Guide Field Magnetic Reconnection Research Articles

Conversion of Magnetic Energy to Plasma Kinetic Energy During Guide Field Magnetic Reconnection in the Laboratory.

We present laboratory measurements showing the two-dimensional (2D) structure of energy conversion during magnetic reconnection with a guide field over the electron and ion diffusion regions, resolving the separate energy deposition on electrons and ions. We find that the electrons are energized by the parallel electric field at two locations, at the X line and around the separatrices. On the other hand, the ions are energized ballistically by the perpendicular electric field in the vicinity of the high-density separatrices. An energy balance calculation by evaluating the terms of the Poynting theorem shows that 40% of the magnetic energy is converted to particle energy, 2/3 of which is transferred to ions and 1/3 to electrons. Further analysis suggests that the energy deposited on particles manifests mostly in the form of thermal kinetic energy in the diffusion regions.

A hard energy spectrum in 3D guide-field magnetic reconnection

Magnetic reconnection has long been known to be the most important mechanism not only for mixing the plasmas by changing the magnetic field topology but also for releasing the magnetic field energy into the plasma kinetic energy. During magnetic energy release, it is possible for some of the heated plasma to be accelerated to energies much higher than the thermal energy. Recently, the energy partitioning of the thermal and the nonthermal energy has been studied by using particle-in-cell (PIC) simulations, and it has been shown that the acceleration efficiency of nonthermal particles increases with increase in the plasma temperature, and the nonthermal energy density occupies more than 90% in the total heated plasma when the Alfvén velocity is close to the speed of light, c. However, the acceleration efficiency decreases as the guide magnetic field increases. So far, the acceleration efficiency has been mainly studied in two-dimensional systems, but it is interesting to study three-dimensional effects where the patchy and turbulent reconnection can dynamically occur. This study explores the effects of three-dimensional relativistic reconnection on a pair plasma with the guide magnetic field, utilizing three-dimensional (3D) PIC simulations. The results indicate that the decrease in nonthermal particle production is smaller in 3D guide-field reconnection compared to 2D. More importantly, contrary to general expectation, 3D reconnection is capable of maintaining a hard nonthermal energy spectrum even in the presence of a strong guide magnetic field.

Evidence of guide field magnetic reconnection in flapping current sheets

Evidence of guide field magnetic reconnection in flapping current sheets

Using Direct Laboratory Measurements of Electron Temperature Anisotropy to Identify the Heating Mechanism in Electron-Only Guide Field Magnetic Reconnection.

Anisotropic electron heating during electron-only magnetic reconnection with a large guide magnetic field is directly measured in a laboratory plasma through insitu measurements of electron velocity distribution functions. Electron heating preferentially parallel to the magnetic field is localized to one separatrix, and anisotropies of 1.5 are measured. The mechanism for electron energization is identified as the parallel reconnection electric field because of the anisotropic nature of the heating and spatial localization. These characteristics are reproduced in a 2D particle-in-cell simulation and are also consistent with numerous magnetosheath observations. A measured increase in the perpendicular temperature along both separatrices is not reproduced by our 2D simulations. This work has implications for energy partition studies in magnetosheath and laboratory reconnection.

Evidence of a Nonorthogonal X-line in Guide-field Magnetic Reconnection

We present observations that suggest the X-line of guide-field magnetic reconnection is not necessarily orthogonal to the plane in which magnetic reconnection is occurring. The plane of magnetic reconnection is often referred to as the L–N plane, where L is the direction of the reversing and reconnecting magnetic field and N is normal to the current sheet. The X-line is often assumed to be orthogonal to the L–N plane (defined as the M-direction) in the majority of theoretical studies and numerical simulations. The four-satellite Magnetospheric Multiscale (MMS) mission, however, observes a guide-field magnetic reconnection event in Earth’s magnetotail in which the X-line may be oblique to the L–N plane. This finding is somewhat opportune as two of the MMS satellites at the same N location report nearly identical observations with no significant time delays in the electron diffusion region (EDR) even though they have substantial separation in L. A minimum directional derivative analysis suggests that the X-line is between 40° and 60° from M, adding support that the X-line is oblique. Furthermore, the measured ion velocity is inconsistent with the apparent motion of the MMS spacecraft in the L-direction through the EDR, which can be resolved if one assumes a shear in the L–N plane and motion in the M-direction. A nonorthogonal X-line, if somewhat common, would call for revisiting theory and simulations of guide-field magnetic reconnection, reexamination of how the reconnection electric field is supported in the EDR, and reconsidering the large-scale geometry of the X-line.

