Kinetic Simulations of Magnetic Reconnection in Partially Ionized Plasmas.

Fast magnetic reconnection occurs in nearly all natural and laboratory plasmas and rapidly releases stored magnetic energy. Although commonly studied in fully ionized plasmas, if and when fast reconnection can occur in partially ionized plasmas, such as the interstellar medium or solar chromosphere, is not well understood. This Letter presents the first fully kinetic particle-in-cell simulations of partially ionized reconnection and demonstrates that fast reconnection can occur in partially ionized systems. In the simulations, the transition to fast reconnection occurs when the current sheet width thins below the ion-inertial length in contrast to previous analytic predictions. The peak reconnection rate is ≥0.08 when normalized to the bulk Alfvén speed (including both ion and neutral mass), consistent with previous experimental results. However, when the bulk Alfvén speed falls below the neutral sound speed, the rate becomes system size dependent. The normalized inflow velocity is ionization fraction dependent, which is shown to be a result of neutral momentum transport. A model for the inflow is developed which agrees well with the simulation results.

Pressure Tensor Elements Breaking the Frozen-In Law During Reconnection in Earth's Magnetotail.

Aided by fully kinetic simulations, spacecraft observations of magnetic reconnection in Earth's magnetotail are analyzed. The structure of the electron diffusion region is in quantitative agreement with the numerical model. Of special interest, the spacecraft data reveal how reconnection is mediated by off-diagonal stress in the electron pressure tensor breaking the frozen-in law of the electron fluid.

Spacecraft Observations of Oblique Electron Beams Breaking the Frozen-In Law During Asymmetric Reconnection.

Fully kinetic simulations of asymmetric magnetic reconnection reveal the presence of magnetic-field-aligned beams of electrons flowing toward the topological magnetic x line. Within the ∼6d_{e} electron-diffusion region, the beams become oblique to the local magnetic field, providing a unique signature of the electron-diffusion region where the electron frozen-in law is broken. The numerical predictions are confirmed by in situ Magnetospheric Multiscale spacecraft observations during asymmetric reconnection at Earth's dayside magnetopause.

Why does Steady-State Magnetic Reconnection have a Maximum Local Rate of Order 0.1?

Simulations suggest collisionless steady-state magnetic reconnection of Harris-type current sheets proceeds with a rate of order 0.1, independent of dissipation mechanism. We argue this long-standing puzzle is a result of constraints at the magnetohydrodynamic (MHD) scale. We predict the reconnection rate as a function of the opening angle made by the upstream magnetic fields, finding a maximum reconnection rate close to 0.2. The predictions compare favorably to particle-in-cell simulations of relativistic electron-positron and nonrelativistic electron-proton reconnection. The fact that simulated reconnection rates are close to the predicted maximum suggests reconnection proceeds near the most efficient state allowed at the MHD scale. The rate near the maximum is relatively insensitive to the opening angle, potentially explaining why reconnection has a similar fast rate in differing models.

Pulsating Magnetic Reconnection Driven by Three-Dimensional Flux-Rope Interactions.

The dynamics of magnetic reconnection is investigated in a laboratory experiment consisting of two magnetic flux ropes, with currents slightly above the threshold for the kink instability. The evolution features periodic bursts of magnetic reconnection. To diagnose this complex evolution, volumetric three-dimensional data were acquired for both the magnetic and electric fields, allowing key field-line mapping quantities to be directly evaluated for the first time with experimental data. The ropes interact by rotating about each other and periodically bouncing at the kink frequency. During each reconnection event, the formation of a quasiseparatrix layer (QSL) is observed in the magnetic field between the flux ropes. Furthermore, a clear correlation is demonstrated between the quasiseparatrix layer and enhanced values of the quasipotential computed by integrating the parallel electric field along magnetic field lines. These results provide clear evidence that field lines passing through the quasiseparatrix layer are undergoing reconnection and give a direct measure of the nonlinear reconnection rate. The measurements suggest that the parallel electric field within the QSL is supported predominantly by electron pressure; however, resistivity may play a role.

Spacecraft Observations and Analytic Theory of Crescent-Shaped Electron Distributions in Asymmetric Magnetic Reconnection.

Supported by a kinetic simulation, we derive an exclusion energy parameter E_{X} providing a lower kinetic energy bound for an electron to cross from one inflow region to the other during magnetic reconnection. As by a Maxwell demon, only high-energy electrons are permitted to cross the inner reconnection region, setting the electron distribution function observed along the low-density side separatrix during asymmetric reconnection. The analytic model accounts for the two distinct flavors of crescent-shaped electron distributions observed by spacecraft in a thin boundary layer along the low-density separatrix.

Role of Ion Kinetic Physics in the Interaction of Magnetic Flux Ropes.

To explain many natural magnetized plasma phenomena, it is crucial to understand how rates of collisionless magnetic reconnection scale in large magnetohydrodynamic (MHD) scale systems. Simulations of isolated current sheets conclude such rates are independent of system size and can be reproduced by the Hall-MHD model, but neglect sheet formation and coupling to MHD scales. Here, it is shown for the problem of flux-rope merging, which includes this formation and coupling, that the Hall-MHD model fails to reproduce the kinetic results. The minimum sufficient model must retain ion kinetic effects, which set the ion diffusion region geometry and give time-averaged rates that reduce significantly with system size, leading to different global evolution in large systems.

