基本情報

所属

• 情報基盤センター　スーパーコンピューティング研究部門

職名

• 教授

学位

• 博士（情報学）（京都大学）

連絡先

• umedaiic.hokudai.ac.jp

科研費研究者番号

• 40432215

ORCID ID

•  https://orcid.org/0000-0001-6882-9200

J-Global ID

• 200901087140936495

研究キーワード

• 計算機シミュレーション   数値スキーム   ハイパフォーマンスコンピューティング   プラズマ科学   宇宙科学   電磁界   

研究分野

• 情報通信 / 高性能計算 / ハイパフォーマンスコンピューティング
• 情報通信 / 計算科学 / 計算機シミュレーション
• 自然科学一般 / 宇宙惑星科学 / 宇宙プラズマ
• エネルギー / プラズマ科学 / プラズマ物理

職歴

• 2024年02月 - 現在 名古屋大学 大学院工学研究科 電気工学専攻 客員教授
• 2024年02月 - 現在 名古屋大学 宇宙地球環境研究所 客員教授
• 2024年02月 - 現在 北海道大学 情報基盤センター 教授
• 2018年09月 - 2024年02月 名古屋大学 宇宙地球環境研究所 附属統合データサイエンスセンター 准教授
• 2015年10月 - 2018年08月 名古屋大学 宇宙地球環境研究所 附属統合データサイエンスセンター 講師
• 2015年08月 - 2015年09月 名古屋大学 太陽地球環境研究所 附属ジオスペース研究センター 講師
• 2007年04月 - 2015年07月 名古屋大学 太陽地球環境研究所 附属ジオスペース研究センター 助教
• 2006年04月 - 2007年03月 名古屋大学 太陽地球環境研究所 附属ジオスペース研究センター 助手
• 2004年04月 - 2006年03月 カリフォルニア大学ロサンゼルス校 地球物理学研究所 大学院研究員
• 2001年04月 - 2004年03月 日本学術振興会特別研究員（ＤＣ１）

学歴

• 2001年04月 - 2004年03月   京都大学   大学院情報学研究科   通信情報システム専攻
• 1999年04月 - 2001年03月   京都大学   大学院情報学研究科   通信情報システム専攻
• 1995年04月 - 1999年03月   京都大学   工学部   電気電子工学科

所属学協会

• アジア太平洋物理学会連合 プラズマ物理学領域   アイ・トリプル・イー   日本地球惑星科学連合   アメリカ地球物理学連合   地球電磁気・地球惑星圏学会   

研究活動情報

論文

• Generation of normal distributions revisited

  Takayuki Umeda

  Computational Statistics 2024年02月23日 [査読有り][通常論文]
   

  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年01月23日 [査読有り][通常論文]
  
• A novel high accuracy finite-difference time-domain method

  Harune Sekido, Takayuki Umeda

  Earth, Planets and Space 76 1 2024年01月05日 [査読有り][通常論文]
   

  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 2023年10月10日 [査読有り][通常論文]
   

  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 2023年04月01日 [査読有り][通常論文]
   

  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 2023年02月 [査読有り][通常論文]
  
• 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 2023年02月01日 [査読有り][通常論文]
   

  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 2023年01月 [査読有り][通常論文]
  
• Multicolor reordering for computing moments in particle-in-cell plasma simulations

  Takayuki Umeda

  Computer Physics Communications 281 108499 - 108499 2022年12月 [査読有り][通常論文]
  
• 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 2022年02月11日 [査読有り][通常論文]
  
• 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 2022年01月01日 [査読有り][通常論文]
   

  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 2020年03月20日 [査読有り][通常論文]
   

  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 2020年03月01日 [査読有り][通常論文]
   

  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年02月17日 [査読有り]
  
• 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 2019年12月02日 [査読有り][通常論文]
  
• 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年12月01日 [査読有り][通常論文]
   

  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 2019年10月01日 [査読有り][通常論文]
   

  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.
• 宇宙プラズマの基礎理論およびその計算機シミュレーション手法

  梅田 隆行

  混相流 33 3 249 - 257 2019年09月15日 [査読無し][招待有り]
  
• Multi-step Boris rotation schemes for Lorentz force equation of charged particles

  Takayuki Umeda

  Computer Physics Communications 237 37 - 41 2019年04月 [査読有り][通常論文]
  
• 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 2019年03月01日 [査読有り][通常論文]
   

  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 2018年12月01日 [査読有り][通常論文]
   

  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 2018年11月01日 [査読有り][通常論文]
   

