Out-of-plane electron currents in magnetic islands formed during collisionless magnetic reconnection【推荐论文】 .doc
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1、精品论文Out-of-plane electron currents in magnetic islands formed during collisionless magnetic reconnection5HUANG Can, LU Quanming, WU Mingyu, LU San, WANG Shui(CAS Key Laboratory of Geospace Environment, Department of Geophysics and PlanetarySciences, University of Science and Technology of China, Hef
2、ei, 230026)Abstract: Secondary islands are considered to play a crucial role in collisionless magnetic reconnection. Based on 2-D particle-in-cell simulations, we investigate the characteristics of the10out-of-plane electron currents in magnetic islands formed during collisionless magnetic reconnect
3、ionwith an initial guide field. In a primary island (formed simultaneously with the appearance of the Xlines), due to the acceleration of the trapped electrons, the direction of the formed out-of-plane electron current is reverse to the original one. In the secondary island (formed in the vicinity o
4、f the X-line), theout-of-plane electron current is generated due to the accelerated electrons by the reconnection electric15field in the vicinity of the X line. In such a way, the direction of the out-of-plane electron current in a secondary island is found to be opposite to that in a primary island
5、. Such characteristics are found to berelated to the evolution of the magnetic islands and then electron dynamics in the islands, which are proposed in this paper to be a possible criterion to identify a secondary island formed duringcollisionless magnetic reconnection, especially in the magnetotail
6、.20Key words: space physics; magnetic island; collisionless magnetic reconnection0IntroductionAs a fundamental physical process in plasma, magnetic reconnection is often invoked to25explain the rapid conversion of magnetic energy into plasma kinetic and thermal energies1.Therefore, magnetic reconnec
7、tion is believed to be the major driving mechanism for many explosive phenomena in solar atmosphere2, the Earths magnetosphere3, laboratory experiments4, and even the magnetotail of a non-magnetized planet5. The characteristics of the ion diffusion region are considered to be the key point to unders
8、tand the fast rate of30collisionless magnetic reconnection, which is determined by the Hall effects caused by ion-electron decoupling6.Recently, numerical studies have shown that the diffusion region is unstable and secondary magnetic islands may be formed in the vicinity of the X line. An extended
9、electron current sheet is found to be formed in the vicinity of the X line in two-dimensional (2-D)35particle-in-cell (PIC) simulations of anti-parallel magnetic reconnection with open boundary condition, where a secondary island may be produced and then ejected out7. A new secondary island may be g
10、enerated again and ejected out again, and such a process occurs periodically. Daughton et al.8 further found that secondary islands may also be generated in Sweet-Parker reconnection layers when the Lundquist number is sufficiently large. The40extension of the electron current sheet and the formatio
11、n of secondary islands have also been found in guide field reconnection, although now the current sheet becomes tilted because the accelerated electrons in the vicinity of the X line stream outward along only two of the four separatrices9. The resulting in secondary islands not only can affect the r
12、econnection rate in both anti-parallel and guide field reconnection7, 10, but can also enhance the electron45acceleration efficiency greatly in magnetic reconnection11. Therefore, it is crucial for us toknow the characteristics of secondary islands. In this paper, based on 2-D PIC simulationFoundati
13、ons: Ph.D. Programs Foundation of Ministry of Education of China (No. 20123402120010) Brief author introduction:HUANG Can (1985-), Male, Associate Researcher, Space plasmas Correspondance author: Lu quanming (1969-), Male, Professor, Space plasmas. E-mail: - 9 -results, we investigate the formation
14、of out-of-plane electron currents in magnetic islands, especially the evolution of the primary islands, which makes it eventually distinct from the secondary ones. It is proposed that the direction of the out-of-plane electron currents may50provide a feasible observation criterion to identify a seco
15、ndary island in collisionless magnetic reconnection.1Simulation Model2-D PIC simulations are employed in this paper. In the PIC simulations, ions and electrons are treated as individual particles, and their trajectories are followed by integrating the Newton55equation. The electromagnetic fields are
16、 defined in the grids, and can be known by solving the Maxwell equations with a full explicit algorithm. Periodic boundary conditions are assumed in the x direction, and the particles leave one boundary will re-enter the other boundary. The ideal conducting boundary conditions for electromagnetic fi
17、elds are employed in the z direction. The1-D Harris current sheet is used as the initial configuration in the present simulations12, and60profile of the magnetic field isB0 ( z) = B0 tanh(z d)e x + By0e y ,(1)whereB0 and d are the asymptotical magnetic field and the initial half-width of the current
18、sheet, respectively. Here,By 0is the amplitude of the initial guide field, set to beBy 0 = -B0 inour simulations. The corresponding profile of the number density is65n( z) = nb + n0 sech2(z d ), (2)where nbrepresents the density of the uniform, background plasma while n0is the peak Harrisdensity. Th
19、e distributions of ions and electrons are assumed to be satisfied the Maxwellian function. The background plasma is non-drifting, while the drift speeds in the y direction of theHarris plasma satisfyVi 0Ti 0 = -Ve0Te0(whereTi 0andTe 0are the initial ion and electron70temperatures, whileVi 0andVe 0ar
20、e the initial ion and electron drift speeds). In our simulations,the initial density ratio is set to ben0 = 5nb . We choosed = 0.5c wpi(wherec wpiis the ioninertial length based on the peak Harris densityn0 ). The mass ratio is set to bemi me =100(where miis the mass of the ion, and meis the rest ma
21、ss of the electron), and the light speed isc = 15vA , where v Ais the Alfven speed based onB0 andn0 . Two simulation cases (Runs 175and 2) are performed in this paper. The initial temperature ratio is set to beRuns 1 and 2, respectively.Ti 0Te0 = 4and 0.5 inThe computation is carried out in a rectan
22、gular domain in the ( x, z)plane. The simulationbox employed in this paper has a size ofLx = 51.2 c wpiin the x direction and a sizeLz = 12.8 c wpiin the z direction withNx Nz = 1024 256grids, so the spatial resolution is80Dx = Dz = 0.05c wpi = 0.5c wpe(wherec wpeis the electron inertial length base
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