Direct Measurement of Electron Cyclotron Emission during High Guide Field Magnetic Reconnection

Direct Measurement of Electron Cyclotron Emission during High Guide Field Magnetic Reconnection

Effects of Guide Field and Background Density and Temperature on Energy Conversion at Magnetic Reconnection Front

AbstractMagnetic reconnection is a fundamental energy converting process changing the topology of the magnetic field in space. We investigate how the guide field and the background plasma density and temperature influence the energy converting process during magnetic reconnection using a two‐dimensional particle‐in‐cell simulation code. It is shown that the guide field reduces both the reconnection rate and the energy conversion rate. Although the downstream Poynting flux from the reconnection site is enhanced during guide field magnetic reconnection, it does not participate in energy conversion as the inflow and outflow of Poynting flux from the left and right boundaries of the reconnection front cancel each other out. The major input of the Poynting flux participating in energy conversion comes from the upper and lower boundaries of the reconnection front. An electrostatic perturbating structure arises in cases with a strong guide field. Apart from the reconnection rate and the energy conversion rate, the energy outflow is altered by changing background plasma density and temperature. Our research shows that the energy input at a well‐developed reconnection front in the case of the guide field does not come from the reconnection site, which implies the reconnection front can be an effective energy converter in the magnetosphere independent of the reconnection site once formed from the transient magnetic reconnection. These simulation results show that the guide field and background density and temperature quantitatively alter the energy conversion at the magnetic reconnection front, but the energy conversion pattern remains unchanged.

Pseudo-Maxwellian Velocity Distribution Formed by the Pickup-like Process in Magnetic Reconnection

Focusing on ring-shaped ion velocity distributions with a finite width formed in magnetic reconnection in the presence of a guide magnetic field, intriguing properties such as the formation mechanism, a significant change in the shape, and necessary conditions for the change are investigated by means of theory and simulations. The width of a ring velocity distribution predominantly originates from velocity variations of seed particles for the pickup-like process. A function exactly representing a ring with a width is analytically formulated, assuming a steady supply of seed particles satisfying a Maxwellian velocity distribution and a mixing of gyration phases. The formulated function indicates that when the ring width is larger than a criterion, the local minimum of the ring’s center is changed into the maximum, and the shape is transformed into a mountain shape. Such a mountain-like distribution is defined as “a pseudo-Maxwellian distribution,” because it is almost indistinguishable in shape from a genuine Maxwellian distribution. Actually, particle simulations demonstrate that mountain-shaped ion velocity distributions are formed during magnetic reconnection with a guide magnetic field, and it is nearly concluded that they are pseudo-Maxwellian distributions. Moreover, two types of evidence for pseudo-Maxwellian distributions are shown by simulations. One is to analyze the dependence of the distribution shape on the guide magnetic field, which is explored by the particle simulation. In cases of slightly different values of the guide field, vague shapes of rings with a width are observed as ion velocity distributions. The other is to observe velocity distributions under a hypothetical condition of an artificial zero temperature in the upstream by utilizing a test particle simulation. In the test particle simulation, ring-shaped distributions with a width are clearly seen, because the velocity variations in the upstream are reduced. From the two types of evidence, it is definitely confirmed that the mountain-shaped distributions found in the particle simulations are pseudo-Maxwellian distribution. These results imply that pseudo-Maxwellian distributions would be created for various cases of guide field magnetic reconnection.

