Researcher Profile and Settings

Master

Affiliation (Master)

• Information Initiative Center　Supercomputing

Affiliation (Master)

• Information Initiative Center　Supercomputing

researchmap

Profile and Settings

Affiliation

• Hokkaido University, Information Initiative Center, Professor

Degree

• Doctor of Informatics（Kyoto University）

Profile and Settings

• Contact Point

  umedaiic.hokudai.ac.jp

• Name (Japanese)

  Umeda

• Name (Kana)

  Takayuki

• Name

  200901087140936495

Affiliation

• Hokkaido University, Information Initiative Center, Professor

Achievement

Research Interests

• Computer simulation   Numerical scheme   High performance Computing   Plasma science   Space science   Electromagnetic fields   

Research Areas

• Informatics / High-performance computing / High-performance Computing
• Informatics / Computational science / Computer Simulation
• Natural sciences / Space and planetary science / Space Plasma
• Energy / Basic plasma science / Plasma Physics

Research Experience

• 2024/02 - Today Nagoya University Graduate School of Engineering, Electrical Engineering Visiting Professor
• 2024/02 - Today Nagoya University Institute for Space and Earth Environmental Research Visiting Professor
• 2024/02 - Today Hokkaido University Information Initiative Center Professor
• 2018/09 - 2024/02 Nagoya University Institute for Space-Earth Environmental Research, Center for Integrated Data Science Associate professor
• 2015/10 - 2018/08 Nagoya University Institute for Space-Earth Environmental Research, Center for Integrated Data Science Lecturer
• 2015/08 - 2015/09 Nagoya University Solar-Terrestrial Environment Laboratory, Geospace Research Center Lecturer
• 2007/04 - 2015/07 Nagoya University Solar-Terrestrial Environment Laboratory, Geospace Research Center Assistant Professor
• 2006/04 - 2007/03 Nagoya University Solar-Terrestrial Environment Laboratory, Geospace Research Center Assistant Professor
• 2004/04 - 2006/03 University of California, Los Angeles Institute of Geophysics and Planetary Physics Postgraduate Researcher
• 2001/04 - 2004/03 JSPS Research Fellow

Education

• 2001/04 - 2004/03  Kyoto University  Graduate School of Informatics  Department of Communications and Computer Engineering
• 1999/04 - 2001/03  Kyoto University  Graduate School of Informatics  Department of Communications and Computer Engineering
• 1995/04 - 1999/03  Kyoto University  Faculty of Engineering  School of Electrical & Electronic Engineering

Committee Memberships

• 2010/10 - Today   Japan Geoscience Union   Session Convener
• 2010/07 - Today   Society of Geomagnetism and Earth, Planetary and Space Sciences   Session convener

Awards

• 2023/11 Division of Plasma Physics, Association of Asia Pacific Physical Societies 7th Asia-Pacific Conference on Plasma Physics (AAPPS-DPP2023) Poster Prize
   Reduction of Anisotropy in Numerical Dispersion in the Explicit Finite-Difference Time-Domain Method with Laplacian 

  受賞者: H. Sekido;T. Umeda;Y. Miyoshi
• 2018/11 Division of Plasma Physics, Association of Asia Pacific Physical Societies 2nd Asia-Pacific Conference on Plasma Physics (AAPPS-DPP2018) Poster Prize
   Non-MHD effects in the nonlinear development of the MHD-scale Rayleigh-Taylor instability 

  受賞者: T. Umeda;Y. Wada
• 2015/04 文部科学省 科学技術分野の文部科学大臣表彰 若手科学者賞
   先端的宇宙プラズマ計算機シミュレーション手法の研究
• 2012/10 地球電磁気・地球惑星圏学会 大林奨励賞
   計算機シミュレーション手法の開発とその宇宙プラズマ現象への応用
• 2008/11 Frontiers in Computational Science 2008 Program Committee Best Presentations Award for International Conference of Frontiers in Computational Science 2008 Grant Prix
   Vlasov code simulation of GEM reconnection challenge 

  受賞者: K. Togano;T. Umeda;T. Ogino
• 2005/10 International Union of Radio Science International Union of Radio Science Young Scientist Award
  

Published Papers

• Generation of normal distributions revisited

  Takayuki Umeda

  Computational Statistics 0943-4062 2024/02/23 [Refereed][Not invited]
   

  Abstract Normally distributed random numbers are commonly used in scientific computing in various fields. It is important to generate a set of random numbers as close to a normal distribution as possible for reducing initial fluctuations. Two types of samples from a uniform distribution are examined as source samples for inverse transform sampling methods. Three types of inverse transform sampling methods with new approximations of inverse cumulative distribution functions are also discussed for converting uniformly distributed source samples to normally distributed samples.
• Relaxation of the Courant Condition in the Explicit Finite-Difference Time-Domain(2,6) Method with Third- and Fifth-degree Differential Terms

  Harune Sekido, Takayuki Umeda

  Progress In Electromagnetics Research M 123 83 - 93 2024/01/23 [Refereed][Not invited]
  
• A novel high accuracy finite-difference time-domain method

  Harune Sekido, Takayuki Umeda

  Earth, Planets and Space 76 (1) 1343-8832 2024/01/05 [Refereed][Not invited]
   

  Abstract The finite-difference time-domain (FDTD) method is widely used for numerical simulations of electromagnetic waves and acoustic waves. It is known, however, that the Courant condition is restricted in higher dimensions and with higher order differences in space. Although it is possible to relax the Courant condition by utilizing the third-degree difference in space, there remains a large anisotropy in the numerical dispersion at large Courant numbers. This study aims to reduce the anisotropy in the numerical dispersion and relax the Courant condition simultaneously. A new third-degree difference operator including the Laplacian is introduced to the time-development equations of FDTD(2,4) with second- and fourth-order accuracies. The present numerical simulations have demonstrated that numerical oscillations due to the anisotropic dispersion relation are reduced with the new operator. Graphical Abstract
• Advanced numerical techniques for time integration of relativistic equations of motion for charged particles

  Takayuki Umeda, Riku Ozaki

  Earth, Planets and Space 75 (1) 1343-8832 2023/10/10 [Refereed][Not invited]
   

  Abstract Advanced numerical techniques for solving the relativistic equations of motion for charged particles are provided. A new fourth-order integrator is developed by combining the Taylor series expansion of the numerical angle of relativistic gyration and the fourth-order Runge–Kutta method for integrating the Lorentz factor. The new integrator gives the exact relativistic E-cross-B drift velocity, but has a numerical accuracy much higher than the classic fourth-order Runge–Kutta integrator. Graphical Abstract
• New Aspects of Energy Conversion in Magnetic Island Dynamics: Particle-in-cell Simulation of Multiple Island Coalescence and MMS Observations

  W.-L. Teh, T. K. M. Nakamura, S. Zenitani, T. Umeda, R. Nakamura

  The Astrophysical Journal 947 (1) 4 - 4 0004-637X 2023/04/01 [Refereed][Not invited]
   

  Abstract Coalescence of multiple magnetic islands is recognized as an effective process to energize particles during magnetic reconnection, while its energy conversion process still remains unclear. Here, a two-dimensional fully kinetic simulation of multiple island coalescence with a small reconnection guide field is studied. In the analysis of energy conversion within a magnetic island, the dot product of ${ { \boldsymbol{V } } }_{e}\cdot \left({\boldsymbol{j } }\times {\boldsymbol{B } }\right)={w}_{1}$ is a useful quantity to compare with j · E = w₂, since the average work done by the Lorentz force on the circulating particles is negligible in the island and ${w}_{2}-{w}_{1}={\boldsymbol{j } }\cdot \left({\boldsymbol{E } }+{ { \boldsymbol{V } } }_{e}\times {\boldsymbol{B } }\right)={\boldsymbol{j } }\cdot { { \boldsymbol{E } } }^{ { \prime} }={w}_{3}$. A bipolar pattern of w₁ is found at a secondary island when the electrons are in circular motion inside the island. Significant energy dynamo (w₃ < 0) resulting from j_∥E_∥ is found at the secondary island, which has not been reported before, where the parallel electric field E_∥ is highly correlated with w₃. Moreover, significant energy dissipation (w₃ > 0) due to ${ { \boldsymbol{j } } }_{\perp }\cdot { { \boldsymbol{E } } }_{\perp }^{ { \prime} }$ is seen in the merging region between two coalescing islands. Both types of energy conversions are accompanied by enhancements in j_∥ and the parallel electron temperature T_e∥. Three ion-scale magnetic islands (FR1, FR2, and FR3) observed by the Magnetospheric Multiscale spacecraft are compared favorably with the simulated signatures of energy dynamo and dissipation of an evolving secondary island. In particular, FR1 displayed a similar energy dynamo signature as that simulated in an early stage of the secondary island. FR2 and FR3 showed a dominant ${ { \boldsymbol{j } } }_{\perp }\cdot { { \boldsymbol{E } } }_{\perp }^{ { \prime} }$ energy conversion similar to that obtained in a later stage of the secondary island.
• Relaxation of the Courant Condition in the Explicit Finite-Difference Time-Domain Method With Higher-Degree Differential Terms

  Harune Sekido, Takayuki Umeda

  IEEE Transactions on Antennas and Propagation 71 (2) 1630 - 1639 0018-926X 2023/02 [Refereed][Not invited]
  
• Spatial and time scaling of coalescing multiple magnetic islands

  T. K. M. Nakamura, W.-L. Teh, S. Zenitani, T. Umeda, M. Oka, H. Hasegawa, A. M. Veronig, R. Nakamura

  Physics of Plasmas 30 (2) 1070-664X 2023/02/01 [Refereed][Not invited]
   

  Magnetic reconnection is a key fundamental process in collisionless plasmas, which converts magnetic energy to plasma kinetic energy. Past observation and simulation studies suggested that this process causes an efficient energy conversion through the formation and coalescence of multiple magnetic islands. In this study, based on a large-scale two-dimensional fully kinetic simulations of coalescing multiple islands with a moderate guide magnetic field, we first examined the spatial dimensions of the internal structures of the coalescing islands. The results show that the dimensions of the structures in the directions normal to and along the initial current sheet depend on the initial thickness of the current sheet and the number of coalescing islands. We then found that the horizontal dimension of the structures controls the evolution time scale of the island coalescence process. We further found that when the vertical dimension of the structures, which corresponds to the length of the reconnection X-line in the reconnection outflow direction at the merging point between the two coalescing islands, is sufficiently longer than the ion inertial length, reconnection in the merging current sheet can well mature and both ions and electrons can be effectively heated around the merging X-line. The obtained scaling predicts that such a strong heating by well-matured reconnection in the island coalescence process would be seen in various plasma environments, such as the Earth's magnetotail and solar flares.
• A new integrator for relativistic E-cross-B motion of charged particles

  Takayuki Umeda

  Journal of Computational Physics 472 111694 - 111694 0021-9991 2023/01 [Refereed][Not invited]
  
• Multicolor reordering for computing moments in particle-in-cell plasma simulations

  Takayuki Umeda

  Computer Physics Communications 281 108499 - 108499 0010-4655 2022/12 [Refereed][Not invited]
  
• High-power laser experiment forming a supercritical collisionless shock in a magnetized uniform plasma at rest

  R. Yamazaki, S. Matsukiyo, T. Morita, S. J. Tanaka, T. Umeda, K. Aihara, M. Edamoto, S. Egashira, R. Hatsuyama, T. Higuchi, T. Hihara, Y. Horie, M. Hoshino, A. Ishii, N. Ishizaka, Y. Itadani, T. Izumi, S. Kambayashi, S. Kakuchi, N. Katsuki, R. Kawamura, Y. Kawamura, S. Kisaka, T. Kojima, A. Konuma, R. Kumar, T. Minami, I. Miyata, T. Moritaka, Y. Murakami, K. Nagashima, Y. Nakagawa, T. Nishimoto, Y. Nishioka, Y. Ohira, N. Ohnishi, M. Ota, N. Ozaki, T. Sano, K. Sakai, S. Sei, J. Shiota, Y. Shoji, K. Sugiyama, D. Suzuki, M. Takagi, H. Toda, S. Tomita, S. Tomiya, H. Yoneda, T. Takezaki, K. Tomita, Y. Kuramitsu, Y. Sakawa

  Physical Review E 105 (2) 2470-0045 2022/02/11 [Refereed][Not invited]
  
• Multi-scale evolution of Kelvin–Helmholtz waves at the Earth's magnetopause during southward IMF periods

  T. K. M. Nakamura, K. A. Blasl, H. Hasegawa, T. Umeda, Y.-H. Liu, S. A. Peery, F. Plaschke, R. Nakamura, J. C. Holmes, J. E. Stawarz, W. D. Nystrom

  Physics of Plasmas 29 (1) 1070-664X 2022/01/01 [Refereed][Not invited]
   

  At the Earth's low-latitude magnetopause, the Kelvin–Helmholtz instability (KHI), driven by the velocity shear between the magnetosheath and magnetosphere, has been frequently observed during northward interplanetary magnetic field (IMF) periods. However, the signatures of the KHI have been much less frequently observed during southward IMF periods, and how the KHI develops under southward IMF has been less explored. Here, we performed a series of realistic 2D and 3D fully kinetic simulations of a KH wave event observed by the Magnetospheric Multiscale (MMS) mission at the dusk-flank magnetopause during southward IMF on September 23, 2017. The simulations demonstrate that the primary KHI bends the magnetopause current layer and excites the Rayleigh–Taylor instability (RTI), leading to penetration of high-density arms into the magnetospheric side. This arm penetration disturbs the structures of the vortex layer and produces intermittent and irregular variations of the surface waves which significantly reduces the observational probability of the periodic KH waves. The simulations further demonstrate that in the non-linear growth phase of the primary KHI, the lower-hybrid drift instability (LHDI) is induced near the edge of the primary vortices and contributes to an efficient plasma mixing across the magnetopause. The signatures of the large-scale surface waves by the KHI/RTI and the small-scale fluctuations by the LHDI are reasonably consistent with the MMS observations. These results indicate that the multi-scale evolution of the magnetopause KH waves and the resulting plasma transport and mixing as seen in the simulations may occur during southward IMF.
• Paradigm Shift in Program Structure of Particle-in-Cell Simulations

