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We compare hybrid (kinetic proton, fluid electron) and particle-in-cell (kinetic proton, kinetic electron) simulations of the solar wind termination shock with parameters similar to those observed by Voyager 2 during its crossing. The steady-state results show excellent agreement between the downstream variations in the density, plasma velocity, and magnetic field. The quasi-perpendicular shock accelerates interstellar pickup ions to a maximum energy limited by the size of the computational domain, with somewhat higher fluxes and maximal energies observed in the particle-in-cell simulation, likely due to differences in the cross-shock electric field arising from electron kinetic-scale effects. The higher fluxes may help address recent discrepancies noted between observations and large-scale hybrid simulations. [ABSTRACT FROM AUTHOR]

We have surveyed 21 reconnection exhaust events observed by Magnetospheric MultiScale in the low-plasma- β and high-Alfvén-speed regime of the Earth's magnetotail to investigate the scaling of electron bulk heating produced by reconnection. The ranges of inflow Alfvén speed and inflow electron β e covered by this study are 800–4000 km s−1 and 0.001–0.1, respectively, and the observed heating ranges from a few hundred electronvolts to several kiloelectronvolts. We find that the temperature change in the reconnection exhaust relative to the inflow, Δ T e, is correlated with the inflow Alfvén speed, V Ax,in, based on the reconnecting magnetic field and the inflow plasma density. Furthermore, Δ T e is linearly proportional to the inflowing magnetic energy per particle, m i V Ax , in 2 , and the best fit to the data produces the empirical relation Δ T e = 0.020 m i V Ax , in 2 , i.e., the electron temperature increase is on average ∼2% of the inflowing magnetic energy per particle. This magnetotail study extends a previous magnetopause reconnection study by two orders of magnitude in both magnetic energy and electron β, to a regime that is comparable to the solar corona. The validity of the empirical relation over such a large combined magnetopause–magnetotail plasma parameter range of V A ∼ 10–4000 km s−1 and β e ∼ 0.001–10 suggests that one can predict the magnitude of the bulk electron heating by reconnection in a variety of contexts from the simple knowledge of a single parameter: the Alfvén speed of the ambient plasma. [ABSTRACT FROM AUTHOR]

We present EUV solar observations showing evidence for omnipresent jetting activity driven by small-scale magnetic reconnection at the base of the solar corona. We argue that the physical mechanism that heats and drives the solar wind at its source is ubiquitous magnetic reconnection in the form of small-scale jetting activity (a.k.a. jetlets). This jetting activity, like the solar wind and the heating of the coronal plasma, is ubiquitous regardless of the solar cycle phase. Each event arises from small-scale reconnection of opposite-polarity magnetic fields producing a short-lived jet of hot plasma and Alfvén waves into the corona. The discrete nature of these jetlet events leads to intermittent outflows from the corona, which homogenize as they propagate away from the Sun and form the solar wind. This discovery establishes the importance of small-scale magnetic reconnection in solar and stellar atmospheres in understanding ubiquitous phenomena such as coronal heating and solar wind acceleration. Based on previous analyses linking the switchbacks to the magnetic network, we also argue that these new observations might provide the link between the magnetic activity at the base of the corona and the switchback solar wind phenomenon. These new observations need to be put in the bigger picture of the role of magnetic reconnection and the diverse form of jetting in the solar atmosphere. [ABSTRACT FROM AUTHOR]

We report Magnetospheric Multiscale four-spacecraft observations of a thin reconnecting current sheet with weakly asymmetric inflow conditions and a guide field of approximately twice the reconnecting magnetic field. The event was observed at the interface of interlinked magnetic field lines at the flank magnetopause when the maximum spacecraft separation was 370 km and the spacecraft covered ∼1.7 ion inertial lengths (di) in the reconnection outflow direction. The ion-scale spacecraft separation made it possible to observe the transition from electron-only super ion-Alfvénic outflow near the electron diffusion region (EDR) to the emergence of sub-Alfvénic ion outflow in the ion diffusion region (IDR). The EDR to IDR evolution over a distance less than 2 di also shows the transition from a near-linear reconnecting magnetic field reversal to a more bifurcated current sheet as well as significant decreases in the parallel electric field and dissipation. Both the ion and electron heating in this diffusion region event were similar to the previously reported heating in the far downstream exhausts. The dimensionless reconnection rate, obtained four different ways, was in the range of 0.13–0.27. This event reveals the rapid spatial evolution of the plasma and electromagnetic fields through the EDR to IDR transition region. [ABSTRACT FROM AUTHOR]

