Hiroaki Ohtani

Two-scale structure of the current layer controlled by meandering motion during steady-state collisionless driven reconnection

A steady two-scale structure of current layer is demonstrated in the collisionless driven reconnections without a guide field by means of two-dimensional full-particle simulations in an open system. The current density profile along the inflow direction consists of two parts. One is a low shoulder controlled by the ion-meandering motion, which is a bouncing motion in a field reversal region. The other is a sharp peak caused mainly by the electron-meandering motion. The shoulder structure is clearly separated from the sharp peak for the case of a large mass ratio calculation mi/me=200 because the ratio of the ion-meandering orbit amplitude to the electron-meandering orbit amplitude is proportional to (mi/me)1/4. Although the ion frozen-in constraint is broken within a distance of the ion skin depth c/ωpi, the violation due to the ion inertia is weak compared to the strong violation caused by the ion-meandering motion. The violation of the electron frozen-in constraint caused by the electron-meandering moti...

Self-generation of hollow current profile and tilt instability in field-reversed configuration

Two-dimensional electromagnetic particle simulation is performed to investigate the profile relaxation from a magnetohydrodynamic (MHD) equilibrium to a kinetic one and the physical property of the kinetic equilibrium in the field-reversed configuration. The radial oscillation is excited in order to relax an excess energy in the MHD equilibrium. After this profile oscillation, the system spontaneously relaxes toward a kinetic equilibrium, in which the electron current profile becomes hollow as a result of the combined effects of the gradient-B drift near the field-null line and the E×B drift generated by the ion finite Larmor radius effect near the magnetic separatrix. On the other hand, the ion current profile becomes peaked due to the effect of the ion meandering orbit near the field-null line. The stability of the obtained kinetic equilibrium against the tilt mode is also studied by means of three-dimensional full electromagnetic particle simulation. It is found that the growth rate of the tilt instabi...

Anomalous resistivity due to kink modes in a thin current sheet

The roles of microscopic plasma instabilities on the violation of the frozen-in constraint are investigated by examining the force balance equation based on explicit electromagnetic particle simulation for a thin current sheet. Wave-particle interactions associated with lower hybrid drift instability and drift kink instability (DKI) contribute to the wavy electric force term at the periphery of the current sheet and the wavy magnetic force term at the neutral sheet, respectively. In the linear growing phase of DKI, the wavy magnetic force term balances with the electric force term due to the dc electric field at the neutral sheet. It is concluded that the growth of DKI can create anomalous resistivity and result in the violation of the frozen-in constraint as well as the diffusion of current density.

Development of multi-hierarchy simulation model with non-uniform space grids for collisionless driven reconnection

A multi-hierarchy simulation model aimed at magnetic reconnection studies has been developed, in which macroscopic and microscopic physics are solved self-consistently and simultaneously. In this work, the previous multi-hierarchy model by these authors is extended to a more realistic one with non-uniform space grids. Based on the domain decomposition method, the multi-hierarchy model consists of three parts: a magnetohydrodynamics algorithm to express the macroscopic global dynamics, a particle-in-cell algorithm to describe the microscopic kinetic physics, and an interface algorithm to interlock macro and micro hierarchies. For its verification, plasma flow injection is simulated in this multi-hierarchy model and it is confirmed that the interlocking method can describe the correct physics. Furthermore, this model is applied to collisionless driven reconnection in an open system. Magnetic reconnection is found to occur in a micro hierarchy by injecting plasma from a macro hierarchy.

