x direction is caused by the Lorentz force,whereas the pressure gradient force tends to decelerate the ion outflow.Although these two terms are important,their effects on ions are almost offset.The Hall electric field force does positive work on ions and is not negligible.The ions are continuously accelerated,and the ion and electron outflow velocities are almost the same near the depolarization front.

Electron acceleration in a coil target-driven low-βmagnetic reconnection simulation

Magnetic reconnection driven by a capacitor coil target is an innovative way to investigate low-βmagnetic reconnection in the laboratory,whereβis the ratio of particle thermal pressure to magnetic pressure.Low-βmagnetic reconnection frequently occurs in the Earth’s magnetosphere,where the plasma is characterized byβ≲0.01.In this paper,we analyze electron acceleration during magnetic reconnection and its effects on the electron energy spectrum via particle-in-cell simulations informed by parameters obtained from experiments.We note that magnetic reconnection starts when the current sheet is down to about three electron inertial lengths.From a quantitative comparison of the different mechanisms underlying the electron acceleration in low-βreconnection driven by coil targets,we find that the electron acceleration is dominated by the betatron mechanism,whereas the parallel electric field plays a cooling role and Fermi acceleration is negligible.The accelerated electrons produce a hardened power-law spectrum with a high-energy bump.We find that injecting electrons into the current sheet is likely to be essential for further acceleration.In addition,we perform simulations for both a double-coil co-directional magnetic field and a single-coil one to eliminate the possibility of direct acceleration of electrons beyond thermal energies by the coil current.The squeeze between the two coil currents can only accelerate electrons inefficiently before reconnection.The simulation results provide insights to guide future experimental improvements in low-βmagnetic reconnection driven by capacitor coil targets.

Numerical Studies of Magnetic Reconnection and Heating Mechanisms for the Ellerman Bomb

An Ellerman Bomb(EB)is a kind of small scale reconnection event,which is ubiquitously formed in the upper photosphere or the lower chromosphere.The low temperature(<10,000 K)and high density(~1019–1022)plasma there makes the magnetic reconnection process strongly influenced by partially ionized effects and radiative cooling.This work studies the highβmagnetic reconnection near the solar temperature minimum region based on high-resolution 2.5D magnetohydrodynamics simulations.The time-dependent ionization degree of hydrogen and helium are included to realize more realistic diffusivities,viscosity and radiative cooling in simulations.Numerical results show that the reconnection rate is smaller than 0.01 and decreases with time during the early quasi-steady stage,then sharply increases to a value above 0.05 in the later stage as the plasmoid instability takes place.Both the large value ofηen(magnetic diffusion caused by the electron-neutral collision)and the plasmoid instability contribute to the fast magnetic reconnection in the EB-like event.The interactions and coalescence of plasmoids strongly enhance the local compression heating effect,which becomes the dominant mechanism for heating in EBs after plasmoid instability appears.However,the Joule heating contributed byηen can play a major role to heat plasmas when the magnetic reconnection in EBs is during the quasi-steady stage with smaller temperature increases.The results also show that the radiative cooling effect suppresses the temperature increase to a reasonable range,and increases the reconnection rate and generation of thermal energy.

Particle-in-cell simulations of low-β magnetic reconnection driven by laser interaction with a capacitor–coil target

The dynamics of low-β magnetic reconnection(MR) driven by laser interaction with a capacitor–coil target are reexamined by simulations in this paper. We compare two cases MR and non-MR(also referred as AP-case and P-case standing for the anti-parallel and parallel magnetic field lines, respectively) to distinguish the different characteristics between them.We find that only in the AP-case the reconnection electric field shows up around the X line and the electron jet is directed toward the X line. The quadruple magnetic fields exist in both cases, however, they distribute in the current sheet area in the AP-case, and out of the squeezing area in the P-case, because electrons are demagnetized in the electron diffusion region in the MR process, which is absent in the P-case. The electron acceleration is dominant by the Fermi-like mechanism before the MR process, and by the reconnection electric field when the MR occurs. A power-law electron energy spectrum with an index of 1.8 is found in the AP-case. This work proves the significant potential of this experimental platform to be applied in the studies of low-β astronomy phenomena.