Identification of intermittent multifractal turbulence in fully kinetic simulations of magnetic reconnection.

Recent fully nonlinear, kinetic three-dimensional simulations of magnetic reconnection [W. Daughton et al., Nat. Phys. 7, 539 (2011)] evolve structures and exhibit dynamics on multiple scales, in a manner reminiscent of turbulence. These simulations of reconnection are among the first to be performed at sufficient spatiotemporal resolution to allow formal quantitative analysis of statistical scaling, which we present here. We find that the magnetic field fluctuations generated by reconnection are anisotropic, have nontrivial spatial correlation, and exhibit the hallmarks of finite range fluid turbulence: they have non-Gaussian distributions, exhibit extended self-similarity in their scaling, and are spatially multifractal. Furthermore, we find that the rate at which the fields do work on the particles, J · E, is also multifractal, so that magnetic energy is converted to plasma kinetic energy in a manner that is spatially intermittent. This suggests that dissipation in this sense in collisionless reconnection on kinetic scales has an analogue in fluidlike turbulent phenomenology, in that it proceeds via multifractal structures generated by an intermittent cascade.

Bifurcated structure of the electron diffusion region in three-dimensional magnetic reconnection.

Three-dimensional kinetic simulations of magnetic reconnection reveal that the electron diffusion region is composed of two or more current sheets in regimes with weak magnetic shear angles ϕ≲80°. This new morphology is explained by oblique tearing modes which produce flux ropes while simultaneously driving enhanced current at multiple resonance surfaces. This physics persists into the nonlinear regime leading to multiple electron layers embedded within a larger Alfvénic inflow and outflow. Surprisingly, the thickness of these layers and the reconnection rate both remain comparable to two-dimensional models. The parallel electric fields are supported predominantly by the electron pressure tensor and electron inertia, while turbulent dissipation remains small.

Regimes of the electron diffusion region in magnetic reconnection.

The electron diffusion region during magnetic reconnection lies in different regimes depending on the pressure anisotropy, which is regulated by the properties of thermal electron orbits. In kinetic simulations at the weakest guide fields, pitch angle mixing in velocity space causes the outflow electron pressure to become nearly isotropic. Above a threshold guide field that depends on a range of parameters, including the normalized electron pressure and the ion-to-electron mass ratio, electron pressure anisotropy develops in the exhaust and supports extended current layers. This new regime with electron current sheets extending to the system size is also reproduced by fluid simulations with an anisotropic closure for the electron pressure. It offers an explanation for recent spacecraft observations.

Laboratory Study of Collisionless Magnetic Reconnection.

A concise review is given on the past two decades' results from laboratory experiments on collisionless magnetic reconnection in direct relation with space measurements, especially by the Magnetospheric Multiscale (MMS) mission. Highlights include spatial structures of electromagnetic fields in ion and electron diffusion regions as a function of upstream symmetry and guide field strength, energy conversion and partitioning from magnetic field to ions and electrons including particle acceleration, electrostatic and electromagnetic kinetic plasma waves with various wavelengths, and plasmoid-mediated multiscale reconnection. Combined with the progress in theoretical, numerical, and observational studies, the physics foundation of fast reconnection in collisionless plasmas has been largely established, at least within the parameter ranges and spatial scales that were studied. Immediate and long-term future opportunities based on multiscale experiments and space missions supported by exascale computation are discussed, including dissipation by kinetic plasma waves, particle heating and acceleration, and multiscale physics across fluid and kinetic scales.

Influence of the lower-hybrid drift instability on magnetic reconnection in asymmetric configurations.

Using fully kinetic 3D simulations of magnetic reconnection in asymmetric antiparallel configurations, we demonstrate that an electromagnetic lower-hybrid drift instability (LHDI) localized near the X line can substantially modify the reconnection mechanism in the regimes with large asymmetry, a moderate ratio of electron to ion temperature, and low plasma ß. However, the mode saturates at a small amplitude in the regimes typical of Earth's magnetopause. In these cases, LHDI-driven turbulence is predominantly localized along the separatrices on the low-ß side of the current sheet, in agreement with spacecraft observations.

Demonstration of anisotropic fluid closure capturing the kinetic structure of magnetic reconnection.

Collisionless magnetic reconnection in high-temperature plasmas has been widely studied through fluid-based models. Here, we present results of fluid simulation implementing new equations of state for guide-field reconnection. The new fluid closure accurately accounts for the anisotropic electron pressure that builds in the reconnection region due to electric and magnetic trapping of electrons. In contrast to previous fluid models, our fluid simulation reproduces the detailed reconnection region as observed in fully kinetic simulations. We hereby demonstrate that the new fluid closure self-consistently captures all the physics relevant to the structure of the reconnection region, providing a gateway to a renewed and deeper theoretical understanding of reconnection in weakly collisional regimes.

