Tooru Sugiyama

The super‐droplet method for the numerical simulation of clouds and precipitation: a particle‐based and probabilistic microphysics model coupled with a non‐hydrostatic model

A novel, particle-based, probabilistic approach for the simulation of cloud microphysics is proposed, which is named the super-droplet method (SDM). This method enables the accurate simulation of cloud microphysics with a less demanding cost in computation. SDM is applied to a warm-cloud system, which incorporates sedimentation, condensation/evaporation and stochastic coalescence. The methodology to couple super-droplets and a non-hydrostatic model is also developed. It is confirmed that the result of our Monte Carlo scheme for the stochastic coalescence of super-droplets agrees fairly well with the solutions of the stochastic coalescence equation. The behaviour of the model is evaluated using a simple test problem, that of a shallow maritime cumulus formation initiated by a warm bubble. Possible extensions of SDM are briefly discussed. A theoretical analysis suggests that the computational cost of SDM becomes lower than the spectral (bin) method when the number of attributes—the variables that identify the state of each super-droplet—becomes larger than some critical value, which we estimate to be in the range . Copyright

Multi-scale plasma simulation by the interlocking of magnetohydrodynamic model and particle-in-cell kinetic model

Many kinds of simulation models have been developed to understand the complex plasma systems. However, these simulation models have been separately performed because the fundamental assumption of each model is different and restricts the physical processes in each spatial and temporal scales. On the other hand, it is well known that the interactions among the multiple scales may play crucial roles in the plasma phenomena (e.g. magnetic reconnection, collisionless shock), where the kinetic processes in the micro-scale may interact with the global structure in the fluid dynamics. To take self-consistently into account such multi-scale phenomena, we have developed a new simulation model by directly interlocking the fluid simulation of the magnetohyrdodynamics (MHD) model and the kinetic simulation of the particle-in-cell (PIC) model. The PIC domain is embedded in a small part of MHD domain. The both simulations are performed simultaneously in each domain and the bounded data are frequently exchanged each other to keep the consistency between the models. We have applied our new interlocked simulation to Alfven wave propagation problem as a benchmark test and confirmed that the waves can propagate smoothly through the boundaries of each domain.

Short note: A note on the dipole coordinates

A couple of orthogonal coordinates for dipole geometry are proposed for numerical simulations of plasma geophysics in the Earths dipole magnetic field. These coordinates have proper metric profiles along field lines in contrast to the standard dipole coordinate system that is commonly used in analytical studies for dipole geometry.

Development and Application of Geospace Environment Simulator for the Analysis of Spacecraft–Plasma Interactions

Space development has been rapidly increasing, and a strong demand should arise regarding the understanding of the spacecraft-plasma interactions, which is one of the very important issues associated with the human activities in space. To evaluate the spacecraft-plasma interactions including plasma kinetics, transient process, and electromagnetic field variation, the authors have started to develop a numerical plasma chamber called Geospace Environment Simulator (GES) by making the most use of the conventional full particle-in-cell plasma simulations. For the development of a proto model of GES, the authors have used the Earth Simulator, which is one of the fastest supercomputers in the world. GES can be regarded as a numerical chamber in which space experiments can be virtually performed and temporal and spatial evolutions of spacecraft-plasma interactions can be analyzed. In this paper, the authors have briefly introduced GES in terms of its concept, modeling, and research targets. As one of the research topics of GES, the authors have investigated the impedance variation of electric field antenna onboard scientific satellites in the photoelectron environment in space. From the preliminary simulation results, the large change of reactance of the antenna impedance below the characteristic frequency corresponding to the local plasma frequency determined by the photoelectron density could be confirmed

Time sequence of energetic particle spectra in quasiparallel shocks in large simulation systems

We have performed collisionless shock simulations using a one-dimensional hybrid particle-in-cell method to investigate the energy spectra of the differential intensity around the quasiparallel shocks. The system size is sufficiently large (200 000 ion inertia length) in order to eliminate the unphysical effect caused by the upstream boundary. The obtained spectrum of the differential intensity have the shape of the power-law with exponentially falling off in higher energy as predicted in previous simulations, however, the power-law indices and e-folding energy do not depend on the shock parameters, the shock Mach number MA (7.1–11.7) and the shock angle θBN (10°–40°). The power-law index is ∼1.0. This number is close to the prediction by the standard diffusive shock acceleration theory. However, the reason for the agreements between the values is not clear because an additional acceleration process is also observed in the present runs; this process was reported in [Sugiyama and Terasawa, Adv. Space Res. ...