The reversibility of the transfer of energy from the magnetic field to the surrounding plasma during magnetic reconnection is examined. Trajectories of test particles in an analytic field model demonstrate that irreversibility is associated with separatrix crossings and passages through regions of weaker magnetic field. Inclusion of a guide field enhances the magnetization of particles and the extent to which forward and reverse trajectories overlap. Full kinetic simulations with a particle-in-cell code support these results and demonstrate that while time-reversed simulations at first "un-reconnect," they eventually evolve into a reconnecting state. [ABSTRACT FROM AUTHOR]

Waves around the lower hybrid frequency are frequently observed at Earth's magnetopause and readily reach very large amplitudes. Determining the properties of lower hybrid waves is crucial because they are thought to contribute to electron and ion heating, cross‐field particle diffusion, anomalous resistivity, and energy transfer between electrons and ions. All these processes could play an important role in magnetic reconnection at the magnetopause and the evolution of the boundary layer. In this paper, the properties of lower hybrid waves at Earth's magnetopause are investigated using the Magnetospheric Multiscale mission. For the first time, the properties of the waves are investigated using fields and direct particle measurements. The highest‐resolution electron moments resolve the velocity and density fluctuations of lower hybrid waves, confirming that electrons remain approximately frozen in at lower hybrid wave frequencies. Using fields and particle moments, the dispersion relation is constructed and the wave‐normal angle is estimated to be close to 90° to the background magnetic field. The waves are shown to have a finite parallel wave vector, suggesting that they can interact with parallel propagating electrons. The observed wave properties are shown to agree with theoretical predictions, the previously used single‐spacecraft method, and four‐spacecraft timing analyses. These results show that single‐spacecraft methods can accurately determine lower hybrid wave properties. Key Points: The velocity and density fluctuations of magnetopause lower hybrid waves are resolved, showing that electrons are approximately frozen inLower hybrid wave dispersion relation and wave‐normal angle are computed from fields and particle measurementsSingle‐ and multi‐spacecraft methods yield consistent lower hybrid wave properties, confirming the accuracy of single‐spacecraft methods [ABSTRACT FROM AUTHOR]

A new computational model, kglobal, is being developed to explore energetic electron production via magnetic reconnection in macroscale systems. The model is based on the discovery that the production of energetic electrons during reconnection is controlled by Fermi reflection in large-scale magnetic fields and not by parallel electric fields localized in kinetic scale boundary layers. Thus, the model eliminates these boundary layers. However, although the parallel electric fields that develop around the magnetic x-line and associated separatrices are not important in producing energetic electrons, there is a large scale electric field that kickstarts the heating of low-energy electrons and drives the cold-electron return current that accompanies escaping energetic electrons in open systems. This macroscale electric field is produced by magnetic-field-aligned gradients in the electron pressure. We have upgraded kglobal to include this large-scale electric field while maintaining energy conservation. The new model is tested by exploring the dynamics of electron acoustic modes which develop as a consequence of the presence of two electron species: hot kinetic and cold fluid electrons. Remarkably, the damping of electron acoustic modes is accurately captured by kglobal. Additionally, it has been established that kglobal correctly describes the dynamics of the interaction of the parallel electric field with escaping hot electrons through benchmarking simulations with the Particle-In-Cell code p3d. [ABSTRACT FROM AUTHOR]