  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 2018年10月18日 [査読有り][通常論文]
  
• 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 2018年10月08日 [査読有り][通常論文]
   

  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 2018年10月01日 [査読有り][通常論文]
   

  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 2018年08月01日 [査読有り][通常論文]
   

  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 2018年07月01日 [査読有り][通常論文]
   

  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 2018年07月 [査読有り]
  
• 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年06月 [査読有り][通常論文]
  
• 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 2018 1 27 - 35 日本計算数理工学会 2018年03月 [査読無し][招待有り]
  
• 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 2018年 [査読有り][通常論文]
   

  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 2017年07月01日 [査読有り][通常論文]
   

  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 2017年06月29日 [査読有り][通常論文]
   

  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 2017年04月26日 [査読有り][通常論文]
   

  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 2016年11月01日 [査読有り]
   

  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年09月25日 [査読有り][通常論文]
  
• 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 2016年08月26日 [査読有り][通常論文]
   

  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 2016年05月01日 [査読有り][通常論文]
   

  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 2016年 [査読有り][通常論文]
  
• 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 2016年 [査読有り][通常論文]
   

  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 2015年12月 [査読有り][通常論文]
  
• 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 2015年12月01日 [査読有り]
   

  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 2015年07月01日 [査読有り][通常論文]
   

  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 2015年04月11日 [査読有り][招待有り]
  
• 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 2014年09月24日 [査読有り][通常論文]
  
• 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 2014年05月08日 [査読有り][通常論文]
  
• 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 2014年04月 [査読有り][通常論文]
  
• Some remarks on the diffusion regions in magnetic reconnection

  Seiji Zenitani, Takayuki Umeda

  Physics of Plasmas 21 3 2014年03月01日 [査読有り][通常論文]
   

  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 2014年02月27日 [査読有り][通常論文]
   

  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 2014年02月 [査読有り][通常論文]
  
• 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 2014年 [査読有り][通常論文]
  
• 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 2014年 [査読有り][通常論文]
   

  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年08月12日 [査読有り][通常論文]
  
• A primitive kinetic-fluid model for quasi-parallel propagating magnetohydrodynamic waves

  Y. Nariyuki, S. Saito, T. Umeda

  Physics of Plasmas 20 7 2013年07月01日 [査読有り][通常論文]
   

  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 2013年 [査読有り][通常論文]
  
• 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 2013年 [査読有り][通常論文]
  
• 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 2012年07月01日 [査読有り][通常論文]
   

  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 2012年05月16日 [査読有り][通常論文]
   

  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 2012年05月 [査読有り][通常論文]
  
• 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 2012年05月 [査読有り][通常論文]
  
• 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 2012年04月01日 [査読有り][通常論文]
   

  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 2012年03月06日 [査読有り][通常論文]
   

  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 2012年02月09日 [査読有り][通常論文]
   

  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 2012年02月 [査読有り][通常論文]
  
• 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 2012年 [査読有り][通常論文]
  
• 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 2011年11月03日 [査読有り][通常論文]
  
• 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 2011年07月01日 [査読有り][通常論文]
  
• 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 2011年06月 [査読有り][通常論文]
  
• Cross-scale coupling at a perpendicular collisionless shock

  Takayuki Umeda, Masahiro Yamao, Ryo Yamazaki

  Planetary and Space Science 59 7 449 - 455 2011年05月 [査読有り][通常論文]
  
• Quasilinear theory and simulation of Buneman instability

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

  Physics of Plasmas 18 4 2011年04月01日 [査読有り][通常論文]
   

  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 2011年02月24日 [査読有り][通常論文]
  
• 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 2011年01月07日 [査読有り][通常論文]
  
• 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年 [査読有り][通常論文]
  
• 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 2011年01月 [査読有り][通常論文]
  
• Nonlinear turbulence theory and simulation of Buneman instability

  P. H. Yoon, T. Umeda

  Physics of Plasmas 17 11 112317 - 112317 2010年11月 [査読有り][通常論文]
  
• 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 2010年10月29日 [査読有り][通常論文]
   

  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 2010年09月 [査読有り][通常論文]
  
• 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 2010年09月 [査読有り][通常論文]
  
• 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 2010年09月 [査読有り][通常論文]
  
• Structures of diffusion regions in collisionless magnetic reconnection

  Takayuki Umeda, Kentaro Togano, Tatsuki Ogino

  Physics of Plasmas 17 5 2010年05月01日 [査読有り][通常論文]
   

