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Gregory Werner of University of Colorado presented initially hot pair plasma acceleration during relativistic 3D reconnection where particles are still robustly accelerated acceleration but via a more complex dynamic behavior.[14] Yi-Min Huang of Princeton University discussed plasmoid-mediated reconnection in Hall MHD, while Andrey Beresnyak of Naval Research Laboratory discussed turbulent reconnection also in Hall MHD. In laboratory plasmas, magnetic reconnection is known to occur during sawtooth oscillations in tokamaks, neoclassical tearing mode growth, disruptions, the startup of Spherical Torus plasmas using coaxial helicity injection, relaxation in reversed field pinches and spheromaks, the formation of field reversed configurations by theta pinch or plasma merging, and possibly in edge-localized modes. Plasmas 4, 1936-1944 (1997).10.1063/1.872336 30 S. Majeski, H. Ji, J. Jara-Almonte, and J. Yoo, " Guide field effects on the distribution of plasmoids in multiple scale reconnection", Phys. Magnetic reconnection[[1]] - the topological rearrangement of magnetic field - underlies many explosive phenomena across a wide range of natural and laboratory plasmas.[3] It plays a pivotal role in electron and ion heating, particle acceleration to high energies, energy transport, and self-organization. [Extracted from the article]

Kinetic simulations have demonstrated that three-dimensional reconnection in collisionless regimes proceeds through the formation and interaction of magnetic flux ropes, which are generated due to the growth of tearing instabilities at multiple resonance surfaces. Since kinetic simulations are intrinsically expensive, it is desirable to explore the feasibility of reduced two-fluid models to capture this complex evolution, particularly, in the strong guide field regime, where two-fluid models are better justified. With this goal in mind, this paper compares the evolution of the collisionless tearing instability in a force-free current sheet with a two-fluid model and fully kinetic simulations. Our results indicate that the most unstable modes are oblique for guide fields larger than the reconnecting field, in agreement with the kinetic results. The standard two-fluid tearing theory is extended to address the tearing instability at oblique angles. The resulting theory yields a flat oblique spectrum and underestimates the growth of oblique modes in a similar manner to kinetic theory relative to kinetic simulations. [ABSTRACT FROM AUTHOR]

In large-scale systems of interest to solar physics, there is growing evidence that magnetic reconnection involves the formation of extended current sheets which are unstable to plasmoids (secondary magnetic islands). Recent results suggest that plasmoids may play a critical role in the evolution of reconnection, and have raised fundamental questions regarding the applicability of resistive MHD to various regimes. In collisional plasmas, where the thickness of all resistive layers remain larger than the ion gyroradius, simulations results indicate that plasmoids permit reconnection to proceed much faster than the slow Sweet-Parker scaling. However, it appears these rates are still a factor of ∼10× slower than observed in kinetic regimes, where the diffusion region current sheet falls below the ion gyroradius and additional physics beyond MHD becomes crucially important. Over a broad range of interesting parameters, the formation of plasmoids may naturally induce a transition into these kinetic regimes. New insights into this scenario have emerged in recent years based on a combination of linear theory, fluid simulations and fully kinetic simulations which retain a Fokker-Planck collision operator to allow a rigorous treatment of Coulomb collisions as the reconnection electric field exceeds the runaway limit. Here, we present some new results from this approach for guide field reconnection. Based upon these results, a parameter space map is constructed that summarizes the present understanding of how reconnection proceeds in various regimes. [ABSTRACT FROM AUTHOR]

Magnetic islands are a common feature in both the onset and nonlinear evolution of magnetic reconnection. In collisionless regimes, the onset typically occurs within ion-scale current layers leading to the formation of magnetic islands when multiple X lines are involved. The nonlinear evolution of reconnection often gives rise to extended electron current layers (ECL) which are also unstable to formation of magnetic islands. Here, we show that the excess negative charge and strong out-of-plane electron velocity in the ECL are passed on to the islands generated therein, and that the corresponding observable distinguishing the islands generated in the ECL is the strongly enhanced in-plane electric fields near the island core. The islands formed in ion-scale current layers do not have these properties of the ECL-generated islands. The above result provides a way to assess the occurrence and importance of extended ECLs that are unstable to island formation in space and laboratory plasmas. [ABSTRACT FROM AUTHOR]