The Formation of Electron Outflow Jets with Power-law Energy Distribution in Guide-field Magnetic Reconnection

Abstract Observationally, electron beams with power-law energy spectra are commonly associated with solar flares. Previous studies have found that during magnetic reconnection with a guide field B g larger than 0.1 times the asymptotic field B 0, electron beams are unable to develop due to the strong deflection caused by the guide field. Using particle-in-cell simulations we show that in force-free reconnection, the development of an electron Kelvin–Helmholz instability can suppress the Hall effect and produce a flute-like outflow exhaust, in which both electrons and ions are nearly frozen-in with the magnetic field. The coupling of a continuously growing electron velocity shear and E × B drift drive the electrons out of magnetic vortices and results in collimated jets with a power-law energy spectrum in the elongated exhaust. The spatial density of electron jets is comparable to the background and is highly inhomogeneous, signifying on asymmetric density structure in guide field reconnection.

Turbulence and Transport During Guide Field Reconnection at the Magnetopause

AbstractWe analyze the development and influence of turbulence in three‐dimensional particle‐in‐cell simulations of guide field magnetic reconnection at the magnetopause with parameters based on observations from the Magnetospheric Multiscale (MMS) mission. Along the separatrices the turbulence is a variant of the lower‐hybrid‐drift instability (LHDI) that produces electric field fluctuations with amplitudes much greater than the reconnection electric field. The turbulence controls the scale length of the density and current profiles while enabling significant transport across the magnetopause, despite the electrons remaining frozen‐in to the magnetic field. Transport of the out‐of‐plane current density exceeds that of the particle density. Near the X‐line the electrons are not frozen‐in and the turbulence, which differs from the LHDI, makes a significant net contribution to the generalized Ohm's law through an anomalous viscosity. The characteristics of the turbulence and associated particle transport are consistent with fluctuation amplitudes in the MMS observations. However, for this event the simulations suggest that the MMS spacecraft were not close enough to the core of the electron diffusion region to identify the region where anomalous viscosity is important.

Hybrid Simulations of Magnetodisc Transport Driven by the Rayleigh‐Taylor Instability

AbstractPlasma transport in the rapidly rotating giant magnetospheres is thought to involve a centrifugally driven flux tube interchange instability, similar to the Rayleigh‐Taylor (RT) instability. In three dimensions, the convective flow patterns associated with the RT instability can produce strong guide field reconnection, allowing plasma mass to move radially outward while conserving magnetic flux (Ma et al., 2016, https://doi.org/10.1002/2015JA022122). We present a set of hybrid (kinetic ion/fluid electron) plasma simulations of the RT instability using high plasma beta conditions appropriate for the inner and middle magnetosphere at Jupiter and Saturn. A density gradient, combined with a centrifugal force, provide appropriate RT onset conditions. Pressure balance requires only a temperature gradient as the magnetic pressure is constant. Pressure balance is achieved with a temperature gradient in a fixed magnetic field. The three‐dimensional simulation domain represents a local volume of the magnetodisc resonant cavity. Simulated RT growth rates compare favorably with linear theory, where the fundamental mode of the resonant cavity determines the largest (stabilizing) parallel wavelength. We suggest that the perpendicular scale of RT structures is determined by the fundamental mode, which limits growth due to magnetic tension. Finally, we investigated strong guide field magnetic reconnection and diffusive processes as plausible mechanisms to facilitate kinetic‐scale radial transport.