  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 2010年05月01日 [査読有り][通常論文]
   

  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 2010年05月01日 [査読有り][通常論文]
   

  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 2010年01月14日 [査読有り][通常論文]
   

  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年09月 [査読有り][通常論文]
  
• 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 2009年07月 [査読有り][通常論文]
   

  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 2009年04月28日 [査読有り][通常論文]
   

  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 2009年04月 [査読有り][通常論文]
   

  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 2009年03月 [査読有り][通常論文]
  
• Vlasov simulation of Langmuir decay instability

  Takayuki Umeda, Tetsuya Ito

  Physics of Plasmas 15 8 2008年08月01日 [査読有り][通常論文]
   

  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 2008年07月 [査読有り][通常論文]
   

  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 2008年06月17日 [査読有り][通常論文]
  
• Generation of low-frequency electrostatic and electromagnetic waves as nonlinear consequences of beam–plasma interactions

  Takayuki Umeda

  Physics of Plasmas 15 6 2008年06月01日 [査読有り][通常論文]
   

  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 2008年01月 [査読有り][通常論文]
  
• Numerical study of electrostatic electron cyclotron harmonic waves due to Maxwellian ring velocity distributions

  Takayuki Umeda

  Earth, Planets and Space 59 11 1205 - 1210 2007年11月30日 [査読有り][通常論文]
   

  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 2007年10月30日 [査読有り][通常論文]
   

  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 2007年10月25日 [査読有り][通常論文]
   

  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 2007年10月01日 [査読有り][通常論文]
   

  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 2007年04月28日 [査読有り][通常論文]
   

  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 2006年12月20日 [通常論文]
  
• 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 2006年10月13日 [査読有り][通常論文]
   

  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 2006年10月07日 [査読有り][通常論文]
   

  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 2006年09月01日 [査読有り][通常論文]
   

  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 2006年06月17日 [査読有り][通常論文]
   

  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年03月 [査読有り][通常論文]
  
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• 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 2004年02月13日 [査読有り][通常論文]
   

  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 2003年12月 [査読有り][通常論文]
  
• 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 2003年06月11日 [査読無し][通常論文]
  
• 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 2003年05月17日 [査読有り]
   

  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 2003年02月01日 [査読有り][通常論文]
   

  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 2003年02月01日 [査読有り][通常論文]
   

  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 2003年02月01日 [査読有り][通常論文]
   

  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 2002年12月18日 [査読有り][通常論文]
   

  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 2001年06月 [査読有り][通常論文]
  
• 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 2001年 [査読有り][通常論文]
   

  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.

書籍

• Computer Physics

  Takayuki Umeda (担当:分担執筆範囲:Simulation of collisionless plasma with the Vlasov method)
  Nova Science 2012年12月 (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 (担当:分担執筆範囲:Electromagnetic waves in plasma)
  In Tech 2011年03月 (ISBN: 9789533072753) 311-330
• Space Plasma Simulation, Lecture Notes in Physics, Volume 615

  Yoshiharu Omura, Takayuki Umeda, Hiroshi Matsumoto (担当:分担執筆範囲:Simulation of electron beam instabilities and nonlinear potential structures)
  Springer Berlin Heidelberg 2003年03月 (ISBN: 9783540006985) 79-92

作品等

• Fortran90 wrapper for HDF5

  Takayuki Umeda  2024年03月 - 現在  Fortran90 wrapper subroutines and sample codes for writing/reading HDF5 datasets
• VLASOV1m

  Takayuki Umeda  2013年07月 - 現在  One-dimensional electrostatic Vlasov code in MATLAB

特許

• 特開2023-170345:FDTD解析方法  2023年12月01日

  梅田 隆行, 関戸 晴宇  国立大学法人東海国立大学機構
• 特開2023-128129:相対論的運動方程式に基づく荷電粒子の運動解析方法  2023年09月14日

  梅田 隆行  国立大学法人東海国立大学機構
• 特開2019-003457:磁場のローレンツ力による荷電粒子の運動解析方法  2019年01月10日

  梅田 隆行  国立大学法人名古屋大学

受賞

• 2023年11月 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年11月 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年04月 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年10月 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年11月 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年10月 国際電波科学連合 国際電波科学連合若手研究者賞
  

共同研究・競争的資金等の研究課題

• 流体シミュレーションで運動論的物理過程はどこまで再現できるか？

  日本学術振興会：科学研究費助成事業

  研究期間 : 2019年04月 -2024年03月 

  代表者 : 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。
• 運動論的乱流形成とその散逸に伴うプラズマ加熱・粒子加速機構

