Numerical Simulations of Magnetic Reconnection in an Asymmetric Current Sheet (original) (raw)
Related papers
Regions associated with electron physics in asymmetric magnetic field reconnection
Geophysical Research Letters, 2009
Spatial relationships between regions containing signatures of electron physics in asymmetric reconnection with a guide field are examined using simulations and space observations that are in excellent agreement. These electron physics regions do not completely overlap, are not confined to sizes ~electron skin depth, and do not surround the X-line. The electron ideal Ohm's law, E+U e xB = 0, is violated over many ion inertia lengths in the outflow direction. Thus, pressure terms and, to a lesser extent, inertia terms in the Generalized Ohm's Law are important in producing electron physics on ion scales. Parallel electric fields that are necessary but not sufficient for reconnection are found in the simulation and in space on ion scales in the outflow direction. They account for ~10% of j·E, averaged over the current sheet. The electron exhaust jet exists over a shorter length for asymmetric reconnection than it does for symmetric reconnection.
Laboratory Study Of Magnetic Reconnection With A Density Asymmetry Across The Current Sheet
2014
The effects of a density asymmetry across the current sheet on anti-parallel magnetic reconnection are studied systematically in a laboratory plasma. Despite a significant density ratio of up to 10, the in-plane magnetic field profile is not significantly changed. On the other hand, the out-of-plane Hall magnetic field profile is considerably modified; it is almost bipolar in structure with the density asymmetry, as compared to quadrupolar in structure with the symmetric configuration. Moreover, the ion stagnation point is shifted to the low-density side, and the electrostatic potential profile also becomes asymmetric with a deeper potential well on the low-density side. Nonclassical bulk electron heating together with electromagnetic fluctuations in the lower hybrid frequency range is observed near the low-density-side separatrix. The dependence of the ion outflow and reconnection electric field on the density asymmetry is measured and compared with theoretical expectations. The measured ion outflow speeds are about 40% of the theoretical values.
Physics of Plasmas, 1999
Generation of anomalous resistivity and dynamical development of collisionless reconnection in the vicinity of a magnetically neutral sheet are investigated by means of a three-dimensional particle simulation. For no external driving source, two different types of plasma instabilities are excited in the current layer. The lower hybrid drift instability ͑LHDI͒ is observed to grow in the periphery of current layer in an early period, while a drift kink instability ͑DKI͒ is triggered at the neutral sheet in a late period as a result of the nonlinear deformation of the current sheet by the LHDI. A reconnection electric field grows at the neutral sheet in accordance with the excitation of the DKI. When an external driving field exists, the convective electric field penetrates into the current layer through the particle kinetic effect and collisionless reconnection is triggered by the convective electric field earlier than the DKI is excited. It is also found that the anisotropic ion distribution is formed through the anomalous ion heating by the DKI.
2005
Large-amplitude (up to $50 mV/m) solitary waves, identified as electron holes, have been observed during waveform captures on two of the four Cluster satellites during several plasma sheet encounters that have been identified as the passage of a magnetotail reconnection x line. The electron holes were seen near the outer edge of the plasma sheet, within and on the edge of a density cavity, at distances on the order of a few ion inertial lengths from the center of the current sheet. The electron holes occur during intervals when there were narrow electron beams but not when the distributions were more isotropic or contained beams that were broad in pitch angle. The region containing the narrow beams (and therefore the electron holes) can extend over thousands of kilometers in the x and y directions, but is very narrow in the z direction. The association with electron beams and the density cavity and the location along the separatrices are consistent with simulations shown herein. The velocities and scale sizes of the electron holes are consistent with the predictions of Drake et al. [2003]. Particle simulations of magnetic reconnection reproduce the observed Cluster data only with the addition of a small (0.2 of the reversed field) ambient guide field. The results suggest that electron holes may sometimes be an intrinsic feature of magnetotail reconnection and that in such cases the traditional neglect of the guide field may not be justified. Very large amplitude lower hybrid waves (hundreds of millivolts per meter), as well as waves at frequencies up to the electron plasma frequency, were also observed during this interval.
Journal of Plasma Physics, 2009
Electron and proton acceleration by a drifted super-Dreicer electric field is investigated in a strongly compressed non-neutral reconnecting current sheet (NRCS). The guiding field is assumed to be constant within an RCS and parallel to the direction of drifted electric field. The other two magnetic field components, transverse and tangential, are considered to vary exponentially and linearly with distances from the X null-point. The proton and electron energy spectra are calculated numerically in a model RCS with different magnetic field topologies by solving a equation of motion in the test particle approach with some test with Particle-in-cell (PIC) approach. Three kinds electric field generated inside an RCS are considered: a drifted electric field caused by the plasma inflows formed during a magnetic reconnection process, a polarization electric field induced by the accelerated protons and electrons and turbulent electric filed induced by instabilities generated by accelerated particles. Electron and proton densities, energy spectra inside an RCS and at ejection are found to be strongly affected by the magnetic field topology: for stronger magnetic fields the spectra are softer having a small higher energy cutoff while for weaker magnetic field the spectra are harder with much bigger upper cutoff energies. Depending on the magnetic component ratios and drifted electric field magnitude, particles are found ejected either as quasi-thermal flows with very high temperatures or as focused power law beams. A polarization field is found to reduce the acceleration time inside an RCS, to increase the energy gained by particles at acceleration by a pure drifted electric field by a few orders of magnitude. The turbulent electric field induced by the two beam instabilities of the same kind particles leads to a significant increase of the number of particles with higher energies resulting in a flattening of their energy spectra.