A long-standing challenge encountered in modeling plasma dynamics is achieving practical Vlasov equation simulation in multiple spatial dimensions over large length and time scales. While direct multi-dimension Vlasov simulation methods using adaptive mesh methods [M. Gutnic et al., Comput. Phys. Commun. 164, 214 (2004)] have recently shown promising results in two dimensions (2D) [J. W. Banks et al., Phys. Plasmas 18, 052102 (2011); B. I. Cohen et al., November 10, 2010, http://meetings.aps.org/link/BAPS.2010.DPP.NP9.142], in this paper, we present an alternative, the Vlasov multi dimensional (VMD) model, that is specifically designed to take advantage of solution properties in regimes when plasma waves are confined to a narrow cone, as may be the case for stimulated Raman scatter in large optic f# laser beams. Perpendicular grid spacing large compared to a Debye length is then possible without instability or loss of accuracy, enabling an order 10 decrease in required computational resources compared to standard particle in cell (PIC) methods in 2D, with another reduction of that order in 3D. Further advantage compared to PIC methods accrues in regimes where particle noise is an issue. VMD and PIC results in a 2D model of localized Langmuir waves are in qualitative agreement. [ABSTRACT FROM AUTHOR]

Recent progress in understanding the physics of magnetic reconnection is conveniently summarized in terms of a phase diagram which organizes the essential dynamics for a wide variety of applications in heliophysics, laboratory, and astrophysics. The two key dimensionless parameters are the Lundquist number and the macrosopic system size in units of the ion sound gyroradius. In addition to the conventional single X-line collisional and collisionless phases, multiple X-line reconnection phases arise due to the presence of the plasmoid instability either in collisional and collisionless current sheets. In particular, there exists a unique phase termed 'multiple X-line hybrid phase' where a hierarchy of collisional islands or plasmoids is terminated by a collisionless current sheet, resulting in a rapid coupling between the macroscopic and kinetic scales and a mixture of collisional and collisionless dynamics. The new phases involving multiple X-lines and collisionless physics may be important for the emerging applications of magnetic reconnection to accelerate charged particles beyond their thermal speeds. A large number of heliophysical and astrophysical plasmas are surveyed and grouped in the phase diagram: Earth's magnetosphere, solar plasmas (chromosphere, corona, wind, and tachocline), galactic plasmas (molecular clouds, interstellar media, accretion disks and their coronae, Crab nebula, Sgr A*, gamma ray bursts, and magnetars), and extragalactic plasmas (active galactic nuclei disks and their coronae, galaxy clusters, radio lobes, and extragalactic jets). Significance of laboratory experiments, including a next generation reconnection experiment, is also discussed. [ABSTRACT FROM AUTHOR]

Based on two-dimensional fully kinetic simulations that resolve the electron diffusion layer in undriven collisionless magnetic reconnection with zero guide field, this paper reports the existence and evolution of an inversion layer of bipolar electric fields, its corresponding phase-space structure (an electron-hole layer), and the implication to collisionless dissipation. The inversion electric field layer is embedded in the layer of bipolar Hall electric field and extends throughout the entire length of the electron diffusion layer. The electron phase-space hole structure spontaneously arises during the explosive growth phase when there exist significant inflows into the reconnection layer, and electrons perform meandering orbits across the layer while being cyclotron-turned toward the outflow directions. The cyclotron turning of meandering electrons by the magnetic field normal to the reconnection layer is shown to be a primary factor limiting the current density in the region where the reconnection electric field is balanced by the gradient (along the current sheet normal) of the off-diagonal electron pressure-tensor. [ABSTRACT FROM AUTHOR]

