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.

Particle Acceleration by Magnetic Reconnection in Geospace

Particles are accelerated to very high, non-thermal energies during explosive energy-release phenomena in space, solar, and astrophysical plasma environments. While it has been established that magnetic reconnection plays an important role in the dynamics of Earth's magnetosphere, it remains unclear how magnetic reconnection can further explain particle acceleration to non-thermal energies. Here we review recent progress in our understanding of particle acceleration by magnetic reconnection in Earth's magnetosphere. With improved resolutions, recent spacecraft missions have enabled detailed studies of particle acceleration at various structures such as the diffusion region, separatrix, jets, magnetic islands (flux ropes), and dipolarization front. With the guiding-center approximation of particle motion, many studies have discussed the relative importance of the parallel electric field as well as the Fermi and betatron effects. However, in order to fully understand the particle acceleration mechanism and further compare with particle acceleration in solar and astrophysical plasma environments, there is a need for further investigation of, for example, energy partition and the precise role of turbulence.

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

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

Properties and Acceleration Mechanisms of Electrons Up To 200 keV Associated With a Flux Rope Pair and Reconnection X-Lines Around It in Earth's Plasma Sheet

The properties and acceleration mechanisms of electrons (<200 keV) associated with a pair of tailward traveling flux ropes and accompanied reconnection X-lines in Earth's plasma sheet are investigated with MMS measurements. Energetic electrons are enhanced on both boundaries and core of the flux ropes. The power-law spectra of energetic electrons near the X-lines and in flux ropes are harder than those on flux rope boundaries. Theoretical calculations show that the highest energy of adiabatic electrons is a few keV around the X-lines, tens of keV immediately downstream of the X-lines, hundreds of keV on the flux rope boundaries, and a few MeV in the flux rope cores. The X-lines cause strong energy dissipation, which may generate the energetic electron beams around them. The enhanced electron parallel temperature can be caused by the curvature-driven Fermi acceleration and the parallel electric potential. Betatron acceleration due to the magnetic field compression is strong on flux rope boundaries, which enhances energetic electrons in the perpendicular direction. Electrons can be trapped between the flux rope pair due to mirror force and parallel electric potential. Electrostatic structures in the flux rope cores correspond to potential drops up to half of the electron temperature. The energetic electrons and the electron distribution functions in the flux rope cores are suggested to be transported from other dawn-dusk directions, which is a 3-dimensional effect. The acceleration and deceleration of the Betatron and Fermi processes appear alternately indicating that the magnetic field and plasma are turbulent around the flux ropes.

Direct observations of pure electron outflow in magnetic reconnection

Magnetic reconnection is a universal process in space, astrophysical, and laboratory plasmas. It alters magnetic field topology and results in energy release to the plasma. Here we report the experimental results of a pure electron outflow in magnetic reconnection, which is not accompanied with ion flows. By controlling an applied magnetic field in a laser produced plasma, we have constructed an experiment that magnetizes the electrons but not the ions. This allows us to isolate the electron dynamics from the ions. Collective Thomson scattering measurements reveal the electron Alfvénic outflow without ion outflow. The resultant plasmoid and whistler waves are observed with the magnetic induction probe measurements. We observe the unique features of electron-scale magnetic reconnection simultaneously in laser produced plasmas, including global structures, local plasma parameters, magnetic field, and waves.

Three-dimensional network of filamentary currents and super-thermal electrons during magnetotail magnetic reconnection

Magnetic reconnection is a fundamental plasma process by which magnetic field lines on two sides of the current sheet flow inward to yield an X-line topology. It is responsible for producing energetic electrons in explosive phenomena in space, astrophysical, and laboratorial plasmas. The X-line region is supposed to be the important place for generating energetic electrons. However, how these energetic electrons are generated in such a limited region is still poorly understood. Here, using Magnetospheric multiscale mission data acquired in Earth’s magnetotail, we present direct evidence of super-thermal electrons up to 300 keV inside an X-line region, and the electrons display a power-law spectrum with an index of about 8.0. Concurrently, three-dimensional network of dynamic filamentary currents in electron scale is observed and leads to electromagnetic turbulence therein. The observations indicate that the electrons are effectively accelerated while the X-line region evolves into turbulence with a complex filamentary current network.

Magnetotail reconnection plays an important role in explosive energy conversion. Here, the authors show direct evidence of super-thermal electrons up to 300 keV within X-line region in Earth’s magnetotail, indicating effective electron acceleration due to turbulence.

