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  • 1. Akhmetova, D.
    et al.
    Kestor, G.
    Gioiosa, R.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Laure, Erwin
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    On the application task granularity and the interplay with the scheduling overhead in many-core shared memory systems2015Inngår i: Proceedings - IEEE International Conference on Cluster Computing, ICCC, IEEE , 2015, s. 428-437Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Task-based programming models are considered one of the most promising programming model approaches for exascale supercomputers because of their ability to dynamically react to changing conditions and reassign work to processing elements. One question, however, remains unsolved: what should the task granularity of task-based applications be? Fine-grained tasks offer more opportunities to balance the system and generally result in higher system utilization. However, they also induce in large scheduling overhead. The impact of scheduling overhead on coarse-grained tasks is lower, but large systems may result imbalanced and underutilized. In this work we propose a methodology to analyze the interplay between application task granularity and scheduling overhead. Our methodology is based on three main points: 1) a novel task algorithm that analyzes an application directed acyclic graph (DAG) and aggregates tasks, 2) a fast and precise emulator to analyze the application behavior on systems with up to 1,024 cores, 3) a comprehensive sensitivity analysis of application performance and scheduling overhead breakdown. Our results show that there is an optimal task granularity between 1.2x10^4 and 10x10^4 cycles for the representative schedulers. Moreover, our analysis indicates that a suitable scheduler for exascale task-based applications should employ a best-effort local scheduler and a sophisticated remote scheduler to move tasks across worker threads.

  • 2.
    Akhmetova, Dana
    et al.
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Cebamanos, L.
    Iakymchuk, Roman
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Rotaru, T.
    Rahn, M.
    Markidis, Stefano
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Laure, Erwin
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Bartsch, V.
    Simmendinger, C.
    Interoperability of GASPI and MPI in large scale scientific applications2018Inngår i: 12th International Conference on Parallel Processing and Applied Mathematics, PPAM 2017, Springer Verlag , 2018, s. 277-287Konferansepaper (Fagfellevurdert)
    Abstract [en]

    One of the main hurdles of a broad distribution of PGAS approaches is the prevalence of MPI, which as a de-facto standard appears in the code basis of many applications. To take advantage of the PGAS APIs like GASPI without a major change in the code basis, interoperability between MPI and PGAS approaches needs to be ensured. In this article, we address this challenge by providing our study and preliminary performance results regarding interoperating GASPI and MPI on the performance crucial parts of the Ludwig and iPIC3D applications. In addition, we draw a strategy for better coupling of both APIs. 

  • 3.
    Al Ahad, Muhammed Abdullah
    et al.
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Simmendinger, Christian
    T Syst Solut Res GmbH, D-70563 Stuttgart, Germany..
    Iakymchuk, Roman
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Laure, Erwin
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Markidis, Stefano
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Efficient Algorithms for Collective Operations with Notified Communication in Shared Windows2018Inngår i: PROCEEDINGS OF PAW-ATM18: 2018 IEEE/ACM PARALLEL APPLICATIONS WORKSHOP, ALTERNATIVES TO MPI (PAW-ATM), IEEE , 2018, s. 1-10Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Collective operations are commonly used in various parts of scientific applications. Especially in strong scaling scenarios collective operations can negatively impact the overall applications performance: while the load per rank here decreases with increasing core counts, time spent in e.g. barrier operations will increase logarithmically with the core count. In this article, we develop novel algorithmic solutions for collective operations such as Allreduce and Allgather(V)-by leveraging notified communication in shared windows. To this end, we have developed an extension of GASPI which enables all ranks participating in a shared window to observe the entire notified communication targeted at the window. By exploring benefits of this extension, we deliver high performing implementations of Allreduce and Allgather(V) on Intel and Cray clusters. These implementations clearly achieve 2x-4x performance improvements compared to the best performing MPI implementations for various data distributions.

