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Title: Particle-in-cell simulation study of the scaling of asymmetric magnetic reconnection with in-plane flow shear

Abstract

We investigate magnetic reconnection in systems simultaneously containing asymmetric (anti-parallel) magnetic fields, asymmetric plasma densities and temperatures, and arbitrary in-plane bulk flow of plasma in the upstream regions. Such configurations are common in the high-latitudes of Earth's magnetopause and in tokamaks. We investigate the convection speed of the X-line, the scaling of the reconnection rate, and the condition for which the flow suppresses reconnection as a function of upstream flow speeds. We use two-dimensional particle-in-cell simulations to capture the mixing of plasma in the outflow regions better than is possible in fluid modeling. We perform simulations with asymmetric magnetic fields, simulations with asymmetric densities, and simulations with magnetopause-like parameters where both are asymmetric. For flow speeds below the predicted cutoff velocity, we find good scaling agreement with the theory presented in Doss et al. [J. Geophys. Res. 120, 7748 (2015)]. Applications to planetary magnetospheres, tokamaks, and the solar wind are discussed.

Authors:
;  [1];  [2]
  1. Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506 (United States)
  2. Department of Physics and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742 (United States)
Publication Date:
OSTI Identifier:
22599936
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 23; Journal Issue: 8; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; ASYMMETRY; COMPUTERIZED SIMULATION; CONVECTION; FLUIDS; MAGNETIC FIELDS; MAGNETIC RECONNECTION; MAGNETOPAUSE; PARTICLES; PLANETARY MAGNETOSPHERES; PLASMA DENSITY; SCALING; SHEAR; SOLAR WIND; TOKAMAK DEVICES; TWO-DIMENSIONAL CALCULATIONS; VELOCITY