Structure of the Current Sheet in the 11 July 2017 Electron Diffusion Region Event

The structure of the current sheet along the Magnetospheric Multiscale (MMS) orbit is examined during the 11 July 2017 Electron Diffusion Region (EDR) event. The location of MMS relative to the X-line is deduced and used to obtain the spatial changes in the electron parameters. The electron velocity gradient values are used to estimate the reconnection electric field sustained by nongyrotropic pressure. It is shown that the observations are consistent with theoretical expectations for an inner EDR in 2-D reconnection. That is, the magnetic field gradient scale, where the electric field due to electron nongyrotropic pressure dominates, is comparable to the gyroscale of the thermal electrons at the edge of the inner EDR. Our approximation of the MMS observations using a steady state, quasi-2-D, tailward retreating X-line was valid only for about 1.4 s. This suggests that the inner EDR is localized; that is, electron outflow jet braking takes place within an ion inertia scale from the X-line. The existence of multiple events or current sheet processes outside the EDR may play an important role in the geometry of reconnection in the near-Earth magnetotail.

Multiscale Currents Observed by MMS in the Flow Braking Region

We present characteristics of current layers in the off-equatorial near-Earth plasma sheet boundary observed with high time-resolution measurements from the Magnetospheric Multiscale mission during an intense substorm associated with multiple dipolarizations. The four Magnetospheric Multiscale spacecraft, separated by distances of about 50 km, were located in the southern hemisphere in the dusk portion of a substorm current wedge. They observed fast flow disturbances (up to about 500 km/s), most intense in the dawn-dusk direction. Field-aligned currents were observed initially within the expanding plasma sheet, where the flow and field disturbances showed the distinct pattern expected in the braking region of localized flows. Subsequently, intense thin field-aligned current layers were detected at the inner boundary of equatorward moving flux tubes together with Earthward streaming hot ions. Intense Hall current layers were found adjacent to the field-aligned currents. In particular, we found a Hall current structure in the vicinity of the Earthward streaming ion jet that consisted of mixed ion components, that is, hot unmagnetized ions, cold E × B drifting ions, and magnetized electrons. Our observations show that both the near-Earth plasma jet diversion and the thin Hall current layers formed around the reconnection jet boundary are the sites where diversion of the perpendicular currents take place that contribute to the observed field-aligned current pattern as predicted by simulations of reconnection jets. Hence, multiscale structure of flow braking is preserved in the field-aligned currents in the off-equatorial plasma sheet and is also translated to ionosphere to become a part of the substorm field-aligned current system.

Anisotropic Electron Tail Generation during Tearing Mode Magnetic Reconnection

The first experimental evidence of anisotropic electron energization during magnetic reconnection that favors a direction perpendicular to the guide magnetic field in a toroidal, magnetically confined plasma is reported in this Letter. Magnetic reconnection plays an important role in particle heating, energization, and transport in space and laboratory plasmas. In toroidal devices like the Madison Symmetric Torus, discrete magnetic reconnection events release large amounts of energy from the equilibrium magnetic field. Fast x-ray measurements imply a non-Maxwellian, anisotropic energetic electron tail is formed at the time of reconnection. The tail is well described by a power-law energy dependence. The expected bremsstrahlung from an electron distribution with an anisotropic energetic tail (v_{⊥}>v_{∥}) spatially localized in the core region is consistent with x-ray emission measurements. A turbulent process related to tearing fluctuations is the most likely cause for the energetic electron tail formation.

Nonthermal Electron Energization from Magnetic Reconnection in Laser-Driven Plasmas

The possibility of studying nonthermal electron energization in laser-driven plasma experiments of magnetic reconnection is studied using two- and three-dimensional particle-in-cell simulations. It is demonstrated that nonthermal electrons with energies more than an order of magnitude larger than the initial thermal energy can be produced in plasma conditions currently accessible in the laboratory. Electrons are accelerated by the reconnection electric field, being injected at varied distances from the X points, and in some cases trapped in plasmoids, before escaping the finite-sized system. Trapped electrons can be further energized by the electric field arising from the motion of the plasmoid. This acceleration gives rise to a nonthermal electron component that resembles a power-law spectrum, containing up to ∼8% of the initial energy of the interacting electrons and ∼24% of the initial magnetic energy. Estimates of the maximum electron energy and of the plasma conditions required to observe suprathermal electron acceleration are provided, paving the way for a new platform for the experimental study of particle acceleration induced by reconnection.

