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Title: Catastrophic onset of fast magnetic reconnection with a guide field

Abstract

It was recently shown that the slow (collisional) Sweet-Parker and the fast (collisionless) Hall magnetic reconnection solutions simultaneously exist for a wide range of resistivities; reconnection is bistable [Cassak, Shay, and Drake, Phys. Rev. Lett., 95, 235002 (2005)]. When the thickness of the dissipation region becomes smaller than a critical value, the Sweet-Parker solution disappears and fast reconnection ensues, potentially explaining how large amounts of magnetic free energy can accrue without significant release before the onset of fast reconnection. Two-fluid numerical simulations extending the previous results for anti-parallel reconnection (where the critical thickness is the ion skin depth) to component reconnection with a large guide field (where the critical thickness is the thermal ion Larmor radius) are presented. Applications to laboratory experiments of magnetic reconnection and the sawtooth crash are discussed.

Authors:
; ;  [1];  [2]
  1. University of Maryland, College Park, Maryland 20742 (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
20975018
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2734948; (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; DEPTH; FREE ENERGY; IONS; LARMOR RADIUS; MAGNETIC RECONNECTION; MAGNETOHYDRODYNAMICS; MATHEMATICAL SOLUTIONS; PLASMA; PLASMA INSTABILITY; PLASMA SIMULATION; SAWTOOTH OSCILLATIONS

Citation Formats

Cassak, P. A., Drake, J. F., Shay, M. A., and University of Delaware, Newark, Delaware 19716. Catastrophic onset of fast magnetic reconnection with a guide field. United States: N. p., 2007. Web. doi:10.1063/1.2734948.
Cassak, P. A., Drake, J. F., Shay, M. A., & University of Delaware, Newark, Delaware 19716. Catastrophic onset of fast magnetic reconnection with a guide field. United States. doi:10.1063/1.2734948.
Cassak, P. A., Drake, J. F., Shay, M. A., and University of Delaware, Newark, Delaware 19716. Tue . "Catastrophic onset of fast magnetic reconnection with a guide field". United States. doi:10.1063/1.2734948.
@article{osti_20975018,
title = {Catastrophic onset of fast magnetic reconnection with a guide field},
author = {Cassak, P. A. and Drake, J. F. and Shay, M. A. and University of Delaware, Newark, Delaware 19716},
abstractNote = {It was recently shown that the slow (collisional) Sweet-Parker and the fast (collisionless) Hall magnetic reconnection solutions simultaneously exist for a wide range of resistivities; reconnection is bistable [Cassak, Shay, and Drake, Phys. Rev. Lett., 95, 235002 (2005)]. When the thickness of the dissipation region becomes smaller than a critical value, the Sweet-Parker solution disappears and fast reconnection ensues, potentially explaining how large amounts of magnetic free energy can accrue without significant release before the onset of fast reconnection. Two-fluid numerical simulations extending the previous results for anti-parallel reconnection (where the critical thickness is the ion skin depth) to component reconnection with a large guide field (where the critical thickness is the thermal ion Larmor radius) are presented. Applications to laboratory experiments of magnetic reconnection and the sawtooth crash are discussed.},
doi = {10.1063/1.2734948},
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}
}
  • The onset and saturation of collisionless magnetic reconnection in the presence of a guide field are investigated using two-dimensional particle-in-cell simulations in which the reconnection evolves out of the initial thermal noise in the current sheet and in which the resolution is sufficient to resolve the electron singular layer. The simulations show that reconnection does not abate when the island width exceeds either the electron singular layer or the initial current sheet width. Instead, reconnection proceeds through an explosive stage which appears to be limited only by the spatial size of the system. The guide-field reconnection dynamics is dominated bymore » the formation of an asymmetric configuration with a deep density cavity along one pair of separatrix arms. In this cavity an electron beam feature is formed which excites the Buneman instability. Near the X line the reconnection electric field is supported by a combination of quasiviscous and bulk inertia effects for the electrons. Around the island perimeter, intense Debye-scale, predominantly perpendicular, electric field structures are formed.« less
  • The onset of the acceleration of ions during magnetic reconnection is explored via particle-in-cell simulations in the limit of a strong ambient guide field that self-consistently and simultaneously follow the motions of protons and ╬▒ particles. Heating parallel to the local magnetic field during reconnection with a guide field is strongly reduced compared with the reconnection of anti-parallel magnetic fields. The dominant heating of thermal ions during guide field reconnection results from pickup behavior of ions during their entry into reconnection exhausts and dominantly produces heating perpendicular rather than parallel to the local magnetic field. Pickup behavior requires that themore » ion transit time across the exhaust boundary (with a transverse scale of the order of the ion sound Larmor radius) be short compared with the ion cyclotron period. This translates into a threshold in the strength of reconnecting magnetic field that favors the heating of ions with high mass-to-charge. A simulation with a broad initial current layer produces a reconnecting system in which the amplitude of the reconnecting magnetic field just upstream of the dissipation region increases with time as reconnection proceeds. The sharp onset of perpendicular heating when the pickup threshold is crossed is documented. A comparison of the time variation of the parallel and perpendicular ion heating with that predicted based on the strength of the reconnecting field establishes the scaling of ion heating with ambient parameters both below and above the pickup threshold. The relevance to observations of ion heating in the solar corona is discussed.« less
  • Nonlinear analytical theory of magnetic reconnection with a large guide field is presented for the first time. We confirm that two distinct steady-state reconnection regimes are possible depending on the relative size of the diffusion region thickness {delta} versus the sound gyroradius {rho}{sub s}. The reconnection is slow (Sweet-Parker-like) for {delta} {ge} {rho}{sub s}, and fast otherwise. However, unlike earlier work, we find that ion viscosity {mu} plays a fundamental role in the fast regime. In particular, for {delta} < {rho}{sub s} we obtain {delta} {infinity} Ha{sup -1}, with Ha {infinity} 1/{radical}{nu}{mu} as the Hartmann number, and the reconnection ratemore » E{sub z} {infinity} Pr{sup -1/2}, with Pr = {mu}/{eta} as the Prandtl number and {eta} as the resistivity. If the perpendicular ion viscosity is employed for {mu}, the reconnection rate becomes independent of plasma {beta} and collision frequencies, and therefore potentially fast.« 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