  • 4. Beck, A.
    et al.
    Innocenti, M. E.
    Lapenta, G.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Multi-level multi-domain algorithm implementation for two-dimensional multiscale particle in cell simulations2014Inngår i: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 271, s. 430-443Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    There are a number of modeling challenges posed by space weather simulations. Most of them arise from the multiscale and multiphysics aspects of the problem. The multiple scales dramatically increase the requirements, in terms of computational resources, because of the need of performing large scale simulations with the proper small-scales resolution. Lately, several suggestions have been made to overcome this difficulty by using various refinement methods which consist in splitting the domain into regions of different resolutions separated by well defined interfaces. The multiphysics issues are generally treated in a similar way: interfaces separate the regions where different equations are solved. This paper presents an innovative approach based on the coexistence of several levels of description, which differ by their resolutions or, potentially, by their physics. Instead of interacting through interfaces, these levels are entirely simulated and are interlocked over the complete extension of the overlap area. This scheme has been applied to a parallelized, two-dimensional, Implicit Moment Method Particle in Cell code in order to investigate its multiscale description capabilities. Simulations of magnetic reconnection and plasma expansion in vacuum are presented and possible implementation options for this scheme on very large systems are also discussed.

  • 5. Cazzola, E.
    et al.
    Curreli, D.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Lapenta, G.
    On the ions acceleration via collisionless magnetic reconnection in laboratory plasmas2016Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 23, nr 11, artikkel-id 112108Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work presents an analysis of the ion outflow from magnetic reconnection throughout fully kinetic simulations with typical laboratory plasma values. A symmetric initial configuration for the density and magnetic field is considered across the current sheet. After analyzing the behavior of a set of nine simulations with a reduced mass ratio and with a permuted value of three initial electron temperatures and magnetic field intensity, the best ion acceleration scenario is further studied with a realistic mass ratio in terms of the ion dynamics and energy budget. Interestingly, a series of shock wave structures are observed in the outflow, resembling the shock discontinuities found in recent magnetohydrodynamic simulations. An analysis of the ion outflow at several distances from the reconnection point is presented, in light of possible laboratory applications. The analysis suggests that magnetic reconnection could be used as a tool for plasma acceleration, with applications ranging from electric propulsion to production of ion thermal beams. © 2016 Author(s).

  • 6. Cazzola, E.
    et al.
    Innocenti, M. E.
    Goldman, M. V.
    Newman, D. L.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Lapenta, G.
    Electrons dynamics in asymmetric magnetic reconnection and rapid island coalescence: Anisotropy and agyrotropywith andwithout a guide field2016Inngår i: 43rd European Physical Society Conference on Plasma Physics, EPS 2016, European Physical Society (EPS) , 2016Konferansepaper (Fagfellevurdert)
  • 7. Cazzola, E.
    et al.
    Innocenti, M. E.
    Goldman, M. V.
    Newman, D. L.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Lapenta, G.
    On the electron agyrotropy during rapid asymmetric magnetic island coalescence in presence of a guide field2016Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, nr 15, s. 7840-7849Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present an analysis of the properties of the electron velocity distribution during island coalescence in asymmetric reconnection with and without guide field. In a previous study, three main domains were identified, in the case without guide field, as X, D, and M regions featuring different reconnection evolutions. These regions are also identified here in the case with guide field. We study the departure from isotropic and gyrotropic behavior by means of different robust detection algorithms proposed in the literature. While in the case without guide field these metrics show an overall agreement, when the guide field is present, a discrepancy in the agyrotropy within some relevant regions is observed, such as at the separatrices and inside magnetic islands. Moreover, in light of the new observations from the Multiscale MagnetoSpheric mission, an analysis of the electron velocity phase-space in these domains is presented.

