We envision a situation where intense current sheets are developed within a magnetically dominated plasma. Earlier work in nonrelativistic low-β plasmas has shown that the gradual evolution of the magnetic field can lead to formation of intense, nearly force-free current layers where magnetic reconnection may be triggered (Galsgaard et al. 2003; Titov et al. 2003). In the present study, the critical parameter is the magnetization parameter defined as 





, which roughly corresponds to the available magnetic energy per particle. The numerical simulations presented in this paper are initialized from a force-free current layer with 





 (Che et al. 2011; Liu et al. 2013, 2014), which corresponds to a magnetic field with magnitude B0 rotating by 180° across the central layer with a half-thickness of λ. No external guide field is included in this study but there is an intrinsic guide field By associated with the central sheet. The plasma consists of electron–positron pairs with mass ratio 





. The initial distributions are Maxwellian with a spatially uniform density n0 and a thermal temperature (





). Particles in the central sheet have a net drift 





 to represent a current density 





 that is consistent with 





. Since the force-free current sheet does not require a hot plasma component to balance the Lorentz force, this initial setup is more suitable to study reconnection in low-β and/or high-σ plasmas. The full particle simulations are performed using the VPIC code (Bowers et al. 2009) and NPIC code (Daughton et al. 2006; Daughton & Karimabadi 2007), both of which solve Maxwell equations and push particles using relativistic approaches. The VPIC code directly evolves electric and magnetic fields, whereas in the NPIC code the fields are advanced using the scalar and vector potentials. Although the two codes have very different algorithms, all of the key results are in good agreement for this study, thus providing additional confidence in our conclusions. In addition, we have developed a particle-tracking module to analyze the detailed physics of the particle energization process. In the simulations, we define and adjust σ by changing the ratio of the electron gyrofrequency 





 to the electron plasma frequency 





, 





. For 2D simulations, we have performed simulations with 





 and box sizes 





, 





, and 





, where di is the inertial length 





. For 3D simulations, the largest case is 





 with σ = 100. For high-σ cases (





), we choose cell sizes 





 and 





, so the particle gyromotion scale 





 is resolved. The time step is chosen to correspond to a Courant number 





, where 





. The half-thickness of the current sheet is 





 for 





, 





 for σ = 400, and 





 for σ = 1600 in order to satisfy the drift velocity 





. For both 2D and 3D simulations, we have more than 100 electron–positron pairs in each cell. The boundary conditions for 2D simulations are periodic for both fields and particles in the x direction, while in the z direction the boundaries are conducting for the field and reflecting for the particles. In the 3D simulations, the boundary conditions are periodic for both fields and particles in the y direction, while the boundary conditions in the x and z directions are the same as in the 2D cases. A weak long-wavelength perturbation (Birn et al. 2001) with 





 is included to initiate reconnection. The parameters for different runs are summarized in Table 1, which also lists key results such as maximum energy of particles, spectral index, the fraction of kinetic energy converted from the magnetic energy, and the portion of energy gain arising from the perpendicular electric fields.
Citation: Daughton & Karimabadi 2007
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