Takahiro Miyoshi

Magnetohydrodynamic Structure of a Plasmoid in Fast Reconnection in Low-Beta Plasmas

Plasmoid structures in fast reconnection in low-beta plasmas are investigated by two-dimensional magnetohydrodynamic simulations. A high-resolution shock-capturing code enables us to explore a variety of shock structures: vertical slow shocks behind the plasmoid, another slow shock in the outer-region, and the shock-reflection in the front side. The Kelvin–Helmholtz-like turbulence is also found inside the plasmoid. It is concluded that these shocks are rigorous features in reconnection in low-beta plasmas, where the reconnection jet speed or the upstream Alfven speed exceeds the sound speed.

Vlasov simulation of the interaction between the solar wind and a dielectric body

The global structure of wake field behind an unmagnetized object in the solar wind is studied by means of a 2.5-dimensional full-electromagnetic Vlasov simulation. The interaction of a plasma flow with an unmagnetized object is quite different from that with a magnetized object such as the Earth. Due to the absence of the global magnetic field, the unmagnetized object absorbs plasma particles that reach the surface, generating a plasma cavity called “wake” on the antisolar side of the object. For numerical simulations of electromagnetic structures around the wake, it is important to include the charging effect in global-scale simulations. The present study is one of the first attempts to study the formation of wake fields via a full-kinetic Vlasov simulation. It has been confirmed that the spatial structures of wake fields depend on the direction of interplanetary magnetic fields as well as the distance from the body.

Performance Measurement of Magnetohydrodynamic Code for Space Plasma on the Various Scalar-Type Supercomputer Systems

The computational performance of magnetohydrodynamic (MHD) code is evaluated on several scalar-type supercomputer systems. We have made performance tuning of a 3-D MHD code for space plasma simulations on the SR16000/L2, FX1, and HX600 supercomputer systems. For parallelization of the MHD code, we use four different methods, i.e., regular 1-D, 2-D, and 3-D domain decomposition methods and a cache-hit type of 3-D domain decomposition method. We found that the regular 3-D decomposition of the MHD model is suitable for HX600 system, and the cache-hit type of 3-D decomposition is suitable for SR16000/L2 and FX1 systems. As results of these runs, we achieved a performance efficiency of almost 20% for MHD code on all systems.

The HLLD Approximate Riemann Solver for Magnetospheric Simulation

A magnetohydrodynamic (MHD) algorithm for global simulations of planetary magnetospheres is developed based on an approximate nonlinear Riemann solver, the so-called Harten-Lax-van Leer-Discontinuities (HLLD) approximate Riemann solver. An approximate nonlinear solution of the MHD Riemann problem, in which the contributions of the background potential magnetic field are subtracted and multispecies plasmas as well as general equation of state are included, can be algebraically obtained under the assumptions that the normal velocity and the background potential magnetic field in the Riemann fan are constant. The theoretical aspects of the HLLD approximate Riemann solver are focused on, in particular.

Boosting magnetic reconnection by viscosity and thermal conduction

Nonlinear evolution of magnetic reconnection is investigated by means of magnetohydrodynamic simulations including uniform resistivity, uniform viscosity, and anisotropic thermal conduction. When viscosity exceeds resistivity (the magnetic Prandtl number Prm>1), the viscous dissipation dominates outflow dynamics and leads to the decrease in the plasma density inside a current sheet. The low-density current sheet supports the excitation of the vortex. The thickness of the vortex is broader than that of the current for Prm>1. The broader vortex flow more efficiently carries the upstream magnetic flux toward the reconnection region, and consequently, boosts the reconnection. The reconnection rate increases with viscosity provided that thermal conduction is fast enough to take away the thermal energy increased by the viscous dissipation (the fluid Prandtl number Pr < 1). The result suggests the need to control the Prandtl numbers for the reconnection against the conventional resistive model.

Comparative Study of Global MHD Simulations of the Terrestrial Magnetosphere With Different Numerical Schemes

We compare recent global MHD simulation models of the terrestrial magnetosphere based on different numerical schemes. The schemes include the finite-difference method based on the modified leapfrog (MLF) scheme, and the semi-Lagrangian scheme based on the constrained interpolation profile (CIP) algorithm. With the two models, we examined the simulation under a northward interplanetary magnetic field (IMF) condition. As a result, we found out that the two simulation models give consistent results on the magnetopause locations at the subsolar point and the terminator, and the overall structures of the cusp in the meridian plane. However, discrepancies are also found in the location and jump conditions of the bow shock. The MLF model showed higher thermal pressure value and weaker magnetic field strength in the downstream than those in the CIP model. The difference in the jump condition across the shock is also reflected in the difference in the length of the magnetotail in the two models. The magnetotail is shorter in the CIP model than that in the MLF model. We conclude that further comparative studies with finite-volume methods are necessary to verify the solution of the bow shock formation and the location of the last closed field line under northward IMF conditions.

Implementation of a non-oscillatory and conservative scheme into magnetohydrodynamic equations

We present a magnetohydrodynamic (MHD) simulation technique with a new non-oscillatory and conservative interpolation scheme. Several high-resolution and stable numerical schemes have recently been proposed for solving the MHD equations. To apply the CIP scheme to the hydrodynamic equations, we need to add a certain diffusion term to suppress numerical oscillations at discontinuities. Although the TVD schemes can automatically avoid numerical oscillations, they are not appropriate for profiles with a local maximum or minimum, such as waves. To deal with the above problems, we implement a new non-oscillatory and conservative interpolation scheme in MHD simulations. Several numerical tests are carried out in order to compare our scheme with other recent high-resolution schemes. The numerical tests suggest that the present scheme can follow long-term evolution of both Alfvén waves and compressive shocks. The present scheme has been used for a numerical modeling of Alfvén waves in the solar wind, in which sinusoidal Alfvén waves decay into compressive sound waves that steepen into shocks.

Simulation study of energy conversion process in magnetohydrodynamic turbulence due to magnetorotational instability

Three dimensional local simulation was performed to investigate the energy conversion and the dynamo action in the magnetohydrodynamic (MHD) turbulence due to the magnetorotational instability (MRI...

Simulation Study on Magneto-Gravity Instabilities in Magnetic Shear Field

We performed two-dimensional nonlinear MHD simulations for magnetic buoyancy instabilities in a sheared magnetic field. This work is motivated by research on solar coronal magnetic loop emergence. The magnetic buoyancy instabilities can be classified into the Parker instability and the interchange instability, depending on whether the wavevector is parallel or perpendicular to the magnetic field line. We consider a highly stratified domain which includes the cooler chromosphere and the hotter corona [1]. Initially, the sheared magnetic flux is embedded in the bottom of the chromosphere, and satisfies the magnetostatic equilibrium condition. The TVD scheme is employed to calculate the MHD equations numerically.