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中国科技论文在线
Numerical simulations of magnetic reconnection in an
asymmetric current sheet
Wang Peiran, HuangCan, Lu Quanming, WangShui
**
(CAS Key Laboratory of Geospace Environment, Department of Geophysics and Planetary 5
Science, University of Science and Technology of China, HeFei 230026)
Brief author introduction: Wang Peiran, (1987-), male, Doctor, solitary wave in plasma.
Correspondance author: Lu Quanming, (1969-), male, Professor, magnetic field reconnection, shock. E-mail:
qmlu@ustc.edu.cn
Abstract: Magnetic reconnection in a symmetric current sheet has been thoroughly investigated in
previous studies. The out-of-plane magnetic field is found to have a quadrupole structure in the
diffusion region. The perpendicular electric field and the parallel electric field with a bipolar structure
exist around the four separatrices. In this paper, by performing two-dimensional (2-D) particle-in-cell
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(PIC) simulations, we investigate the electric and magnetic structures of magnetic reconnection in an
asymmetric current sheet. Compared with magnetic reconnection in a symmetric current sheet, the
quadrupole structure of the out-of-plane magnetic field now disappears in the diffusion region, and
both the parallel and perpendicular electric field only exist in the upper separatrices. The parallel
electric field with a bipolar structure is considered to be the results of the nonlinear evolution of the
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Buneman instability, which is excited by the high-speed electron flow formed after their acceleration
around the X line. The disappearance of the parallel electric field around the lower separatrices may be
due to the transverse instability, which is unstable in a weak magnetized plasma.
Keywords: Magnetic reconnection; electron hole; Buneman instability
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0 Introduction Numerical simulations of magnetic reconnection in
an asymmetric current sheet
Magnetic reconnection rapidly converts magnetic energy into plasma energy [1,2], which is
used to explain many explosive phenomena in space and laboratory plasma, such as solar flares,
magnetospheric substorms and tokamak disruptions [3-7]. The structures of anti-parallel 25
collisionless reconnection in a symmetric current sheet has been thoroughly studied [8-11]. In the
ion diffusion region, ions are demagnetized, while electrons are magnetized and move toward the
X line along the separatrices because of the effect of the magnetic mirror. These electrons are
accelerated and then directed away from the X line along the magnetic field lines just inside the
separatrices after they reach the vicinity of the X line. The resulting Hall currents are directed 30
toward the X line along the magnetic field lines just inside the separatrices and away from the X
line along the separatrices, and the Hall currents lead to the quadrupolar structure of the
out-of-plane magnetic field[12-14]. At the same time, the Hall electric field, which points toward
the center of the current sheet[15-17], and electron holes with bipolar structures of the parallel
electric field, are also formed around the separatrices[18,19]. These electron holes are considered 35
to be generated during the nonlinear evolution of the Buneman instability, which is excited by the
electron beam around the separatrices[20,21]. The structures of both the magnetic field and
electric field during anti-parallel magnetic reconnection in a symmetric current sheet have good
symmetry.
However, magnetic reconnection may also occur at an asymmetric current sheet. For example, 40
magnetic reconnection at the earth's magnetopause occurs between two topologically distinct
regions with quite different properties[22,23]. In this paper, with 2-D PIC simulations, we
investigate the structures of the magnetic field and electric field, as well as electron holes near the
separatrices during anti-parallel collisionless reconnection in an asymmetric current
sheet.First-order headline. 45