Observations from the Earth’s geomagnetic tail have established that the current sheet is often bifurcated with two peaks in the current density. These so-called bifurcated current sheets have also been reported in a variety of simulations and often occur in conjunction with significant temperature anisotropy. In this work, a new self-consistent Vlasov equilibrium is developed that permits both the current profile and temperature anisotropy to be independently adjusted. The stability of these layers with respect to the collisionless tearing mode is examined using both standard analytic techniques and a formally exact treatment involving a numerical evaluation of the full orbit integral. The resulting linear growth rate and mode structure are verified with fully kinetic particle-in-cell simulations. These results demonstrate that a bifurcated current profile has a strong stabilizing influence on the tearing mode in comparison to centrally peaked layers with a similar thickness. In contrast, electron temperature anisotropy is strongly destabilizing in the limit Te⊥>Te∥ and strongly stabilizing when Te⊥>Te∥. The simplified analytic theory is reasonably accurate in capturing these dependencies for long-wavelength modes, but the short-wavelength regime generally requires the full numerical treatment to accurately compute the growth rate. [ABSTRACT FROM AUTHOR]

One of the most fundamental questions in reconnection physics is how the dynamical evolution will scale to macroscopic systems of physical relevance. This issue is examined for electron-positron plasmas using two-dimensional fully kinetic simulations with both open and periodic boundary conditions. The resulting evolution is complex and highly dynamic throughout the entire duration. The initial phase is distinguished by the coalescence of tearing islands to larger scale while the later phase is marked by the expansion of diffusion regions into elongated current layers that are intrinsically unstable to plasmoid generation. It appears that the repeated formation and ejection of plasmoids plays a key role in controlling the average structure of a diffusion region and preventing the further elongation of the layer. The reconnection rate is modulated in time as the current layers expand and new plasmoids are formed. Although the specific details of this evolution are affected by the boundary and initial conditions, the time averaged reconnection rate remains fast and is remarkably insensitive to the system size for sufficiently large systems. This dynamic scenario offers an alternative explanation for fast reconnection in large-scale systems. [ABSTRACT FROM AUTHOR]

Kinetic simulations of magnetic reconnection typically employ periodic boundary conditions that limit the duration in which the results are physically meaningful. To address this issue, a new model is proposed that is open with respect to particles, magnetic flux, and electromagnetic radiation. The model is used to examine undriven reconnection in a neutral sheet initialized with a single x-point. While at early times the results are in excellent agreement with previous periodic studies, the evolution over longer intervals is entirely different. In particular, the length of the electron diffusion region is observed to increase with time resulting in the formation of an extended electron current sheet. As a consequence, the electron diffusion region forms a bottleneck and the reconnection rate is substantially reduced. Periodically, the electron layer becomes unstable and produces a secondary island, breaking the diffusion region into two shorter segments. After growing for some period, the island is ejected and the diffusion region again expands until a new island is formed. Fast reconnection may still be possible provided that the generation of secondary islands remains sufficiently robust. These results indicate that reconnection in a neutral sheet may be inherently unsteady and raise serious questions regarding the standard model of Hall mediated reconnection. [ABSTRACT FROM AUTHOR]

We performed single-channel, single-component random phase approximation calculations for dense He and Li plasmas for an electron temperature of kT=10 eV. Results are presented for excitation energies and oscillator strengths for the (1,2)s−np (n=2−5) transitions using both the ion-sphere and ion-correlation (IC) models. Our results indicate that ion correlations are not a significant effect for these systems and conditions. [Copyright &y& Elsevier]