We report Magnetospheric Multiscale observations of reconnection in a thin current sheet at the interface of interlinked flux tubes carried by converging reconnection jets at Earth's magnetopause. The ion skin depth‐scale width of the interface current sheet and the non‐frozen‐in ions indicate that Magnetospheric Multiscale crossed the reconnection layer near the X‐line, through the ion diffusion region. Significant pileup of the reconnecting component of the magnetic field in this and three other events on approach to the interface current sheet was accompanied by an increase in magnetic shear and decrease in Δβ, leading to conditions favorable for reconnection at the interface current sheet. The pileup also led to enhanced available magnetic energy per particle and strong electron heating. The observations shed light on the evolution and energy release in 3‐D systems with multiple reconnection sites. Plain Language Summary: The Earth and the solar wind magnetic fields interconnect through a process called magnetic reconnection. The newly reconnected magnetic field lines are strongly bent and accelerate particles, similar to a rubber band in a slingshot. In this paper we have used observations from NASA's Magnetospheric MultiScale spacecraft to investigate what happens when two of these slingshot‐like magnetic field lines move toward each other and get tangled up. We found that the two bent magnetic field lines tend to orient themselves perpendicular to each other as they become interlinked and stretched, similar to what rubber bands would do. This reorientation allows the interlinked magnetic fields to reconnect again, releasing part of the built‐up magnetic energy as strong electron heating. The results are important because they show how interlinked magnetic fields, which occur in many solar and astrophysics contexts, reconnect and produce enhanced electron heating, something that was not understood before. Key Points: Magnetic flux pileup observed upstream of reconnecting current sheet at the interface of converging reconnection jetsMagnetic flux pileup was accompanied by increase in magnetic shear and decrease in Δβ, leading to conditions favorable for reconnectionMagnetic flux pileup leads to enhanced available magnetic energy per particle and strong electron heating [ABSTRACT FROM AUTHOR]

The structure of dipolarization jets with finite width in the dawn‐dusk direction relevant to magnetic reconnection in the Earth's magnetotail is explored with particle‐in‐cell simulations. We carry out Riemann simulations of the evolution of the jet in the dawn‐dusk, north‐south plane to investigate the dependence of the jet structure on the jet width in the dawn‐dusk direction. We find that the magnetic field and Earth‐directed ion flow structure depend on the dawn‐dusk width. A reversal in the usual Hall magnetic field near the center of the current sheet on the duskside of larger jets is observed. For small widths, the maximum velocity of the earthward flow is significantly reduced below the theoretical limit of the upstream Alfvén speed. However, the ion flow speed approaches this limit once the width exceeds the ion Larmor radius based on the normal magnetic field, Bz. Plain Language Summary: Magnetic reconnection is a phenomenon occurring in the Earth's magnetotail in which energy stored in the magnetic field is converted into kinetic energy. This process creates a high‐speed dipolarization jet of plasma. However, the physical dimension of these jets in the dawn‐dusk direction is not well constrained. We study how the structure of dipolarization jets depends on their width in the dawn‐dusk direction and find that both the shape of the magnetic field and the profile of the ion outflow speed across the jet vary as a function of the jet width. In particular, the maximum ion outflow can fall well short of the theoretically expected outflow speed for small widths. Our results can be used by satellites to determine their positions relative to observed dipolarization jets and place limits on the jet widths. Key Points: The dependence of the structure of magnetotail dipolarization jets on cross‐tail width is studied through 2‐D Riemann simulationsFor small widths the jet velocity falls well below expectations based on predictions of conventional reconnection modelsIn wide jets the self‐generated Hall magnetic field reverses sign compared with traditional predictions on the dawn edge of the jet [ABSTRACT FROM AUTHOR]

Electron holes are electrostatic non-linear structures widely observed in the space plasma. In the present paper, we analyze the process of energy exchange between electrons trapped within electron hole, untrapped electrons, and an electron hole propagating in a weakly inhomogeneous magnetic field. We show that as the electron hole propagates into the region with stronger magnetic field, trapped electrons are heated due to the conservation of the first adiabatic invariant. At the same time, the electron hole amplitude may increase or decrease in dependence on properties of distribution functions of trapped and untrapped resonant electrons. The energy gain of trapped electrons is due to the energy losses of untrapped electrons and/or decrease of the electron hole energy. We stress that taking into account the energy exchange with untrapped electrons increases the lifetime of electron holes in inhomogeneous magnetic field. We illustrate the suggested mechanism for small-amplitude Schamel's [Phys. Scr. T2, 228-237 (1982)] electron holes and show that during propagation along a positive magnetic field gradient their amplitude should grow. Neglect of the energy exchange with untrapped electrons would result in the electron hole dissipation with only modest heating factor of trapped electrons. The suggested mechanism may contribute to generation of suprathermal electron fluxes in the space plasma. [ABSTRACT FROM AUTHOR]

