William Scott Daughton

Fully kinetic simulations of undriven magnetic reconnection with open boundary conditions

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 eject...

Collisionless magnetic reconnection in large-scale electron-positron plasmas

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.

Current sheets and pressure anisotropy in the reconnection exhaust

A particle-in-cell simulation shows that the exhaust during anti-parallel reconnection in the collisionless regime contains a current sheet extending 100 inertial lengths from the X line. The current sheet is supported by electron pressure anisotropy near the X line and ion anisotropy farther downstream. Field-aligned electron currents flowing outside the magnetic separatrices feed the exhaust current sheet and generate the out-of-plane, or Hall, magnetic field. Existing models based on different mechanisms for each particle species provide good estimates for the levels of pressure anisotropy. The ion anisotropy, which is strong enough to reach the firehose instability threshold, is also important for overall force balance. It reduces the outflow speed of the plasma.

Do dispersive waves play a role in collisionless magnetic reconnection

Using fully kinetic simulations, we demonstrate that the properly normalized reconnection rate is fast ∼0.1 for guide fields up to 80× larger than the reconnecting field and is insensitive to both the system size and the ion to electron mass ratio. These results challenge conventional explanations of reconnection based on fast dispersive waves, which are completely absent for sufficiently strong guide fields. In this regime, the thickness of the diffusion layer is set predominantly by the electron inertial length with an inner sublayer that is controlled by finite gyro-radius effects. As the Alfven velocity becomes relativistic for very strong guide fields, the displacement current becomes important and strong deviations from charge neutrality occur, resulting in the build-up of intense electric fields which absorb a portion of the magnetic energy release. Over longer time scales, secondary magnetic islands are generated near the active x-line while an electron inertial scale Kelvin-Helmholtz instability is driven within the outflow. These secondary instabilities give rise to time variations in the reconnection rate but do not alter the average value.

Phase space structure of the electron diffusion region in reconnection with weak guide fields

Kinetic simulations of magnetic reconnection provide detailed information about the electric and magnetic structure throughout the simulation domain, as well as high resolution profiles of the essential fluid parameters including the electron and ion densities, flows, and pressure tensors. However, the electron distribution function, f(v), within the electron diffusion region becomes highly structured in the three dimensional velocity space and is not well resolved by the data available from the particle-in-cell (PIC) simulations. Here, we reconstruct the electron distribution function within the diffusion region at enhanced resolution. This is achieved by tracing electron orbits in the fields taken from PIC simulations back to the inflow region where an analytic form of the magnetized electron distribution is known. For antiparallel reconnection, the analysis reveals the highly structured nature of f(v), with striations corresponding to the number of times electrons have been reflected within the reconne...

New role of the lower-hybrid drift instability in the magnetic reconnection

Kinetic simulation results reveal that the growth of the lower-hybrid drift instability (LHDI) in current sheets has an important effect on the onset and nonlinear development of magnetic reconnection. The LHDI does this by heating electrons anisotropically, by increasing the peak current density, by producing current bifurcation, and by causing ion velocity shear. The role of these in magnetic reconnection is explained. Confidence in the results is strongly enhanced by agreement between implicit and massively-parallel-explicit particle-in-cell simulations.

Kinetic theory and simulation of collisionless tearing in bifurcated current sheets

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 temper...

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.

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

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

Electron energization during magnetic island coalescence

Radio emission from colliding coronal mass ejection flux ropes in the interplanetary medium suggested the local generation of superthermal electrons. Inspired by those observations, a fully kinetic particle-in-cell simulation of magnetic island coalescence models the magnetic reconnection between islands as a source of energetic electrons. When the islands merge, stored magnetic energy is converted into electron kinetic energy. The simulation demonstrates that a mechanism for electron energization originally applied to open field line reconnection geometries also operates near the reconnection site of merging magnetic islands. The electron heating is highly anisotropic, and it results mainly from an electric field surrounding the reconnection site that accelerates electrons parallel to the magnetic field. A detailed theory predicts the maximum electron energies and how they depend on the plasma parameters. In addition, the global motion of the magnetic islands launches low-frequency waves in the surrounding plasma, which induce large-amplitude, anisotropic fluctuations in the electron temperature.