The linear and nonlinear properties of the lower-hybrid drift instability are examined in a thin current sheet with thickness comparable to a thermal ion gyroradius ρ[sub i]∼L. The linear Vlasov stability is calculated using a formally exact technique in which the orbit integrals are treated numerically and the eigenvalue problem for the resulting system of integrodifferential equations is solved using a finite element representation of the eigenfunction. For the fastest growing lower-hybrid modes with wavelength on the electron gyroscale (k[sub y]ρ[sub e]∼1), the resulting mode structure is localized on the edge of the current sheet. However, for modes with wavelengths intermediate between the electron and ion gyroscale k[sub y]&ρ[sub i]ρ[sub e]radic; ∼ 1, the lower-hybrid instability has a significant electromagnetic component to the mode structure which is localized in the central region of the sheet. The addition of a weak guide field complicates the mode structure and gives rise to fluctuations in all three components of the magnetic field. These new predictions from linear Vlasov theory are confirmed using fully kinetic particle-in-cell simulations which indicate the modes saturate at large amplitude in the central region of the sheet. These results suggest the possibility that the electromagnetic fluctuations may potentially influence the development of magnetic reconnection. [ABSTRACT FROM AUTHOR]

Observations indicate that the current sheet in the Earth's geomagnetic tail may compress to a thickness comparable to an ion gyro-radius prior to substorm onset. In recent years, there has been considerable controversy regarding the kinetic stability of these thin structures. In particular, the growth rate of the kink instability and its relevance to magnetotail dynamics is still being debated. In this work, a series of fully kinetic particle-in-cell simulations are performed for a thin Harris sheet. The ion to electron mass ratio is varied between m[sub i]/m[sub e]=4 → 400 and careful comparisons are made with a formally exact approach to the linear Vlasov theory. At low mass ratio m[sub i]/m[sub e] < 64, the simulations are in excellent agreement with the linear theory, but at high mass ratio the kink instability is observed to grow more rapidly in the kinetic simulations than predicted by theory. The resolution to this apparent discrepancy involves the lower hybrid instability which is active on the edge of the sheet and rapidly produces nonlinear modifications to the initial equilibrium. The nature of this nonlinear deformation is characterized and a simple model is proposed to explain the physics. After the growth and saturation of the lower hybrid fluctuations, the deformed current sheet is similar in structure to a Harris equilibrium with an additional background population. This may explain the large growth rate of the kink instability at later times, since this type of modification to the Harris sheet has been shown to greatly enhance the growth rate of the kink mode. [ABSTRACT FROM AUTHOR]

Examines the linear stability of a Harris current sheet using the Vlasov description for ions and electrons. Solution of the eigenvalue problem for the system of integro-differential equations; Relevance to the stability of the Earth's geomagnetic tail.

In strongly magnetized astrophysical plasma systems, magnetic reconnection is believed to be the primary process during which explosive energy release and particle acceleration occur, leading to significant high-energy emission. Past years have witnessed active development of kinetic modeling of relativistic magnetic reconnection, supporting this magnetically dominated scenario. A much less explored issue in studies of relativistic reconnection is the consequence of three-dimensional dynamics, where turbulent structures are naturally generated as various types of instabilities develop. This paper presents a series of three-dimensional, fully kinetic simulations of relativistic turbulent magnetic reconnection (RTMR) in positronâ€"electron plasmas with system domains much larger than kinetic scales. Our simulations start from a force-free current sheet with several different modes of long-wavelength magnetic field perturbations, which drive additional turbulence in the reconnection region. Because of this, the current layer breaks up and the reconnection region quickly evolves into a turbulent layer filled with coherent structures such as flux ropes and current sheets. We find that plasma dynamics in RTMR is vastly different from its 2D counterpart in many aspects. The flux ropes evolve rapidly after their generation, and can be completely disrupted by the secondary kink instability. This turbulent evolution leads to superdiffusive behavior of magnetic field lines as seen in MHD studies of turbulent reconnection. Meanwhile, nonthermal particle acceleration and the timescale for energy release can be very fast and do not depend strongly on the turbulence amplitude. The main acceleration mechanism is a Fermi-like acceleration process supported by the motional electric field, whereas the nonideal electric field acceleration plays a subdominant role. We also discuss possible observational implications of three-dimensional RTMR in high-energy astrophysics. [ABSTRACT FROM AUTHOR]

Describes the linear stability and eventual nonlinear saturation of electrostatic instabilities due to the shear in a plasma flow in the direction parallel to the magnetic field. Use of collisional fluid equations; View of saturation amplitude as a function of the ratio of the ideal growth rate to the diamagnetic frequency.