  日本学術振興会：科学研究費助成事業

  研究期間 : 2014年04月 -2019年03月 

  代表者 : 齊藤 慎司, 成行 泰裕, 梅田 隆行

   

  本研究課題では、プラズマ中の個々の荷電粒子の運動の性質が重要になる運動論的プラズマ乱流の複雑な発展過程について研究を行った。運動論的乱流の散逸によって生じるプラズマの加熱や加速機構について注目し、どのようにして運動論的乱流からプラズマ粒子にエネルギーを輸送するのかについて研究を進めた。本研究ではホイッスラー波動と呼ばれるプラズマ波動によって構成される波動乱流に注目し、プラズマ粒子、特にイオンの散乱過程について新しい知見を得た。これまでこの波動乱流では加熱や加速が起こりにくいと考えられてきたイオンが、乱流中に生じる不安定性や非線形発展に伴う局所的な性質によって、加速や加熱を受けることがわかった。
• 電磁プラズマ流体シミュレーションの共通数値解法の開発：MHDから多流体まで

  日本学術振興会：科学研究費助成事業

  研究期間 : 2015年04月 -2018年03月 

  代表者 : 梅田 隆行

   

  ケルヴィン・ヘルムホルツ不安定性およびレイリー・テイラー不安定性の非線形発展について、2次元ブラソフシミュレーションにより研究を行った。初期勾配層の幅に対するイオン慣性長やイオンジャイロ半径を変化させたシミュレーションを行い、ホール効果およびジャイロ運動効果などの非MHD効果の寄与を議論した。非MHD効果があまり効かない場合にはMHDシミュレーションと同様の対称的な不安定性の発達が見られ、ホール効果が影響する場合には、非対称に発達することが分かった。またジャイロ運動効果が影響する場合には、イオンジャイロ半径スケールの新たな不安定性が生じ、不安定性の構造の成長を阻害することが分かった。
• ハイブリッド粒子コードを用いた無衝突衝撃波における宇宙線陽子の生成過程の研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 2014年04月 -2018年03月 

  代表者 : 梅田 隆行

   

  計数ソートを用いた粒子データの並べ替えと座標配列に従属した粒子データ構造への変更により、Particle-In-Cell型のプラズマ粒子コードの高速化を行った。高速化したコードによる大規模２次元粒子シミュレーションにより、垂直衝撃波の非定常性である再形成について研究を行い、その存在が衝撃波遷移層における微視的不安定性の成長モードに依存することを示唆した。 衝撃波遷移層における大振幅ホイッスラーモード波の発展について、1次元および2次元粒子シミュレーションを行った。1次元では波の発展ともなって電子が相対論的に加速されたが、2次元では新たな不安定性により波が急速に減衰し、電子加速は起こらなかった。
• パルサー終端衝撃波における新しい粒子加速過程の探求

  日本学術振興会：科学研究費助成事業

  研究期間 : 2013年04月 -2016年03月 

  代表者 : 銭谷 誠司, 篠原 育, 長井 嗣信, 梅田 隆行, 和田 智秀

   

  磁気リコネクションと衝撃波のそれぞれの素過程について、基礎的な研究を行った。磁気リコネクション系を議論するために必要な重要領域の定義を提案し、さらに粒子軌道との関係について考察した。次に、粒子シミュレーションで相対論的高速で流れる粒子分布を扱うためのアルゴリズムを提案し、相対論的磁気リコネクションの重要領域の電磁流体的な力学関係を議論した。公開磁気流体コードを開発してシミュレーション研究を行い、リコネクションと衝撃波との相互作用を議論した。磁気流体コードを併用して、かにパルサー系で起きるガンマ線爆発（フレア）現象を説明するモデルを考案した。
• 6次元プラズマシミュレーション

  日本学術振興会：科学研究費助成事業

  研究期間 : 2013年04月 -2015年03月 

  代表者 : 梅田 隆行

   

  6次元電磁ブラソフコードの開発を行った。実空間の移流計算の際に、６次元分布関数のデータの並びを速度－位置から位置－速度に転置することにより、使用メモリ量を大幅に削減した。また転置により、インテル系Xeonプロセッサにおいては、従来のコードよりも大幅に性能向上できた。一方で、富士通FX10及び京コンピュータにおいては、従来の5次元コードに比べて1割ほど実効効率が落ちたため、今後更なるチューニングを施す必要がある。 本研究ではさらに、ケルビン・ヘルムホルツ不安定性や弱磁化小天体の大規模シミュレーションを行い、磁気流体スケールとイオンスケール及び電子スケールについて、物理過程の分離を行った。
• 大規模ブラソフシミュレーションによる宇宙プラズマのスケール間結合の研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 2011年 -2012年 