  Takayuki Umeda

  Parallel Computing: Technology Trends, Advances in Parallel Computing 36 455 - 464 0927-5452 2020/03/20 [Refereed][Not invited]
   

  Performance measurement of the particle-in-cell (PIC) method for collisionless plasma is made on the strong scaling of the thread-level parallelism with OpenMP. The conventional program structure of the PIC method, in which a single loop statement involves an iteration through the list of particles, is compared with the new program structure, in which outer multiple loop statements involve iterations through spatial grid cells and the most inner single loop statement involves an iteration through the list of particles. The present strong scaling measurement shows that the new program structure improves both performance and scalability of the PIC code from the conventional program structure. The new code runs about three times faster than the conventional code without sorting of the list of particles.
• Evaluating higher moments in the transverse Kelvin–Helmholtz instability by full kinetic simulation

  Takayuki Umeda

  Physics of Plasmas 27 (3) 1070-664X 2020/03/01 [Refereed][Not invited]
   

  Approximated forms of the third and fourth moments of a velocity distribution function are derived by using a perturbed velocity distribution function around a characteristic spatial scale on the gyroradius derived by Thompson [Rep. Prog. Phys. 24, 363–424 (1961)]. Then, they are evaluated by using a two-dimensional full kinetic Vlasov simulation result of the transverse Kelvin–Helmholtz instability. It is shown that the derived form of the fourth moment is in agreement with the one calculated from the distribution function data of the Vlasov simulation. On the other hand, the derived form of the third moment is quite different from the one (i.e., heat flux tensor) calculated from the distribution function data of the Vlasov simulation. The results suggest that the perturbed velocity distribution function of Thompson needs an improvement.
• Stability of Contact Discontinuities in Electrostatic Hybrid- and Full-Vlasov Simulations

  Naru TSUJINE, Takayuki HARUKI, Takayuki UMEDA, Yasuhiro NARIYUKI, Masahiro SATO

  Plasma and Fusion Research 15 1401002 - 1401002 2020/02/17 [Refereed]
  
• Disturbance of the Front Region of Magnetic Reconnection Outflow Jets due to the Lower-Hybrid Drift Instability

  T. K. M. Nakamura, T. Umeda, R. Nakamura, H. S. Fu, M. Oka

  Physical Review Letters 123 (23) 0031-9007 2019/12/02 [Refereed][Not invited]
  
• Mach number and plasma beta dependence of the ion temperature perpendicular to the external magnetic field in the transition region of perpendicular collisionless shocks

  Ryo Yamazaki, Ayato Shinoda, Takayuki Umeda, Shuichi Matsukiyo

  AIP Advances 9 (12) 2019/12/01 [Refereed][Not invited]
   

  Ion temperature anisotropy is a common feature for (quasi-)perpendicular collisionless shocks. By using two-dimensional full particle simulations, it is shown that the ion temperature component perpendicular to the shock magnetic field at the shock foot region is proportional to the square of the Alfvén Mach number divided by the plasma beta. This result is also explained by a simple analytical argument in which the reflected ions get energy from an upstream plasma flow. By comparing our analytic and numerical results, it is also confirmed that the fraction of the reflected ions hardly depends on the plasma beta and the Alfvén Mach number when the square of the Alfvén Mach number divided by the plasma beta is larger than about 20.
• Vlasov code simulation of contact discontinuities

  Takayuki Umeda, Naru Tsujine, Yasuhiro Nariyuki

  Physics of Plasmas 26 (10) 1070-664X 2019/10/01 [Refereed][Not invited]
   

  The stability of contact discontinuities formed by the relaxation of two Maxwellian plasmas with different number densities but the same plasma thermal pressure is studied by means of a one-dimensional electrostatic full Vlasov simulation. Our simulation runs with various combinations of ion-to-electron ratios of the high-density and low-density regions showed that transition layers of density and temperature without jump in the plasma thermal pressure are obtained when the electron temperatures in the high-density and low-density regions are almost equal to each other. However, the stable structure of the contact discontinuity with a sharp transition layer on the Debye scale is not maintained. It is suggested that non-Maxwellian velocity distributions are necessary for the stable structure of contact discontinuities.
• Introduction to Basic Space Plasma Physics and Its Methods for Computer Simulation

  Takayuki UMEDA

  JAPANESE JOURNAL OF MULTIPHASE FLOW 33 (3) 249 - 257 0914-2843 2019/09/15 [Not refereed][Invited]
  
• Multi-step Boris rotation schemes for Lorentz force equation of charged particles

  Takayuki Umeda

  Computer Physics Communications 237 37 - 41 0010-4655 2019/04 [Refereed][Not invited]
  
• Full particle-in-cell simulation of the interaction between two plasmas for laboratory experiments on the generation of magnetized collisionless shocks with high-power lasers

  Takayuki Umeda, Ryo Yamazaki, Yutaka Ohira, Natsuki Ishizaka, Shin Kakuchi, Yasuhiro Kuramitsu, Shuichi Matsukiyo, Itaru Miyata, Taichi Morita, Youichi Sakawa, Takayoshi Sano, Shuto Sei, Shuta J. Tanaka, Hirohumi Toda, Sara Tomita

  Physics of Plasmas 26 (3) 1070-664X 2019/03/01 [Refereed][Not invited]
   

  A preliminary numerical experiment is conducted for laboratory experiments on the generation of magnetized collisionless shocks with high-power lasers by using one-dimensional particle-in-cell simulation. The present study deals with the interaction between a moving aluminum plasma and a nitrogen plasma at rest. In the numerical experiment, the nitrogen plasma is unmagnetized or magnetized by a weak external magnetic field. Since the previous study suggested the generation of a spontaneous magnetic field in the piston (aluminum) plasma due to the Biermann battery, the effect of the magnetic field is of interest. Sharp jumps of the electron density and magnetic field are observed around the interface between the two plasmas as long as one of the two plasmas is magnetized, which indicates the formation of tangential electron-magneto-hydro-dynamic discontinuity. When the aluminum plasma is magnetized, strong compression of both the density and the magnetic field takes place in the pure aluminum plasma during the gyration of nitrogen ions in the aluminum plasma region. The formation of a shock downstream is obtained from the shock jump condition. The results suggest that the spontaneous magnetic field in the piston (aluminum) plasma plays an essential role in the formation of a perpendicular collisionless shock.
• Magnetosonic/whistler mode turbulence influences on ion dynamics

  S. Saito, Y. Nariyuki, T. Umeda

  Physics of Plasmas 25 (12) 1070-664X 2018/12/01 [Refereed][Not invited]
   

  The development of decaying magnetosonic/whistler mode turbulence is investigated, employing a two-dimensional, fully kinetic, particle-in-cell simulation that covers scales at the end of the inertial range. The anisotropic wavenumber spectrum of magnetic fluctuations reaches electron kinetic scales through the forward cascade of the turbulence. The magnetic fluctuations have intermittency at scales shorter than the ion inertial length. The intermittent nature leads to a localized steepening of magnetic fluctuations which accelerates ions nonlinearly in the direction perpendicular to the background magnetic field. The non-thermal ions have the speed of several times faster than the ion thermal speed. The fully kinetic particle-in-cell simulation shows important implications for a fundamental understanding of the non-thermal ion production in magnetosonic/whistler mode turbulence.
• On the Boris solver in particle-in-cell simulation

  Seiji Zenitani, Takayuki Umeda

  Physics of Plasmas 25 (11) 1070-664X 2018/11/01 [Refereed][Not invited]
   

  A simple form of the Boris solver in particle-in-cell (PIC) simulation is proposed. It employs an exact solution of the Lorentz-force part, and it is equivalent to the Boris solver with a gyrophase correction. As a favorable property for stable schemes, this form preserves a volume in the phase space. Numerical tests of the Boris solvers are conducted by test-particle simulations and by PIC simulations. The proposed form provides better accuracy than the popular form, while it only requires few additional computation time.
• Performance Comparison of Eulerian Kinetic Vlasov Code Between Xeon Phi KNL and Xeon Broadwell

  Takayuki Umeda, Keiichiro Fukazawa

  Methods and Applications for Modeling and Simulation of Complex Systems, AsiaSim 2018, Communications in Computer and Information Science 946 143 - 150 1865-0929 2018/10/18 [Refereed][Not invited]
  
• Oblique Ion‐Scale Magnetotail Flux Ropes Generated by Secondary Tearing Modes

  Wai‐Leong Teh, Takuma Nakamura, Rumi Nakamura, Takayuki Umeda

  Journal of Geophysical Research: Space Physics 123 (10) 8122 - 8130 2169-9380 2018/10/08 [Refereed][Not invited]
   

  Abstract We examine Magnetospheric Multiscale observations of three ion‐scale magnetic flux ropes (diameter ~6–9 ion inertial lengths) in the Earth's magnetotail reconnection diffusion region with a small guide field. During the event, the electron reconnection exhaust reversed direction together with the reversal of normal magnetic field component. After that, the three ion‐scale flux ropes were observed near the ion diffusion region, in which flat‐top distributions were found at the field‐aligned electrons (<2 keV). The axis orientation and motion velocity of the flux ropes are determined by minimum variance analysis of the electric field component along the flux rope axis, using single‐spacecraft measurements. Results show that the three flux rope axes form 13°, 55°, and 55° to the guide field direction and are mostly tilted toward the direction of the reconnecting field. These results are also validated by multiple‐spacecraft analysis methods. It is found that the tilted angles agree reasonably well with the predicted angles for secondary tearing modes, suggesting that these ion‐scale flux ropes are likely generated by secondary tearing instabilities as those recently found in the three‐dimensional kinetic simulations of turbulent magnetic reconnection.
• Electromagnetic linear dispersion relation for plasma with a drift across magnetic field revisited

  Takayuki Umeda, Takuma K. M. Nakamura

  Physics of Plasmas 25 (10) 1070-664X 2018/10/01 [Refereed][Not invited]
   

  A current across the magnetic field is formed in various situations in plasma. The relative drift between ions and electrons due to the cross-field current becomes a source of various microscopic instabilities. A fully electromagnetic and kinetic linear dispersion relation for plasma with a drift across magnetic field is derived by assuming a uniform background plasma. The dielectric permittivity tensor for shifted Maxwellian velocity distributions is also presented. Linear dispersion relations obtained by using the new dielectric permittivity tensor were confirmed by comparison with the previous studies and with particle-in-cell simulation results.
• Periodic self-reformation of rippled perpendicular collisionless shocks in two dimensions

  Takayuki Umeda, Yuki Daicho

  Annales Geophysicae 36 (4) 1047 - 1055 0992-7689 2018/08/01 [Refereed][Not invited]
   

  Abstract. Large-scale two-dimensional (2-D) full particle-in-cell (PIC) simulations arecarried out for studying periodic self-reformation of a supercriticalcollisionless perpendicular shock with an Alfvén–Mach number MA∼6. Previous self-consistent one-dimensional (1-D) hybrid and full PICsimulations have demonstrated that the periodic reflection of upstream ionsat the shock front is responsible for the formation and vanishing of theshock-foot region on a timescale of the local ion cyclotron period, which wasdefined as the reformation of (quasi-)perpendicular shocks. The present 2-D full PIC simulations with different ion-to-electron massratios show that the dynamics at the shock front is strongly modified bylarge-amplitude ion-scale fluctuations at the shock overshoot, which areknown as ripples. In the run with a small mass ratio, the simultaneous enhancement of the shockmagnetic field and the reflected ions take place quasi-periodically, which isidentified as the reformation. In the runs with large mass ratios, thesimultaneous enhancement of the shock magnetic field and the reflected ionsoccur randomly in time, and the shock magnetic field is enhanced on atimescale much shorter than the ion cyclotron period. These results indicate a coupling between the shock-front ripples andelectromagnetic microinstabilities in the foot region in the runs with largemass ratios. Keywords. Space plasma physics (wave–particle interactions)
• Decay of nonlinear whistler mode waves: 1D versus 2D

  Takayuki Umeda, Shinji Saito, Yasuhiro Nariyuki

  Physics of Plasmas 25 (7) 1070-664X 2018/07/01 [Refereed][Not invited]
   

  Direct comparison between one-dimensional (1D) and two-dimensional (2D) models for the development of a nonlinear, short-wavelength, and monochromatic electromagnetic whistler mode wave is made by means of fully electromagnetic particle-in-cell simulations. The 1D and 2D simulations are performed for low beta conditions in which the plasma pressure is much lower than the magnetic pressure, although the plasma kinetic energy in the direction perpendicular to the ambient magnetic field is highly dominant due to the velocity field of the imposed parent whistler mode wave. A three-wave parametric decay of the parent whistler mode wave was reconfirmed in the 1D simulation. On the other hand, a rapid decay of the parent whistler mode wave thorough a five-wave interaction or double three-wave interactions was seen in the 2D simulation. Electron heating processes in the 2D simulation are also different from those in the 1D simulation. It is suggested that the present 2D decay process is a new instability which is quite different from velocity space instabilities driven by temperature/energy anisotropy.
• A three-step Boris integrator for Lorentz force equation of charged particles

  Takayuki Umeda

  Computer Physics Communications 228 1 - 4 0010-4655 2018/07 [Refereed]
  
• Performance Analysis of CPU and DRAM Power Constrained Systems with Magnetohydrodynamic Simulation Code

  Keiichiro Fukazawa, Masatsugu Ueda, Yuichi Inadomi, Mutsumi Aoyagi, Takayuki Umeda, Koji Inoue

  2018 IEEE 20th International Conference on High Performance Computing and Communications; IEEE 16th International Conference on Smart City; IEEE 4th International Conference on Data Science and Systems (HPCC/SmartCity/DSS) 626 - 631 2018/06 [Refereed][Not invited]
  
• 宇宙プラズマの運動論シミュレーション

  梅田 隆行

  計算数理工学レビュー 日本計算数理工学会 2018 (1) 27 - 35 2018/03 [Not refereed][Invited]
  
• Performance Evaluation and Optimization of MagnetoHydroDynamic Simulation for Planetary Magnetosphere with Xeon Phi KNL