Citation Formats

Doss, C. E., Cassak, P. A., E-mail: Paul.Cassak@mail.wvu.edu, and Swisdak, M.. Particle-in-cell simulation study of the scaling of asymmetric magnetic reconnection with in-plane flow shear. United States: N. p., 2016. Web. doi:10.1063/1.4960324.
Doss, C. E., Cassak, P. A., E-mail: Paul.Cassak@mail.wvu.edu, & Swisdak, M.. Particle-in-cell simulation study of the scaling of asymmetric magnetic reconnection with in-plane flow shear. United States. doi:10.1063/1.4960324.
Doss, C. E., Cassak, P. A., E-mail: Paul.Cassak@mail.wvu.edu, and Swisdak, M.. 2016. "Particle-in-cell simulation study of the scaling of asymmetric magnetic reconnection with in-plane flow shear". United States. doi:10.1063/1.4960324.
@article{osti_22599936,
title = {Particle-in-cell simulation study of the scaling of asymmetric magnetic reconnection with in-plane flow shear},
author = {Doss, C. E. and Cassak, P. A., E-mail: Paul.Cassak@mail.wvu.edu and Swisdak, M.},
abstractNote = {We investigate magnetic reconnection in systems simultaneously containing asymmetric (anti-parallel) magnetic fields, asymmetric plasma densities and temperatures, and arbitrary in-plane bulk flow of plasma in the upstream regions. Such configurations are common in the high-latitudes of Earth's magnetopause and in tokamaks. We investigate the convection speed of the X-line, the scaling of the reconnection rate, and the condition for which the flow suppresses reconnection as a function of upstream flow speeds. We use two-dimensional particle-in-cell simulations to capture the mixing of plasma in the outflow regions better than is possible in fluid modeling. We perform simulations with asymmetric magnetic fields, simulations with asymmetric densities, and simulations with magnetopause-like parameters where both are asymmetric. For flow speeds below the predicted cutoff velocity, we find good scaling agreement with the theory presented in Doss et al. [J. Geophys. Res. 120, 7748 (2015)]. Applications to planetary magnetospheres, tokamaks, and the solar wind are discussed.},
doi = {10.1063/1.4960324},
journal = {Physics of Plasmas},
number = 8,
volume = 23,
place = {United States},
year = 2016,
month = 8
}
  • Effects of out-of-plane shear flows on asymmetric magnetic reconnect are investigated in a two-dimensional (2D) hybrid model with an initial Harris sheet equilibrium. It is found that the out-of-plane flow with an in-plane shear can significantly change the asymmetric reconnection process as well as the related geometry. The magnetic flux, out-of-plane magnetic field, in-plane flow vorticity, plasma density, and the reconnection rate are discussed in detail. The results are in comparison with the cases without the shear flows to further understand the effect.
  • The scaling of magnetic reconnection in the presence of an oppositely directed sub-Alfvenic shear flow parallel to the reconnecting magnetic field is studied using analytical scaling arguments and two-dimensional two-fluid numerical simulations of collisionless (Hall) reconnection. Previous studies noted that the reconnection rate falls and the current sheet tilts with increasing flow speed, but no quantitative theory was presented. This study presents a physical model of the effect of shear flow on reconnection, resulting in expressions for the scaling of properties such as the reconnection rate, outflow speed, and thickness and length of the dissipation region, which are verified numerically.more » Differences between Hall and Sweet-Parker reconnection are pointed out. The tilting of the current sheet is explained physically and a quantitative prediction is presented and verified.« less
  • The scaling of the reconnection rate during (fast) Hall magnetic reconnection in the presence of an oppositely directed bulk shear flow parallel to the reconnecting magnetic field is studied using two-dimensional numerical simulations of Hall reconnection with two different codes. Previous studies noted that the reconnection rate falls with increasing flow speed and shuts off entirely for super-Alfvenic flow, but no quantitative expression for the reconnection rate in sub-Alfvenic shear flows is known. An expression for the scaling of the reconnection rate is presented.
  • Electron bulk heating during magnetic reconnection with symmetric inflow conditions is examined using kinetic particle-in-cell simulations. Inflowing plasma parameters are varied over a wide range of conditions, and the increase in electron temperature is measured in the exhaust well downstream of the x-line. The degree of electron heating is well correlated with the inflowing Alfvén speed c{sub Ar} based on the reconnecting magnetic field through the relation ΔT{sub e}=0.033 m{sub i} c{sub Ar}{sup 2}, where ΔT{sub e} is the increase in electron temperature. For the range of simulations performed, the heating shows almost no correlation with inflow total temperature T{sub tot}=T{sub i}+T{submore » e} or plasma β. An out-of-plane (guide) magnetic field of similar magnitude to the reconnecting field does not affect the total heating, but it does quench perpendicular heating, with almost all heating being in the parallel direction. These results are qualitatively consistent with a recent statistical survey of electron heating in the dayside magnetopause (Phan et al., Geophys. Res. Lett. 40, 4475, 2013), which also found that ΔT{sub e} was proportional to the inflowing Alfvén speed. The net electron heating varies very little with distance downstream of the x-line. The simulations show at most a very weak dependence of electron heating on the ion to electron mass ratio. In the antiparallel reconnection case, the largely parallel heating is eventually isotropized downstream due a scattering mechanism, such as stochastic particle motion or instabilities. The simulation size is large enough to be directly relevant to reconnection in the Earth's magnetosphere, and the present findings may prove to be universal in nature with applications to the solar wind, the solar corona, and other astrophysical plasmas. The study highlights key properties that must be satisfied by an electron heating mechanism: (1) preferential heating in the parallel direction; (2) heating proportional to m{sub i} c{sub Ar}{sup 2}; (3) at most a weak dependence on electron mass; and (4) an exhaust electron temperature that varies little with distance from the x-line.« less
  • Magnetic reconnection during collisionless, stressed, X-point collapse was studied using a kinetic, 2.5D, fully electromagnetic, relativistic particle-in-cell numerical code. Two cases of weakly and strongly stressed X-point collapse were considered. Here the descriptors ''weakly'' and ''strongly'' refer to 20% and 124% unidirectional spatial compression of the X-point, respectively. In the weakly stressed case, the reconnection rate, defined as the out-of-plane electric field in the X-point (the magnetic null) normalized by the product of external magnetic field and Alfven speeds, peaks at 0.11, with its average over 1.25 Alfven times being 0.04. During the peak of the reconnection, electron inflow intomore » the current sheet is mostly concentrated along the separatrices until they deflect from the current sheet on the scale of electron skin depth, with the electron outflow speeds being of the order of the external Alfven speed. Ion inflow starts to deflect from the current sheet on the ion skin depth scale with the outflow speeds about four times smaller than that of electrons. Electron energy distribution in the current sheet, at the high-energy end of the spectrum, shows a power-law distribution with the index varying in time, attaining a maximal value of -4.1 at the final simulation time step (1.25 Alfven times). In the strongly stressed case, the magnetic reconnection peak occurs 3.8 times faster and is more efficient. The peak reconnection rate now attains the value 2.5, with the average reconnection rate over 1.25 Alfven times being 0.5. Plasma inflow into the current sheet is perpendicular to it, with the electron outflow seeds reaching 1.4 Alfven external Mach number and ions again being about four times slower than electrons. The power-law energy spectrum for the electrons in the current sheet attains now a steeper index of -5.5, a value close to those observed near the X-type region in the Earth's magnetotail. Within about one Alfven time, 2% and 20% of the initial magnetic energy is converted into heat and accelerated particle energy in the case of weak and strong stress, respectively. In both cases, during the peak of the reconnection, the quadruple out-of-plane magnetic field is generated, hinting possibly at the Hall regime of the reconnection. These results strongly suggest the importance of the collisionless, stressed X-point collapse as an efficient mechanism of converting magnetic energy into heat and superthermal particle energy.« less