Session CM10: Mini-Conference on Reconnection: Basic

Live

Magnetic reconnection is a primary driver of particle acceleration processes in space and astrophysical plasmas. One of the major unsolved problems in reconnection studies is nonthermal particle acceleration. Here we present results from 3D PIC simulations of low-$\beta$ magnetic reconnection. We find that a clear power-law energy spectrum with a power-law index 4 can form and persist throughout the 3D simulations. We show that 3D effects such as self-generated turbulence and chaotic magnetic field lines enable the transport of high-energy electrons across the reconnection layer and allow them to access several main acceleration regions. This leads to a sustained and nearly constant acceleration rate for electrons at different energies. We will present the results from simulations with different guide field and discuss the properties of the self-generated turbulence in 3D reconnection (e.g. spectrum and anisotropy). These results are important for understanding particle acceleration during magnetic reconnection in low-$\beta$ plasmas, such as solar corona and near-Sun solar wind.   

Live

A recent trend in reconnection research has been the study of the trans-relativistic regime where the energy density of the magnetic field is smaller or comparable to the energy density associated with the rest mass of protons, but much larger then the energy density associated with the rest mass of electrons. This parameter range is relevant for radiatively inefficient, geometrically thick, optically thin accretion discs discs around black holes that accrete well below the Eddington limit. Electrons are quickly accelerated to high energies and form non-thermal distributions. This talk focuses on the underlying mechanisms that determine the properties of this distribution and their relative importance. To investigate these we have performed a number of fully-kinetic simulations using different system sizes, guide fields and mass ratios.   

Live

We used particle-in-cell simulation to study magnetic reconnection in 3D collisionless relativistic electron-positron plasmas relevant to astrophysical sources such as pulsar wind nebulae and blazar jets. Although 3D reconnection is similar to 2D when the upstream magnetization is large (low plasma beta), significant differences emerge in the regime of moderate magnetization (beta near 1). For large magnetization, plasmoids grow and merge hierarchically in reconnection outflows for both 2D and 3D, and energy conversion rates and nonthermal particle acceleration (NTPA) are nearly identical in 2D and 3D. This holds for both weak and strong guide magnetic fields. However, for moderate magnetization and weak guide field, 3D reconnection is different; small plasmoids (flux ropes) form as in 2D, but instead of growing and merging, they dissipate in an increasingly turbulent current sheet. Magnetic energy release is slower in 3D, but eventually upstream magnetic energy is converted more completely to plasma energy, because little magnetic energy winds up in flux ropes. Despite the lower reconnection rate, NTPA remains robust in 3D, and possibly slightly enhanced. A stronger guide magnetic field, however, causes 3D reconnection to behave more similarly to 2D reconnection.   

Live

The plasmoid instability is known to mediate the transition to fast reconnection in collisional plasmas described by resistive magnetohydrodynamics (MHD). Due to the plasmoid instability, the reconnection layer becomes a chain of plasmoids connected by secondary current sheets in 2D and self-generated turbulent reconnection in 3D. For many large-scale systems of interest, e.g., the solar atmosphere, the transition may occur in the collisional regime at the early phase, but become collisionless as the current sheet fragmentation proceeds. It is crucial to capture this change in physics regimes during the transition. The Hall MHD model is generally considered the middle ground between the collisional MHD and the collisionless plasmas modeled by kinetic particle-in-cell(PIC) simulations. However, although Hall MHD captures some key physics of collisionless reconnection, it often leads to single-X-line reconnection in 2D after the onset of the plasmoid instability, significantly different from PIC simulation results. In this work, we will present the results of 3D Hall MHD simulations. We show that single-X-line reconnection is less likely in 3D compared to that in 2D. We will contrast the 3D Hall MHD results with the corresponding resistive MHD results and discuss the implications.   

