Periodic Current Sheet Research Articles

PARTICLE ACCELERATION IN FRAGMENTING PERIODIC RECONNECTING CURRENT SHEETS IN SOLAR FLARES

Proton and electron acceleration in a fragmenting periodic current sheet (CS) is investigated, based on the forced magnetic reconnection scenario. The aim is to understand the role of CS fragmentation in high-energy beam generation in solar flares. We combine magnetohydrodynamics and test-particle models to consider particle trajectories consistent with a time-dependent reconnection model. It is shown that accelerated particles in such a model form two distinct populations. Protons and electrons moving in open magnetic field have energy spectra that are a combination of the initial Maxwellian distribution and a power-law high-energy (E>20 keV) part. The second population contains particles moving in a closed magnetic field around O-points. These particles move predominantly along the guiding field and their energies fall within quite a narrow range between ∼1 MeV and ∼10 MeV. It is also found that particles moving in an open magnetic field have a considerably wider pitch-angle distribution.

Relativistic filamentary equilibria

AbstractPlasma filamentation is often encountered in collisionless shocks and inertial confinement fusion. We develop a general analytical description of the two-dimensional relativistic filamentary equilibrium and derive the conditions for existence of potential-free equilibria. A pseudopotential equation for the vector-potential is constructed for cold and relativistic Maxwellian distributions. The role of counter-streaming is explained. We present single current sheet and periodic current sheet solutions, and analyze the equilibria with electric potential. These solutions can be used to study linear and nonlinear evolution of the relativistic filamentation instability.

Neoclassical tearing mode saturation in periodic current sheets

The saturation of Neoclassical Tearing Mode islands in a periodic slab configuration is investigated. Several theoretical models, all based on a generalization of Rutherford’s procedure, that aim at reducing the complete system to a single equation of the magnetic island width, are compared against numerical simulations. When the effects of the bootstrap current and of the second derivative of the equilibrium current profile are included, the numerical saturation levels are well matched with the predictions of this equation in a wide region of the stability diagram. However, the numerical results diverge from the standard theory when evaluating the threshold for nonlinear destabilization, since the theoretical value appears to be strongly conservative. In other words, the standard generalization of Rutherford’s equation is not able to capture the minimum value of the linear stability parameter and of the island width such that below them the Neoclassical Tearing Mode is always suppressed. To correct this discrepancy, a new theoretical model in which the transverse propagation of the island affects the bootstrap current term is proposed.

Properties of the tearing mode in periodic current sheets

The tearing mode in multiple periodic current sheet systems with a hyperbolic tangent profile of magnetic shear has been investigated. It is found that the linear growth rate of the antisymmetric and symmetric mode tearing instability increases and decreases, respectively, with the current sheet separation decreasing, which is similar to the situation with the step function profile of current density [A. Otto and G. T. Birk, Phys Fluids B 4, 3811 (1992)]. But, the results here also show that the linear growth rate of the antisymmetric mode tearing instability maximizes at certain current sheet separation. The linear growth of the tearing instability in multiple current sheet systems is not enhanced as greatly as has usually been expected.

Unsteady magnetic reconnection in non-periodic multiple current sheets

The process of magnetic reconnection in non-periodic three-layer current sheets is studied numerically by using two-dimensional magnetohydrodynamic simulation. The results show that unlike periodic current sheets, it is complex unsteady magnetic reconnection. It may be important for solar flare and corona heating.

An eddy current method for flaw characterization from spatially periodic current sheets : Nair, S.M.; Rose, J.H. Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), 23–28 Jul. 1989. Vol. 9A, pp. 265–272. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1990)

An eddy current method for flaw characterization from spatially periodic current sheets : Nair, S.M.; Rose, J.H. Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), 23–28 Jul. 1989. Vol. 9A, pp. 265–272. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1990)

Tearing mode instability in a multiple current sheet system

The tearing mode and magnetic reconnection are studied for multiple current sheet systems by two‐dimensional magnetohydrodynamic (MHD) simulations. Both the linear and nonlinear evolution of this process are analysed for laminar perturbations. The results illustrate the existence of a linear regime with a symmetric and an antisymmetric mode and agree with previous analytic results (Otto and Birk, 1992). The nonlinear evolution shows a number of interesting new features and may explain some properties in corresponding studies of turbulent reconnection. For wavelengths larger than twice the current sheet separation the evolution of antisymmetric modes leads to an entire reconfiguration of the magnetic field and converts a major portion of the magnetic energy into kinetic energy. Antisymmetric modes with smaller wavelengths and symmetric modes are found to saturate. The influence of the value of the resistivity on the reconnection rate decreases in the nonlinear evolution, and the ratio of current sheet separation to wavelength seems to be of major importance. A comparison of the dynamics of periodic current sheets with the evolution of only two current sheets indicates that some of the results for the periodic system also apply to the evolution of only two interacting current sheets. The results are discussed with respect to observations of large‐scale plasma and magnetic field reconfigurations in the magnetosheath and near the Earth's bow shock.

Resistive instability of periodic current sheets

The dispersion relation of the resistive tearing instability is derived and numerically solved for configurations characterized by periodic current sheets where a parameter-dependent resistivity can be included. Two unstable modes of different symmetry properties and different growth rates are identified, where the growth rates depend on the distance between the current layers. The results are relevant to a number of space and laboratory plasmas and the corresponding numerical simulations.