Kinetic simulations of 3D collisionless magnetic reconnection with a guide field show a dramatic enhancement of energetic electron production when compared with 2D systems. In the 2D systems, electrons are trapped in magnetic islands that limit their energy gain, whereas in the 3D systems the filamentation of the current layer leads to a stochastic magnetic field that enables the electrons to access volume-filling acceleration regions. The dominant accelerator of the most energetic electrons is a Fermi-like mechanism associated with reflection of charged particles from contracting field lines. [ABSTRACT FROM AUTHOR]

A new regime of fast magnetic reconnection with an out-of-plane (guide) magnetic field is reported in which the key role is played by an electron pressure anisotropy described by the Chew-Goldberger-Low gyrotropic equations of state in the generalized Ohm's law, which even dominates the Hall term. A description of the physical cause of this behavior is provided and two-dimensional fluid simulations are used to confirm the results. The electron pressure anisotropy causes the out of- plane magnetic field to develop a quadrupole structure of opposite polarity to the Hall magnetic field and gives rise to dispersive waves. In addition to being important for understanding what causes reconnection to be fast, this mechanism should dominate in plasmas with low plasma beta and a high in-plane plasma beta with electron temperature comparable to or larger than ion temperature, so it could be relevant in the solar wind and some tokamaks. [ABSTRACT FROM AUTHOR]

The onset of the acceleration of ions during magnetic reconnection is explored via particle-in-cell simulations in the limit of a strong ambient guide field that self-consistently and simultaneously follow the motions of protons and a particles. Heating parallel to the local magnetic field during reconnection with a guide field is strongly reduced compared with the reconnection of anti-parallel magnetic fields. The dominant heating of thermal ions during guide field reconnection results from pickup behavior of ions during their entry into reconnection exhausts and dominantly produces heating perpendicular rather than parallel to the local magnetic field. Pickup behavior requires that the ion transit time across the exhaust boundary (with a transverse scale of the order of the ion sound Larmor radius) be short compared with the ion cyclotron period. This translates into a threshold in the strength of reconnecting magnetic field that favors the heating of ions with high mass-tocharge. A simulation with a broad initial current layer produces a reconnecting system in which the amplitude of the reconnecting magnetic field just upstream of the dissipation region increases with time as reconnection proceeds. The sharp onset of perpendicular heating when the pickup threshold is crossed is documented. A comparison of the time variation of the parallel and perpendicular ion heating with that predicted based on the strength of the reconnecting field establishes the scaling of ion heating with ambient parameters both below and above the pickup threshold. The relevance to observations of ion heating in the solar corona is discussed. [ABSTRACT FROM AUTHOR]

Magnetic islands or flux ropes produced by magnetic reconnection have been observed on the magnetopause, in the magnetotail, and in coronal current sheets. Particle-in-cell simulations of magnetic reconnection with a guide field produce elongated electron current layers that spontaneously produce secondary islands. Here, we explore the seed mechanism that gives birth to these islands. The most commonly suggested theory for island formation is the tearing instability. We demonstrate that in our simulations these structures typically start out, not as magnetic islands, but as electron flow vortices within the electron current sheet. When some of these vortices first form, they do not coincide with closed magnetic field lines, as would be the case if they were islands. Only after they have grown larger than the electron skin depth do they couple to the magnetic field and seed the growth of finite-sized islands. The streaming of electrons along the magnetic separatrix produces the flow shear necessary to drive an electron Kelvin-Helmholtz instability and produce the initial vortices. The conditions under which this instability is the dominant mechanism for seeding magnetic islands are explored. [ABSTRACT FROM AUTHOR]

The structure of shocks that form at the exhaust boundaries during collisionless reconnection of anti-parallel fields is studied using particle-in-cell (PIC) simulations and modeling based on the anisotropic magnetohydrodynamic equations. Large-scale PIC simulations of reconnection and companion Riemann simulations of shock development demonstrate that the pressure anisotropy produced by counterstreaming ions within the exhaust prevents the development of classical Petschek switch-off-slow shocks (SSS). The shock structure that does develop is controlled by the firehose stability parameter &eh;=1-μ0(P|-P⊥)/B2 through its influence on the speed order of the intermediate and slow waves. Here, P| and P⊥ are the pressure parallel and perpendicular to the local magnetic field. The exhaust boundary is made up of a series of two shocks and a rotational wave. The first shock takes &eh; from unity upstream to a plateau of 0.25 downstream. The condition &eh;=0.25 is special because at this value, the speeds of nonlinear slow and intermediate waves are degenerate. The second slow shock leaves &eh;=0.25 unchanged but further reduces the amplitude of the reconnecting magnetic field. Finally, in the core of the exhaust, &eh; drops further and the transition is completed by a rotation of the reconnecting field into the out-of-plane direction. The acceleration of the exhaust takes place across the two slow shocks but not during the final rotation. The result is that the outflow speed falls below that expected from the Walén condition based on the asymptotic magnetic field. A simple analytic expression is given for the critical value of &eh; within the exhaust below which SSSs no longer bound the reconnection outflow. [ABSTRACT FROM AUTHOR]

