S. Dorfman

New insights into dissipation in the electron layer during magnetic reconnection

ELECTRON DISSIPATION IN RECONNECTION Detailed comparisons are reported between laboratory observations of electron scale dissipation layers near a reconnecting X-line and direct two-dimensional full-particle simulations. Many experimental features of the electron layers, such as insensitivity to the ion mass, are reproduced by the simulations; the layer thickness, however, is about 3 - 5 times larger than the predictions. Consequently, the leading candidate 2D mechanism based on collisionless electron nongyrotropic pressure is insuffcient to explain the observed reconnection rates. These results suggest that, in addition to the residual collisions, 3D effects play an important role in electron-scale dissipation during fast reconnection.

Three‐dimensional, impulsive magnetic reconnection in a laboratory plasma

Impulsive, local, 3-D reconnection is identified for the first time in a laboratory current sheet. The events observed in the Magnetic Reconnection Experiment (MRX) are characterized by large local gradients in the third direction and cannot be explained by 2-D models. Detailed measurements show that the ejection of flux rope structures from the current sheet plays a key role in these events. By contrast, even though electromagnetic fluctuations in the lower hybrid frequency range are also observed concurrently with the impulsive behavior, they are not the key physics responsible. A qualitative, 3-D, two-fluid model is proposed to explain the observations. The experimental results may be particularly applicable to space and astrophysical plasmas where impulsive reconnection occurs.

Experimental study of the Hall effect and electron diffusion region during magnetic reconnection in a laboratory plasma

The Hall effect during magnetic reconnection without an external guide field has been extensively studied in the laboratory plasma of the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)] by measuring its key signature, an out-of-plane quadrupole magnetic field, with magnetic probe arrays whose spatial resolution is on the order of the electron skin depth. The in-plane electron flow is deduced from out-of-plane magnetic field measurements. The measured in-plane electron flow and numerical results are in good agreement. The electron diffusion region is identified by measuring the electron outflow channel. The width of the electron diffusion region scales with the electron skin depth (∼5.5–7.5c∕ωpe) and the peak electron outflow velocity scales with the electron Alfven velocity (∼0.12–0.16VeA), independent of ion mass. The measured width of the electron diffusion region is much wider and the observed electron outflow is much slower than those obtained in 2D numerical simulati...

Two-dimensional fully kinetic simulations of driven magnetic reconnection with boundary conditions relevant to the Magnetic Reconnection Experiment

Two-dimensional fully kinetic simulations are performed using global boundary conditions relevant to model the Magnetic Reconnection Experiment (MRX) [M. Yamada et al., Phys Plasmas 4, 1936 (1997)]. The geometry is scaled in terms of the ion kinetic scales in the experiment, and a reconnection layer is created by reducing the toroidal current in the flux cores in a manner similar to the actual experiment. The ion-scale features in these kinetic simulations are in remarkable agreement with those observed in MRX, including the reconnection inflow rate and quadrupole field structure. In contrast, there are significant discrepancies in the simulated structure of the electron layer that remain unexplained. In particular, the measured thickness of the electron layers is 3–5 times thicker in MRX than in the kinetic simulations. The layer length is highly sensitive to downstream boundary conditions as well as the time over which the simulation is driven. However, for a fixed set of chosen boundary conditions, an ...

Electromagnetic instability of thin reconnection layers: Comparison of three-dimensional simulations with MRX observations

The influence of current-aligned instabilities on magnetic reconnection in weakly collisional regimes is investigated using experimental observations from Magnetic Reconnection Experiment (MRX) [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)] and large-scale fully kinetic simulations. In the simulations as well as in the experiment, the dominant instability is localized near the center of the reconnection layer, produces large perturbations of the magnetic field, and is characterized by the wavenumber that is a geometric mean between electron and ion gyroradii k∼(ρeρi)−1/2. However, both the simulations and the experimental observations suggest the instability is not the dominant reconnection mechanism under parameters typical of MRX.