Numerical simulations of separatrix instabilities in collisionless magnetic reconnection
Physics of Plasmas, 2012
Graf von der Pahlen and Tsiklauri [Phys. Plasmas 21, 060705 (2014)] established that the generation of octupolar out-of-plane magnetic field structure in a stressed X-point collapse is due to ion currents. The field has a central region, comprising of the well-known quadrupolar field (quadrupolar components), as well as four additional poles of reversed polarity closer to the corners of the domain (octupolar components). In this extended work, the dependence of the octupolar structure on domain size and ion mass variation is investigated. Simulations show that the strength and spatial structure of the generated octupolar magnetic field is independent of ion to electron mass ratio; thus showing that ion currents play a significant role in out-of-plane magnetic structure generation in physically realistic scenarios. Simulations of different system sizes show that the width of the octupolar structure remains the same and has a spacial extent of the order of the ion inertial length. The width of the structure thus appears to be independent on boundary condition effects. The length of the octupolar structure, however, increases for greater domain sizes, prescribed by the external system size. This was found to be a consequence of the structure of the in-plane magnetic field in the outflow region halting the particle flow and thus terminating the in-plane currents that generate the out-of-plane field. The generation of octupolar magnetic field structure is also established in a tearing-mode reconnection scenario. The differences in the generation of the octupolar field and resulting qualitative differences between X-point collapse and tearing-mode are discussed. V
Bifurcated Structure of the Electron Diffusion Region in Three-Dimensional Magnetic Reconnection
Physical Review Letters, 2013
Three-dimensional kinetic simulations of magnetic reconnection reveal that the electron diffusion region is composed of two or more current sheets in regimes with weak magnetic shear angles φ < ∼ 80 •. This new morphology is explained by oblique tearing modes which produce flux ropes while simultaneously driving enhanced current at multiple resonance surfaces. This physics persists into the nonlinear regime leading to multiple electron layers embedded within a larger Alfvénic inflow and outflow. Surprisingly, the thickness of these layers and the reconnection rate both remain comparable to two-dimensional models. The parallel electric fields are supported predominantly by the electron pressure tensor and electron inertia, while turbulent dissipation remains small.
2014
Magnetic reconnection is a rapid rearrangement of magnetic line topology in a plasma that can allow magnetic energy to heat, drive macroscopic flows, or accelerate particles in space and laboratory plasmas. Though reconnection affects global plasma dynamics, it depends intimately on small-scale electron physics. In weakly-collisional plasmas, electron pressure anisotropy resulting from the electric and magnetic trapping of electrons strongly affects the structure surrounding the electron diffusion region and the electron current layer. Previous fluid models and simulations fail to account for this anisotropy. In this thesis, new equations of state that accurately describe the electron pressure anisotropy in cases of sufficiently strong guide magnetic field are implemented in fluid simulations and are compared to previous fluid models and kinetic simulations. Elongated current layers in the reconnection region, driven, in part, by this pressure anisotropy, appear as part of a self-re...
Electron and proton acceleration by a drifted super-Dreicer electric field is investigated in a strongly compressed non-neutral reconnecting current sheet (NRCS). The guiding field is assumed to be constant within an RCS and parallel to the direction of drifted electric field. The other two magnetic field components, transverse and tangential, are considered to vary exponentially and linearly with distances from the X null-point. The proton and electron energy spectra are calculated numerically in a model RCS with different magnetic field topologies by solving a equation of motion in the test particle approach with some test with Particle-in-cell (PIC) approach. Three kinds electric field generated inside an RCS are considered: a drifted electric field caused by the plasma inflows formed during a magnetic reconnection process, a polarization electric field induced by the accelerated protons and electrons and turbulent electric filed induced by instabilities generated by accelerated particles. Electron and proton densities, energy spectra inside an RCS and at ejection are found to be strongly affected by the magnetic field topology: for stronger magnetic fields the spectra are softer having a small higher energy cutoff while for weaker magnetic field the spectra are harder with much bigger upper cutoff energies. Depending on the magnetic component ratios and drifted electric field magnitude, particles are found ejected either as quasi-thermal flows with very high temperatures or as focused power law beams. A polarization field is found to reduce the acceleration time inside an RCS, to increase the energy gained by particles at acceleration by a pure drifted electric field by a few orders of magnitude. The turbulent electric field induced by the two beam instabilities of the same kind particles leads to a significant increase of the number of particles with higher energies resulting in a flattening of their energy spectra.
Asymmetric magnetic reconnection in the presence of a guide field
Journal of Geophysical Research: Space Physics, 2009
The properties of asymmetric magnetic reconnection in the presence of a guide magnetic field are investigated using two‐dimensional particle‐in‐cell simulations. The reconnection process is initiated by applying a spatially localized and temporally steady convection electric field at the high‐density/low‐magnetic‐field (magnetosheath) side of the current layer. The in‐plane Hall currents are dominated by the electron flows along the separatrices from the high‐density to low‐density side of the layer, and they strongly enhance the out‐of‐plane magnetic field in one hemisphere and decrease it in the other. On the enhanced magnetic field side are situated a bipolar pair of parallel electric fields and an electron velocity shear flow layer, both of which extend several ion inertia lengths (di) away from the X line. The shear flow layer is unstable to the generation of small‐scale (≪di) electron vortices which propagate away from the X line and produce a reduction of the order of 30% in ...