The anomalous drag, D, due to large amplitude plasma waves is used for the first time, in place of η*j, to estimate dissipation at the sub-solar magnetopause and to determine the extent to which this drag accounts for the reconnection electric field. This anomalous drag is determined by measuring correlations of the fluctuations in the electric field and plasma density. Large amplitude electric fields occurred more than 60% of the time in the more than 100 sub-solar, low latitude magnetopause crossings of the THEMIS satellite. They occurred mainly near the magnetospheric separatrix in the form of electrostatic lower hybrid and whistler waves. The anomalous drag at the separatrix was generally <10% of the average reconnection electric field, and it was <1% of the field in the current sheet. Thus, anomalous drag due to waves is not a significant driver of reconnection or of the required dissipation at the sub-solar magnetopause. [ABSTRACT FROM AUTHOR]

The propagation of reconnection signatures and their associated energy are examined using kinetic particle-in-cell simulations and Cluster satellite observations. It is found that the quadrupolar out-of-plane magnetic field near the separatrices is associated with a kinetic Alfvén wave. For magnetotail parameters, the parallel propagation of this wave is super-Alfvénic (V∥ ~ 1500-5500 km/s) and generates substantial Poynting flux (S -~ 10~-10~ W/m2) consistent with Cluster observations of magnetic reconnection. This Poynting flux substantially exceeds that due to frozen-in ion bulk outflows and is sufficient to generate white light aurora in Earth's ionosphere. [ABSTRACT FROM AUTHOR]

All current global models of the heliosphere are based on the assumption that the magnetic field in the heliosheath, in the region close to the heliopause (HP), is laminar. We argue that in that region the heliospheric magnetic field is not laminar but instead consists of magnetic bubbles. We refer to it as the bubble-dominated heliosheath region. Recently, we proposed that the annihilation of the "sectored" magnetic field within the heliosheath as it is compressed on its approach to the HP produces anomalous cosmic rays and also energetic electrons. As a product of the annihilation of the sectored magnetic field, densely packed magnetic islands (which further interact to form magnetic bubbles) are produced. These magnetic islands/bubbles will be convected with ambient flows as the sector region is carried to higher latitudes filling the heliosheath. We further argue that the magnetic islands/bubbles will develop upstream within the heliosheath. As a result, the magnetic field in the heliosheath sector region will be disordered well upstream of the HP. We present a three-dimensional MHD simulation with very high numerical resolution that captures the north-south boundaries of the sector region. We show that due to the high pressure of the interstellar magnetic field a north-south asymmetry develops such that the disordered sectored region fills a large portion of the northern part of the heliosphere with a smaller extension in the southern hemisphere. We suggest that this scenario is supported by the following changes that occurred around 2008 and from 2009.16 onward: (1) the sudden decrease in the intensity of low energy electrons (0.02-1.5 MeV) detected by Voyager 2, (2) a sharp reduction in the intensity of fluctuations of the radial flow, and (3) the dramatic differences in intensity trends between galactic cosmic ray electrons (3.8-59 MeV) at Voyager 1 and 2. We argue that these observations are a consequence of Voyager 2 leaving the sector region of disordered field during these periods and crossing into a region of unipolar laminar field. [ABSTRACT FROM AUTHOR]

It was recently shown that magnetic reconnection exhibits bistability, where the Sweet–Parker (collisional) and Hall (collisionless) reconnection solutions are both attainable for the same set of system parameters. Here, a dynamical model based on saddle-node bifurcations is presented which reproduces the slow to fast transition. It is argued that the properties of the dynamical model are a result of the Hall effect and the dispersive physics associated with it. Evidence from resistive two-fluid and Hall magnetohydrodynamics simulations are presented that show that the time evolution agrees with the dynamical model, the outflow speed is correlated with the dispersive physics due to the Hall effect, and bistability persists in the absence of electron inertia. [ABSTRACT FROM AUTHOR]