The effect of the guide field on energy conversion during collisionless magnetic reconnection

Magnetic reconnection is well known as an efficient mechanism for transferring magnetic energy into plasma energy.However,how the energy conversion and partition between different species is influenced by the shear angle of the reconnecting magnetic component(i.e.,the guide field strength)is not well understood.Using 2.5-dimensional particle-in-cell simulations,we investigated the energy conversion in reconnection with different guide fields.We found that the overall energy conversion first decreases steeply and then increases slowly when the guide field increases fromB_(g)=0 toB_(g)=4.The increase in energy conversion in the large guide field regime is due to the electron energy gain through the perpendicular channelJ_(⊥)·E_(⊥).The overall energy conversion is predominantly contributed byJ_(⊥)·E_(⊥) rather thanJ||E||.We further find that energy conversion mainly occurs within the reconnection front and the flux pileup region.However,the contribution from the fore reconnection front becomes important in large guide field regimes(3<B_(g)≤4)because of the enhanced electron energy gain.

New Evidence for Magnetic Reconnection in the Tail of Interplanetary Magnetic Cloud

We analyse the WIND data of an interplanetary magnetic cloud （MC） on 2 November 2001, and find new evidences for magnetic reconnection in the tail of this MC. In the MC tail, the largely dip and the large change of the orientation of the magnetic field occurred simultaneously, △θ≈45° and △φ changed from 90° to 320°. Correspondingly, the number density of ions increased, and the superthermal electrons were heated and accelerated, however its number density decreased. Meanwhile, inverse jets and Hall term were observed. The pitch-angle distributions of the electrons with lower energy and higher energy showed strong turbulence and bi-direction flow, respectively. The plasma wave activity enhanced near the electron plasma frequency, fpe and 2fpe. These important physical characteristics are new evidences for magnetic reconnection existing in interplanetary space.

Conceptual design of the three-dimensional magnetic field configuration relevant to the magnetopause reconnection in the SPERF

A new ground-based expenmental device,the Space Plasma Environment Research Facility（SPERF）,is being designed at Harbin Institute of Technology in China,with Asymmetric REconnection eXperiment-3 Dimensional（AREX-3D） as one of the experimental components to study the asymmetric reconnection dynamics relevant to the interaction between the interplanetary and magnetospheric plasmas.The asymmetry in the designed magnetic reconnection process not only refers to the distinct plasma parameters designed for the two upstream regions across the current sheet,but also refers to the inhomogeneity in the direction along the current sheet resulting from the designed 3D magnetic field geometry.These two asymmetries are fundamental features of the reconnection process at the Earth＇s magnetopause.In experiment,the reconnection process is driven by a set of flux cores through coil-currentramp-up from the ＇magnetosheath-side＇ to interact with a dipole magnetic field generated by the Dipole Research Experiment（DREX） coil on the ＇magnetosphere-side＇.The AREX-3D will be able to investigate a range of important reconnection issues in 3D magnetic field geometry that is relevant to the Earth＇s magnetopause.A wide range of plasma parameters can be achieved through inductive plasma generation with flux cores on the ＇magnetosheath-side＇ and electron cyclotron resonance（ECR） with microwave sources on the ＇magnetosphere-side＇,e.g.high（low）plasma density at experimental magnetosheath（dipole） side.Different reconnection regimes and geometries can be produced by adjusting plasma parameters and coil setups as well as coil current waveforms.The three-dimensional magnetic field configurations in the SPERF relevant to the dayside magnetopause reconnection are discussed in detail.