  Keiichiro Fukazawa, Takeshi Soga, Takayuki Umeda, Takeshi Nanri

  Parallel Computing is Everywhere, Advances in Parallel Computing 32 178 - 187 1879-808X 2018 [Refereed][Not invited]
   

  The magnetohydrodynamic (MHD) simulation is often applied to study the global dynamics and configuration of a planetary magnetosphere for the space weather. In this paper, the computational performance of MHD code is evaluated with 128 nodes Xeon Phi KNL of Cray XC40. As the results, the 2D and 3D domain decompositions of SoA (structure of array) make the effective performances and AoS (array of structure) and hybrid parallel computation become low performances. Adding the performance optimizations for Xeon Phi to our MHD simulation code, then we have obtained 2.4 % increase of execution efficiency in total and we achieved 3 TFlops performance gain using 128 nodes.
• Non-MHD effects in the nonlinear development of the MHD-scale Rayleigh-Taylor instability

  Takayuki Umeda, Yasutaka Wada

  Physics of Plasmas 24 (7) 1070-664X 2017/07/01 [Refereed][Not invited]
   

  The nonlinear evolution of the Rayleigh-Taylor instability (RTI) at a density shear layer transverse to magnetic field in a collisionless plasma is investigated by means of a fully kinetic Vlasov simulation with two spatial and two velocity dimensions. The primary RTI in the MHD regime develops symmetrically in a coordinate axis parallel to gravity as seen in the previous MHD simulations. The primary RTI in the Hall-MHD regime develops asymmetrically in a coordinate axis parallel to gravity. A compressible flow is formed at the secondary density shear layer by the Hall effect, which generates a strong scalar pressure gradient of ions. A Hall electric field due to the diamagnetic current results in the asymmetric flow at the tip of the finger structure. In the primary RTI with the ion gyro kinetic effect, secondary RTI with a wavelength shorter than the wavelength of the primary RTI is generated at the saturation stage of the primary RTI. A seed perturbation for the secondary RTI is excited by another secondary instability due to the coupling between the electron stress tensor and the Hall electric field. The heat flux term plays an important role in the time development of the total pressure. On the other hand, the contribution of the ion stress tensor is small in both the electric current and the total pressure.
• Generation of intermittent ion acoustic waves in whistler-mode turbulence

  S. Saito, Y. Nariyuki, T. Umeda

  Physics of Plasmas 24 (7) 1070-664X 2017/06/29 [Refereed][Not invited]
   

  A two-dimensional, fully kinetic, electromagnetic, particle-in-cell simulation in a magnetized collisionless plasma has been performed, demonstrating the generation of intermittent ion acoustic waves in finite-amplitude whistler-mode turbulence. The self-consistent simulation shows that ion/ion acoustic instability can be driven as a consequence of the nonlinear evolution of whistler-mode turbulence. The instability triggering the generation of ion acoustic waves occurs intermittently in several local regions. We propose that the nonlinear development of the phase-space density that drives kinetic instabilities must be analyzed with greater care if the dissipation of plasma turbulence is to be understood.
• Rapid decay of nonlinear whistler waves in two dimensions: Full particle simulation

  Takayuki Umeda, Shinji Saito, Yasuhiro Nariyuki

  Physics of Plasmas 24 (5) 1070-664X 2017/04/26 [Refereed][Not invited]
   

  The decay of a nonlinear, short-wavelength, and monochromatic electromagnetic whistler wave is investigated by utilizing a two-dimensional (2D) fully relativistic electromagnetic particle-in-cell code. The simulation is performed under a low-beta condition in which the plasma pressure is much lower than the magnetic pressure. It has been shown that the nonlinear (large-amplitude) parent whistler wave decays through the parametric instability in a one-dimensional (1D) system. The present study shows that there is another channel for the decay of the parent whistler wave in 2D, which is much faster than in the timescale of the parametric decay in 1D. The parent whistler wave decays into two sideband daughter whistlers propagating obliquely with respect to the ambient magnetic field with a frequency close to the parent wave and two quasi-perpendicular electromagnetic modes with a frequency close to zero via a 2D decay instability. The two sideband daughter oblique whistlers also enhance a nonlinear longitudinal electrostatic wave via a three-wave interaction as a secondary process.
• Secondary instabilities in the collisionless Rayleigh-Taylor instability: Full kinetic simulation

  Takayuki Umeda, Yasutaka Wada

  Physics of Plasmas 23 (11) 1070-664X 2016/11/01 [Refereed]
   

  The nonlinear evolution of the Rayleigh-Taylor instability (RTI) at a density shear layer transverse to magnetic field in collisionless plasma is investigated by means of a fully kinetic Vlasov simulation with two spatial and two velocity dimensions. The primary RTI in the MHD regime develops symmetrically in a coordinate axis parallel to gravity as seen in the previous MHD simulations. Small-scale secondary instabilities are generated due to secondary velocity shear layers formed by the nonlinear development of the primary RTI. The secondary instabilities take place asymmetrically in the coordinate axis parallel to gravity. It is suggested that these secondary instabilities correspond to the electron Kelvin-Helmholtz instability generated by the electron velocity shear, whose development depends on the polarity of the inner product between the magnetic field and the vorticity of the velocity shear layer.
• Performance comparison of Eulerian kinetic Vlasov code between flat-MPI parallelism and hybrid parallelism on Fujitsu FX100 supercomputer

  Takayuki Umeda, Keiichiro Fukazawa

  Proceedings of the 23rd European MPI Users' Group Meeting 218 - 221 2016/09/25 [Refereed][Not invited]
  
• Three‐dimensional development of front region of plasma jets generated by magnetic reconnection

  T. K. M. Nakamura, R. Nakamura, W. Baumjohann, T. Umeda, I. Shinohara

  Geophysical Research Letters 43 (16) 8356 - 8364 0094-8276 2016/08/26 [Refereed][Not invited]
   

  Abstract A three‐dimensional fully kinetic particle‐in‐cell simulation of antiparallel magnetic reconnection is performed to investigate the three‐dimensional development of reconnection jet fronts treating three instabilities: the lower hybrid drift instability (LHDI), the ballooning/interchange instability (BICI), and the ion‐ion kink instability. Sufficiently large system size and high ion‐to‐electron mass ratio of the simulation allow us to see the coupling among the three instabilities in the fully kinetic regime for the first time. As the jet fronts develop, the LHDI and BICI become dominant over the ion‐ion kink instability. The rapid growth of the LHDI enhances the BICI growth and the resulting formation of finger‐like structures. The small‐scale front structures produced by these instabilities are similar to recent high‐resolution field observations of the dipolarization fronts in the near‐Earth magnetotail using Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Cluster spacecraft and pose important questions for a future full high‐resolution observation by the Magnetospheric Multiscale (MMS) mission.
• Evaluating gyro-viscosity in the Kelvin-Helmholtz instability by kinetic simulations

  Takayuki Umeda, Natsuki Yamauchi, Yasutaka Wada, Satoshi Ueno

  Physics of Plasmas 23 (5) 1070-664X 2016/05/01 [Refereed][Not invited]
   

  In the present paper, the finite-Larmor-radius (gyro-viscous) term [K. V. Roberts and J. B. Taylor, Phys. Rev. Lett. 8, 197–198 (1962)] is evaluated by using a full kinetic Vlasov simulation result of the Kelvin-Helmholtz instability (KHI). The velocity field and the pressure tensor are calculated from the high-resolution data of the velocity distribution functions obtained by the Vlasov simulation, which are used to approximate the Finite-Larmor-Radius (FLR) term according to Roberts and Taylor [Phys. Rev. Lett. 8, 197–198 (1962)]. The direct comparison between the pressure tensor and the FLR term shows an agreement. It is also shown that the anisotropic pressure gradient enhanced the linear growth of the KHI when the inner product between the vorticity of the primary velocity shear layer and the magnetic field is negative, which is consistent with the previous FLR-magnetohydrodynamic simulation result. This result suggests that it is not sufficient for reproducing the kinetic simulation result by fluid simulations to include the FLR term (or the pressure tensor) only in the equation of motion for fluid.
• Toward the Generation of Magnetized Collisionless Shocks with High-Power Lasers

  Yoshitaka SHOJI, Ryo YAMAZAKI, Sara TOMITA, Yushiro KAWAMURA, Yutaka OHIRA, Satoshi TOMIYA, Yoichi SAKAWA, Takayoshi SANO, Yukiko HARA, Sarana KONDO, Hiroshi SHIMOGAWARA, Shuichi MATSUKIYO, Taichi MORITA, Kentaro TOMITA, Hitoki YONEDA, Kazunori NAGAMINE, Yasuhiro KURAMITSU, Toseo MORITAKA, Naofumi OHNISHI, Takayuki UMEDA, Hideaki TAKABE

  Plasma and Fusion Research 11 3401031 - 3401031 1880-6821 2016 [Refereed][Not invited]
  
• Hybrid parallelization of hyper-dimensional Vlasov code with OpenMP loop collapse directive

  Takayuki Umeda, Keiichiro Fukazawa

  Parallel Computing: On the Road to Exascale, Advances in Parallel Computing 27 265 - 274 1879-808X 2016 [Refereed][Not invited]
   

  Space plasma is a collisionless, multi-scale, and highly nonlinear medium. Computer simulations with the first-principle kinetic equation are essential for space plasma studies. In the present study, a hyper-dimensional Vlasov (collisionless Boltzmann) simulation code, which is a first-principle method for collisionless space plasma, is parallelized with two-level hybrid MPI and OpenMP and is benchmarked on massively-parallel supercomputer systems. The benchmark result shows that the loop "collapse" directive option of the OpenMP reduces threading overhead of multiple loops. It is also shown that the hybrid parallelism with MPI and OpenMP is useful to reduce the communication time of MPI collective communication subroutines.
• Performance Comparison of Open-Source Parallel Sorting with OpenMP

  Takayuki Umeda, Shuhei Oya

  2015 Third International Symposium on Computing and Networking (CANDAR) 334 - 340 2379-1888 2015/12 [Refereed][Not invited]
  
• Note on one-fluid modeling of low-frequency Alfvénic fluctuations in a solar wind plasma with multi-ion components

  Y. Nariyuki, T. Umeda, T. K. Suzuki, T. Hada

  Physics of Plasmas 22 (12) 1070-664X 2015/12/01 [Refereed]
   

  A simple point of view that non-zero Alfvén ratio (residual energy) appears as a consequence of one-fluid modeling of uni-directional Alfvén waves in a solar wind plasma is presented. Since relative speeds among ions are incorporated into the one-fluid model as a pressure anisotropy, the Alfvén ratio can be finite due to the decrease in the phase velocity. It is shown that a proton beam component typically found in the solar wind plasma can contribute to generating non-zero Alfvén ratio observed in the solar wind plasma. Local equilibrium velocity distribution functions of each ion component are also discussed by using maximum entropy principle.
• Nonlinear damping of a finite amplitude whistler wave due to modified two stream instability

  Shinji Saito, Yasuhiro Nariyuki, Takayuki Umeda

  Physics of Plasmas 22 (7) 1070-664X 2015/07/01 [Refereed][Not invited]
   

  A two-dimensional, fully kinetic, particle-in-cell simulation is used to investigate the nonlinear development of a parallel propagating finite amplitude whistler wave (parent wave) with a wavelength longer than an ion inertial length. The cross field current of the parent wave generates short-scale whistler waves propagating highly oblique directions to the ambient magnetic field through the modified two-stream instability (MTSI) which scatters electrons and ions parallel and perpendicular to the magnetic field, respectively. The parent wave is largely damped during a time comparable to the wave period. The MTSI-driven damping process is proposed as a cause of nonlinear dissipation of kinetic turbulence in the solar wind.
• A high-resolution global Vlasov simulation of a small dielectric body with a weak intrinsic magnetic field on the K computer

  Takayuki Umeda, Keiichiro Fukazawa

  Earth, Planets and Space 67 (1) 1880-5981 2015/04/11 [Refereed][Invited]
  
• ELECTRON ACCELERATION DURING THE DECAY OF NONLINEAR WHISTLER WAVES IN LOW-BETA ELECTRON-ION PLASMA

  Takayuki Umeda, Shinji Saito, Yasuhiro Nariyuki

  The Astrophysical Journal 794 (1) 63 - 63 0004-637X 2014/09/24 [Refereed][Not invited]
  
• Ion kinetic effects on nonlinear processes of the Kelvin–Helmholtz instability

  Takayuki Umeda, Satoshi Ueno, Takuma K M Nakamura

  Plasma Physics and Controlled Fusion 56 (7) 075006 - 075006 0741-3335 2014/05/08 [Refereed][Not invited]
  
• Entry of solar-wind ions into the wake of a small body with a magnetic anomaly: A global Vlasov simulation

  Takayuki Umeda, Yosuke Ito

  Planetary and Space Science 93-94 35 - 40 0032-0633 2014/04 [Refereed][Not invited]
  
• Some remarks on the diffusion regions in magnetic reconnection

  Seiji Zenitani, Takayuki Umeda

  Physics of Plasmas 21 (3) 1070-664X 2014/03/01 [Refereed][Not invited]
   

  The structure of the diffusion regions in antiparallel magnetic reconnection is investigated by means of a theory and a Vlasov simulation. The magnetic diffusion is considered as relaxation to the frozen-in state, which depends on a reference velocity field. A field-aligned component of the frozen-in condition is proposed to evaluate a diffusion-like process. Diffusion signatures with respect to ion and electron bulk flows indicate the ion and electron diffusion regions near the reconnection site. The electron diffusion region resembles the energy dissipation region. These results are favorable to a previous expectation that an electron-scale dissipation region is surrounded by an ion-scale Hall-physics region.
• Ion acceleration by parallel propagating nonlinear Alfvén wave packets in a radially expanding plasma

  Y. Nariyuki, T. Umeda, T. K. Suzuki, T. Hada

  Nonlinear Processes in Geophysics 21 (1) 339 - 346 1023-5809 2014/02/27 [Refereed][Not invited]
   