Kinetic simulation as well as in situ measurement of reconnecting current sheets in the Earth’s magnetosphere show strong electron temperature anisotropy, with the parallel electron temperature becoming several times greater than the perpendicular temperature. This anisotropy is accounted for in a solution of the Vlasov equation recently derived for general reconnection geometries with magnetized electrons in the limit of fast transit time. A necessary ingredient is a magnetic field-aligned electric field extending over the ion inertial length scale. The parallel electric field maintains quasineutrality by regulating the electron density, traps a large fraction of thermal electrons, and heats electrons in the parallel direction. Based on the expression for the electron phase-space density, equations of state provide a fluid closure for the electrons that relates the parallel and perpendicular pressures to the density and magnetic field strength. The resulting fluid model agrees well with fully kinetic simulations of guide-field reconnection, accurately predicting the electron temperature anisotropy. In addition, the equations of state impose strong constraints on the electron Hall currents and magnetic fields that develop during antiparallel reconnection. The model provides scaling laws for the Hall magnetic fields and predicts the magnitude of the current in the electron layer. [ABSTRACT FROM AUTHOR]

Sweet–Parker (collisional) magnetic reconnection at high Lundquist number is modified by secondary islands. Daughton et al. [Phys. Rev. Lett. 103, 065004 (2009)] suggested the Sweet–Parker model governs the fragmented current sheet segments. If true, the reconnection rate would increase by the square root of the number of secondary islands. High Lundquist number resistive magnetohydrodynamic simulations are presented which agree, in a time-averaged sense, with the predicted scaling. This result may have important implications for energy storage before a solar eruption and its subsequent release. [ABSTRACT FROM AUTHOR]

Recent full-particle simulations of electron-positron reconnection revealed that the Weibel instability plays an active role in controlling the dynamics of the current layer and maintaining fast reconnection. A four-beam model is developed to explore the development of the instability within a narrow current layer characteristic of reconnection. The problem is reduced to two coupled second-order differential equations, whose growing eigenmodes are obtained via both asymptotic approximations and finite difference methods. Full particle simulations confirm the linear theory and help probe the nonlinear development of the instability. The current layer broadening in the reconnection outflow jet is linked to the scattering of high-velocity streaming particles in the Weibel-generated, out-of-plane magnetic field. [ABSTRACT FROM AUTHOR]

The results of large-scale, particle-in-cell simulations are presented on the role of Hall electric and magnetic fields on the structure of the electron dissipation region and outflow exhaust during the collisionless magnetic reconnection of antiparallel fields. The simulations reveal that the whistler wave plays the key role in driving the electrons away from the magnetic x-line. Further downstream the electron outflow exhaust consists of a narrow super-Alfvénic jet, which remains collimated far downstream of the x-line, flanked by a pedestal whose width increases monotonically with increasing distance downstream. The open outflow exhaust, which is required for fast reconnection in large systems, is driven by the Hall electric and magnetic fields. Finally, it is the whistler that ultimately facilitates fast reconnection by diverting the electrons flowing toward the current layer into the outflow direction and thereby limiting the length of this layer. The results are contrasted with reconnection in an electron-positron plasma where the Hall fields are absent. The consequence of the expanding outflow exhaust is that, consistent with recent observations, the extended super-Alfvénic electron outflow jet carries a smaller and smaller fraction of the outflowing electrons with increasing distance downstream of the x-line. The results suggest that the structure of the electron current layer and exhaust in simulations might be sensitive to boundary conditions unless the simulation boundary along the outflow direction is sufficiently far from the x-line. [ABSTRACT FROM AUTHOR]

It was recently shown that the slow (collisional) Sweet-Parker and the fast (collisionless) Hall magnetic reconnection solutions simultaneously exist for a wide range of resistivities; reconnection is bistable [Cassak, Shay, and Drake, Phys. Rev. Lett., 95, 235002 (2005)]. When the thickness of the dissipation region becomes smaller than a critical value, the Sweet-Parker solution disappears and fast reconnection ensues, potentially explaining how large amounts of magnetic free energy can accrue without significant release before the onset of fast reconnection. Two-fluid numerical simulations extending the previous results for anti-parallel reconnection (where the critical thickness is the ion skin depth) to component reconnection with a large guide field (where the critical thickness is the thermal ion Larmor radius) are presented. Applications to laboratory experiments of magnetic reconnection and the sawtooth crash are discussed. [ABSTRACT FROM AUTHOR]

