Keizo Fujimoto

Time evolution of the electron diffusion region and the reconnection rate in fully kinetic and large system

Time evolutions of the electron diffusion region embedded in the ion-scale diffusion region and the reconnection rate associated with magnetic reconnection are investigated using 2-1/2 dimensional full kinetic simulations in a large system, so that any effects of the downstream boundary conditions are negligible. The simulation code employs the adaptive mesh refinement technique and the particle splitting algorithm to the conventional particle-in-cell code, which enable us to perform large-scale full particle simulations. It is shown that the reconnection rate increases associated with magnetic reconnection and reaches a peak value large enough for fast reconnection, but then it decreases as time goes on, even though the effects of the system boundary are negligible. The key process responsible for slowing the reconnection processes is the extension of the electron diffusion region in association with the enhancement of the polarization electric field directing toward the neutral sheet in the electron inf...

Wave activities in separatrix regions of magnetic reconnection

Two-dimensional particle-in-cell simulations have reproduced the waves consistent with those observed frequently in the separatrix regions of magnetic reconnection in the Earths magnetotail. The key process generating the waves is intense parallel acceleration of the electrons due to an electrostatic potential hump formed in the inflow side of the separatrices. The intense electron beams trigger the electron two-stream instability (ETSI) and the beam-driven whistler instability (WI). The Buneman instability (BI) is also excited by moderate electron beams arising upstream the potential hump. The ETSI generates the Langmuir waves, while the BI gives lower hybrid waves. Both modes evolve the electrostatic solitary waves in the nonlinear phases. The ETSI traps the electrons in the parallel direction and forms a flat-top distribution with high-energy cutoff. On the other hand, the WI scatters the electrons in the perpendicular direction, producing isotropic distribution with nonthermal high-energy tail.

Particle description of the electron diffusion region in collisionless magnetic reconnection

The present study clarifies the dissipation mechanism of collisionless magnetic reconnection in two-dimensional system based on particle dynamics. The electrons are accelerated without thermalization in the electron diffusion region, carry out the meandering oscillation, and are ejected away from the X-line. This electron behavior not only generates the electron inertia resistivity based on the particle description, but also it can be the origin of the electron viscosity resulting in the off-diagonal pressure tensor term in the generalized Ohm’s law near the X-line. We derive an analytical profile for the electron pressure tensor term and confirm that the profile is consistent with the particle-in-cell simulation. The present results demonstrate that the magnetic dissipation due to the electron viscosity in the fluid picture is equivalent to that due to the inertia resistivity in the particle description. It is also suggested that the width of the electron current sheet is on the order of the electron ine...

Electromagnetic full particle code with adaptive mesh refinement technique: Application to the current sheet evolution

We have developed a new two and a half dimensional electromagnetic particle code with adaptive mesh refinement (AMR) technique in an effort to give a self-consistent description of the dynamic change of the plasma sheet in association with magnetic reconnection, which includes multi-scale processes from the electron scale to the magnetohydrodynamic scale. The AMR technique subdivides and removes cells dynamically in accordance with a refinement criterion and it is quite effective to achieve high-resolution simulations of phenomena that locally include micro-scale processes. Since the number of particles per cell decreases in the subdivided cells and a numerical noise increases, we subdivide not only cells but also particles therein and control the number of particles per cell. Our code is checked against several well-known processes such as the Landau damping of the Langmuir waves and we show that the AMR technique and particle splitting algorithm are successfully applied to the conventional particle codes. We have also examined the nonlinear evolution of the Harris-type current sheet and realized basically the same properties as those in other full particle simulations. We show that the numbers of cells and particles are greatly reduced so that the time to complete the simulation is significantly shortened in our AMR code, which enables us to conduct large-scale simulations on the current sheet evolution.

Fast magnetic reconnection in a kinked current sheet

Magnetic reconnection processes in a kinked current sheet are investigated using three-dimensional electromagnetic particle-in-cell simulations in a large system where both the tearing and kink modes are able to be captured. The spatial resolution is efficiently enhanced using the adaptive mesh refinement and particle splitting-coalescence method. The kink mode scaled by the current sheet width such as kyL∼1 is driven by the ions that are accelerated due to the reconnection electric field in the ion-scale diffusion region. Although the kink mode deforms the current sheet structure drastically, the gross rate of reconnection is almost identical to the case without the kink mode and fast magnetic reconnection is achieved. The magnetic dissipation mechanism is, however, found very different between the cases with and without the kink mode. The kink mode broadens the current sheet width and reduces the electron flow velocity, so that the electron inertia resistivity is decreased. Nevertheless, anomalous dissi...