  代表者 : 梅田 隆行

   

  ポストペタフロップスコンピュータ用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。超多次元保存則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、10,000コア以上のスカラ型超並列計算機に向けた並列化を行った。また宇宙プラズマ中の様々なマルチスケール現象に対して適用を行い、世界で初めて磁化天体のグローバルブラソフシミュレーションに成功するとともに、速度シア層におけるケルビン-ヘルムホルツ不安定性において、流体スケールの境界層渦構造の発展が、粒子スケールのイオンジャイロ運動に大きく影響を受ける結果を得た。
• 磁気流体波による太陽風非平衡プラズマの生成過程

  日本学術振興会：科学研究費助成事業

  研究期間 : 2010年 -2012年 

  代表者 : 成行 泰裕, 梅田 隆行, 齊藤 慎司, 鈴木 建, 羽田 亨, 成田 康人

   

  本研究では、太陽風中の非平衡な速度分布が磁気流体波によって生成される過程について、数値計算・理論解析を用いた議論を行った。その結果、(1)磁気流体波が存在する場合に現れる「見かけの」非平衡速度分布が磁気流体系の平衡状態に対応していること、(2)太陽コロナから伝搬する磁気流体波が伝搬過程で生じる急峻化の過程で非平衡な速度分布が生成されること、(3)非平衡な速度分布によって励起される短波長の波動によって低周波の磁気流体波の減衰が促進されること、などが明らかになった
• 次世代第一原理粒子シミュレーションによる無衝突衝撃波の粒子加速機構の解明

  日本学術振興会：科学研究費助成事業

  研究期間 : 2009年 -2011年 

  代表者 : 梅田 隆行

   

  大規模な2次元全粒子シミュレーションにより、垂直衝撃波の非定常性について研究を行い、以下の成果を得た。衝撃波面におけるイオンの反射によってイオンと電子に相対速度が生じ、電子スケールの変形不安定性によって電磁ホイッスラー波動が励起する。このホイッスラー波動はリップルと呼ばれるイオンスケールの衝撃波面の変動と磁気的相互作用し、互いに強め合う。その結果、衝撃波面に入射する上流イオンと波面で反射するイオンが激しく散乱され、周期的かつコヒーレントなイオンの反射を抑制し、衝撃波の再形成が消滅する。以上により、垂直衝撃波の非定常性の1つである再形成の存在の有無が、電子スケールの微視的不安定性にコントロールされていることを明らかにし、電子-イオン-流体間のスケール間結合過程が存在することを示した。
• 実証型ジオスペースモデリングに向けた内部磁気圏基本モデルの構築による宇宙嵐の研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 2008年 -2011年 

  代表者 : 関 華奈子, 三好 由純, 天野 孝伸, 齊藤 慎司, 梅田 隆行, 松本 洋介, 海老原 祐輔, 高田 拓

   

  本研究では、マクスウェル方程式とボルツマン方程式とを同時に数値的に解くことで、内部磁気圏の場と粒子の時間発展を自己無撞着にシミュレートすることが可能な数値コードを開発した。具体的には、内部磁気圏の環電流粒子のグローバルなダイナミクスを現実的な計算機リソースで再現するために、本研究では粒子軌道をドリフト近似したブラソフ方程式(ドリフト運動論的方程式)とマクスウェル方程式を連立させ、場と粒子の自己無撞着な時間発展を追跡する方程式系を新たに導いた。またこの連立方程式系を数値的に解く3次元(分布関数は5次元)のシミュレーションコードを開発し、実際に磁気流体波動が伝播する様子を再現することを確認した。本研究で開発した新モデルにより、宇宙嵐時の粒子加速に重要な役割を果たすと思われている低周波波動や惑星間空間衝撃波到達に伴う磁気圏の急激な圧縮現象SC等の速い現象を、物理モデルに基づき自己無撞着にシミュレートしその物理機構を詳細に研究することが可能となった。
• 次世代スーパーコンピュータに向けたブラソフシミュレーション手法の研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 2009年 -2010年 

  代表者 : 梅田 隆行

   