The structure of the electron diffusion region (EDR), fundamental to a comprehensive theory of magnetic reconnection, is highly sensitive to asymmetry in the plasma parameters across the reconnection current layer. Using known scaling laws for the inflow region electron pressure anisotropy, a model is developed to predict the formation of extended electron jets within the EDR. Confirmed by kinetic simulation, the analysis shows how embedded electron jets are driven for scenarios fulfilling a generalized symmetry conditions based on the marginal firehose condition. Key Points: Formation of reconnection region electron jets requires the electron firehose parameter to be symmetric across the EDRA small density asymmetry eliminates electron jets within the EDRThe electron jets can be restored by an asymmetric temperature enhancing the asymmetry in the electron pressure, while restoring symmetry in the firehose parameter [ABSTRACT FROM AUTHOR]

Plasma energization through magnetic reconnection in the magnetically dominated regime featured by low plasma beta (β = 8πnkT0/B2 ⪡ 1) and/or high magnetization (σ = B2/(4πnmc2) ⪢ 1) is important in a series of astrophysical systems such as solar flares, pulsar wind nebula, and relativistic jets from black holes. In this paper, we review the recent progress on kinetic simulations of this process and further discuss plasma dynamics and particle acceleration in a low-β reconnection layer that consists of electron-positron pairs. We also examine the effect of different initial thermal temperatures on the resulting particle energy spectra. While earlier papers have concluded that the spectral index is smaller for higher σ, our simulations show that the spectral index approaches p=1 for sufficiently low plasma β, even if σ ∼ 1. Since this predicted spectral index in the idealized limit is harder than most observations, it is important to consider effects that can lead to a softer spectrum such as open boundary simulations. We also remark that the effects of three-dimensional reconnection physics and turbulence on reconnection need to be addressed in the future. [ABSTRACT FROM AUTHOR]

Magnetospheric Multiscale observed an extended current layer concurrent with a strong parallel electric field. The current layer is embedded in the reconnection exhaust and is consistent with an extended magnetized jet expected in roughly symmetric moderate guide field reconnection. The strong parallel electric field observed at the jet balances a strong gradient in the parallel electron pressure caused by a transition between magnetized electrons with Te‖ > Te⊥ on one side of the jet and demagnetized, roughly isotropic electrons across the thin current layer. Simulation results show how this transition can occur and how the associated electron pressure gradients in Ohm's law balance the parallel electric field. Plain Language Summary: NASA's Magnetospheric Multiscale (MMS) mission is a group of four satellites taking measurements in the plasma surrounding Earth to study magnetic reconnection, among other things. Magnetic reconnection is a topological change in magnetic field lines that occurs across a current sheet. MMS observed an extended current layer concurrent with a strong electric field parallel to the local magnetic field. The current layer is embedded in the reconnection exhaust and is consistent with an extended magnetized jet previously shown in computational studies. Using a kinetic simulation, we show that the strong parallel electric field observed at the jet location can be the result of a transition between two electron populations on either side of the jet. On one side, the electrons are well‐magnetized, and their energies are preferentially aligned with the magnetic field. On the other side, electrons become demagnetized at some point, making the direction of their velocities roughly uniform. The parallel electric field arises to balance the transition between these two populations. Key Points: Magnetospheric Multiscale observed an elongated current sheet that is regulated by the marginal electron firehose conditionA strong parallel E field concurrent with the current sheet is caused by a transition between isotropic and anisotropic electron populationsA kinetic simulation shows that parallel pressure divergence balances a strong parallel electric field across an extended current layer [ABSTRACT FROM AUTHOR]