  Abstract. The numerical simulation of the nonlinear evolution of the parallel propagating Alfvén waves in a radially expanding plasma is performed by using a kinetic-fluid model (the Vlasov–MHD model). In our study, both the nonlinear evolution of the Alfvén waves and the radial evolution of the velocity distribution function (VDF) are treated simultaneously. On the other hand, important ion kinetic effects such as ion cyclotron damping and instabilities driven by the non-equilibrium ion velocity distributions are not included in the present model. The results indicate that the steepened Alfvén wave packets outwardly accelerate ions, which can be observed as the beam components in the interplanetary space. The energy of imposed Alfvén waves is converted into the longitudinal fluctuations by the nonlinear steepening and the nonlinear Landau damping. The wave shoaling due to the inhomogeneity of the phase velocity is also observed.
• Dynamics and microinstabilities at perpendicular collisionless shock: A comparison of large-scale two-dimensional full particle simulations with different ion to electron mass ratio

  Takayuki Umeda, Yoshitaka Kidani, Shuichi Matsukiyo, Ryo Yamazaki

  Physics of Plasmas 21 (2) 022102 - 022102 1070-664X 2014/02 [Refereed][Not invited]
  
• Performance Tuning of Vlasov Code for Space Plasma on the K Computer

  Takayuki Umeda, Keiichiro Fukazawa

  AsiaSim 2014, Communications in Computer and Information Science 474 127 - 138 1865-0929 2014 [Refereed][Not invited]
  
• Performance measurements of MHD simulation for planetary magnetosphere on peta-scale computer FX10

  Keiichiro Fukazawa, Takeshi Nanri, Takayuki Umeda

  Parallel Computing: Accelerating Computational Science and Engineering (CSE),Advances in Parallel Computing 25 387 - 394 0927-5452 2014 [Refereed][Not invited]
   

  Magnetohydrodynamic (MHD) simulations are often applied to study the global dynamics and configuration of the planetary magnetosphere. The computational performance of an MHD code is evaluated on a massive parallel scalar type supercomputer system with one PFlops ideal performance. We have made the performance tuning of our three-dimensional MHD code for the planetary magnetosphere on the FX10 which has 76,800 cores, distributed on 4,800 SPARC64 IXfx nodes. For the parallelization of the MHD code, we use four different methods, i.e. one-dimensional, two-dimensional, three-dimensional regular domain decomposition methods and a cache-hit type of three-dimensional domain decomposition method. We found that the cache-hit type of three-dimensional decomposition of the MHD model is suitable for the FX10 system. We also found the pack/unpack operation for the inter-node communications decreases the execution efficiency by 2 %. After asynchronous communication is introduced and the pack/unpack operation is overlapped, we achieved a computing performance of 230 TFlops and an efficiency of almost 20 % for the MHD code. © 2014 The authors and IOS Press.
• Global Vlasov simulation on magnetospheres of astronomical objects

  Takayuki Umeda, Yosuke Ito, Keiichiro Fukazawa

  Journal of Physics: Conference Series 454 012005 - 012005 2013/08/12 [Refereed][Not invited]
  
• A primitive kinetic-fluid model for quasi-parallel propagating magnetohydrodynamic waves

  Y. Nariyuki, S. Saito, T. Umeda

  Physics of Plasmas 20 (7) 1070-664X 2013/07/01 [Refereed][Not invited]
   

  The extension and limitation of the existing one-dimensional kinetic-fluid model (Vlasov-MHD (magnetohydrodynamic) model), which has been used to analyze parametric instabilities of parallel propagating Alfvén waves, are discussed. The inconsistency among the given velocity distribution functions in the past studies is resolved through the systematic derivation of the multi-dimensional Vlasov-MHD model. The linear dispersion analysis of the present model indicates that the collisionless damping of the slow modes is adequately evaluated in low beta plasmas, although the deviation between the present model and the full-Vlasov theory increases with increasing plasma beta and increasing propagation angle. This is because the transit-time damping is not correctly evaluated in the present model. It is also shown that the ponderomotive density fluctuations associated with the envelope-modulated quasi-parallel propagating Alfvén waves derived from the present model is not consistent with those derived from the other models such as the Landau-fluid model, except for low beta plasmas. The result indicates the present model would be useful to understand the linear and nonlinear development of the Alfvénic turbulence in the inner heliosphere, whose condition is relatively low beta, while the existing model and the present model are insufficient to discuss the parametric instabilities of Alfvén waves in high beta plasmas and the obliquely propagating waves.
• Performance Measurement of Parallel Vlasov Code for Space Plasma on Scalar-Type Supercomputer Systems with Large Number of Cores

  Takayuki Umeda, Keiichiro Fukazawa

  AsiaSim 2013, Communications in Computer and Information Science 402 561 - 569 1865-0929 2013 [Refereed][Not invited]
  
• Performance Evaluation of Magnetohydrodynamics Simulation for Magnetosphere on K Computer

  Keiichiro Fukazawa, Takeshi Nanri, Takayuki Umeda

  AsiaSim2013, Communications in Computer and Information Science 402 570 - 576 1865-0929 2013 [Refereed][Not invited]
  
• A numerical electromagnetic linear dispersion relation for Maxwellian ring-beam velocity distributions

  Takayuki Umeda, Shuichi Matsukiyo, Takanobu Amano, Yoshizumi Miyoshi

  Physics of Plasmas 19 (7) 1070-664X 2012/07/01 [Refereed][Not invited]
   

  A positive slope in a velocity distribution function perpendicular to the ambient magnetic field, such as due to a loss cone or ring velocity distribution, can become a free energy source for the excitation of various plasma waves. Since there exists no analytic expression for integrals of Maxwellian ring velocity distribution functions, their linear properties have previously been studied using several approximations or modeled distributions. In this paper, a numerical method for analyzing the linear dispersion relation for Maxwellian ring-beam velocity distributions is developed. The obtained linear properties are confirmed by direct comparison with full particle simulation results.
• Vlasov simulation of electrostatic solitary structures in multi‐component plasmas

  Takayuki Umeda, Maha Ashour‐Abdalla, Jolene S. Pickett, Melvyn L. Goldstein

  Journal of Geophysical Research: Space Physics 117 (A5) 0148-0227 2012/05/16 [Refereed][Not invited]
   

  Electrostatic solitary structures have been observed in the Earth's magnetosheath by the Cluster spacecraft. Recent theoretical work has suggested that these solitary structures are modeled by electron acoustic solitary waves existing in a four‐component plasma system consisting of core electrons, two counter‐streaming electron beams, and one species of background ions. In this paper, the excitation of electron acoustic waves and the formation of solitary structures are studied by means of a one‐dimensional electrostatic Vlasov simulation. The present result first shows that either electron acoustic solitary waves with negative potential or electron phase‐space holes with positive potential are excited in four‐component plasma systems. However, these electrostatic solitary structures have longer duration times and higher wave amplitudes than the solitary structures observed in the magnetosheath. The result indicates that a high‐speed and small free energy source may be needed as a fifth component. An additional simulation of a five‐component plasma consisting of a stable four‐component plasma and a weak electron beam shows the generation of small and fast electron phase‐space holes by the bump‐on‐tail instability. The physical properties of the small and fast electron phase‐space holes are very similar to those obtained by the previous theoretical analysis. The amplitude and duration time of solitary structures in the simulation are also in agreement with the Cluster observation.
• A non-oscillatory and conservative semi-Lagrangian scheme with fourth-degree polynomial interpolation for solving the Vlasov equation

  Takayuki Umeda, Yasuhiro Nariyuki, Daichi Kariya

  Computer Physics Communications 183 (5) 1094 - 1100 0010-4655 2012/05 [Refereed][Not invited]
  
• A Scalable Full-Electromagnetic Vlasov Solver for Cross-Scale Coupling in Space Plasma

  Takayuki Umeda, Keiichiro Fukazawa, Yasuhiro Nariyuki, Tatsuki Ogino

  IEEE Transactions on Plasma Science 40 (5) 1421 - 1428 0093-3813 2012/05 [Refereed][Not invited]
  
• Microinstabilities at perpendicular collisionless shocks: A comparison of full particle simulations with different ion to electron mass ratio

  Takayuki Umeda, Yoshitaka Kidani, Shuichi Matsukiyo, Ryo Yamazaki

  Physics of Plasmas 19 (4) 1070-664X 2012/04/01 [Refereed][Not invited]
   

  A full particle simulation study is carried out for studying microinstabilities generated at the shock front of perpendicular collisionless shocks. The structure and dynamics of shock waves are determined by Alfvén Mach number and plasma beta, while microinstabilities are controlled by the ratio of the upstream bulk velocity to the electron thermal velocity and the plasma-to-cyclotron frequency. Thus, growth rates of microinstabilities are changed by the ion-to-electron mass ratio, even with the same Mach number and plasma beta. The present two-dimensional simulations show that the electron cyclotron drift instability is dominant for a lower mass ratio, and electrostatic electron cyclotron harmonic waves are excited. For a higher mass ratio, the modified two-stream instability is dominant and oblique electromagnetic whistler waves are excited, which can affect the structure and dynamics of collisionless shocks by modifying shock magnetic fields.
• Modified two‐stream instability at perpendicular collisionless shocks: Full particle simulations

  Takayuki Umeda, Yoshitaka Kidani, Shuichi Matsukiyo, Ryo Yamazaki

  Journal of Geophysical Research: Space Physics 117 (A3) 0148-0227 2012/03/06 [Refereed][Not invited]
   

  A full particle simulation study is carried out for studying microinstabilities generated in self‐consistently excited perpendicular collisionless shocks. The present two‐dimensional simulation with a high ion‐to‐electron mass ratio shows that the modified two‐stream instability can be generated by the interaction of incoming and reflected ions with electromagnetic whistler mode waves propagating in the direction quasi‐perpendicular to the ambient magnetic field. Properties of the excited whistler mode waves are consistent with the linear dispersion analysis based on the simulated velocity distribution functions. It is also found that the excited waves have an electron‐scale wavelength in the shock‐normal direction and an ion‐scale wavelength in the shock‐tangential direction, suggesting that the electron‐scale microturbulences and the ion‐scale shock structures are coupled with each other through the modified two‐stream instability.
• Performance measurement of magnetohydrodynamic code for space plasma on typical scalar-type supercomputer systems with a large number of cores

  Keiichiro Fukazawa, Takayuki Umeda

  The International Journal of High Performance Computing Applications 26 (3) 310 - 318 1094-3420 2012/02/09 [Refereed][Not invited]
   

  The computational performance of magnetohydrodynamic (MHD) code is evaluated on two typical scalar-type supercomputer systems. We have carried out performance tuning of a three-dimensional MHD code for space plasma simulations on the HA8000 (with 8192 cores) and SR16000/L2 (with 1344 cores) supercomputer systems. For parallelization of the MHD code, we use four different methods, that is, regular one-dimensional, two-dimensional and three-dimensional domain decomposition methods and a cache-hit type of three-dimensional domain decomposition method. We found that the regular three-dimensional decomposition of the MHD model is suitable for the HA8000 system, and the cache-hit type of three-dimensional decomposition is suitable for SR16000/L2 systems. As a result of these runs, we achieved a performance efficiency of almost 15% on the HA8000 and 20% on the SR16000/L2 for MHD code.
• Effect of ion cyclotron motion on the structure of wakes: A Vlasov simulation

  Takayuki Umeda

  Earth, Planets and Space 64 (2) 231 - 236 1343-8832 2012/02 [Refereed][Not invited]
  
• Performance Measurement of Parallel Vlasov Code for Space Plasma on Various Scalar-Type Supercomputer Systems

  Takayuki Umeda, Keiichiro Fukazawa

  Algorithms and Architectures for Parallel Processing, ICA3PP 2012, Lecture Notes in Computer Science 7439 233 - 240 0302-9743 2012 [Refereed][Not invited]
  
• RELATIVISTIC ELECTRON SHOCK DRIFT ACCELERATION IN LOW MACH NUMBER GALAXY CLUSTER SHOCKS

  S. Matsukiyo, Y. Ohira, R. Yamazaki, T. Umeda

  The Astrophysical Journal 742 (1) 47 - 47 0004-637X 2011/11/03 [Refereed][Not invited]
  
• SUPPRESSION OF REFLECTED ELECTRONS BY KINETIC ALFVÉN TURBULENCE IN A QUASI-PERPENDICULAR SHOCK: PARTICLE-IN-CELL SIMULATIONS

  S. Saito, T. Umeda

  The Astrophysical Journal 736 (1) 35 - 35 0004-637X 2011/07/01 [Refereed][Not invited]
  
• A new numerical method for simulating the solar wind Alfvén waves: Development of the Vlasov–MHD model

  Y. Nariyuki, T. Umeda, T. Kumashiro, T. Hada

  Planetary and Space Science 59 (8) 767 - 771 0032-0633 2011/06 [Refereed][Not invited]
  
• Cross-scale coupling at a perpendicular collisionless shock

  Takayuki Umeda, Masahiro Yamao, Ryo Yamazaki

  Planetary and Space Science 59 (7) 449 - 455 0032-0633 2011/05 [Refereed][Not invited]
  
• Quasilinear theory and simulation of Buneman instability

  J. Pavan, P. H. Yoon, T. Umeda

  Physics of Plasmas 18 (4) 1070-664X 2011/04/01 [Refereed][Not invited]
   

  In a recently developed nonlinear theory of Buneman instability, a simplifying assumption of self-similarity was imposed for the electron distribution function, based upon which, a set of moment kinetic equations was derived and solved together with nonlinear wave kinetic equation [P. H. Yoon and T. Umeda, Phys. Plasmas 17, 112317 (2010)]. It was found that the theoretical result compared reasonably against one-dimensional electrostatic Vlasov simulation. In spite of this success, however, the simulated distribution deviated appreciably from the assumed self-similar form during the late stages of nonlinear evolution. In order to rectify this shortcoming, in this paper, the distribution function is computed on the basis of rigorous velocity space diffusion equation. A novel theoretical scheme is developed so that both the quasilinear particle diffusion equation and the adiabatic dispersion relation can be solved for an arbitrary particle distribution function. Comparison with Vlasov simulation over relatively early quasilinear phase of the instability shows a reasonable agreement, despite the fact that quasilinear theory lacks coherent nonlinear effects as well as mode–mode coupling effects.
• Self-consistent kinetic numerical simulation model for ring current particles in the Earth's inner magnetosphere