  次々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。超多次元保存則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た。スカラ型超並列計算機に向けた並列化を行い、1,000以上のコアを用いた場合でも80%を超える並列計算効率を得ることに成功した。また宇宙プラズマ中の様々なマルチスケール現象に対して適用を行い、流体スケールと粒子スケール間の結合を示唆する結果を得た。
• 革新的なブラソフシミュレーション手法の研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 2007年 -2008年 

  代表者 : 梅田 隆行

   

  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

  研究期間 : 2001年 -2003年 

  代表者 : 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。

大学運営

委員歴

• 2020年04月 - 現在   名古屋大学宇宙地球環境研究所   統合データサイエンスセンター運営委員
• 2019年10月 - 現在   高度情報科学技術研究機構   HPCIシステム利用研究課題審査委員会レビュアー
• 2012年04月 - 現在   京都大学生存圏研究所   KDK共同利用専門委員
• 2010年10月 - 現在   日本地球惑星科学連合   連合大会レギュラーセッション「宇宙プラズマ理論・シミュレーション」コンビーナ
• 2010年07月 - 現在   地球電磁気・地球惑星圏学会   レギュラーセッション「宇宙プラズマ理論・シミュレーション」コンビーナ
• 2023年09月 - 2024年03月   名古屋大学情報連携推進本部   次期スパコン検討WG研究会委員
• 2023年09月 - 2024年01月   名古屋大学工学部   G30自動車工学プログラム運営委員
• 2023年04月 - 2024年01月   名古屋大学大学院工学研究科   大学院推薦入試広報委員
• 2021年04月 - 2024年01月   名古屋大学宇宙地球環境研究所   共同利用・共同研究委員
• 2020年04月 - 2024年01月   名古屋大学情報連携推進本部   情報セキュリティ連絡協議会組織責任者
• 2020年04月 - 2024年01月   名古屋大学高等研究院   若手育成プログラム(YLC)審査委員
• 2020年04月 - 2023年03月   名古屋大学工学部   工学部電気電子情報工学科令和２年度入学生学年担当
• 2020年11月 - 2022年10月   サイエンティフィック・システム研究会   A64FXシステムアプリ性能検討WG 推進委員
• 2021年04月 - 2022年03月   日本学術振興会   特別研究員等審査委員会専門委員
• 2019年11月 - 2021年03月   名古屋大学宇宙地球環境研究所   宇宙地球環境情報処理システム仕様策定委員長
• 2016年08月 - 2019年03月   名古屋大学情報連携推進本部   次期スパコン検討WG委員および次期スーパーコンピュータシステム仕様策定委員
• 2017年01月 - 2018年12月   サイエンティフィック・システム研究会   メニーコア時代のアプリ性能検討WG 推進委員
• 2014年06月 - 2018年02月   生存圏フォーラム   運営委員
• 2014年05月 - 2016年03月   名古屋大学太陽地球環境研究所   太陽地球環境情報処理システム仕様策定委員
• 2014年03月 - 2016年02月   サイエンティフィック・システム研究会   ポストペタアプリ性能WG 推進委員
• 2012年05月 - 2015年08月   名古屋大学情報連携統括本部   教育研究用高性能コンピュータシステム仕様策定委員
• 2010年12月 - 2013年05月   サイエンティフィック・システム研究会   マルチコアクラスタ性能WG 推進委員
• 2009年07月 - 2010年11月   名古屋大学太陽地球環境研究所   太陽地球環境情報処理システム仕様策定委員
• 2008年10月 - 2009年03月   名古屋大学情報連携推進本部   スーパーコンピュータシステム技術審査委員

社会貢献活動

• 宇宙プラズマの第一原理ブラソフシミュレーション

  期間 : 2017年03月

  役割 : 講師

  主催者・発行元 : 神戸大学計算科学教育センター

  イベント・番組・新聞雑誌名 : 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

  期間 : 2015年12月

  役割 : 講師

  主催者・発行元 : 高度情報科学技術研究機構

  イベント・番組・新聞雑誌名 : 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

  期間 : 2013年10月

  役割 : 講師

  主催者・発行元 : サイエンティフィック・システム研究会

  イベント・番組・新聞雑誌名 : 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

  期間 : 2010年08月

  役割 : 講師

  主催者・発行元 : 高知工業高等専門学校

  イベント・番組・新聞雑誌名 : 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

  期間 : 2009年09月

  役割 : 講師

  主催者・発行元 : 名古屋大学太陽地球環境研究所

  イベント・番組・新聞雑誌名 : 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

メディア報道

• 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 -- 」

  報道 : 2020年08月30日

  発行元・放送局 : 青山学院大学

  　インターネットメディア