Magnetic island equilibria resulting from reconnection in magnetohydrodynamic (MHD) simulations are explored in a two-dimensional slab geometry. Magnetic islands are evolved to finite amplitude with a nonzero resistivity. The resistivity and flows are then set to zero and the system is allowed to relax toward equilibrium. A y-type singular current layer in the equilibrium state is identified for all but systems with the smallest values of the tearing mode stability parameter Δ′. It is shown that the length of the equilibrium y line tracks the length of the Sweet–Parker current layer that develops during reconnection. This suggests that the formation of Sweet–Parker current layers during magnetic reconnection in the resistive MHD model is a consequence of the presence of a singularity in post-reconnection state. A threshold in Δ′ for singular behavior is also identified. [ABSTRACT FROM AUTHOR]

The scaling of the reconnection rate is examined in situations in which the equilibrium current supporting a reversed magnetic field has a spatial scale length that is much greater than all nonmagnetohydrodynamic (non-MHD) kinetic scales. In this case, denoted as embedded reconnection, the narrow non-MHD region around the x-line where dissipation is important is embedded inside of a much larger equilibrium current sheet. In this system, the magnetic field just upstream of this non-MHD region, Bd changes significantly during the reconnection process. This wide equilibrium current sheet is contrasted with the very thin equilibrium current sheets of width c/ωpi used in previous simulations to establish the importance of the Hall term in Ohm's law in allowing fast reconnection in large scale collisionless systems. In the present study we lay out a procedure for determining Bd directly from simulation data and use this value to renormalize the reconnection rate using Sweet-Parker-like scaling arguments. Using two-dimensional two-fluid simulations, we find that the time evolution of the reconnection process can he broken into two phases: A developmental phase that is quite long and strongly dependent on system size and presumably the dissipation mechanisms, and a fast asymptotic phase in which the flow velocity into the x-line is on the order of 0.1 of the Alfvén speed based on Bd. The reconnection rate during the asymptotic phase is independent of system size and the majority of island growth and flux reconnection occurs during this phase. The time to reconnect a significant amount of magnetic flux is roughly consistent with solar flare timescales. [ABSTRACT FROM AUTHOR]

Studies a self-consistent nonlinear evolution and saturation of the dynamo, including the back reaction of the magnetic field on the flow through the Lorentz JxB force. Simulation of the fully compressible magnetohydrodynamic equations; Turbulence of the saturated states; Ratio of magnetic to kinetic energy in the saturated state.

Demonstrates how pressure forces acting on electrons dramatically alter magnetic field line reconnection in high temperature plasmas. Physical scale length; Ion gyroradius based on the electron temperature; Resistive magnetohydrodynamic equations; Single dissipation layer of resistive MHD.

An analytical and numerical investigation has been completed of the nonlinear growth and saturation of long-wavelength ion-temperature-gradient-driven turbulence in a sheared magnetic field for the case of an isolated rational surface. The radial correlation length of the turbulence is found to be of order ρiLs/ Lt, with ρi the ion Larmor radius, Ls the magnetic shear length, and LT the temperature scale length. The scaling of the resulting anomalous cross-field diffusivity is χ⊥∝g(ηi)(ρ2i/LTLy) (Ls/LT)2ρivti, where g(ηi) is obtained from the numerical results. In the poloidal direction, the spectrum collapses to the longest wavelength, Ly, available. Explicit results for the function g(ηi) are presented for values of ηi ranging from just above marginal stability ηic to ηi=∞. Although the quasilinear temperature profile is held fixed, the transport rates are very small as a result of local flattening of the temperature profile near the rational surface. The implications of these results for understanding anomalous transport in tokamaks are discussed. [ABSTRACT FROM AUTHOR]