Driven magnetic reconnection near the Dreicer limit

The influence of Coulomb collisions on the dynamics of driven magnetic reconnection in geometry mimicking the Magnetic Reconnection eXperiment (MRX) [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)] is investigated using two-dimensional (2D) fully kinetic simulations with a Monte Carlo treatment of the collision operator. For values of collisionality typical of MRX, the reconnection mechanism is shown to be a combination of collisionless effects, represented by off-diagonal terms in the electron stress tensor, and collisional momentum exchange between electrons and ions. The ratio of the reconnection electric field ER to the critical runaway field Ecrit provides a convenient measure of the relative importance of these two mechanisms. The structure of electron-scale reconnection layers in the presence of collisions is investigated in light of the previously reported [S. Dorfman et al., Phys. Plasmas 15, 102107 (2008)] discrepancy in the width of the electron reconnection layers between collisionless simulat...

Current disruption and its spreading in collisionless magnetic reconnection

Recent magnetic reconnection experiments (MRX) [Dorfman et al., Geophys. Res. Lett. 40, 233 (2013)] have disclosed current disruption in the absence of an externally imposed guide field. During current disruption in MRX, both the current density and the total observed out-of-reconnection-plane current drop simultaneous with a rise in out-of-reconnection-plane electric field. Here, we show that current disruption is an intrinsic property of the dynamic formation of an X-point configuration of magnetic field in magnetic reconnection, independent of the model used for plasma description and of the dimensionality (2D or 3D) of reconnection. An analytic expression for the current drop is derived from Amperes Law. Its predictions are verified by 2D and 3D electron-magnetohydrodynamic (EMHD) simulations. Three dimensional EMHD simulations show that the current disruption due to localized magnetic reconnection spreads along the direction of the electron drift velocity with a speed which depends on the wave number of the perturbation. The implications of these results for MRX are discussed.

Nonlinear Excitation of Acoustic Modes by Large-Amplitude Alfvén Waves in a Laboratory Plasma

The nonlinear three-wave interaction process at the heart of the parametric decay process is studied by launching counterpropagating Alfvén waves from antennas placed at either end of the Large Plasma Device. A resonance in the beat wave response produced by the two launched Alfvén waves is observed and is identified as a damped ion acoustic mode based on the measured dispersion relation. Other properties of the interaction including the spatial profile of the beat mode and response amplitude are also consistent with theoretical predictions for a three-wave interaction driven by a nonlinear ponderomotive force.

Experimental observation of 3-D, impulsive reconnection events in a laboratory plasma

Fast, impulsive reconnection is commonly observed in laboratory, space, and astrophysical plasmas. In this work, impulsive, local, 3-D reconnection is identified for the first time in a laboratory current sheet. The two-fluid, impulsive reconnection events observed on the Magnetic Reconnection Experiment (MRX) [Yamada et al., Phys Plasmas 4, 1936 (1997)] cannot be explained by 2-D models and are therefore fundamentally three-dimensional. Several signatures of flux ropes are identified with these events; 3-D high current density regions with O-point structure form during a slow buildup period that precedes a fast disruption of the reconnecting current layer. The observed drop in the reconnection current and spike in the reconnection rate during the disruption are due to ejection of these flux ropes from the layer. Underscoring the 3-D nature of the events, strong out-of-plane gradients in both the density and reconnecting magnetic field are found to play a key role in this process. Electromagnetic fluctuat...

Observation of an Alfvén Wave Parametric Instability in a Laboratory Plasma

A shear Alfvén wave parametric instability is observed for the first time in the laboratory. When a single finite ω/Ω_{i} kinetic Alfvén wave (KAW) is launched in the Large Plasma Device above a threshold amplitude, three daughter modes are produced. These daughter modes have frequencies and parallel wave numbers that are consistent with copropagating KAW sidebands and a low frequency nonresonant mode. The observed process is parametric in nature, with the frequency of the daughter modes varying as a function of pump wave amplitude. The daughter modes are spatially localized on a gradient of the pump wave magnetic field amplitude in the plane perpendicular to the background field, suggesting that perpendicular nonlinear forces (and therefore k_{⊥} of the pump wave) play an important role in the instability process. Despite this, modulational instability theory with k_{⊥}=0 has several features in common with the observed nonresonant mode and Alfvén wave sidebands.