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Title: Fast collisionless reconnection in electron-positron plasmas

Abstract

Magnetic reconnection in electron-positron (pair) plasmas is studied by two-dimensional, electromagnetic, particle-in-cell simulations. In pair plasmas, the Hall current does not appear because there is no scale separation between ion and electron motion. Simulations show that fast reconnection is realized in both nonrelativistic and relativistic regimes. It is demonstrated that the divergence of the electron and positron pressure tensors essentially plays the role of an effective collisionless resistivity in realizing fast reconnection. Since there is no Hall current in pair plasmas, there is no quadrupolar structure in the out-of-plane magnetic field. Particle acceleration by reconnection is investigated. Major sites of particle acceleration are the regions in the vicinity of X lines, where ultrarelativistic particles are generated by the reconnection electric field.

Authors:
;  [1]
  1. Space Science Center and Center for Magnetic Self-Organization, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire 03824 (United States)
Publication Date:
OSTI Identifier:
20975076
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2714020; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ACCELERATION; ELECTRIC FIELDS; ELECTRONS; MAGNETIC FIELDS; MAGNETIC RECONNECTION; MAGNETOHYDRODYNAMICS; PLASMA PRESSURE; PLASMA SIMULATION; POSITRONS; RELATIVISTIC PLASMA; RELATIVISTIC RANGE; TENSORS; TWO-DIMENSIONAL CALCULATIONS

Citation Formats

Bessho, Naoki, and Bhattacharjee, A. Fast collisionless reconnection in electron-positron plasmas. United States: N. p., 2007. Web. doi:10.1063/1.2714020.
Bessho, Naoki, & Bhattacharjee, A. Fast collisionless reconnection in electron-positron plasmas. United States. doi:10.1063/1.2714020.
Bessho, Naoki, and Bhattacharjee, A. Tue . "Fast collisionless reconnection in electron-positron plasmas". United States. doi:10.1063/1.2714020.
@article{osti_20975076,
title = {Fast collisionless reconnection in electron-positron plasmas},
author = {Bessho, Naoki and Bhattacharjee, A.},
abstractNote = {Magnetic reconnection in electron-positron (pair) plasmas is studied by two-dimensional, electromagnetic, particle-in-cell simulations. In pair plasmas, the Hall current does not appear because there is no scale separation between ion and electron motion. Simulations show that fast reconnection is realized in both nonrelativistic and relativistic regimes. It is demonstrated that the divergence of the electron and positron pressure tensors essentially plays the role of an effective collisionless resistivity in realizing fast reconnection. Since there is no Hall current in pair plasmas, there is no quadrupolar structure in the out-of-plane magnetic field. Particle acceleration by reconnection is investigated. Major sites of particle acceleration are the regions in the vicinity of X lines, where ultrarelativistic particles are generated by the reconnection electric field.},
doi = {10.1063/1.2714020},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • 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 plasmoidsmore » 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.« less
  • In this Letter, we put forth (and validate numerically) a fluid-based analytical theory, which predicts that fast reconnection in nonrelativistic, low-{beta} pair plasmas is possible in collisionless regimes. This novel theoretical result complements recent kinetic computational evidence and challenges the accepted understanding, which considers fast dispersive waves (not supported in pair plasmas) as the key enabling physics ingredient for fast reconnection. The implications of this theory for the understanding of fast reconnection in standard electron-proton plasmas are discussed.
  • Two-dimensional particle-in-cell simulations have been performed to study magnetic reconnection in low-density electron-positron plasmas without a guide magnetic field. Impulsive reconnection rates become of the order of unity when the background density is much smaller than 10% of the density in the initial current layer. It is demonstrated that the outflow speed is less than the upstream Alfven speed, and that the time derivative of the density must be taken into account in the definition of the reconnection rate. The reconnection electric fields in the low-density regime become much larger than the ones in the high-density regime, and it ismore » possible to accelerate the particles to high energies more efficiently. The inertial term in the generalized Ohm's law is the most dominant term that supports a large reconnection electric field. An effective collisionless resistivity is produced and tracks the extension of the diffusion region in the late stage of the reconnection dynamics, and significant broadening of the diffusion region is observed. Because of the broadening of the diffusion region, no secondary islands, which have been considered to play a role to limit the diffusion region, are generated during the extension of the diffusion region in the outflow direction.« less
  • Magnetic reconnection and particle acceleration in relativistic Harris sheets in low-density electron-positron plasmas with no guide field have been studied by means of two-dimensional particle-in-cell simulations. Reconnection rates are of the order of one when the background density in a Harris sheet is of the order of 1% of the density in the current sheet, which is consistent with previous results in the non-relativistic regime. It has been demonstrated that the increase of the Lorentz factors of accelerated particles significantly enhances the collisionless resistivity needed to sustain a large reconnection electric field. It is shown analytically and numerically that themore » energy spectrum of accelerated particles near the X-line is the product of a power law and an exponential function of energy, {gamma}{sup -1/4}exp (- a{gamma}{sup 1/2}), where {gamma} is the Lorentz factor and a is a constant. However, in the low-density regime, while the most energetic particles are produced near X-lines, many more particles are energized within magnetic islands. Particles are energized in contracting islands by multiple reflection, but the mechanism is different from Fermi acceleration in magnetic islands for magnetized particles in the presence of a guide field. In magnetic islands, strong core fields are generated and plasma beta values are reduced. As a consequence, the fire-hose instability condition is not satisfied in most of the island region, and island contraction and particle acceleration can continue. In island coalescence, reconnection between two islands can accelerate some particles, however, many particles are decelerated and cooled, which is contrary to what has been discussed in the literature on particle acceleration due to reconnection in non-relativistic hydrogen plasmas.« less
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