Using fully kinetic simulations, we study the scaling of the inflow speed of collisionless magnetic reconnection in electron-positron plasmas from the nonrelativistic to ultrarelativistic limit. In the antiparallel configuration, the inflow speed increases with the upstream magnetization parameter o and approaches the speed of light when σ > O(100), leading to an enhanced reconnection rate. In all regimes, the divergence of the pressure tensor is the dominant term responsible for breaking the frozen-in condition at the x line. The observed scaling agrees well with a simple model that accounts for the Lorentz contraction of the plasma passing through the diffusion region. The results demonstrate that the aspect ratio of the diffusion region, modified by the compression factor of proper density, remains ∼0.1 in both the nonrelativistic and relativistic limits. [ABSTRACT FROM AUTHOR]

Using fully kinetic simulations, we demonstrate that magnetic reconnection in relativistic plasmas is highly efficient at accelerating particles through a first-order Fermi process resulting from the curvature drift of particles in the direction of the electric field induced by the relativistic flows. This mechanism gives rise to the formation of hard power-law spectra in parameter regimes where the energy density in the reconnecting field exceeds the rest mass energy density σ = B2/ (4πnmec2) > 1 and when the system size is sufficiently large. In the limit σ » 1, the spectral index approaches p = 1 and most of the available energy is converted into nonthermal particles. A simple analytic model is proposed which explains these key features and predicts a general condition under which hard power-law spectra will be generated from magnetic reconnection. [ABSTRACT FROM AUTHOR]

The concept of reconnection is found in many fields of physics with the closest analogue to magnetic reconnection being the reconnection of vortex tubes in hydrodynamics. In plasmas, magnetic reconnection plays an important role in release of energy associated with the magnetic shear into particle energy. Although most studies to date have focused on 2D reconnection, the availability of 3D petascale kinetic simulations have brought the complexity of 3D reconnection to the forefront in collisionless reconnection studies. Here we briefly review the latest advances in 2D and compare and contrast the results with recent 3D studies that address role of anomalous transport in reconnection, effects of turbulence on the rate and structure, among others. Another outcome of recent research is the realization of a deeper link between turbulence and reconnection where the common denominator is the generic formation of electron scale sheets which dissipate the energy through reconnection. Finally, we close the review by listing some of the major outstanding problems in reconnection physics. [ABSTRACT FROM AUTHOR]

From spacecraft data, it is evident that electron pressure anisotropy develops in collisionless plasmas. This is in contrast to the results of theoretical investigations, which suggest this anisotropy should be limited. Common for such theoretical studies is that the effects of electron trapping are not included; simply speaking, electron trapping is a non-linear effect and is, therefore, eliminated when utilizing the standard methods for linearizing the underlying kinetic equations. Here, we review our recent work on the anisotropy that develops when retaining the effects of electron trapping. A general analytic model is derived for the electron guiding center distribution [formula] of an expanding flux tube. The model is consistent with anisotropic distributions observed by spacecraft, and is applied as a fluid closure yielding anisotropic equations of state for the parallel and perpendicular components (relative to the local magnetic field direction) of the electron pressure. In the context of reconnection, the new closure accounts for the strong pressure anisotropy that develops in the reconnection regions. It is shown that for generic reconnection in a collisionless plasma nearly all thermal electrons are trapped, and dominate the properties of the electron fluid. A new numerical code is developed implementing the anisotropic closure within the standard two-fluid framework. The code accurately reproduces the detailed structure of the reconnection region observed in fully kinetic simulations. These results emphasize the important role of pressure anisotropy for the reconnection process. In particular, for reconnection geometries characterized by small values of the normalized upstream electron pressure, βe∞, the pressure anisotropy becomes large with p∥>p⊥ and strong parallel electric fields develop in conjunction with this anisotropy. The parallel electric fields can be sustained over large spatial scales and, therefore, become important for electron acceleration. [ABSTRACT FROM AUTHOR]

Abstract: We construct and investigate a set of stochastic differential equations that incorporate the physics of velocity-dependent small-angle Coulomb collisions among the plasma particles in a particle-in-cell simulation. Each particle is scattered stochastically from all the other particles in a simulation cell modeled as one or more Maxwellians. Total energy and momentum are conserved by linear transformation of the velocity increments. In two test simulations the proposed “particle-moment” collision algorithm performs well with time steps as large as 10% of the relaxation time – far larger than a particle-pairing collision algorithm, in which pairs of particles are scattered from one another, requires to achieve the same accuracy. [Copyright &y& Elsevier]