Macro-micro Interlocked Simulation for Multiscale Phenomena

A new methodology for the simulation of multiscale pro-cesses, called Macro-Micro Interlocked (MMI) Simulation, is introduced. The MMI simulation is carried out by the two-way connection of different numerical models, which may handle macroscopic and microscopic dynamics, respectively. The MMI simulation are applied to several multiscale phenomena, for instance, cloud formation, gas detonation, and plasma dynamics. The results indicate that the MMI simulation provide us an effective and prospective framework for multiscale simulation.

Model experiment of magnetic field amplification in laser-produced plasmas via the Richtmyer-Meshkov instability

A model experiment of magnetic field amplification (MFA) via the Richtmyer-Meshkov instability (RMI) in supernova remnants (SNRs) was performed using a high-power laser. In order to account for very-fast acceleration of cosmic rays observed in SNRs, it is considered that the magnetic field has to be amplified by orders of magnitude from its background level. A possible mechanism for the MFA in SNRs is stretching and mixing of the magnetic field via the RMI when shock waves pass through dense molecular clouds in interstellar media. In order to model the astrophysical phenomenon in laboratories, there are three necessary factors for the RMI to be operative: a shock wave, an external magnetic field, and density inhomogeneity. By irradiating a double-foil target with several laser beams with focal spot displacement under influence of an external magnetic field, shock waves were excited and passed through the density inhomogeneity. Radiative hydrodynamic simulations show that the RMI evolves as the density inhomogeneity is shocked, resulting in higher MFA.

Energy Spectra of Energetic Ions Around Quasi‐Parallel Shocks

We have performed a number of simulations of quasi-parallel shocks to investigate how the energy spectra of non-thermal components are controlled by the presence of alpha (He 2+ ) particles. The simulations are done for the Earths bow shock-like situation by an one-dimensional hybrid code that treats ions as particles but electrons as a massless fluid. The upstream conditions are modified by varying the He 2+ ions content while keeping the proton content unchanged. The range of the He 2+ ion density ratio (R) variation relative to proton is from 0.1% to 30% including the typical ratio of 4 ∼ 5% for the solar wind. As observed in the upstream of the bow shock, the differential flux spectra of the two ion species obtained in the simulations from the downstream region are found to be well represented by exponential shapes. When the energy scale is presented in the energy-per-charge (E/Q) unit, the two spectra have the same characteristic energy (E C ) that increases in time. While E C is a function of the density ratio R and increases with increasing R, the equality between the two species holds throughout. Detailed analyses show that larger magnetic fluctuations brought about by the presence of He 2+ particles enable more efficient acceleration at the shock front. Thus it is via exciting, stronger turbulence that the additional upstream ram energy carried by He 2+ ions is poured into the process of hardening the energy spectrum of the non-thermal particles of both species.

Power-law spatial profile in an upstream region of CME-driven interplanetary shock

We study the density decay profile of energetic particles in the upstream region of an interplanetary shock on 14 Dec 2006 observed by the ACE spacecraft at 1 AU. The spatial decay profile of the energetic particle flux does not exhibit an exponential behavior as expected for the standard diffusive shock acceleration process but a power-law behavior in anomalous or superdiffusive transport. The power-law profiles are observed for not only the energetic ions reported in Sugiyama & Shiota (2011) but also heavier ions of He2+, CNO, and Fe. We observe the relation 〈Δx2〉 ∝ tα for α ~ 1.24–1.72, where Δx is the particle displacement within the time scale t, and the bracket denotes an ensemble average. This implies that particle propagation around a near-earth orbit can be intermediate between normal diffusion (α = 1) and ballistic motion (α equals 2).

Spacecraft plasma environment analysis via large scale 3D plasma particle simulation

Geospace environment simulator (GES) has started as one of the advanced computing research projects at the Earth Simulator Center in Japan Marine Science and Technology Center since 2002: [1]. By using this computing resource, a large scale simulation which reproduces a realistic physical model can be utilized not only for studying the geospace environment but also for various human activities in space. GES project aims to reproduce fully kinetic environment around a spacecraft by using the 3-dimensional full-particle electromagnetic simulation code which could include spacecraft model inside (NuSPACE). NuSPACE can model interaction between space plasma and a spacecraft by the unstructuredgrid 3D plasma particle simulation code embedded in the NuSPACE.We will report current status of the project and our concept of achieving the spacecraft environment in conjunction with the space weather.