Kinetic turbulence in fast three-dimensional collisionless guide-field magnetic reconnection

Although turbulence has been conjectured to be important for magnetic reconnection, still very little is known about its role in collisionless plasmas. Previous attempts to quantify the effect of turbulence on reconnection usually prescribed Alfv\'enic or other low-frequency fluctuations or investigated collisionless kinetic effects in just two-dimensional configurations and antiparallel magnetic fields. In view of this, we analyzed the kinetic turbulence self-generated by three-dimensional guide-field reconnection through force-free current sheets in frequency and wavenumber spaces, utilizing 3D particle-in-cell code numerical simulations. Our investigations reveal reconnection rates and kinetic turbulence with features similar to those obtained by current in-situ spacecraft observations of MMS as well as in the laboratory reconnection experiments MRX, VTF and \textsc{Vineta}-II. In particular we found that the kinetic turbulence developing in the course of 3D guide-field reconnection exhibits a broadband power-law spectrum extending beyond the lower-hybrid frequency and up to the electron frequencies. In the frequency space the spectral index of the turbulence appeared to be close to -2.8 at the reconnection X-line. In the wavenumber space it also becomes -2.8 as soon as the normalized reconnection rate reaches 0.1. The broadband kinetic turbulence is mainly due to current-streaming and electron-flow-shear instabilities excited in the sufficiently thin current sheets of kinetic reconnection. The growth of the kinetic turbulence corresponds to high reconnection rates which exceed those of fast laminar, non-turbulent reconnection.

Two-stage Electron Acceleration by 3D Collisionless Guide-field Magnetic Reconnection

Abstract We report a newly found two-stage mechanism of electron acceleration near X-lines of 3D collisionless guide-field magnetic reconnection in the nonrelativistic regime typical, e.g., for stellar coronae. We found that after electrons are first pre-accelerated during the linear growth of reconnection, they become additionally accelerated in the course of the nonlinear stage of 3D guide-field magnetic reconnection. This additional acceleration is due to the filamentation of electric and magnetic fields caused by streaming instabilities. In addition to enhanced parallel electric fields, the filamentation leads to additional curvature-driven electron acceleration in the guide-field direction. As a result, part of the accelerated electron spectra becomes a power law with a spectral index of ∼−1.6 near the X-line. This second stage of acceleration due to nonlinear reconnection is relevant for the production of energetic electrons in, e.g., thin current sheets of stellar coronae.

Tripolar electric field Structure in guide field magnetic reconnection

Abstract. It has been shown that the guide field substantially modifies the structure of the reconnection layer. For instance, the Hall magnetic and electric fields are distorted in guide field reconnection compared to reconnection without guide fields (i.e., anti-parallel reconnection). In this paper, we performed 2.5-D electromagnetic full particle simulation to study the electric field structures in magnetic reconnection under different initial guide fields (Bg). Once the amplitude of a guide field exceeds 0.3 times the asymptotic magnetic field B0, the traditional bipolar Hall electric field is clearly replaced by a tripolar electric field, which consists of a newly emerged electric field and the bipolar Hall electric field. The newly emerged electric field is a convective electric field about one ion inertial length away from the neutral sheet. It arises from the disappearance of the Hall electric field due to the substantial modification of the magnetic field and electric current by the imposed guide field. The peak magnitude of this new electric field increases linearly with the increment of guide field strength. Possible applications of these results to space observations are also discussed. Keywords. Space plasma physics (magnetic reconnection)

Population Mixing in Asymmetric Magnetic Reconnection with a Guide Field.

We investigate how population mixing leads to structured electron distribution functions in asymmetric guide-field magnetic reconnection based on particle-in-cell simulations. The change of magnetic connectivity patches populations from different inflow regions to form multicomponent distributions in the exhaust, illustrating the direct consequence of the breaking and rejoining of magnetic flux tubes. Finite Larmor radius (FLR) effects of electrons accelerated by the perpendicular electric fields result in crescent-type nongyrotropic distributions. A new type of nongyrotropy is found to be caused by the combined effects of the FLR and velocity dispersion of electrons accelerated by the parallel electric field. The patching together of populations and the effects of acceleration and the FLR form the first steps of mixing in the exhaust and separatrix regions.

