Toseo Moritaka

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.

Momentum transfer of solar wind plasma in a kinetic scale magnetosphere

Solar wind interaction with a kinetic scale magnetosphere and the resulting momentum transfer process are investigated by 2.5-dimensional full kinetic particle-in-cell simulations. The spatial scale of the considered magnetosphere is less than or comparable to the ion inertial length and is relevant for magnetized asteroids or spacecraft with mini-magnetosphere plasma propulsion. Momentum transfer is evaluated by studying the Lorentz force between solar wind plasma and a hypothetical coil current density that creates the magnetosphere. In the zero interplanetary magnetic field (IMF) limit, solar wind interaction goes into a steady state with constant Lorentz force. The dominant Lorentz force acting on the coil current density is applied by the thin electron current layer at the wind-filled front of the magnetosphere. Dynamic pressure of the solar wind balances the magnetic pressure in this region via electrostatic deceleration of ions. The resulting Lorentz force is characterized as a function of the scal...

Roles of ion and electron dynamics in the onset of magnetic reconnection due to current sheet instabilities

Roles of ion and electron kinetic effects in the trigger mechanism of magnetic reconnection due to current sheet instabilities are investigated by means of (2+1∕2)D explicit particle simulation. The simulation is performed for the Harris equilibrium without guide fields in the plane perpendicular to the antiparallel magnetic fields. The instabilities excited in the vicinity of the neutral sheet are classified into two modes, i.e., one is a longer wavelength kink mode and the other is a shorter wavelength kink mode. The growth of the longer kink mode depends only on the ion mass, while the growth of the shorter one depends only on the electron mass. Before the growth of these kink modes, the lower hybrid drift instability leads to two types of plasma diffusion: diffusion at the periphery controlled by ions and diffusion in the vicinity of the neutral sheet controlled by electrons. The diffusion at the periphery affects the ion distribution function at the neutral sheet through the ion meandering motion, an...

Full Particle-in-Cell Simulation Study on Magnetic Inflation Around a Magneto Plasma Sail

In order to consider a next-generation space propulsion system referred to as the “magneto plasma sail,” the magnetic inflation mechanism of a small artificial magnetosphere is investigated. We carry out a two-and-half-dimensional full particle-in-cell simulation, and magnetic inflation mediated by the gyration motion of injected ions is observed. As a result of the gyration motion, an ion-rich region is formed near the direction-reversal position of the injected ions. Magnetic inflation takes place due to the flow of electrons toward the ion-rich region, which carries the field lines of the original magnetosphere. This inflation process is effective for a magnetosphere with a scale comparable to the gyration radius of the injected ions. If the original magnetosphere is much smaller than this, background electrons flow into the ion-rich region outside the magnetosphere, and the inflated magnetosphere is confined to a smaller region. In addition, the thermal effects of background electrons have a similar impact on the inflation process, even if the direction-reversal position is located inside the magnetosphere.

A Multi-Scale Electromagnetic Particle Code with Adaptive Mesh Refinement and Its Parallelization

Abstract Space plasma phenomena occur in multi-scale processes from the electron scale to the magnetohydrodynamic scale. In order to investigate such multi-scale phenomena including plasma kinetic effects, we started to develop a new electromagnetic Particle-In-Cell (PIC) code with Adaptive Mesh Refinement (AMR) technique. AMR can realize high-resolution calculation saving computer resources by generating and removing hierarchical cells dynamically. In the parallelization, we adopt domain decomposition method and for good locality preserving and dynamical load balancing, we will use the Morton ordered curve. In the PIC method, particle calculation occupies most of the total calculation time. In our AMR-PIC code, time step intervals are also refined. To realize the load balancing between processes in the domain decomposition scheme, it is the most essential to consider the number of particle calculation loops for each cell among all hierarchical levels as a work weight for each processor. Therefore, we calculate the work weights based on the cost of particle calculation and hierarchical levels of each cell. Then we decompose the domain according to the Morton curve and the work weight, so that each processor has approximately the same amount of work. By performing a simple one-dimensional simulation, we confirmed that the dynamic load balancing is achieved and the computation time is reduced by introducing the dynamic domain decomposition scheme.