The structure and growth rate of the radiative instability in a sheared magnetic field B have been calculated analytically using the Braginskii fluid equations. In a shear layer, temperature and density perturbations are linked by the propagation of sound waves parallel to the local magnetic field. As a consequence, density clumping or condensation plays an important role in driving the instability. Parallel thermal conduction localizes the mode to a narrow layer where k|| =k·B/|B| is small and stabilizes short wavelengths k>kc, where kc depends on the local radiation and conduction rates. Thermal coupling to ions also limits the width of the unstable spectrum. It is shown that a broad spectrum of modes is typically unstable in tokamak edge plasmas and it is argued that this instability is sufficiently robust to drive the large-amplitude density fluctuations often measured there. [ABSTRACT FROM AUTHOR]

The dynamics of weakly magnetized collisionless plasmas in the presence of an imposed temperature gradient along an ambient magnetic field is explored with particle-in-cell simulations and modeling. Two thermal reservoirs at different temperatures drive an electron heat flux that destabilizes off-angle whistler-type modes. The whistlers grow to large amplitude, δB/B0≃1, and resonantly scatter the electrons, significantly reducing the heat flux. Surprisingly, the resulting steady-state heat flux is largely independent of the thermal gradient. The rate of thermal conduction is instead controlled by the finite propagation speed of the whistlers, which act as mobile scattering centers that convect the thermal energy of the hot reservoir. The results are relevant to thermal transport in high-ß astrophysical plasmas such as hot accretion flows and the intracluster medium of galaxy clusters. [ABSTRACT FROM AUTHOR]

Since April of 2010, Voyager 1 has been immersed in a region of near zero radial flows, where the solar wind seems to have stopped. The existence of this region contradicts current models that predict that the radial flows will go to zero only at the heliopause. These models, however, do not include the sector region (or include it in a kinematic fashion), where the solar magnetic field periodically reverses polarity. Here we show that the presence of the sector region in the heliosheath, where reconnection occurs, fundamentally alters the flows, giving rise to a Flow Transition Region (FTR), where the flow abruptly turns and the radial velocity becomes near zero or negative. We estimate, based on a simulation, that at the Voyager 1 location, the thickness of the FTR is around 7-11 AU. [ABSTRACT FROM AUTHOR]

A stationary equilibrium of a liquid metal flowing past a cylindrical magnetic cavity is presented. The cavity has an azimuthal magnetic field and can also have an axial field. The liquid metal flow can be maintained by a sufficiently high pressure head. The scheme could be used to support a flowing liquid wall for systems producing high heat fluxes. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Recent in situ observations of interplanetary coronal mass ejections (ICMEs) found signatures of reconnection exhausts in their interior or trailing edge. Whereas reconnection on the leading edge of an ICME would indicate an interaction with the coronal or interplanetary environment, this result suggests that the internal magnetic field reconnects with itself. In light of this data, we consider the stability properties of flux ropes first developed in the context of astrophysics, then further elaborated upon in the context of reversed field pinches (RFPs). It was shown that the lowest energy state of a flux rope corresponds to ∇ × B = λB with λ a constant, the so-called Taylor state. Variations from this state will result in the magnetic field trying to reorient itself into the Taylor state solution, subject to the constraints that the toroidal flux and magnetic helicity are invariant. In reversed field pinches, this relaxation is mediated by the reconnection of the magnetic field, resulting in a sawtooth crash. If we likewise treat the ICME as a flux rope, any deviation from the Taylor state will result in reconnection within the interior of the flux tube, in agreement with the observations by Gosling et al. Such a departure from the Taylor state takes place as the flux tube cross section alytically that this elongation results in a state which is no longer in the minimum energy Taylor state. We then present magnetohydrodynamic simulations of an elongated flux tube which has evolved away from the Taylor state and show that reconnection at many surfaces produces a complex stochastic magnetic field as the system evolves back to a minimum energy state configuration. [ABSTRACT FROM AUTHOR]

The partition of energy flux in magnetic reconnection is examined experimentally using Cluster satellite observations of collisionless reconnection in Earth's magnetotail. In this plasma regime, the dominant component of the energy flux is ion enthalpy flux, with smaller contributions from the electron enthalpy and heat flux and the ion kinetic energy flux. However, the Poynting flux is not negligible, and in certain parts of the ion diffusion region the Poynting flux in fact dominates. Evidence for earthward-tailward asymmetry is ascribed to the presence of Earth's dipole fields. [ABSTRACT FROM AUTHOR]