The relaxation of field-line tension during magnetic reconnection gives rise to a universal Fermi acceleration process involving the curvature drift of particles. However, the efficiency of this mechanism is limited by the trapping of energetic particles within flux ropes. Using 3D fully kinetic simulations, we demonstrate that the flux-rope kink instability leads to strong field-line chaos in weak-guide-field regimes where the Fermi mechanism is most efficient, thus allowing particles to transport out of flux ropes and undergo further acceleration. As a consequence, both ions and electrons develop clear power-law energy spectra that contain a significant fraction of the released energy. The low-energy bounds are determined by the injection physics, while the high-energy cutoffs are limited only by the system size. These results have strong relevance to observations of nonthermal particle acceleration in space and astrophysics. [ABSTRACT FROM AUTHOR]

Analyzes plasma and magnetic field data from February and March 1995 of the Ulysses mission in terms of the core/halo electron model to yield scaling relations of dimensionless electron parameters and empirical upper bounds on the dimensional heat flux. Similarity of empirical bounds and theoretical thresholds.

Magnetic reconnection in strongly magnetized astrophysical plasma environments is believed to be the primary process for fast energy release and particle energization. Currently, there is strong interest in relativistic magnetic reconnection in that it may provide a new explanation for high-energy particle acceleration and radiation in strongly magnetized astrophysical systems. We review recent advances in particle acceleration and reconnection physics in the magnetically dominated regime. Much discussion is focused on the physics of particle acceleration and power-law formation as well as the reconnection rate problem. In addition, we provide an outlook for studying reconnection acceleration mechanisms and kinetic physics in the next step. [ABSTRACT FROM AUTHOR]

A valid fluid model for electrons in collisionless space plasmas is desirable for understanding the structure and evolution of magnetic reconnection geometries. Additionally, such a fluid model would be useful for the simulation of systems too large to be tractable in a fully kinetic model. Using Magnetospheric Multiscale spacecraft observations, we provide direct confirmation of the Lê et al., 2009 (https://doi.org/10.1103/PhysRevLett.102.085001) equations of state for the electron pressure tensor during guide field reconnection and demonstrate how the closure can be applied in efficient numerical simulation, yielding new physical insight to the electron heating problem. Furthermore, we use the Lê et al., 2009 equations of state to predict a scaling of electron heating in the exhaust comparable to the available observational data. Key Points: The anisotropic equations of state of Lê et al. are validated for the first time with spacecraft data of a reconnection eventHybrid simulations using the Lê et al., 2009 EoS for the electron fluid reproduce MMS measurements better than low mass ratio PIC simulationsScalings for peak electron heating in the outflow are presented for strong guide field reconnection [ABSTRACT FROM AUTHOR]

Kinetic plasma turbulence cascade spans multiple scales ranging from macroscopic fluid flow to subelectron scales. Mechanisms that dissipate large scale energy, terminate the inertial range cascade, and convert kinetic energy into heat are hotly debated. Here, we revisit these puzzles using fully kinetic simulation. By performing scale-dependent spatial filtering on the Vlasov equation, we extract information at prescribed scales and introduce several energy transfer functions. This approach allows highly inhomogeneous energy cascade to be quantified as it proceeds down to kinetic scales. The pressure work, -(P . ∇) . u, can trigger a channel of the energy conversion between fluid flow and random motions, which contains a collision-free generalization of the viscous dissipation in collisional fluid. Both the energy transfer and the pressure work are strongly correlated with velocity gradients. [ABSTRACT FROM AUTHOR]