Electromagnetic particle-in-cell simulations on magnetic reconnection with adaptive mesh refinement

The electromagnetic particle-in-cell (EM-PIC) model using the adaptive mesh refinement (AMR) is reconsidered so that it is properly and efficiently applied to the current sheet evolution associated with magnetic reconnection. It is very important to adequately select the refinement criteria for cell splitting. It is demonstrated that fine cells have to be distributed not only in the region where the electron Debye length is small, but also in the region where the electron-scale structure is expected to be significant. While the AMR reduces the number of cells drastically, the total simulation cost is also controlled by the number of particles. In order to reduce the total number of particles in the entire system, the present code controls the local number of particles per cell by splitting or coalescing particles. It is shown that the particle splitting and coalescence are quite effective as well as the AMR to enhance the efficiency of the EM-PIC simulations. A new 3D code extended from the 2D code is also introduced. The code is checked against the tearing instability and the lower hybrid drift instability, and it is confirmed that the code has been successfully developed. It is also found that the 3D simulations can gain more efficiency by using the AMR than the 2D simulations.

A new electromagnetic particle-in-cell model with adaptive mesh refinement for high-performance parallel computation

A new electromagnetic particle-in-cell (EMPIC) model with adaptive mesh refinement (AMR) has been developed to achieve high-performance parallel computation in distributed memory system. For minimizing the amount and frequency of inter-processor communications, the present study uses the staggering grid scheme with the charge conservation method, which consists only of the local operations. However, the scheme provides no numerical damping for electromagnetic waves regardless of the wavenumber, which results in significant noise in the refinement region that eventually covers over physical signals. In order to suppress the electromagnetic noise, the present study introduces a smoothing method which gives numerical damping preferentially for short wavelength modes. The test simulations show that only a weak smoothing results in drastic reduction in the noise, so that the implementation of the AMR is possible in the staggering grid scheme. The computational load balance among the processors is maintained by a new method termed the adaptive block technique for the domain decomposition parallelization. The adaptive block technique controls the subdomain (block) structure dynamically associated with the system evolution, such that all the blocks have almost the same number of particles. The performance of the present code is evaluated for the simulations of the current sheet evolution. The test simulations demonstrate that the usage of the adaptive block technique as well as the staggering grid scheme enhances significantly the parallel efficiency of the AMR-EMPIC model.

Dissipation mechanism in 3D magnetic reconnection

Dissipation processes responsible for fast magnetic reconnection in collisionless plasmas are investigated using 3D electromagnetic particle-in-cell simulations. The present study revisits the two simulation runs performed in the previous study (Fujimoto, Phys. Plasmas 16, 042103 (2009)); one with small system size in the current density direction, and the other with larger system size. In the case with small system size, the reconnection processes are almost the same as those in 2D reconnection, while in the other case a kink mode evolves along the current density and deforms the current sheet structure drastically. Although fast reconnection is achieved in both the cases, the dissipation mechanism is very different between them. In the case without kink mode, the electrons transit the electron diffusion region without thermalization, so that the magnetic dissipation is supported by the inertia resistivity alone. On the other hand, in the kinked current sheet, the electrons are not only accelerated in bu...

Ion and electron dynamics generating the Hall current in the exhaust far downstream of the reconnection x-line

We have investigated the ion and electron dynamics generating the Hall current in the reconnection exhaust far downstream of the x-line where the exhaust width is much larger than the ion gyro-radius. A large-scale particle-in-cell simulation shows that most ions are accelerated through the Speiser-type motion in the current sheet formed at the center of the exhaust. The transition layers formed at the exhaust boundary are not identified as slow mode shocks. (The layers satisfy mostly the Rankine-Hugoniot conditions for a slow mode shock, but the energy conversion hardly occurs there.) We find that the ion drift velocity is modified around the layer due to a finite Larmor radius effect. As a result, the ions are accumulated in the downstream side of the layer, so that collimated ion jets are generated. The electrons experience two steps of acceleration in the exhaust. The first is a parallel acceleration due to the out-of-plane electric field Ey which has a parallel component in most area of the exhaust. ...

Three‐dimensional outflow jets generated in collisionless magnetic reconnection

The present study proposes a new theoretical model generating three dimensional (3D) outflow jets in collisionless magnetic reconnection by means of a large-scale particle-in-cell simulation. The key mechanism is the formation of 3D flux ropes arising in the turbulent electron current layer formed around the magnetic x-line. The scale of the flux ropes along the current density is determined by the wavelength of an electron flow shear mode which is a macroscopic scale larger than the typical kinetic scales. The 3D flux ropes are intermittently ejected from the current layer and regulates the outflow jets into fully three dimensions. The gross reconnection rate is sufficiently large, since reconnection takes place almost uniformly along the x-line.