  • 8. Cazzola, E.
    et al.
    Innocenti, M. E.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Goldman, M. V.
    Newman, D. L.
    Lapenta, G.
    On the electron dynamics during island coalescence in asymmetric magnetic reconnection2015Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 22, nr 9, artikkel-id 092901Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present an analysis of the electron dynamics during rapid island merging in asymmetric magnetic reconnection. We consider a doubly periodic system with two asymmetric transitions. The upper layer is an asymmetric Harris sheet of finite width perturbed initially to promote a single reconnection site. The lower layer is a tangential discontinuity that promotes the formation of many X-points, separated by rapidly merging islands. Across both layers, the magnetic field and the density have a strong jump, but the pressure is held constant. Our analysis focuses on the consequences of electron energization during island coalescence. We focus first on the parallel and perpendicular components of the electron temperature to establish the presence of possible anisotropies and non-gyrotropies. Thanks to the direct comparison between the two different layers simulated, we can distinguish three main types of behavior characteristic of three different regions of interest. The first type represents the regions where traditional asymmetric reconnections take place without involving island merging. The second type of regions instead shows reconnection events between two merging islands. Finally, the third regions identify the regions between two diverging island and where typical signature of reconnection is not observed. Electrons in these latter regions additionally show a flat-top distribution resulting from the saturation of a two-stream instability generated by the two interacting electron beams from the two nearest reconnection points. Finally, the analysis of agyrotropy shows the presence of a distinct double structure laying all over the lower side facing the higher magnetic field region. This structure becomes quadrupolar in the proximity of the regions of the third type. The distinguishing features found for the three types of regions investigated provide clear indicators to the recently launched Magnetospheric Multiscale NASA mission for investigating magnetopause reconnection involving multiple islands.

  • 9. Chen, Yuxi
    et al.
    Toth, Gabor
    Cassak, Paul
    Jia, Xianzhe
    Gombosi, Tamas I.
    Slavin, James A.
    Markidis, Stefano
    KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Peng, Ivy Bo
    KTH.
    Jordanova, Vania K.
    Henderson, Michael G.
    Global Three-Dimensional Simulation of Earth's Dayside Reconnection Using a Two-Way Coupled Magnetohydrodynamics With Embedded Particle-in-Cell Model: Initial Results2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 10, s. 10318-10335Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We perform a three-dimensional (3-D) global simulation of Earth's magnetosphere with kinetic reconnection physics to study the flux transfer events (FTEs) and dayside magnetic reconnection with the recently developed magnetohydrodynamics with embedded particle-in-cell model. During the 1 h long simulation, the FTEs are generated quasi-periodically near the subsolar point and move toward the poles. We find that the magnetic field signature of FTEs at their early formation stage is similar to a "crater FTE," which is characterized by a magnetic field strength dip at the FTE center. After the FTE core field grows to a significant value, it becomes an FTE with typical flux rope structure. When an FTE moves across the cusp, reconnection between the FTE field lines and the cusp field lines can dissipate the FTE. The kinetic features are also captured by our model. A crescent electron phase space distribution is found near the reconnection site. A similar distribution is found for ions at the location where the Larmor electric field appears. The lower hybrid drift instability (LHDI) along the current sheet direction also arises at the interface of magnetosheath and magnetosphere plasma. The LHDI electric field is about 8 mV/m, and its dominant wavelength relative to the electron gyroradius agrees reasonably with Magnetospheric Multiscale (MMS) observations.