  Takanobu Amano, Kanako Seki, Yoshizumi Miyoshi, Takayuki Umeda, Yosuke Matsumoto, Yusuke Ebihara, Shinji Saito

  Journal of Geophysical Research: Space Physics 116 (A2) n/a - n/a 0148-0227 2011/02/24 [Refereed][Not invited]
  
• Modeling nonlinear development of Buneman instability with linear dispersion theory

  Neeraj Jain, Takayuki Umeda, Peter H Yoon

  Plasma Physics and Controlled Fusion 53 (2) 025010 - 025010 0741-3335 2011/01/07 [Refereed][Not invited]
  
• Vlasov Code Simulation of Cross-scale Coupling in Space Plasmas

  T. Umeda, Y. Matsumoto, T. K. M. Nakamura, K. Fukazawa, T. Ogino

  5th International Conference of Numerical Modeling of Space Plasma Flows (ASTRONUM 2010) 444 182 - + 2011 [Refereed][Not invited]
  
• Vlasov simulation of the interaction between the solar wind and a dielectric body

  Takayuki Umeda, Tetsuya Kimura, Kentaro Togano, Keiichiro Fukazawa, Yosuke Matsumoto, Takahiro Miyoshi, Naoki Terada, Takuma K. M. Nakamura, Tatsuki Ogino

  Physics of Plasmas 18 (1) 012908 - 012908 1070-664X 2011/01 [Refereed][Not invited]
  
• Nonlinear turbulence theory and simulation of Buneman instability

  P. H. Yoon, T. Umeda

  Physics of Plasmas 17 (11) 112317 - 112317 1070-664X 2010/11 [Refereed][Not invited]
  
• On the reformation at quasi‐ and exactly perpendicular shocks: Full particle‐in‐cell simulations

  Takayuki Umeda, Yoshitaka Kidani, Masahiro Yamao, Shuichi Matsukiyo, Ryo Yamazaki

  Journal of Geophysical Research: Space Physics 115 (A10) 0148-0227 2010/10/29 [Refereed][Not invited]
   

  A full particle‐in‐cell (PIC) simulation study is carried out on the reformation at quasi‐ and exactly perpendicular collisionless shocks with a relatively low Alfven Mach number (M_A = 5). Previous self‐consistent one‐dimensional (1‐D) hybrid and full PIC simulations have demonstrated that ion kinetics are essential for the nonstationarity of perpendicular collisionless shocks. These results showed that reflection of ions at the shock front is responsible for the periodic collapse and redevelopment of a new shock front on a timescale of the ion cyclotron period, which is called the shock reformation. Recent 2‐D hybrid and full PIC simulations, however, suggested that the shock reformation does not take place at exactly perpendicular shocks with M_A ∼ 5. By contrast, another 2‐D hybrid PIC simulation showed that the shock reformation persists at quasi‐perpendicular shocks with M_A ∼ 5. Although these two works seem to be inconsistent with each other, the reason is not well understood because of several differences in numerical simulation conditions. Thus this paper gives a direct comparison between full PIC simulations of quasi‐ and exactly perpendicular shocks with almost the same condition. It is found that the time development of the shock magnetic field averaged over the shock‐tangential direction shows the transition from the reformation to no‐reformation phase. On the other hand, local shock magnetic field shows the evident appearance and disappearance of the shock front, and the period becomes longer in the no‐reformation phase than in the reformation phase.
• The HLLD Approximate Riemann Solver for Magnetospheric Simulation

  Takahiro Miyoshi, Naoki Terada, Yosuke Matsumoto, Keiichiro Fukazawa, Takayuki Umeda, Kanya Kusano

  IEEE Transactions on Plasma Science 38 (9) 2236 - 2242 0093-3813 2010/09 [Refereed][Not invited]
  
• Performance Measurement of Magnetohydrodynamic Code for Space Plasma on the Various Scalar-Type Supercomputer Systems

  Keiichiro Fukazawa, Takayuki Umeda, Takahiro Miyoshi, Naoki Terada, Yosuke Matsumoto, Tatsuki Ogino

  IEEE Transactions on Plasma Science 38 (9) 2254 - 2259 0093-3813 2010/09 [Refereed][Not invited]
  
• Comparative Study of Global MHD Simulations of the Terrestrial Magnetosphere With Different Numerical Schemes

  Yosuke Matsumoto, Naoki Terada, Takahiro Miyoshi, Keiichiro Fukazawa, Takayuki Umeda, Tatsuki Ogino, Kanako Seki

  IEEE Transactions on Plasma Science 38 (9) 2229 - 2235 0093-3813 2010/09 [Refereed][Not invited]
  
• Structures of diffusion regions in collisionless magnetic reconnection

  Takayuki Umeda, Kentaro Togano, Tatsuki Ogino

  Physics of Plasmas 17 (5) 1070-664X 2010/05/01 [Refereed][Not invited]
   

  Detailed structures of diffusion regions in two-dimensional collisionless magnetic reconnection are studied by using an electromagnetic Vlasov simulation. It has been well known that plasma number density decreases near the X-point of the reconnection. However, numerical thermal fluctuations exist in particle-in-cell simulations, and there is a possibility that detailed structures near the X-point diffuse numerically when the number of particles per cell is not enough. In the present study, a high-resolution two-dimensional Vlasov simulation is performed. It is found that electron number density in the electron diffusion region decreases to a hundredth of the initial value. Structures of electron diffusion region are determined by the local electron inertial length.
• On the nonlinearity of the Langmuir turbulence excited by a weak electron beam-plasma interaction

  Y. Nariyuki, T. Umeda

  Physics of Plasmas 17 (5) 1070-664X 2010/05/01 [Refereed][Not invited]
   

  In the present study, we analyze the data sets produced by a one-dimensional Vlasov–Poisson simulation of the weak electron beam-plasma instability to clarify the nonlinearity of the Langmuir turbulence excited by the weak-beam interaction. The growth of wave number modes is analyzed by using the momentum equation of the whole electrons. The analysis shows that the primary Langmuir wave mode is almost linear, while the nonlinear terms play important roles in the growth of the lower harmonic mode and the secondary higher harmonic mode. After the linear growth saturates, while the wave power of the primary mode is much larger than the other modes, linear and nonlinear interactions occurring in both lower harmonic and secondary higher harmonic modes are more active than those in the primary mode. Nonlinearity in the system comes from the advection rather than the ponderomotive forces.
• Full electromagnetic Vlasov code simulation of the Kelvin–Helmholtz instability

  Takayuki Umeda, Jun-ichiro Miwa, Yosuke Matsumoto, Takuma K. M. Nakamura, Kentaro Togano, Keiichiro Fukazawa, Iku Shinohara

  Physics of Plasmas 17 (5) 1070-664X 2010/05/01 [Refereed][Not invited]
   

  Recent advancement in numerical techniques for Vlasov simulations and their application to cross-scale coupling in the plasma universe are discussed. Magnetohydrodynamic (MHD) simulations are now widely used for numerical modeling of global and macroscopic phenomena. In the framework of the MHD approximation, however, diffusion coefficients such as resistivity and adiabatic index are given from empirical models. Thus there are recent attempts to understand first-principle kinetic processes in macroscopic phenomena, such as magnetic reconnection and the Kelvin–Helmholtz (KH) instability via full kinetic particle-in-cell and Vlasov codes. In the present study, a benchmark test for a new four-dimensional full electromagnetic Vlasov code is performed. First, the computational speed of the Vlasov code is measured and a linear performance scaling is obtained on a massively parallel supercomputer with more than 12 000 cores. Second, a first-principle Vlasov simulation of the KH instability is performed in order to evaluate current status of numerical techniques for Vlasov simulations. The KH instability is usually adopted as a benchmark test problem for guiding-center Vlasov codes, in which a cyclotron motion of charged particles is neglected. There is not any full electromagnetic Vlasov simulation of the KH instability; this is because it is difficult to follow E⃗×B⃗ drift motion accurately without approximations. The present first-principle Vlasov simulation has successfully represented the formation of KH vortices and its secondary instability. These results suggest that Vlasov code simulations would be a powerful approach for studies of cross-scale coupling on future Peta-scale supercomputers.
• Electromagnetic plasma emission during beam‐plasma interaction: Parametric decay versus induced scattering

  Takayuki Umeda

  Journal of Geophysical Research: Space Physics 115 (A1) 0148-0227 2010/01/14 [Refereed][Not invited]
   

  Two‐dimensional electromagnetic particle‐in‐cell simulations are performed for examination of electromagnetic plasma emission at twice the electron plasma frequency. Electromagnetic “2fp” waves are considered to be excited by nonlinear three‐wave processes in beam‐plasma interactions. In this paper, nonlinear development of an electron‐beam‐plasma instability is studied as an initial value problem. The present simulation result confirmed that electromagnetic 2fp waves are strongly enhanced by the wave‐wave interaction between forward and backward Langmuir waves, which is in agreement with the previous studies. It is also demonstrated that large‐amplitude forward Langmuir waves decay into backward Langmuir waves and ion acoustic waves via a parametric decay and that electromagnetic 2fp waves are also enhanced by the decay of Langmuir waves. However, the growth rate of the electromagnetic 2fp waves due to the parametric decay of Langmuir waves is not as high. It should be noted that induced backscattering of Langmuir waves by enhanced thermal fluctuations of ions cannot be neglected in the excitation of backward Langmuir waves. Hence, low‐noise simulations are necessary to suppress the effect of enhanced thermal fluctuations in the particle‐in‐cell method.
• Vlasov simulation of finite amplitude magnetohydrodynamic waves in the solar wind: Development of Vlasov-Hall-MHD code

  T. Kumashiro, T. Hada, Y. Nariyuki, T. Umeda

  Journal of Plasma and Fusion Research SERIES 8 831 - 836 2009/09 [Refereed][Not invited]
  
• Implementation of a non-oscillatory and conservative scheme into magnetohydrodynamic equations

  Shin Tanaka, Takayuki Umeda, Yosuke Matsumoto, Takahiro Miyoshi, Tatsuki Ogino

  Earth, Planets and Space 61 (7) 895 - 903 1343-8832 2009/07 [Refereed][Not invited]
   

  Abstract 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.
• Cluster observations and numerical modeling of energy‐dispersed ionospheric H⁺ ions bouncing at the plasma sheet boundary layer

  J. M. Bosqued, M. Ashour‐Abdalla, T. Umeda, M. El Alaoui, V. Peroomian, H. U. Frey, A. Marchaudon, H. Laakso

  Journal of Geophysical Research: Space Physics 114 (A4) 0148-0227 2009/04/28 [Refereed][Not invited]
   

  The Cluster mission offers a unique opportunity to investigate the origin of the energy‐dispersed ion structures frequently observed at 4.5–5 R_E altitude in the auroral region. We present a detailed study of the 14 February 2001 northern pass, characterized by the successive observation by three spacecraft of a series of energy‐dispersed structures at ∼72–75° ILAT in a region of poleward convection. Equatorward, the satellites also observed a localized, steady, and intense source of outflowing energetic (3–10 keV) H⁺ and O⁺ ions. These substructures were modeled by launching millions of H⁺ ions from this ionospheric source and following them through time‐dependent electric and magnetic fields obtained from a global MHD simulation of this event. Despite the complexity of ion orbits, the simulations showed that a large number of ions returned to the Cluster location, poleward of their source, in a number of adjacent or overlapping energy‐latitude substructures with the correct dispersion. The first dispersed echo was unexpectedly generated by “half‐bouncing” ions that interacted with the current sheet to return to the same hemisphere. The time‐shifted observations made by two Cluster (SC1 and SC3) spacecrafts were correctly reproduced. Almost all the ions returning to the spacecraft underwent a ∼2–5 keV nonadiabatic acceleration at each interaction with the current sheet in a very confined resonant region. This acceleration explains the overall energy increase from one structure to the next. This event confirms the importance of the ionospheric source in populating bouncing ion clusters within the magnetosphere, even at high latitudes.
• ELECTRON ACCELERATION AT A LOW MACH NUMBER PERPENDICULAR COLLISIONLESS SHOCK

  Takayuki Umeda, Masahiro Yamao, Ryo Yamazaki

  ASTROPHYSICAL JOURNAL 695 (1) 574 - 579 0004-637X 2009/04 [Refereed][Not invited]
   

  A full particle simulation study is carried out on the electron acceleration at a collisionless, relatively low Alfven Mach number (M-A = 5), perpendicular shock. Recent self-consistent hybrid shock simulations have demonstrated that the shock front of perpendicular shocks has a dynamic rippled character along the shock surface of low Mach number perpendicular shocks. In this paper, the effect of the rippling of perpendicular shocks on the electron acceleration is examined by means of large-scale (ion-scale) two-dimensional full particle simulations. It has been shown that a large-amplitude electric field is excited at the shock front in association with the ion-scale rippling, and that reflected ions are accelerated upstream at a localized region where the shock-normal electric field of the rippled structure is polarized upstream. The current-driven instability caused by the highly accelerated reflected ions has a high growth rate of up to large-amplitude electrostatic waves. Energetic electrons are then generated by the large-amplitude electrostatic waves via electron surfing acceleration at the leading edge of the shock-transition region. The present result suggests that the electron surfing acceleration is also a common feature at low Mach number perpendicular collisionless shocks.
• Two-dimensional full-electromagnetic Vlasov code with conservative scheme and its application to magnetic reconnection

  Takayuki Umeda, Kentaro Togano, Tatsuki Ogino

  Computer Physics Communications 180 (3) 365 - 374 0010-4655 2009/03 [Refereed][Not invited]
  
• Vlasov simulation of Langmuir decay instability

  Takayuki Umeda, Tetsuya Ito

  Physics of Plasmas 15 (8) 1070-664X 2008/08/01 [Refereed][Not invited]
   