As modeling of collisionless magnetic reconnection in most space plasmas with realistic parameters is beyond the capability of today's simulations, due to the separation between global and kinetic length scales, it is important to establish scaling relations in model problems so as to extrapolate to realistic scales. Recently, large scale particle-in-cell simulations of island coalescence have shown that the time averaged reconnection rate decreases with system size, while fluid systems at such large scales in the Hall regime have not been studied. Here, we perform the complementary resistive magnetohydrodynamic (MHD), Hall MHD, and two fluid simulations using a ten-moment model with the same geometry. In contrast to the standard Harris sheet reconnection problem, Hall MHD is insufficient to capture the physics of the reconnection region. Additionally, motivated by the results of a recent set of hybrid simulations which show the importance of ion kinetics in this geometry, we evaluate the efficacy of the ten-moment model in reproducing such results. [ABSTRACT FROM AUTHOR]

While the most important feature of magnetic reconnection is that it energizes plasma particles by converting magnetic energy to particle energy, the exact mechanisms by which this happens are yet to be determined despite a long history of reconnection research. Recently, we have reported our results on the energy conversion and partitioning in a laboratory reconnection layer in a short communication [Yamada et al., Nat. Commun. 5, 4474 (2014)]. The present paper is a detailed elaboration of this report together with an additional dataset with different boundary sizes. Our experimental study of the reconnection layer is carried out in the two-fluid physics regime where ions and electrons move quite differently. We have observed that the conversion of magnetic energy occurs across a region significantly larger than the narrow electron diffusion region. A saddle shaped electrostatic potential profile exists in the reconnection plane, and ions are accelerated by the resulting electric field at the separatrices. These accelerated ions are then thermalized by re-magnetization in the downstream region. A quantitative inventory of the converted energy is presented in a reconnection layer with a well-defined, variable boundary. We have also carried out a systematic study of the effects of boundary conditions on the energy inventory. This study concludes that about 50% of the inflowing magnetic energy is converted to particle energy, 2/3 of which is ultimately transferred to ions and 1/3 to electrons. Assisted by another set of magnetic reconnection experiment data and numerical simulations with different sizes of monitoring box, it is also observed that the observed features of energy conversion and partitioning do not depend on the size of monitoring boundary across the range of sizes tested from 1.5 to 4 ion skin depths. [ABSTRACT FROM AUTHOR]

This paper presents an alternative technique for solving the linearized Vlasov-Maxwell set of equations, in which the velocity dependence of the perturbed distribution function is described by means of an infinite series of orthogonal functions, chosen as Hermite polynomials. The orthogonality properties of such functions allow us to decompose the Vlasov equation into a set of infinite coupled linear equations. With a suitable truncation relation, the problem is transformed in an eigenvalue problem. This technique is based on solid but easy concepts, not attempting to evaluate the integration over the unperturbed trajectories and can be applied to any equilibrium. Although the solutions are approximate, because they neglect contributions of higher order coefficients of the series, the physical meaning of the low-order coefficients is clear. Furthermore the accuracy of the solution, which depends on the number of terms taken into account in the Hermite series, appears to be merely a problem of computational power. The method has been tested for a 1D Harris equilibrium, known to give rise to several instabilities like tearing, drift kink, and lower hybrid. The results are shown in agreement with those obtained by Daughton with a traditional technique based on the integration over unperturbed orbits. [ABSTRACT FROM AUTHOR]

Quiet direct simulation Monte Carlo (QDSMC) is a new particle simulation technique that is applicable to a broad range of applications where the underlying system dynamics obey Fokker Planck equations. These include hydrodynamics, radiation transport, magnetohydrodynamics, diffusion, and collisional kinetic plasmas. At the beginning of each time step in QDSMC, the weights and abscissas of Gaussian-Hermite quadrature are used to deterministically create particles to sample the random process. At the end of the time step, particles are gathered to the computational mesh to obtain updated distributions of conserved quantities on the mesh and then the particles are destroyed. The creation and destruction of particles allows arbitrary dynamical range to be accessed quiescently with only a small number of particles per computational cell. The application of QDSMC to the simulation of Coulomb collisions is considered in this report, and the method is demonstrated on problems involving the collisional relaxation of non-Maxwellian distributions. [ABSTRACT FROM AUTHOR]