  • 10. Chien, Steven W. D.
    et al.
    Markidis, Stefano
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Sishtla, Chaitanya Prasad
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Santos, Luis
    Herman, Pawel
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Nrasimhamurthy, Sai
    Laure, Erwin
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Characterizing Deep-Learning I/O Workloads in TensorFlow2018Inngår i: Proceedings of PDSW-DISCS 2018: 3rd Joint International Workshop on Parallel Data Storage and Data Intensive Scalable Computing Systems, Held in conjunction with SC 2018: The International Conference for High Performance Computing, Networking, Storage and Analysis, Institute of Electrical and Electronics Engineers (IEEE), 2018, s. 54-63Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The performance of Deep-Learning (DL) computing frameworks rely on the rformance of data ingestion and checkpointing. In fact, during the aining, a considerable high number of relatively small files are first aded and pre-processed on CPUs and then moved to accelerator for mputation. In addition, checkpointing and restart operations are rried out to allow DL computing frameworks to restart quickly from a eckpoint. Because of this, I/O affects the performance of DL plications. this work, we characterize the I/O performance and scaling of nsorFlow, an open-source programming framework developed by Google and ecifically designed for solving DL problems. To measure TensorFlow I/O rformance, we first design a micro-benchmark to measure TensorFlow ads, and then use a TensorFlow mini-application based on AlexNet to asure the performance cost of I/O and checkpointing in TensorFlow. To prove the checkpointing performance, we design and implement a burst ffer. find that increasing the number of threads increases TensorFlow ndwidth by a maximum of 2.3 x and 7.8 x on our benchmark environments. e use of the tensorFlow prefetcher results in a complete overlap of mputation on accelerator and input pipeline on CPU eliminating the fective cost of I/O on the overall performance. The use of a burst ffer to checkpoint to a fast small capacity storage and copy ynchronously the checkpoints to a slower large capacity storage sulted in a performance improvement of 2.6x with respect to eckpointing directly to slower storage on our benchmark environment.

  • 11.
    Chien, Steven Wei Der
    et al.
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST). KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Sishtla, Chaitanya Prasad
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST). KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Markidis, Stefano
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST). KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Jun, Zhang
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST). KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Peng, Ivy Bo
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC. KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Laure, Erwin
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC. KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    An Evaluation of the TensorFlow Programming Model for Solving Traditional HPC Problems2018Inngår i: Proceedings of the 5th International Conference on Exascale Applications and Software, The University of Edinburgh , 2018, s. 34-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Computational intensive applications such as pattern recognition, and natural language processing, are increasingly popular on HPC systems. Many of these applications use deep-learning, a branch of machine learning, to determine the weights of artificial neural network nodes by minimizing a loss function. Such applications depend heavily on dense matrix multiplications, also called tensorial operations. The use of Graphics Processing Unit (GPU) has considerably speeded up deep-learning computations, leading to a Renaissance of the artificial neural network. Recently, the NVIDIA Volta GPU and the Google Tensor Processing Unit (TPU) have been specially designed to support deep-learning workloads. New programming models have also emerged for convenient expression of tensorial operations and deep-learning computational paradigms. An example of such new programming frameworks is TensorFlow, an open-source deep-learning library released by Google in 2015. TensorFlow expresses algorithms as a computational graph where nodes represent operations and edges between nodes represent data flow. Multi-dimensional data such as vectors and matrices which flows between operations are called Tensors. For this reason, computation problems need to be expressed as a computational graph. In particular, TensorFlow supports distributed computation with flexible assignment of operation and data to devices such as GPU and CPU on different computing nodes. Computation on devices are based on optimized kernels such as MKL, Eigen and cuBLAS. Inter-node communication can be through TCP and RDMA. This work attempts to evaluate the usability and expressiveness of the TensorFlow programming model for traditional HPC problems. As an illustration, we prototyped a distributed block matrix multiplication for large dense matrices which cannot be co-located on a single device and a Conjugate Gradient (CG) solver. We evaluate the difficulty of expressing traditional HPC algorithms using computational graphs and study the scalability of distributed TensorFlow on accelerated systems. Our preliminary result with distributed matrix multiplication shows that distributed computation on TensorFlow is extremely scalable. This study provides an initial investigation of new emerging programming models for HPC.