  A parametric decay process of beam-generated Langmuir waves is examined by a one-dimensional Vlasov code simulation in the open system. It is confirmed that pump Langmuir waves decay into backscattered Langmuir and ion acoustic waves when the wave energy is higher than the thermal energy of background electrons. In the present Vlasov simulation, a large-amplitude pump Langmuir wave is excited by an electron beam. Then a backscattered Langmuir wave and a forward ion acoustic wave are excited by the parametric decay instability. When the wave energy of the backscattered Langmuir waves is also higher than the thermal energy of background electrons, backward ion acoustic waves are also excited. As a result, the wavenumber spectra of Langmuir and ion acoustic waves are broadened in both forward and backward directions, suggesting possible multiple parametric decay processes of Langmuir waves in a beam-plasma system.
• A conservative and non-oscillatory scheme for Vlasov code simulations

  Takayuki Umeda

  Earth, Planets and Space 60 (7) 773 - 779 1880-5981 2008/07 [Refereed][Not invited]
   

  Abstract A new numerical positive interpolation technique for conservation laws and its application to Vlasov code simulations are presented. In recent Vlasov simulation codes, the Vlasov equation is solved based on the numerical interpolation method because of its simplicity of algorithm and its ease of programming. However, a large number of grid points are needed in both configuration and velocity spaces to suppress numerical diffusion. In this paper we propose a new high-order interpolation scheme for Vlasov simulations. The current scheme is non-oscillatory and conservative and is well-designed for Vlasov simulations. This is compared with the latest interpolation schemes by performing one-dimensional electrostatic Vlasov simulations.
• Two-dimensional Full Particle Simulation of a Perpendicular Collisionless Shock with a Shock-Rest-Frame Model

  Takayuki Umeda, Masahiro Yamao, Ryo Yamazaki

  The Astrophysical Journal 681 (2) L85 - L88 0004-637X 2008/06/17 [Refereed][Not invited]
  
• Generation of low-frequency electrostatic and electromagnetic waves as nonlinear consequences of beam–plasma interactions

  Takayuki Umeda

  Physics of Plasmas 15 (6) 1070-664X 2008/06/01 [Refereed][Not invited]
   

  Nonlinear evolution of the electron two-stream instability in a current-carrying plasma is examined by using a two-dimensional electromagnetic particle-in-cell simulation. Formation of electron phase-space holes is observed as an early nonlinear consequence of electron–beam–plasma interactions. Lower-hybrid waves, electrostatic, and electromagnetic whistler mode waves are also excited by different mechanisms during the ensuing nonlinear wave–particle interactions. It is shown by the present computer simulation with a large simulation domain and a long simulation time that these low-frequency waves can disturb the electrostatic equilibrium of electron phase-space holes, suggesting that the lifetime of electron phase-space holes sometimes becomes shorter in a current-carrying plasma.
• Modeling PSBL high speed ion beams observed by Cluster and Double Star

  Maha Ashour-Abdalla, Jean-Michel Bosqued, Mostafa El-Alaoui, Vahé Peroomian, Takayuki Umeda, Raymond J. Walker

  Advances in Space Research 41 (10) 1598 - 1610 0273-1177 2008/01 [Refereed][Not invited]
  
• Numerical study of electrostatic electron cyclotron harmonic waves due to Maxwellian ring velocity distributions

  Takayuki Umeda

  Earth, Planets and Space 59 (11) 1205 - 1210 1343-8832 2007/11/30 [Refereed][Not invited]
   

  Abstract Excitation of electrostatic electron cyclotron harmonic (ECH) waves is studied by performing linear dispersion analysis and particle-in-cell computer simulation. The ECH wave emissions can be excited by a positive slope in a velocity distribution function perpendicular to the ambient magnetic field, such as that due to a loss cone or ring velocity distribution. However, there exists no analytic expression for integration of Maxwellian ring velocity distribution functions. Here we present a method to solve the linear dispersion relations of Maxwellian ring velocity distribution functions with numerical integration. The obtained dispersion relations are confirmed by particle-in-cell simulation.
• Parametric decay of circularly polarized Alfvén waves in the radially expanding solar wind

  Shin Tanaka, Tatsuki Ogino, Takayuki Umeda

  Journal of Geophysical Research: Space Physics 112 (A10) 0148-0227 2007/10/30 [Refereed][Not invited]
   

  We present a magnetohydrodynamic (MHD) simulation study of the parametric decay of the circularly polarized Alfvén waves propagating in the radial outflow of the solar wind. Assuming the transonic wind solution as an initial condition, we continuously injected monochromatic circularly polarized Alfvén waves from the inner boundary at the lower corona, and simulated the wave propagation. The injected Alfvén waves are subject to the parametric decay, and density fluctuations in the solar wind plasma grow rapidly at a specific region. The location of the most unstable region depends on the amplitude and frequency of injected Alfvén waves. We found that the unstable region of simulation results can be well estimated by considering a localized dispersion relation in the frame of reference moving with the background solar wind.
• Vlasov simulation of Langmuir wave packets

  T. Umeda

  Nonlinear Processes in Geophysics 14 (5) 671 - 679 1023-5809 2007/10/25 [Refereed][Not invited]
   

  Abstract. Amplitude modulation and packet formation of Langmuir waves are commonly observed during a nonlinear interaction between electron beams and plasmas. In this paper, we briefly review the history of Langmuir wave packets as developed by recent spacecraft observations and computer simulations. New one-dimensional electrostatic Vlasov simulations are performed to study their formation processes. It is found that the formation of Langmuir wave packets involves both an incoherent turbulent process and a coherent nonlinear trapping process. Existence of cold ions does not affect nonlinear processes of the weak-electron-beam instability in which the ion distribution is hardly modified by the excited Langmuir wave packets.
• Turbulent acceleration of superthermal electrons

  C.-M. Ryu, T. Rhee, T. Umeda, P. H. Yoon, Y. Omura

  Physics of Plasmas 14 (10) 1070-664X 2007/10/01 [Refereed][Not invited]
   

  In a recent paper it was suggested on the basis of weak turbulence theory that the collisionality of a plasma, coupled with nonlinear wave-particle interaction, is crucial for the acceleration of electrons by Langmuir turbulence to a superthermal energy level. In this Letter, fully nonlinear Vlasov and particle-in-cell (PIC) simulation techniques are employed to further verify this potentially important finding. The previous conclusion is fully confirmed by observing the expected difference between the Vlasov and PIC simulation results in the weak beam regime. However, in the strong beam regime, both the Vlasov and PIC simulations are found to produce a high-energy tail population, which indicates that there may be other mechanisms in the high beam speed situation, that are responsible for the generation the superthermal electrons.
• Particle‐in‐cell simulation of Maxwellian ring velocity distribution

  Takayuki Umeda, Maha Ashour‐Abdalla, David Schriver, Robert L. Richard, Ferdinand V. Coroniti

  Journal of Geophysical Research: Space Physics 112 (A4) 0148-0227 2007/04/28 [Refereed][Not invited]
   

  Electrostatic electron cyclotron harmonic (ECH) and electromagnetic whistler wave emissions are sometimes observed simultaneously in the near‐Earth nightside equatorial magnetotail region. The excitation mechanism for these two wave emissions, however, is quite different with ECH waves excited by a positive slope in the velocity distribution function perpendicular to the ambient magnetic field (such as due to a loss cone or ring velocity distribution), while whistler waves are excited by a temperature anisotropy whereby the perpendicular temperature is larger than the parallel temperature. Here we examine, using a two‐dimensional electromagnetic particle‐in‐cell computer simulation, the excitation of both waves as a consequence of a warm electron ring distribution in the presence of cold background electrons. Initially, ECH waves are excited by the ring distribution in the linear stage, which results in heating of the cold electrons and smearing of the ring distribution. During a nonlinear consequence of the ECH excitation and ensuing electron wave‐particle interactions, whistler waves are also excited by the different instability. Thus both waves are excited during the same simulation run, which may account for the magnetospheric observations of both ECH and whistler waves at the same location where electron loss cone distributions form in the equatorial magnetotail at about 6–9 RE from the Earth.
• Comparison of numerical interpolation schemes for one-dimensional electrostatic Vlasov code

  Takayuki Umeda, Maha Ashour-Abdalla, David Schriver

  Journal of Plasma Physics 72 (06) 1057 - 1057 0022-3778 2006/12/20 [Not invited]
  
• Full particle simulation of a perpendicular collisionless shock: A shock-rest-frame model

  Takayuki Umeda, Ryo Yamazaki

  Earth, Planets and Space 58 (10) e41 - e44 1343-8832 2006/10/13 [Refereed][Not invited]
   

  Abstract The full kinetic dynamics of a perpendicular collisionless shock is studied by means of a one-dimensional electromagnetic full particle simulation. The present simulation domain is taken in the shock rest frame in contrast to the previous full particle simulations of shocks. Preliminary results show that the downstream state falls into a unique cyclic reformation state for a given set of upstream parameters through the self-consistent kinetic processes.
• Nonlinear evolution of the electron two‐stream instability: Two‐dimensional particle simulations

  Takayuki Umeda, Yoshiharu Omura, Taketoshi Miyake, Hiroshi Matsumoto, Maha Ashour‐Abdalla

  Journal of Geophysical Research: Space Physics 111 (A10) 0148-0227 2006/10/07 [Refereed][Not invited]
   

  We studied nonlinear evolution of the electron two‐stream instability in a two‐dimensional system. Electron two‐stream and bump‐on‐tail instabilities are considered to be the most probable generation mechanisms for electrostatic solitary waves and electron holes observed in various regions of the Earth's magnetosphere. We performed two‐dimensional particle‐in‐cell simulations for various sets of electron cyclotron frequencies and initial electron thermal velocities. We found that the nonlinear evolution falls into four groups. When the electron cyclotron frequency is smaller than the bounce frequency of electrons trapped by electron holes, the electron holes become unstable as reported in previous simulations. When the electron cyclotron frequency is larger than the bounce frequency, the stability of electron holes is controlled by their amplitude. In the case of the cold two‐stream instability where the potential energy of excited electrostatic waves becomes larger than the thermal energy of background electrons, electron holes decay into electrostatic whistler waves. In another case, the two‐stream instability develops to form electron holes in runs with high initial electron thermal velocities. When the electron cyclotron frequency is much larger than the electron bounce frequency, we found the formation of stable one‐dimensional electron holes through coalescence. When the electron cyclotron frequency is smaller than twice the bounce frequency, we found formation of two‐dimensional electron holes isolated in both directions parallel and perpendicular to the ambient magnetic field.
• Vlasov simulation of amplitude-modulated Langmuir waves

  Takayuki Umeda

  Physics of Plasmas 13 (9) 1070-664X 2006/09/01 [Refereed][Not invited]
   

  Mechanisms for the generation of Langmuir wave packets are studied by performing a one-dimensional electrostatic Vlasov simulation. The present simulation of a weak-electron-beam instability without ion dynamics suggests two new processes for the amplitude modulation of Langmuir waves. The beam instability excites Langmuir modes over a wide wave number range, but the saturation of the most unstable Langmuir mode “filters” the growth of sideband modes. Specific upper and lower sideband modes linearly grow to a high saturation level. Then the primary Langmuir mode is amplified and strongly modulated through interaction with the sideband modes.
• Cluster observations of energetic ionospheric ion beams in the auroral region: Acceleration and associated energy‐dispersed precipitation

  J. M. Bosqued, M. Ashour‐Abdalla, A. Marchaudon, H. Laakso, T. Umeda, M. El Alaoui, V. Peroomian, H. Rème, G. Paschmann, M. Dunlop, A. Fazakerley

  Geophysical Research Letters 33 (12) 0094-8276 2006/06/17 [Refereed][Not invited]
   

  This paper presents a detailed study of the Feb. 14, 2001 Cluster northern auroral pass at mid‐altitudes (4–5 R_E), characterized by observations of a series of energy‐dispersed ion structures in a region of poleward convection. In contradiction with one current view, that ions populating these energy‐dispersed signatures originate sporadically in the magnetotail, Cluster directly observed energetic (0.2–15 keV), field‐aligned H⁺ ions of ionospheric origin. The ions were ejected at the top of a steady auroral acceleration region near 72.5° ILAT, then bounced on closed field‐lines, and were finally dispersed poleward in latitude by the E × B drift effect. Simple but realistic latitudinal drift computations demonstrate that the anticipated location of successive bouncing echoes coincides rather well with the Cluster observations. Best agreement is reached when the particles are further accelerated (presumably nonadiabatically) by 1–2 keV, as they periodically cross the tail neutral sheet.
• Study on Nonlinear Processes of Electron Beam Instabilities via Computer Simulations

  Takayuki Umeda

  Ph.D. Thesis, Graduate School of Informatics, Kyoto University 2004/03 [Refereed][Not invited]
  
  ダウンロード
• Two‐dimensional particle simulation of electromagnetic field signature associated with electrostatic solitary waves

  Takayuki Umeda, Yoshiharu Omura, Hiroshi Matsumoto

  Journal of Geophysical Research: Space Physics 109 (A2) 0148-0227 2004/02/13 [Refereed][Not invited]
   

  We present particle simulations of electrostatic solitary waves (ESW) observed by the Geotail spacecraft and recent spacecraft in the Earth's magnetosphere. Recent particle simulations have demonstrated that ESW correspond to Bernstein‐Greene‐Kruskal electron holes formed through nonlinear evolution of electron beam instabilities. Since an electron hole is a coherent electrostatic potential structure, electron beam instabilities were conventionally studied by electrostatic particle simulations. However, the Polar spacecraft and FAST spacecraft observed electromagnetic field signatures associated with ESW. To study interaction between coherent electrostatic potentials and electromagnetic waves, we extend the previous electrostatic particle model to an electromagnetic particle model. In the present two‐dimensional simulations of an electron beam instability, electromagnetic field components are enhanced around two‐dimensional electron holes. We found that the enhancement of electromagnetic fields is due to a current formed by electrons undergoing the E × B₀ drift, where the electric field is a perpendicular electrostatic field at the edge of a two‐dimensional electron hole. An electromagnetic beam mode is excited by the current due to the drifting electrons moving with the electron hole. The amplitude ratio of the electric field to the magnetic field is estimated on the basis of the present simulation result, and it is in agreement with those of the Polar and FAST observations.
• A new charge conservation method in electromagnetic particle-in-cell simulations