Development of electromagnetic particle simulation code in an open system

In an electromagnetic particle simulation for magnetic reconnection in an open system, which has a free boundary condition, particles go out and come into the system through the boundary and the number of particles depends on time. Besides, particles are locally attracted due to physical condition. Accordingly, it is hard to realize an adequate load balance with domain decomposition. Furthermore, a vector performance does not become efficient without a large memory size due to a recurrence of array access. In this paper, we parallelise the code with High Performance Fortran. For data layout, all field data are duplicated on each parallel process, but particle data are distributed among them. We invent an algorithm for the open boundary of particles, in which an operation for outgoing and incoming particles is performed in each processor, and the only reduction operation for the number of particles is executed in data transfer. This adequate treatment makes the amount and frequency of data transfer small, and the load balance among processes relevant. Furthermore, a compiler-directive listvec in the gather process dramatically decreases the memory size and improves the vector performance. Vector operation ratio becomes about 99.5% and vector length turns 240 and over. It becomes possible to perform the simulation with 800 million particles in 512 × 128 × 64 meshes. We succeed in opening a path for a large-scale simulation.

Towards cross-hierarchy simulation of collisionless driven reconnection in an open system

The basic idea of a cross-hierarehy model for magnetic reconnection in an open system is proposed, where a microscopic system is surrounded by a macroscopic system and the interaction between the two systems is expressed by the plasma inflow and outflow through the system boundary. Collisionless driven reconnection in two-dimensional and three-dimensional open systems is demonstrated using an open particle simulation model developed as a microscopic part of a cross-hierarchy model. It is found that the openness of the system and scale-coupling effects play crucial roles in collisionless driven reconnection.

Three-dimensional particle simulation on structure formation and plasma instabilities in collisionless driven reconnection

Structure formation and plasma instabilities in collisionless driven reconnection are investigated by three-dimensional electromagnetic (EM) particle simulation in an open system. In the early period, the lower-hybrid drift instability is excited in tlie periphery of the current layer. In the intermediate period, magnetic islands are created in the downstream due to magnetic reconnection. and they become unstable against a kink instability. In the late period, after the magnetic islands go out through the boundary, a wide, thin current sheet is generated and a low-frequency EM mode is excited in the central region. This mode has a frequency comparable to the ion gyration frequency, and thus it is considered to be the drift kink instability.

Structure formation and dynamical behavior of kinetic plasmas controlled by magnetic reconnection

Structure formation and dynamical behavior of kinetic plasmas controlled by magnetic reconnection is investigated by means of electromagnetic particle simulations. Two-dimensional simulation in a long time scale reveals that there are two evolving regimes in the temporal behavior of current layer structure, dependently on the spatial size of plasma inflow through the upstream boundary, i.e., a steady regime and an intermittent regime. In three-dimensional case the spatial structure of current sheet is dynamically modified by plasma instabilities excited through wave-particle interaction.

Multi-Hierarchy Simulation of Collisionless Driven Reconnection by Real-Space Decomposition

The first results on analysis of collisionless driven reconnection with a multihierarchy simulation model are reported. In the multi-hierarchy simulation model, real space in a simulation consists of three parts: a magnetohydrodynamics (MHD) domain to deal with macroscopic dynamics, a particle-in-cell (PIC) domain to solve microscopic kinetic physics from the first principle, and an interface domain to interlock the two domains. By means of multi-hierarchy simulations, the influence of macroscopic dynamics on microscopic physics of magnetic reconnection is investigated. Dynamical behaviors of collisionless reconnection in the PIC domain depend strongly on plasma inflows from the MHD domain. It is found that if the width of an MHD inflow increases as vw vA0, where vA0 is the Alfven speed at the upstream boundary, magnetic reconnection has only a single X-point., while in cases of vw2.0vA0, reconnection with multiple X-points takes place.

IMMERSIVE VR VISUALIZATIONS BY VFIVE PART 2: APPLICATIONS

VFIVE is a scientific visualization application for CAVE-type immersive virtual reality systems. The source codes are freely available. VFIVE is used as a research tool in various VR systems. It also lays the groundwork for developments of new visualization software for CAVEs. In this paper, we pick up five CAVE systems in four different institutions in Japan. Applications of VFIVE in each CAVE system are summarized. Special emphases will be placed on scientific and technical achievements made possible by VFIVE.