MPI Parallelization of PIC Simulation with Adaptive Mesh Refinement

With the prevalence of massively parallel computer architecture, MPI parallelization of existing simulation codes for stand-alone system and its parallel optimization to achieve feasible scalability are in critical need. Of many numerical approaches, adaptive mesh refinement(AMR) is known to be one of the particular cases in which MPI parallelization is challenging. In this manuscript, our ongoing project and roadmap to port the AMR-enabled Particle-in-Cell code onto distributed environments is exhibited. Our present technique is characterized by two main features. First, remote memory access is employed for inter-process data transfer in attempt to access the data with less intermediate processes. Secondly, in order to achieve better load balance, the domain decomposition according to the modified Morton ordering is introduced.

Plasma instabilities and anomalous resistivity in the current sheet

The role of microscopic plasma instabilities in the current sheet in the violation of the ideal magnetohydrodynamic (MHD) condition is investigated by means of a 21 2-dimensional explicit particle simulation code. Two instabilities, lower hybrid drift instabilities (LHDIs) and drift kink instabilities (I)KIs) are found to evolve in the current sheet. In the LHDI growing phase, nonlinear coupling of the fluctuations of the electric field and particle density generates the outward flow and deforms the current sheet profile. The DC electric field leading to the magnetic flux reduction and the damping of equilibrium current are observed in the DK1 growing phase. The force balance between the averaged electric force term and the additional wavy component associated with the fluctuations of magnetic field and current density holds at the neutral sheet. This suggests that the DKI can contribute to creating anomalous resistivity and triggering the collisionless magnetic reconnection.

Full particle-in-cell simulation study on the solar wind interaction with small-scale magnetic dipole field

We have been investigating the solar wind interaction with a small-scale dipole magnetic field structure comparable to or less than the ion inertial length by performing full particle-in-cell electromagnetic simulation. Such a micro-scale magnetosphere would be used for the next-generation interplanetary flight system called Magneto Plasma Sail (MPS) which has been proposed as one of the innovative interplanetary flight systems by JAXA. In the current paper, we focus on the analysis of current layer caused by the interaction of the solar wind at the boundary of the small dipole field. The current layer is very important for the MPS thrust which can be evaluated with the Lorentz force obtained with the magnetic field component induced by the current layer and the current by a superconducting coil at the satellite. In a situation where the ion inertia length is larger than the dipole field region, it turns out that electron interaction with the local magnetic field becomes important. The ions, which are basically unmagnetized in such as a situation, can be indirectly influenced by the presence of the dipole field due to the electrostatic force cause by the difference from the electron dynamics. We will examine the formation of a small-scale magnetosphere in such a situation as well as the features of the current layer in terms of location, width and intensity. In addition, IMF effect such as the formation of shock structure and magnetic field reconnection can affect the formation of the current layer. In the preliminary two dimensional PIC simulations, magnetic reconnection takes place at the night side of the magnetosphere even in the northward IMF case. A current density peak is formed inside the magnetosphere due to the electron backflow from the reconnection region, in addition to the induced current density at the front boundary layer where the solar wind momentum is primarily diverted. Consequently, when we consider the IMF effect, we could observe expansion of the dipole field structure and the increase of the MPS thrust at the satellite. In parallel, we have been developing a new simulation code by incorporating adaptive mesh refinement for multi-scale PIC simulation. A prototype of the code is completed and currently we have been working on the code parallelization with adaptive domain-decomposition scheme using MPI. In the presentation, we will show some of the results obtained with the newly developed multi-scale simulation code and examine the detailed process of the solar wind interaction with a small scale dipole field. We particularly focus on the boundary region where the magnetic field variation and the current density is large.