  T Umeda, Y Omura, T Tominaga, H Matsumoto

  Computer Physics Communications 156 (1) 73 - 85 0010-4655 2003/12 [Refereed][Not invited]
  
• Two-Dimensional Particle Simulation of Electrostatic Solitary Waves with an Open Boundary Condition

  T. Umeda, Y. Omura, H. Matsumoto, H. Usui

  PLASMA PHYSICS: 11th International Congress on Plasma Physics: ICPP2002, AIP Conference Proceedings 669 735 - 738 0094-243X 2003/06/11 [Not refereed][Not invited]
  
• Particle simulation of plasma response to an applied electric field parallel to magnetic field lines

  Y. Omura, W. J. Heikkila, T. Umeda, K. Ninomiya, H. Matsumoto

  Journal of Geophysical Research: Space Physics 108 (A5) 0148-0227 2003/05/17 [Refereed]
   

  We study response of thermal plasmas to an induction electric field via one‐dimensional particle simulations. The induction electric field is assumed to be uniform in space and constant in time. Because of acceleration of electrons and ions in the opposite directions, there arise counter streaming electrons and ions that cause the Buneman instability. Depending on the ratio of the ion temperature T_i to the electron temperature T_e, responses to the electric field are different. For a case with hot ions (T_i ≫ T_e) the Buneman instability leads to formation of large isolated electrostatic potentials which trap some electrons to move with ions. For a case with colder ions (T_i ≪ T_e) the Buneman instability is taken over by excitation of ion acoustic waves, which diffuse the low‐energy part of the accelerated electrons to stabilize the instability. However, a substantial part of the electrons are grouped together at the high‐energy part, forming a distinct bump in the electron distribution. In the present simulations we have found that the induction electric field can form an electron beam along the magnetic field line. Since the electron beam leaves the region of the induction electric field and moves into an unperturbed plasma, the accelerated electrons can cause a bump‐on‐tail instability. This can lead to formation of electrostatic solitary waves as frequently observed by the GEOTAIL spacecraft in the plasma sheet boundary layer (PSBL). The persistent observation of the electrostatic solitary waves indicates their association with the induction electric field that results from meandering motion of the current sheet in the magnetotail.
• Harmonic Langmuir waves. I. Nonlinear dispersion relation

  P. H. Yoon, R. Gaelzer, T. Umeda, Y. Omura, H. Matsumoto

  Physics of Plasmas 10 (2) 364 - 372 1070-664X 2003/02/01 [Refereed][Not invited]
   

  Generation of electrostatic multiple harmonic Langmuir modes during beam–plasma interaction process has been observed in laboratory and spaceborne active experiments, as well as in computer simulation experiments. Despite earlier efforts, such a phenomenon has not been completely characterized both theoretically and in terms of numerical simulations. This paper is a first in a series of three papers in which analytic expressions for harmonic Langmuir mode dispersion relations are derived and compared against the numerical simulation result.
• Harmonic Langmuir waves. II. Turbulence spectrum

  R. Gaelzer, P. H. Yoon, T. Umeda, Y. Omura, H. Matsumoto

  Physics of Plasmas 10 (2) 373 - 381 1070-664X 2003/02/01 [Refereed][Not invited]
   

  The Langmuir wave turbulence generated by a beam–plasma interaction has been studied since the early days of plasma physics research. In particular, mechanisms which lead to the quasi-power-law spectrum for Langmuir waves have been investigated, since such a spectrum defines the turbulence characteristics. Meanwhile, the generation of harmonic Langmuir modes during the beam–plasma interaction has been known for quite some time, and yet has not been satisfactorily accounted for thus far. In paper I of this series, nonlinear dispersion relations for these harmonics have been derived. In this paper (paper II), generalized weak turbulence theory which includes multiharmonic Langmuir modes is formulated and the self-consistent particle and wave kinetic equations are solved. The result shows that harmonic Langmuir mode spectra can indeed exhibit a quasi-power-law feature, implying multiscale structure in both frequency and wave number space spanning several orders of magnitude.
• Harmonic Langmuir waves. III. Vlasov simulation

  T. Umeda, Y. Omura, P. H. Yoon, R. Gaelzer, H. Matsumoto

  Physics of Plasmas 10 (2) 382 - 391 1070-664X 2003/02/01 [Refereed][Not invited]
   

  Generation of harmonic Langmuir modes during beam–plasma interaction is studied by means of nonlinear theoretical calculations and computer simulations. The present Vlasov simulation of multiple harmonic Langmuir modes (up to 12th harmonics), generalizes the previously available simulations which were restricted to the second harmonic only. The frequency-wave-number spectrum obtained by taking the Fourier transformation of simulated electric field both in time and space shows an excellent agreement with the theoretical nonlinear dispersion relations for harmonic Langmuir waves. The saturated wave amplitude features a quasi-power-law spectrum which reveals that the harmonic generation process may be an integral part of the Langmuir turbulence.
• Formation of electrostatic solitary waves in space plasmas: Particle simulations with open boundary conditions

  T. Umeda, Y. Omura, H. Matsumoto, H. Usui

  Journal of Geophysical Research: Space Physics 107 (A12) 0148-0227 2002/12/18 [Refereed][Not invited]
   

  We study formation process of electrostatic solitary waves (ESW) observed by recent spacecraft via one‐ and two‐dimensional electrostatic particle simulations with open boundaries. The previous simulations have demonstrated that ESW correspond to Bernstein‐Greene‐Kruskal electron holes formed by electron beam instabilities. However, since the previous simulations were performed in uniform periodic systems, wave‐particle interaction of an electron beam instability was taking place uniformly in the systems. In the present study, we inject a weak electron beam from an open boundary into the background plasma to study spatial and temporal development of a bump‐on‐tail instability from a localized source. In the open system, spatial structures of electron holes vary depending on the distance from the source of the electron beam. In an early phase of the simulation run, electron holes that are initially uniform in the direction perpendicular to the magnetic field become twisted through modulation by oblique electron beam modes. As the electron holes propagate along the magnetic field, they are aligned in the perpendicular direction through coalescence. Spatial structures of electron holes in a distant region from the source become one‐dimensional. In a long‐time evolution of the instability, ion dynamics becomes important in determining spatial structures of electron holes. A lower hybrid mode is excited locally in the region close to the source of the electron beam through coupling with electron holes at the same parallel phase velocity. The lower hybrid mode modulates electron holes excited in later phases, resulting in formation of modulated one‐dimensional potentials. Since the perpendicular electric fields of electron holes are carried by the electron holes at the drift velocity of the electron holes, they can be observed even at a distant place from the source.
• An improved masking method for absorbing boundaries in electromagnetic particle simulations

  Takayuki Umeda, Yoshiharu Omura, Hiroshi Matsumoto

  Computer Physics Communications 137 (2) 286 - 299 0010-4655 2001/06 [Refereed][Not invited]
  
• Observational evidence of dissipative small scale processes: Geotail spacecraft observation and simulation of electrostatic solitary waves

  Y Omura, H Kojima, T Umeda, H Matsumoto

  Astrophysics and Space Science 277 (1-2) 45 - 57 0004-640X 2001 [Refereed][Not invited]
   

  In recent spacecraft observations, coherent microscale structures such as electrostatic solitary waves are observed in various regions of the magnetosphere. The Geotail spacecraft observation has shown that these solitary waves are associated with high energy non-thermal electrons flowing along the magnetic field. The solitary structures are generated as a result of a long time evolution of coherent nonlinear trapping of electrons as found in bump-on-tail, bi-stream and Buneman instabilities. It is noted that these solitary waves can be generated at distant regions far away from the spacecraft locations, because these trapped electrons, or electron holes, are drifting much faster than the local thermal plasmas. Some of the solitary waves are accompanied by perpendicular electric fields indicating that two- or three-dimensional potential structures are passing by the spacecraft. Depending on the local plasma parameters, these multi-dimensional solitary structures couple with perpendicular modes such as electrostatic whistler modes and lower-hybrid modes. In a long time evolution, these perpendicular modes are dissipated via self-organization of small solitary potentials, leading to formation of one-dimensional potential troughs as observed in the deep magnetotail. The above dissipative small-scale processes are reproduced in particle simulations, and they can be used for diagnostics of electron dynamics from spacecraft observation of multi-dimensional solitary waves in various regions of the magnetosphere.

Books etc

• Computer Physics

  Takayuki Umeda (ContributorSimulation of collisionless plasma with the Vlasov method)
  Nova Science 2012/12 (ISBN: 9781613247907) 315-332 

  Numerical schemes for solving the Vlasov-Maxwell system of equations are presented. Our Universe is filled with collisionless plasma, which is a dielectric medium with nonlinear interactions between charged particles and electromagnetic fields. Thus computer simulations play an essential role in studies of such highly nonlinear systems. The full kinetics of collisionless plasma is described by the Vlasov-Maxwell equations. Since the Vlasov equation treats charged particles as position-velocity phase-space distribution functions in hyper dimensions, huge supercomputers and highly-scalable parallel codes are essential. Recently, a new parallel Vlasov-Maxwell solver is developed by adopting a stable but less-dissipative scheme for time integration of conservation laws, which has successfully achieved a high scalability on massively parallel supercomputers with multi-core scalar processors. The new code has applied to 2P3V (two dimensions for position and three dimensions for velocity) problems of cross-scale plasma processes such as magnetic reconnection, Kelvin-Helmholtz instability and the interaction between the solar wind and an asteroid. © 2012 Nova Science Publishers, Inc. All rights reserved.
• Wave Propagation

  Takayuki Umeda (ContributorElectromagnetic waves in plasma)
  In Tech 2011/03 (ISBN: 9789533072753) 311-330
• Space Plasma Simulation, Lecture Notes in Physics, Volume 615

  Yoshiharu Omura, Takayuki Umeda, Hiroshi Matsumoto (ContributorSimulation of electron beam instabilities and nonlinear potential structures)
  Springer Berlin Heidelberg 2003/03 (ISBN: 9783540006985) 79-92

Teaching Experience

Introduction to Electrical/ Electronic and Information Engineering for AutomobilesIntroduction to Electrical/ Electronic and Information Engineering for Automobiles Nagoya University
Advanced Theory of Space Information EngineeringAdvanced Theory of Space Information Engineering Nagoya University
Theory of Electric Circuits with ExercisesTheory of Electric Circuits with Exercises Nagoya University
Theory of Data Processing and Analysis with ExcerisesTheory of Data Processing and Analysis with Excerises Nagoya University
Tours in Industrial Plants BTours in Industrial Plants B Nagoya University
Theory of Digital Circuits with ExercisesTheory of Digital Circuits with Exercises Nagoya University

Association Memberships

• Association of Asia Pacific Physical Societies, Division of Plasma Physics   IEEE   Japan Geoscience Union   Americal Geophysical Union   Society of Geomagnetism and Earth, Planetary and Space Sciences   

Works

• Fortran90 wrapper for HDF5

  Takayuki Umeda 2024/03 - Today 

  Fortran90 wrapper subroutines and sample codes for writing/reading HDF5 datasets
• VLASOV1m

  Takayuki Umeda 2013/07 - Today 

  One-dimensional electrostatic Vlasov code in MATLAB

Research Projects

• How do fluid simulations reproduce kinetic simulation results?

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2019/04 -2024/03 

  Author : 梅田 隆行

   

  宇宙プラズマ環境変動の予測において計算機シミュレーションは今後不可欠な手段となる。既存の磁気流体力学(MHD)方程式に基づくプラズマ流体シミュレーションでは、慣性効果、圧力テンソルの非対角成分、熱輸送テンソルや四次以上の高次モーメント量などの非MHD項は無視されている。本研究では、第一原理運動論プラズマシミュレーションと流体ミュレーションとの直接比較によって、非MHD項である高次モーメント量が従来のMHD量に対してどのくらいの強度を持つのかを定量的に評価し、非MHD項の各項の重要性を議論する。また高次モーメント量を導入した新たな電磁流体方程式系を導出し、その数値シミュレーション手法を開発する。 Thompson [1961]のジャイロ平均した速度分布関数の近似式に基づいて、三次及び四次モーメント量の近似式を導出した。また、ケルヴィン・ヘルムホルツ不安定性の2次元full-Vlasovシミュレーション結果より三次及び四次モーメント量を計算し、導出した近似式との直接比較を行った。その結果、四次モーメント量はよく近似できていることに対し、三次モーメント量はほどんど近似できていないことを示した。本研究グループの過去の研究において二次モーメント量もよく近似できていることを示した[Umeda et al. 2016]ため、この結果はThompson [1961]の奇数モーメント量の近似式を見直す必要があることを示唆する。 1次元full-Vlasovシミュレーションによる接触不連続のパラメータサーベイを行い、Maxwell速度分布を持つプラズマ中ではMHD平衡解としての接触不連続が存在しないことを示唆した。更に、full-Vlasovシミュレーションと全く同一パラメータを用いた1次元hybrid-Vlasovシミュレーションとの直接比較を行い、接触不連続の崩壊の要因が三次モーメント量である熱輸送量であることを示した。この結果は、流体と運動論の差を作る要因の１つが三次モーメント量であることを示唆する。 高次モーメント量を含む電磁流体方程式を導出し、差分化のための格子系の検討を行った。また、staggered格子系の方程式についての高次精度化スキームの検討を行い、1次元Maxwell方程式について従来のFDTD法を四次精度に拡張した。 令和元年度の課題として設定した３つのうち、境界層の多次元full-Vlasovシミュレーションは順調に進んでいる。また磁場に沿った円柱座標系の回転方向に平均化したVlasov方程式を用いたThompson[1961]のモーメント近似法について、3次モーメント量である熱輸送テンソルの近似精度が悪いことを示した研究結果を出版した。4次モーメントまでを含んだ電磁流体方程式を導出し、計算手法の検討を行っている最中である。 Vlasovシミュレーションについては計算を継続する。また磁場に沿った円柱座標系の回転方向に平均化したVlasov方程式を用いたThompson 1961のモーメント近似法について、平均化したVlasov方程式そのものについて、導出過程から見直す。四次モーメントまでを含んだ電磁流体方程式については、計算手法の検討を引き続き行い、また磁場に対するガウスの法則（磁場の無発散）を満たすため、staggered格子系の導入を試みる。
• Plasma heating/acceleration in kinetic plasma turbulence and its turbulence evolution