Stochastic acceleration by multi-island contraction during turbulent magnetic reconnection

The acceleration of charged particles in magnetized plasmas is considered during turbulent multi-island magnetic reconnection. The particle acceleration model is constructed for an ensemble of islands which produce adiabatic compression of the particles. The model takes into account the statistical fluctuations in the compression rate experienced by the particles during their transport in the acceleration region. The evolution of the particle distribution function is described as a simultaneous first- and second-order Fermi acceleration process. While the efficiency of the first-order process is controlled by the average rate of compression, the second-order process involves the variance in the compression rate. Moreover, the acceleration efficiency associated with the second-order process involves both the Eulerian properties of the compression field and the Lagrangian properties of the particles. The stochastic contribution to the acceleration is nonresonant and can dominate the systematic part in the case of a large variance in the compression rate. The model addresses the role of the second-order process, how the latter can be related to the large-scale turbulent transport of particles, and explains some features of the numerical simulations of particle acceleration by multi-island contraction during magnetic reconnection.

Energetic electrons associated with magnetic reconnection in the magnetic cloud boundary layer

Here is reported in situ observation of energetic electrons (∼100-500 keV) associated with magnetic reconnection in the solar wind by the ACE and Wind spacecraft. The properties of this magnetic cloud driving reconnection and the associated energetic electron acceleration problem are discussed. Further analyses indicate that the electric field acceleration and Fermi-type mechanism are two fundamental elements in the electron acceleration processes and the trapping effect of the specific magnetic field configuration maintains the acceleration status that increases the totally gained energy.

Comment on "Tail reconnection triggering substorm onset"

Angelopoulos et al. (Research Articles, 15 August 2008, p. 931) reported that magnetic reconnection in Earth's magnetotail triggered the onset of a magnetospheric substorm. We provide evidence that (i) near-Earth current disruption, occurring before the conventional tail reconnection signatures, triggered the onset; (ii) the observed auroral intensification and tail reconnection are not causally linked; and (iii) the onset they identified is a continuation of earlier substorm activities.

Two-scale structure of the electron dissipation region during collisionless magnetic reconnection

Particle-in-cell simulations of collisionless magnetic reconnection are presented that demonstrate that reconnection remains fast in very large systems. The electron dissipation region develops a distinct two-scale structure along the outflow direction. Consistent with fast reconnection, the length of the electron current layer stabilizes and decreases with decreasing electron mass, approaching the ion inertial length for a proton-electron plasma. Surprisingly, the electrons form a super-Alfvénic outflow jet that remains decoupled from the magnetic field and extends large distances downstream from the x line.

Formation of inner structure of a reconnection separatrix region

We present multipoint spacecraft observations at the dayside magnetopause of a magnetic reconnection separatrix region. This region separates two plasmas with significantly different temperatures and densities, at a large distance from the X line. We identify which terms in the generalized Ohm's law balance the observed electric field throughout the separatrix region. The electric field inside a thin approximately c/omega pi Hall layer is balanced by the j x B/ne term while other terms dominate elsewhere. On the low density side of the region we observe a density cavity which forms due to the escape of magnetospheric electrons along the newly opened field lines. The perpendicular electric field inside the cavity constitutes a potential jump of several kV. The observed potential jump and field aligned currents can be responsible for strong aurora.

Production of energetic electrons during magnetic reconnection

The production of energetic electrons during magnetic reconnection is explored with full particle simulations and analytic analysis. Density cavities generated along separatrices bounding growing magnetic islands support parallel electric fields that act as plasma accelerators. Electrons because of their low mass are fast enough to make multiple passes through these acceleration cavities and are therefore capable of reaching relativistic energies.

Evidence for electron acceleration up to approximately 300 keV in the magnetic reconnection diffusion region of earth's magnetotail

We report direct measurements of high-energy particles in a rare crossing of the diffusion region in Earth's magnetotail by the Wind spacecraft. The fluxes of energetic electrons up to approximately 300 keV peak near the center of the diffusion region and decrease monotonically away from this region. The diffusion region electron flux spectrum obeys a power law with an index of -3.8 above approximately 2 keV, and the electron angular distribution displays strong field-aligned bidirectional anisotropy at energies below approximately 2 keV, becoming isotropic above approximately 6 keV. These observations indicate significant electron acceleration inside the diffusion region. Ions show no such energization.