Contribution of patchy reconnection to the ion-to-electron temperature ratio in the Earth’s magnetotail

The ion-to-electron temperature ratio is a good indicator of the processes involved in the plasma sheet.Observations have suggested that patchy reconnection and the resulting earthward bursty bulk flows(BBFs)transport may be involved in causing the lower temperature ratios at smaller radial distances during southward IMF periods.In this paper,we estimate theoretically how a patchy magnetic reconnection electric field can accelerate ions and electrons differently.If both ions and electrons are non-adiabatically accelerated only once within each reconnection,the temperature ratio would be preserved.However,when reconnection occurs closer to the Earth where magnetic field lines are shorter,particles mirrored back from the ionosphere can cross the reconnection region more than once within one reconnection;and electrons,moving faster than ions,can have more crossings than do ions,leading to electrons being accelerated more than ions.Thus as particles are transported from tail to the near-Earth by BBFs through multiple reconnection,electrons should be accelerated by the reconnection electric field more times than are ions,which can explain the lower temperature ratios observed closer to the Earth.

In situ evidence of resonant interactions between energetic electrons and whistler waves in magnetopause reconnection

In this paper,we analyze one reconnection event observed by the Magnetospheric Multiscale(MMS)mission at the earth’s magnetopause.In this event,the spacecraft crossed the reconnection current sheet from the magnetospheric side to the magnetosheath side,and whistler waves were observed on both the magnetospheric and magnetosheath sides.On the magnetospheric side,the whistler waves propagated quasi-parallel to the magnetic field and toward the X-line,while on the magnetosheath side they propagated almost anti-parallel to the magnetic field and away from the X-line.Associated with the enhancement of the whistler waves,we find that the fluxes of energetic electrons are concentrated around the pitch angle 90°when their energies are higher than the minimum energy that is necessary for the resonant interactions between the energetic electrons and whistler waves.This observation provides in situ observational evidence of resonant interactions between energetic electrons and whistler waves in the magnetic reconnection.

In situ detection of the electron diffusion region of collisionless magnetic reconnection at the high-latitude magnetopause

Magnetic reconnection is the most fundamental energy-transfer mechanism in the universe that converts magnetic energy into heat and kinetic energy of charged particles.For reconnection to occur,the frozen-in condition must break down in a localized region,commonly called the ‘diffusion region'.In Earth's magnetosphere,ion diffusion regions have already been observed,while electron diffusion regions have not been detected due to their small scales(of the order of a few km)(Paschmann,2008).In this paper we report,for the first time,in situ observations of an active electron diffusion region by the four Cluster spacecraft at the Earth's highlatitude magnetopause.The electron diffusion region is characterized by nongyrotropic electron distribution,strong field-aligned currents carried by electrons and bi-directional super-Alfvénic electron jets.Also observed were multiple micro-scale flux ropes,with a scale size of about 5 c/ω_(pe)(12 km,with c/ωpe the electron inertial length),that are crucial for electron acceleration in the guide-field reconnection process(Drake et al.,2006 a).The data demonstrate the existence of the electron diffusion region in collisionless guide-field reconnection at the magnetopause.

Generation of Electric Field and Net Charge in Hall Reconnection

Generation of Hall electric field and net charge associated initial conditions of plasma density and magnetic field. with magnetic reconnection is studied under different With inclusion of the Hall effects, decoupling of the electron and ion motions leads to the formation of a narrow layer with strong electric field and large net charge density along the separatrix. The asymmetry of the plasma density or magnetic field or both across the current sheet will largely increase the magnitude of the electric field and net charge. The results indicate that the asymmetry of the magnetic field is more effective in producing larger electric field and charge density. The electric field and net charge are always much larger in the low density or/and high magnetic field side than those in the high density or/and low magnetic field side. Both the electric field and net charge density are linearly dependent on the ratios of the plasma density or the square of the magnetic field across the current sheet. For the case with both initial asymmetries of the magnetic field and density, rather large Hall electric field and charge density are generated.

Characteristics of Plasma Jets in Laser-Driven Magnetic Reconnection

The jets driven by magnetic reconnection in laser-plasma interactions are investi- gated experimentally. The diagnostics in the optical and X-ray ranges provide detailed information about the jet characteristics. The plasma jets perpendicular to and along the target surface are observed clearly, which is evident signatures of laser driven magnetic reconnection. The jet formation is also investigated for different experimental parameters.