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2014/04 -2019/03 

  Author : Saito Shinji, UMEDA takayuki

   

  We study kinetic plasma turbulence, where kinetic effect of charged particles are important in its dynamics, by using fully kinetic particle-in-cell simulations. We focus on heating/acceleration processes associated with dissipation of the kinetic turbulence. In this study we investigate nonlinear dynamics of whistler mode turbulence. In linear theory, whistler mode does not resonate with ions. However, the fully kinetic particle-in-cell simulations demonstrate that ions can be accelerated and/or heated by whistler mode turbulence through its nonlinear developments that include plasma instability and nonlinear intermittent nature of the kinetic turbulence.
• Study of unified numerical schemes for numerical simulations of electromagnetic fluid plasma: From MHD to multi fluids

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2015/04 -2018/03 

  Author : UMEDA TAKAYUKI

   

  The nonlinear evolution of the Kelvin-Helmholtz instability (KHI) and the Rayleigh-Taylor instability (RTI) at a density shear layer transverse to magnetic field in collisionless plasma is investigated by means of a fully kinetic Vlasov simulation with two spatial and two velocity dimensions. Several simulation runs with different ratio of the ion inertial length and/or ion gyro radius to the thickness of the initial shear layer have been performed to discuss “non-MHD effects” such as the ion inertial (Hall) effect and the ion finite Larmor radius (FLR) effect. The result shows that the instabilities develop symmetrically when neither Hall effect or FLR effect are available. An asymmetric development of the instabilities is seen when the Hall effect is available. When the FLR effect is available, a different type of instability with a spatial scale of ion gyro radius is generated, which disturbs the development of the primary instabilities.
• Generation of cosmic-ray protons at collisionless shocks via hybrid particle-in-cell simulations

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2014/04 -2018/03 

  Author : UMEDA TAKAYUKI

   

  The computational speed of the particle-in-cell-type plasma simulation codes is accelerated by using the counting sort algorithm and the array-particle data structure. Large-scale two-dimensional particle-in-cell simulations are carried out by using the improved code for studying non-stationarity of perpendicular shocks called the reformation. The result implies that the existence of the reformation is controlled by types of micro-instabilities at the shock transition layer. One- and two-dimensional particle simulations are also carried out on the development of a large-amplitude whistler-mode waves in the shock transition layer. Relativistic electron acceleration is found in the 1D run. On the other hand, a rapid decay of whistler-mode waves through a new instability is found in the 2D run, which suppress the relativistic electron acceleration.
• Basic plasma processes in the termination shock in a striped pulsar wind

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2013/04 -2016/03 

  Author : Zenitani Seiji, SHINOHARA Iku, NAGAI Tsugunobu, UMEDA Takayuki, WADA Tomohide

   

  We conducted basic researches on magnetic reconnection and magnetohydrodynamic (MHD) shocks. We proposed a new definition of the central region of magnetic reconnection and then further discussed particle dynamics in the reconnection magnetic geometry. Next we organized a fundamental algorithm to deal with relativistically-moving plasma distribution in particle-in-cell (PIC) simulations. By using a relativistic PIC simulation, we studied the electromagnetic balance in the central region of relativistic magnetic reconnection. We also developed a shock-capturing MHD code, which is publicly available. We studied the interaction between magnetic reconnection and MHD shocks. We also proposed a new mechanism to explain gamma-ray flares in a Crab pulsar system, by using MHD simulations.
• Six-dimensional plasma simulation

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2013/04 -2015/03 

  Author : UMEDA Takayuki

   

  A fully six-dimensional electromagnetic Vlasov code has been developed. Work arrays and memory space for the computation of the advection in the configuration space are substantially saved by the transposition of the data arrays for distribution functions. It is confirmed that the transposition efficiently accelerates the computational speed on x86 CPUs. While on SPARC CPUs, the computational efficiency becomes slightly worse. We also performed large-scale Vlasov simulation of the Kelvin-Helmholtz instability (KHI) and a weakly-magnetized small body. It is shown that ion-scale secondary instabilities taking place during the nonlinear development of MHD-scale primary KHI are affected by the structure of secondary velocity-shear layers of primary KH vortices. On the other hand, electron-scale charge separation does not affect the structure of MHD-scale magnetosphere but affects wave activities inside the magnetosphere.
• Study of cross-scale coupling in space plasma via large-scale Vlasov simulations

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2011 -2012 

  Author : UMEDA Takayuki

   

  We study numerical schemes for the first-principle Vlasov code as a post-Peta-scale computer simulation technique of space plasma. We developed numerical algorithms for solving the collisionless Boltzmann (Vlasov) equation which is a hyper-dimensional conservation law. The new simulation code is parallelized for scalar massively parallel computers with more than 10,000 CPU cores. The new code is applied to various multi-scale processes in space plasma. The first global Vlasov simulation of a magnetized astronomical body is succeeded. Also, it is shown that temporal evolution of fluid-scale Kelvin-Helmholtz vortices generated at a velocity-shear layer is strongly affected by the particle-scale ion gyro motion.
• Non-equilibrium plasmas associated with magnetohydrodynamic waves in solar wind plasmas

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2010 -2012 

  Author : NARIYUKI Yasuhiro, UMEDA Takayuki, SAITO Shinji, SUZUKI Takeru, HADA Tohru, NARITA Yasuhito

   

  In the present study, the solar wind non-equilibrium plasmas associated with magnetohydrodynamic （MHD) waves are discussed. The main results are as follows: （1) The “apparent” temperature anisotropy associated with MHD waves corresponds to the magnetohydrodynamic equilibrium state （Alfven-Beltrami field), （2)Ion beams can be formed by the nonlinear steepening of the MHD waves propagating from the solar corona, （3)The low-frequency MHD waves are rapidly dissipated due to the presence of the beam-induced short-wavelength waves.
• Study of particle acceleration processes at collisionless shocks via next-generation and first-principle particle simulations

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2009 -2011 

  Author : UMEDA Takayuki

   

  Large-scale two-dimensional full particle-in-cell simulations are carried out for studying non-stationarity of perpendicular shocks. It is confirmed that the persistence and absence of the self-reformation is controlled by micro-instabilities at the shock front. When electromagnetic whistler mode waves are strongly excited by the modified two-stream instability, both incoming and reflected ions are strongly scattered and the shock reformation becomes absent. On the other hand, when electrostatic waves are weakly excited or there is no micro-instabilities, reflected ions show a coherent behavior and the shock reformation is persistent. In this case, however, the reformation period is modified essentially due to the shock front ripples. The present simulation result suggests that the structure and dynamics of shocks are affected by the cross-scale coupling between electron-scale micro-instabilities and ion-scale shock-front ripples.
• Study of geospace storms based on a new observation-based model of the inner magnetosphere

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2008 -2011 

  Author : SEKI Kanako, MIYOSHI Yoshizumi, AMANO Takanobu, SAITO Shinji, UMEDA Takayuki, MATSUMOTO Yosuke, EBIHARA Yusuke, TAKADA Taku, VASSILIS Angelopoulos

   

  A new self-consistent and kinetic model for ring current particles in the inner magnetosphere is presented. A closed set of nonlinear time evolution equations is derived that incorporates kinetic particle dynamics and self-consistent development of the electromagnetic field. The particle transport is described by a five-dimensional collisionless drift kinetic equation, in which particle trajectories are approximated by their guiding centers under the influence of a time-dependent electromagnetic field. The time evolution of the electromagnetic field follows the Maxwell equations with the feedback from particles through electric currents. A numerical simulation code solving the system of equations in a global inner magnetosphere in three spatial dimensions(or five dimensions in phase space) is developed. It is demonstrated that the propagation of low-frequency plasma waves can successfully be described by the developed model. It is also found that the self-consistent coupling could affect the transport of energetic particles especially at low energies as well as the intensity and spatial distribution of field-aligned currents. These preliminary results suggest the importance of the self-consistent coupling in the global development of geomagnetic storms. Our approach extends the capability of modeling storm time phenomena occurring in the inner magnetosphere. These include sudden commencements(SCs), substorms, and ULF waves, and other associated phenomena.
• Study of Vlasov simulation techniques for next-generation supercomputers

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2009 -2010 

  Author : UMEDA Takayuki

   

  We study numerical schemes for the first-principle Vlasov code as a second-next-generation computer simulation technique of space plasma. We developed numerical algorithms for solving the collisionless Boltzmann (Vlasov) equation which is a hyper-dimensional conservation law. The new simulation code is parallelized for scalar massively parallel computers and has achieved more than 80% parallel efficiency on more than 1,000 CPU cores. The new code is applied to various multi-scale processes in space plasma. The numerical results imply the existence of cross-scale coupling between fluid scale and particle scale.
• Study of Revolutionary Vlasov simulation techniques

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2007 -2008 

  Author : UMEDA Takayuki

   

  次世代宇宙プラズマシミュレーション技術である第一原理ブラソフコードの研究開発を行った。連続体の多次元発展方程式である無衝突ボルツマン(ブラソフ)方程式を高精度かつ安定に解き進めることかできる数値解法を開発し、宇宙プラズマ中の様々なマルチスケール現象に対してベンチマークテストを行った。その結果、新たに開発したコードは従来のブラソフコードよりもはるかにロバストであることがわかった。次世代スーパーコンピュータに向けた高速化および超並列化が今後の課題である
• 開放系プラズマモデルによる静電孤立波の計算機実験

  日本学術振興会：科学研究費助成事業

  Date (from‐to) : 2001 -2003 

  Author : 梅田 隆行

   

  本年度の研究によって得られた以下の成果を国内・国際会議および学会誌に論文発表をしたとともに、膨大なデータの保存のためにハードディスク等を購入した。 1.二次元及び、三次元電磁粒子コードにおいて、電流密度の計算を従来の方法より高速に行う計算アルゴリズムを開発し、コンピュータ科学雑誌に発表した。この手法を用いて、数値的熱雑音がより少なく計算効率の良い三次元電磁粒子コードを新たに開発した。 2.従来電磁界を無視した静電粒子コードを用いて行われていた電子ビーム不安定性の計算機実験を、世界に先駆けて電磁粒子モデルへ拡張した。背景磁場と二次元構造を持つ静電孤立波の磁場に垂直な電界を受けて電子がE×Bドリフトすることにより二次元静電孤立波の垂直方向の端において電流が生じ、二次元静電孤立波の周辺に電磁界が励起することを明らかにした。また、計算機実験結果がオーロラ領域で観測されている静電孤立波に伴った電磁界成分の特性とよく一致していることを示した。さらに、孤立した静電ポテンシャル内において形成された電流構造が電磁波の放射源となり得るという新しい物理過程について明らかにした。これは、従来の電子ビームや連続的な波動からの直接的な電磁放射とは全く異なる物理過程であり、宇宙プラズマ物理のみならず、実験室プラズマや天体プラズマへの適応が可能な新たな電磁放射過程である。この成果を、2件の国内会議、国際測地学・地球物理学連合会札幌大会及びアメリカ地球物理学連合秋大会において研究発表を行い、論文としてJGR誌に発表した。また、上記の最新の結果を含む静電孤立波の国内外の研究に関して、実験室及び宇宙空間におけるプラズマ実験に関する国際研究集会において招待講演を行った。

Industrial Property Rights

• 特開2023-170345:The FDTD analysis method  2023/12/01

  Takayuki UMEDA, Harune Sekido  国立大学法人東海国立大学機構
• 特開2023-128129:METHOD FOR ANALYZING MOTION OF CHARGED PARTICLES BASED ON RELATIVISTIC EQUATION OF MOTION  2023/09/14

  梅田 隆行  国立大学法人東海国立大学機構
• 特開2019-003457:METHOD OF ANALYZING CHARGED PARTICLE KINETICS BY LORENTZ FORCE OF MAGNETIC FIELD  2019/01/10

  Takayuki UMEDA  Nagoya University

Social Contribution

• 宇宙プラズマの第一原理ブラソフシミュレーション

  Date (from-to) : 2017/03

  Role : Lecturer

  Sponser, Organizer, Publisher  : 神戸大学計算科学教育センター

  Event, Program, Title : KOBE HPC Spring School 2017

  神戸大学計算科学教育センター
• 宇宙プラズマの第一原理ブラソフコードの性能評価

  Date (from-to) : 2015/12

  Role : Lecturer

  Sponser, Organizer, Publisher  : 高度情報科学技術研究機構

  Event, Program, Title : 平成27年度「京」における高速化ワークショップ

  秋葉原UDX 宇宙プラズマの第一原理ブラソフコードの性能評価
• 京コンピュータを用いた宇宙プラズマの第一原理ブラソフシミュレーション

  Date (from-to) : 2013/10

  Role : Lecturer

  Sponser, Organizer, Publisher  : サイエンティフィック・システム研究会

  Event, Program, Title : 科学技術計算分科会 2013年度会合「ペタからエクサへの課題」

  ホテルオークラ神戸
• 宇宙科学とコンピュータシミュレーション

  Date (from-to) : 2010/08

  Role : Lecturer

  Sponser, Organizer, Publisher  : 高知工業高等専門学校

  Event, Program, Title : STP教育研究セミナー公開講座

  高知工業高等専門学校
• STPシミュレーションの過去・現在・未来

  Date (from-to) : 2009/09

  Role : Lecturer

  Sponser, Organizer, Publisher  : 名古屋大学太陽地球環境研究所

  Event, Program, Title : 宇宙地球惑星科学若手会 2009年夏の学校

  伊勢青少年研修センター

Media Coverage

• 青山学院大学ら8大学の共同研究グループが「大型レーザー装置で実験室に宇宙プラズマ衝撃波を生成 -- 宇宙線の生成メカニズム解明に向け新たな研究手段を確立 -- 」

  Date : 2020/08/30

  Publisher, broadcasting station: 青山学院大学

  Internet