  • 12. Daldorff, Lars K. S.
    et al.
    Toth, Gabor
    Gombosi, Tamas I.
    Lapenta, Giovanni
    Amaya, Jorge
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Brackbill, Jeremiah U.
    Two-way coupling of a global Hall magnetohydrodynamics model with a local implicit particle-in-cell model2014Inngår i: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 268, s. 236-254Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Computational models based on a fluid description of the plasma, such as magnetohydrodynamic (MHD) and extended magnetohydrodynamic (XMHD) codes are highly efficient, but they miss the kinetic effects due to the assumptions of small gyro radius, charge neutrality, and Maxwellian thermal velocity distribution. Kinetic codes can properly take into account the kinetic effects, but they are orders of magnitude more expensive than the fluid codes due to the increased degrees of freedom. If the fluid description is acceptable in a large fraction of the computational domain, it makes sense to confine the kinetic model to the regions where kinetic effects are important. This coupled approach can be much more efficient than a pure kinetic model. The speed up is approximately the volume ratio of the full domain relative to the kinetic regions assuming that the kinetic code uses a uniform grid. This idea has been advocated by [1] but their coupling was limited to one dimension and they employed drastically different grid resolutions in the fluid and kinetic models. We describe a fully two-dimensional two-way coupling of a Hall MHD model BATS-R-US with an implicit Particle-in-Cell (PIC) model iPIC3D. The coupling can be performed with identical grid resolutions and time steps. We call this coupled computational plasma model MHD-EPIC (MHD with Embedded PIC regions). Our verification tests show that MHD-EPIC works accurately and robustly. We show a two-dimensional magnetosphere simulation as an illustration of the potential future applications of MHD-EPIC.

  • 13. Deca, J.
    et al.
    Divin, A.
    Lapenta, G.
    Lembège, B.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Horányi, M.
    Electromagnetic Particle-in-Cell Simulations of the Solar Wind Interaction with Lunar Magnetic Anomalies2014Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, nr 15, s. 151102-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present the first three-dimensional fully kinetic and electromagnetic simulations of the solar wind interaction with lunar crustal magnetic anomalies (LMAs). Using the implicit particle-in-cell code IPIC3D, we confirm that LMAs may indeed be strong enough to stand off the solar wind from directly impacting the lunar surface forming a mini-magnetosphere, as suggested by spacecraft observations and theory. In contrast to earlier magnetohydrodynamics and hybrid simulations, the fully kinetic nature of IPIC3D allows us to investigate the space charge effects and in particular the electron dynamics dominating the near-surface lunar plasma environment. We describe for the first time the interaction of a dipole model centered just below the lunar surface under plasma conditions such that only the electron population is magnetized. The fully kinetic treatment identifies electromagnetic modes that alter the magnetic field at scales determined by the electron physics. Driven by strong pressure anisotropies, the mini-magnetosphere is unstable over time, leading to only temporal shielding of the surface underneath. Future human exploration as well as lunar science in general therefore hinges on a better understanding of LMAs.

  • 14. Deca, J.
    et al.
    Lapenta, G.
    Marchand, R.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Spacecraft charging analysis with the implicit particle-in-cell code iPic3D2013Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, nr 10, s. 102902-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present the first results on the analysis of spacecraft charging with the implicit particle-in-cell code iPic3D, designed for running on massively parallel supercomputers. The numerical algorithm is presented, highlighting the implementation of the electrostatic solver and the immersed boundary algorithm; the latter which creates the possibility to handle complex spacecraft geometries. As a first step in the verification process, a comparison is made between the floating potential obtained with iPic3D and with Orbital Motion Limited theory for a spherical particle in a uniform stationary plasma. Second, the numerical model is verified for a CubeSat benchmark by comparing simulation results with those of PTetra for space environment conditions with increasing levels of complexity. In particular, we consider spacecraft charging from plasma particle collection, photoelectron and secondary electron emission. The influence of a background magnetic field on the floating potential profile near the spacecraft is also considered. Although the numerical approaches in iPic3D and PTetra are rather different, good agreement is found between the two models, raising the level of confidence in both codes to predict and evaluate the complex plasma environment around spacecraft.