First resolved observations of the demagnetized electron-diffusion region of an astrophysical magnetic-reconnection site.

Spatially resolved, diagnostic signatures across the X-line and electron-diffusion region (EDR) by the Polar spacecraft are reported at Earth's magnetopause. The X-line traversal has a local electron's skin depth scale. First, resolved EDR profiles are presented with peak electron thermal Mach numbers >1.5, anisotropy >7, calibrated electron agyrotropy >1, and misordered expansion parameters indicative of demagnetization and strong (150 eV) increases in electron temperature. The amplitude and phase of these profiles correlate well with a guide geometry kinetic simulation of collisionless magnetic reconnection. Such high resolution diagnosis has been made possible by data processing techniques that afford an 11-fold reduction in the aliasing time for the electron moments.

Intermittent dissipation at kinetic scales in collisionless plasma turbulence.

High resolution kinetic simulations of collisionless plasma driven by shear show the development of turbulence characterized by dynamic coherent sheetlike current density structures spanning a range of scales down to electron scales. We present evidence that these structures are sites for heating and dissipation, and that stronger current structures signify higher dissipation rates. Evidently, kinetic scale plasma, like magnetohydrodynamics, becomes intermittent due to current sheet formation, leading to the expectation that heating and dissipation in astrophysical and space plasmas may be highly nonuniform and patchy.

Kinetic structure of the electron diffusion region in antiparallel magnetic reconnection.

Strong electron pressure anisotropy has been observed upstream of electron diffusion regions during reconnection in Earth's magnetotail and kinetic simulations. For collisionless antiparallel reconnection, we find that the anisotropy drives the electron current in the electron diffusion region, and that this current is insensitive to the reconnection electric field. Reconstruction of the electron distribution function within this region at enhanced resolutions reveals its highly structured nature and the mechanism by which the pressure anisotropy sets the structure of the region.

Flux pileup in collisionless magnetic reconnection: bursty interaction of large flux ropes.

Using fully kinetic simulations of the island coalescence problem for a range of system sizes greatly exceeding kinetic scales, the phenomenon of flux pileup in the collisionless regime is demonstrated. While small islands on the scale of λ ≤ 5 ion inertial length (d(i)) coalesce rapidly and do not support significant flux pileup, coalescence of larger islands is characterized by large flux pileup and a weaker time averaged reconnection rate that scales as √(d(i)/λ) while the peak rate remains nearly independent of island size. For the largest islands (λ = 100d(i)), reconnection is bursty and nearly shuts off after the first bounce, reconnecting ~20% of the available flux.

Direct evidence for a three-dimensional magnetic flux rope flanked by two active magnetic reconnection X lines at Earth's magnetopause.

We report the direct detection by three THEMIS spacecraft of a magnetic flux rope flanked by two active X lines producing colliding plasma jets near the center of the flux rope. The observed density depletion and open magnetic field topology inside the flux rope reveal important three-dimensional effects. There was also evidence for nonthermal electron energization within the flux rope core where the fluxes of 1-4 keV superthermal electrons were higher than those in the converging reconnection jets. The observed ion and electron energizations differ from current theoretical predictions.

The two-fluid dynamics and energetics of the asymmetric magnetic reconnection in laboratory and space plasmas.

Magnetic reconnection is a fundamental process in magnetized plasma where magnetic energy is converted to plasma energy. Despite huge differences in the physical size of the reconnection layer, remarkably similar characteristics are observed in both laboratory and magnetosphere plasmas. Here we present the comparative study of the dynamics and physical mechanisms governing the energy conversion in the laboratory and space plasma in the context of two-fluid physics, aided by numerical simulations. In strongly asymmetric reconnection layers with negligible guide field, the energy deposition to electrons is found to primarily occur in the electron diffusion region where electrons are demagnetized and diffuse. A large potential well is observed within the reconnection plane and ions are accelerated by the electric field toward the exhaust region. The present comparative study identifies the robust two-fluid mechanism operating in systems over six orders of magnitude in spatial scales and over a wide range of collisionality.

Turbulent mass transfer caused by vortex induced reconnection in collisionless magnetospheric plasmas.

Magnetic reconnection is believed to be the main driver to transport solar wind into the Earth's magnetosphere when the magnetopause features a large magnetic shear. However, even when the magnetic shear is too small for spontaneous reconnection, the Kelvin-Helmholtz instability driven by a super-Alfvénic velocity shear is expected to facilitate the transport. Although previous kinetic simulations have demonstrated that the non-linear vortex flows from the Kelvin-Helmholtz instability gives rise to vortex-induced reconnection and resulting plasma transport, the system sizes of these simulations were too small to allow the reconnection to evolve much beyond the electron scale as recently observed by the Magnetospheric Multiscale (MMS) spacecraft. Here, based on a large-scale kinetic simulation and its comparison with MMS observations, we show for the first time that ion-scale jets from vortex-induced reconnection rapidly decay through self-generated turbulence, leading to a mass transfer rate nearly one order higher than previous expectations for the Kelvin-Helmholtz instability.