Live

Large-scale reconnection, like the one causing solar flares, involves multiple scales. The size of the system is many orders of magnitude larger than electron skin depth or electron Larmor radius. Do micro-scales fundamentally affect large-scale reconnection rate? If yes, our methodologies to describe plasma using reduced descriptions, such as MHD, will have to change and MHD will have to be replaced with special treatment near current layers. If no, we can get away with using MHD in many large-scale physical systems, such as the Sun and solar corona. Resolving this question directly with numerical simulations is very challenging and may involve boxes up to ten times bigger than we can currently afford. We use simplistic planar current layers in a periodic box, unstable to oblique tearing and developing turbulent current layer. We look at the current layer evolution and change the box size with respect to the ion skin depth. Independently we change the mass ratio. Looking at this two-parameter space allows us to elucidate general trends in multi-scale turbulent reconnection. As the current layer thickness increases, the mixing rate of ion fluid approaches that of an electron fluid. The mixing surface for electron fluid is bi-fractal, on small scales it corresponds to a more vigorous Hall reconnection, dominated more by 2D X-points, while on large scales it looks more like 3D MHD type. Our best guess is that on large scales we may eventually recover MHD behavior.   

Live

Large-scale 3D kinetic simulations suggest that reconnection layers may fragment into a turbulent spectrum of interacting flux ropes, leading to many interacting reconnection sites. However, often the global flux changes connectivity across a single kinetic-scale layer, which may be unrealistic for very large applications. Furthermore, most simulations are initialized with highly extended kinetic-scale current sheets, which is not physically realistic, and precludes the possibility of reconnection occurring in much thicker layers. Here, we investigate several new approaches for driving turbulent reconnection in layers much thicker than the inertial scale. These simulations suggest several possible regimes, including slow turbulent diffusion within thicker layers, and the spontaneous collapse of these layers to a dominant separator at the kinetic scale.   

Live

We present results from experiments at the National Ignition Facility to study reconnection in large and highly-extended current sheets. Two highly-elongated plasma plumes were produced by tiling two rows of lasers, with magnetic fields generated in each plume by the Biermann battery effect. X-ray measurements provided estimates of local electron temperature and density scale length, which were also used to benchmark simulations. Detailed magnetic field observations, obtained from proton radiography using a DHe3 capsule implosion, reveal reconnection occurring in an extended, quasi-1D current sheet with large aspect ratio $\sim$100. The 1-D geometry allowed a rigorous and unique reconstruction of the magnetic field, which showed a reconnection current sheet that thinned down to a half-width close to the electron gyro-scale. Despite the large aspect ratio, a large fraction of the magnetic flux reconnected, suggesting fast reconnection supported by the non-gyrotropic electron pressure tensor.   

Live

We present simulations of recent experiments on magnetic reconnection between magnetized laser-produced plasmas at the National Ignition Facility. Two highly-elongated plasma plumes are produced by tiling two rows of lasers, with magnetic field generated in each plume by the Biermann battery effect, and collision of the two plumes drives magnetic reconnection. The experimental evolution is simulated with the particle-in-cell code PSC, which models the experiment ab initio, from the initial magnetic field generation by the Biermann effect at early times, through the formation of a thin current sheet when two plumes collide. Simulations were used to design the experiments, and predictions are compared to experimental observations of plasma and magnetic field evolution, including the formation of a thin current sheet close to the electron gyro-scale.   

Live

Referring to collisionless or weakly collisional plasma regimes such as those characteristic of Earth's magnetotail regions where the ratios of the plasma microscopic scale distances to the macroscopic scale distances are extremely small, current theories of magnetic reconnection have the problem of involving unrealistically small reconnection layers. Given this issue and that relatively steep electron temperature gradients have been observed in these regions, recent developments in the theory of magneto-thermal reconnection [1] can be proposed as being of relevance to the physics of these regions. In this context purely oscillatory modes involving endogenous [1] magnetic reconnection have been identified for regimes where the longitudinal electron thermal conductivity is relatively large and the transverse (to the magnetic field) thermal conductivity is not negligible. In this case the width of the reconnection layer depends weakly on the ratio of the two thermal conductivities and increases with the involved macroscopic lengths as neither the electrical resistivity nor the electron inertia affect it directly. [1] B. Coppi and B. Basu, Phys. Plasmas \textbf{26}, 042115 (2019).