  • 15. Deca, Jan
    et al.
    Divin, Andrey
    Henri, Pierre
    Eriksson, Anders
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Olshevsky, Vyacheslav
    Horanyi, Mihaly
    Electron and Ion Dynamics of the Solar Wind Interaction with a Weakly Outgassing Comet2017Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 118, nr 20, artikkel-id 205101Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using a 3D fully kinetic approach, we disentangle and explain the ion and electron dynamics of the solar wind interaction with a weakly outgassing comet. We show that, to first order, the dynamical interaction is representative of a four-fluid coupled system. We self-consistently simulate and identify the origin of the warm and suprathermal electron distributions observed by ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko and conclude that a detailed kinetic treatment of the electron dynamics is critical to fully capture the complex physics of mass-loading plasmas.

  • 16. Deca, Jan
    et al.
    Divin, Andrey
    Lembege, Bertrand
    Horanyi, Mihaly
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Lapenta, Giovanni
    General mechanism and dynamics of the solar wind interaction with lunar magnetic anomalies from 3-D particle-in-cell simulations2015Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 8, s. 6443-6463Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a general model of the solar wind interaction with a dipolar lunar crustal magnetic anomaly (LMA) using three-dimensional full-kinetic and electromagnetic simulations. We confirm that LMAs may indeed be strong enough to stand off the solar wind from directly impacting the lunar surface, forming a so-called minimagnetosphere, as suggested by spacecraft observations and theory. We show that the LMA configuration is driven by electron motion because its scale size is small with respect to the gyroradius of the solar wind ions. We identify a population of back-streaming ions, the deflection of magnetized electrons via the E x B drift motion, and the subsequent formation of a halo region of elevated density around the dipole source. Finally, it is shown that the presence and efficiency of the processes are heavily impacted by the upstream plasma conditions and, on their turn, influence the overall structure and evolution of the LMA system. Understanding the detailed physics of the solar wind interaction with LMAs, including magnetic shielding, particle dynamics and surface charging is vital to evaluate its implications for lunar exploration.

  • 17. Deca, Jan
    et al.
    Divin, Andrey
    Wang, Xu
    Lembege, Bertrand
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST). KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Horanyi, Mihaly
    Lapenta, Giovanni
    Three-dimensional full-kinetic simulation of the solar wind interaction with a vertical dipolar lunarmagnetic anomaly2016Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, nr 9, s. 4136-4144Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A detailed understanding of the solar wind interaction with lunar magnetic anomalies (LMAs) is essential to identify its implications for lunar exploration and to enhance our physical understanding of the particle dynamics in a magnetized plasma. We present the first three-dimensional full-kinetic electromagnetic simulation case study of the solar wind interaction with a vertical dipole, resembling a medium-size LMA. In contrast to a horizontal dipole, we show that a vertical dipole twists its field lines and cannot form a minimagnetosphere. Instead, it creates a ring-shaped weathering pattern and reflects up to 21% (four times more as compared to the horizontal case) of the incoming solar wind ions electrostatically through the normal electric field formed above the electron shielding region surrounding the cusp. This work delivers a vital piece to fully comprehend and interpret lunar observations, as we find the amount of reflected ions to be a tracer for the underlying field structure.

  • 18. Divin, A.
    et al.
    Khotyaintsev, Y. V.
    Vaivads, Andris
    André, M.
    Toledo-Redondo, S.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Lapenta, G.
    Three-scale structure of diffusion region in the presence of cold ions2016Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, nr 12, s. 12,001-12,013Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Kinetic simulations and spacecraft observations typically display the two-scale structure of collisionless diffusion region (DR), with electron and ion demagnetization scales governing the spatial extent of the DR. Recent in situ observations of the nightside magnetosphere, as well as investigation of magnetic reconnection events at the Earth's magnetopause, discovered the presence of a population of cold (tens of eV) ions of ionospheric origin. We present two-dimensional particle-in-cell simulations of collisionless magnetic reconnection in multicomponent plasma with ions consisting of hot and cold populations. We show that a new cold ion diffusion region scale is introduced in between that of hot ions and electrons. Demagnetization scale of cold ion population is several times (∼4–8) larger than the initial cold ion gyroradius. Cold ions are accelerated and thermalized during magnetic reconnection and form ion beams moving with velocities close to the Alfvén velocity.