Nonlinear evolution of electron shear flow instabilities in the presence of an external guide magnetic field

The dissipation mechanism by which the magnetic field reconnects in the presence of an external (guide) magnetic field in the direction of the main current is not well understood. In thin electron current sheets (half thickness close to an electron inertial length) formed in a quasi-steady state of collisionless magnetic reconnection, electron shear flow instabilities are potential candidates for providing an anomalous dissipation mechanism which can break the frozen-in condition of the magnetic field affecting the structure and rate of reconnection. We present the results of investigations of the evolution of electron shear flow instabilities, from linear to nonlinear state, in guide field magnetic reconnection. The properties of the plasma turbulence resulting from the growth of instability and their dependence on the strength of the guide field are studied. For this sake, we utilize the three dimensional electron-magnetohydrodynamic simulations of electron current sheets. We show that, unlike the case of current sheets self-consistently embedded in anti-parallel magnetic fields, the evolution of thin electron current sheets in the presence of a finite external guide field (equal to the asymptotic value of the reconnecting magnetic field or larger) is dominated by high wave number non-tearing mode instabilities. The latter causes the development of, first, a wavy structure of the current sheet. The turbulence, developed later, consists of current filaments and electron flow vortices. As a result of the nonlinear evolution of instability, the current sheet broadens simultaneously with its flattening in the central region mimicking a viscous-like turbulent dissipation. Later, the flattened current sheet bifurcates. During the time of bifurcation, the rate of the change of mean electron flow velocity is proportional to the magnitude of the flow velocity, suggesting a resistive-like dissipation. The turbulence energy cascades to shorter wavelengths preferentially in the direction perpendicular to the guide magnetic field. The degree of anisotropy of the turbulence was found to increase with the increasing strength of the guide field.

Magnetospheric Multiscale Observations of the Electron Diffusion Region of Large Guide Field Magnetic Reconnection.

We report observations from the Magnetospheric Multiscale (MMS) satellites of a large guide field magnetic reconnection event. The observations suggest that two of the four MMS spacecraft sampled the electron diffusion region, whereas the other two spacecraft detected the exhaust jet from the event. The guide magnetic field amplitude is approximately 4 times that of the reconnecting field. The event is accompanied by a significant parallel electric field (E_{∥}) that is larger than predicted by simulations. The high-speed (∼300 km/s) crossing of the electron diffusion region limited the data set to one complete electron distribution inside of the electron diffusion region, which shows significant parallel heating. The data suggest that E_{∥} is balanced by a combination of electron inertia and a parallel gradient of the gyrotropic electron pressure.

Energy Flux due to Electromagnetic Fluctuations during Guide Field Magnetic Reconnection

Energy Flux due to Electromagnetic Fluctuations during Guide Field Magnetic Reconnection

Localized electron heating by strong guide-field magnetic reconnection

Localized electron heating of magnetic reconnection was studied under strong guide-field using two merging spherical tokamak plasmas in the University of Tokyo Spherical Tokamak experiment. Our new slide-type two-dimensional Thomson scattering system is documented for the first time the electron heating localized around the X-point. Shape of the high electron temperature area does not agree with that of energy dissipation term Et·jt. If we include a guide-field effect term Bt/(Bp+αBt) for Et·jt, the energy dissipation area becomes localized around the X-point, suggesting that the electrons are accelerated by the reconnection electric field parallel to the magnetic field and thermalized around the X-point.

Magnetic reconnection in the auroral acceleration region

AbstractEvidence is presented for the operation of extreme guide field magnetic reconnection in the presence of sheared flows in a narrow current sheet through the auroral acceleration region. Using auroral imagery, magnetically conjugate particle measurements and a 3‐D reduced MHD simulation, vortical motions in a narrow, rapidly evolving auroral arc indicative of the operation of magnetic tearing are identified. An estimate of the contribution of this process to the observed rate of energy deposition at the ionosphere shows that while this contribution can be significant it remains a minor fraction of the total. These observations are placed in the context of low magnetic shear reconnection made possible in plasmas with low plasma beta.