An Experimental Platform for the Study on Magnetic Reconnection in Laboratory Plasmas

In order to investigate electron dynamics near the electron diffusion region in mag- netic reconnection process, an upgrade in the linear magnetized plasma （LMP） device is accom- plished at the University of Science and Technology of China. Radio frequency （RF） helicon discharge is used to generate a quasi-stationary plasma, and a time-dependent magnetic field is applied to the plasma, which exhibits an X-type neutral point in vacuum. A two-dimensional sophisticated mobile platform is built up, providing a high spatial resolution, below 0.5 ram, for the diagnostics.

Electron acceleration in interaction of magnetic islands in large temporal-spatial turbulent magnetic reconnection

A new combined Fermi, betatron, and turbulent electron acceleration mechanism is proposed in interaction of magnetic islands during turbulent magnetic reconnection evolution in explosive astrophysical phenomena at large temporal-spatial scale(LTSTMR), the ratio of observed current sheets thickness to electron characteristic length, electron Larmor radius for low-β and electron inertial length for high-β, is on the order of 10^(10)–10^(11); the ratio of observed evolution time to electron gyroperiod is on the order of 10~7–10~9).The original combined acceleration model is known to be one of greatest importance in the interaction of magnetic islands; it assumes that the continuous kinetic-dynamic temporal-spatial scale evolution occurs as two separate independent processes.In this paper, we reconsider the combined acceleration mechanism by introducing a kinetic-dynamic-hydro full-coupled model instead of the original micro-kinetic or macro-dynamic model.We investigate different acceleration mechanisms in the vicinity of neutral points in magnetic islands evolution, from the stage of shrink and breakup into smaller islands(kinetic scale), to the stage of coalescence and growth into larger islands(dynamic scale), to the stages of constant and quasi-constant(contracting-expanding) islands(hydro scale).As a result, we give for the first time the acceleration efficiencies of different types of acceleration mechanisms in magnetic islands' interactions in solar atmosphere LTSTMR activities(pico-, 10^(–2)–10~5 m; nano-, 10~5–10~6 m; micro-, 10~6–10~7 m; macro-, 10~7–10~8 m; large-,10~8–10~9 m).

Electron Magnetohydrodynamics Magnetic Reconnection Experiment on Keda Linear Magnetized Plasma Device

We conduct an electron magnetohydrodynamics magnetic reconnection experiment with guide-field in our Keda linear magnetized plasma device, in which two pulsed currents with the same direction are conducted in parallel with the axial direction of the main chamber of the device using two long aluminum sticks. After approximately 5μs, an X-type magnetic field line topology is formed at the center of the chamber. With the formation of the X-type topology of magnetic field lines, we can also find the rapid increase of the current and ratio of the common flux to the private flux in this area. Additionally, a reduction in the plasma density and the plasma density concentration along one pair of separatrices can also be found.

Chaos-induced resistivity of collisionless magnetic reconnection in the presence of a guide field

One of the most puzzling problems in astrophysics is to understand the anomalous resistivity in collisionless magnetic reconnection that is believed extensively to be responsible for the energy re- lease in various eruptive phenomena. The magnetic null point in the reconnecting current sheet, acting as a scattering center, can lead to chaotic motions of particles in the current sheet, which is one of the possible mechanisms for anomalous resistivity and is called chaos-induced resistivity. In many interest- ing cases, however, instead of the magnetic null point, there is a nonzero magnetic field perpendicular to the merging field lines, usually called the guide field, whose effect on chaos-induced resistivity has been an open problem. By use of the test particle simulation method and statistical analysis, we investigate chaos-induced resistivity in the presence of a constant guide field. The characteristics of particle motion in the reconnecting region, in particular, the chaotic behavior of particle orbits and evolving statistical features, are analyzed. The results show that as the guide field increases, the radius of the chaos region increases and the Lyapunov index decreases. However, the effective collision frequency, and hence the chaos-induced resistivity, reach their peak values when the guide field approaches half of the character- istic strength of the reconnection magnetic field. The presence of a guide field can significantly influence the chaos of the particle orbits and hence the chaos-induced resistivity in the reconnection sheet, which decides the collisionless reconnection rate. The present result is helpful for us to understand the micro- physics of anomalous resistivity in collisionless reconnection with a guide field.