  • 19. Divin, A.
    et al.
    Khotyaintsev, Yu. V.
    Vaivads, A.
    Andre, M.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Lapenta, G.
    Evolution of the lower hybrid drift instability at reconnection jet front2015Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 4, s. 2675-2690Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate current-driven modes developing at jet fronts during collisionless reconnection. Initial evolution of the reconnection is simulated using conventional 2-D setup starting from the Harris equilibrium. Three-dimensional PIC calculations are implemented at later stages, when fronts are fully formed. Intense currents and enhanced wave activity are generated at the fronts because of the interaction of the fast flow plasma and denser ambient current sheet plasma. The study reveals that the lower hybrid drift instability develops quickly in the 3-D simulation. The instability produces strong localized perpendicular electric fields, which are several times larger than the convective electric field at the front, in agreement with Time History of Events and Macroscale Interactions during Substorms observations. The instability generates waves, which escape the front edge and propagate into the undisturbed plasma ahead of the front. The parallel electron pressure is substantially larger in the 3-D simulation compared to that of the 2-D. In a time similar to Omega(-1)(ci), the instability forms a layer, which contains a mixture of the jet plasma and current sheet plasma. The results confirm that the lower hybrid drift instability is important for the front evolution and electron energization.

  • 20. Divin, A.
    et al.
    Lapenta, G.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Newman, D. L.
    Goldman, M. V.
    Numerical simulations of separatrix instabilities in collisionless magnetic reconnection2012Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 19, nr 4, s. 042110-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Electron scale dynamics of magnetic reconnection separatrix jets is studied in this paper. Instabilities developing in directions both parallel and perpendicular to the magnetic field are investigated. Implicit particle-in-cell simulations with realistic electron-to-ion mass ratio are complemented by a set of small scale high resolution runs having the separatrix force balance as the initial condition. A special numerical procedure is developed to introduce the force balance into the small scale runs. Simulations show the development of streaming instabilities and consequent formation of electron holes in the parallel direction. A new electron jet instability develops in the perpendicular direction. The instability is closely related to the electron MHD Kelvin-Helmholtz mode and is destabilized by a flow, perpendicular to magnetic field at the separatrix. Tearing instability of the separatrix electron jet is modulated strongly by the electron MHD Kelvin-Helmholtz mode.

  • 21. Divin, A.
    et al.
    Lapenta, Giovanni
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Semenov, V. S.
    Erkaev, N. V.
    Korovinskiy, D. B.
    Biernat, H. K.
    Scaling of the inner electron diffusion region in collisionless magnetic reconnection2012Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, s. A06217-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Sweet-Parker analysis of the inner electron diffusion region of collisionless magnetic reconnection is presented. The study includes charged particles motion near the X-line and an appropriate approximation of the off-diagonal term for the electron pressure tensor. The obtained scaling shows that the width of the inner electron diffusion region is equal to the electron inertial length, and that electrons are accelerated up to the electron Alfven velocity in X-line direction. The estimated effective plasma conductivity is based on the electron gyrofrequency rather than the binary collision frequency, and gives the extreme (minimal) value of the plasma conductivity similar to Bohm diffusion. The scaling properties are verified by means of Particle-in-Cell simulations. An ad hoc parameter needs to be introduced to the scaling relations in order to better match the theory and simulations.

  • 22. Divin, A.
    et al.
    Semenov, V.
    Korovinskiy, D.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC. KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Deca, J.
    Olshevsky, V.
    Lapenta, G.
    A new model for the electron pressure nongyrotropy in the outer electron diffusion region2016Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, nr 20, s. 10565-10573Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a new model to describe the electron pressure nongyrotropy inside the electron diffusion region (EDR) in an antiparallel magnetic reconnection scenario. A combination of particle-in-cell simulations and analytical estimates is used to identify such a component of the electron pressure tensor in the rotated coordinates, which is nearly invariant along the outflow direction between the X line and the electron remagnetization points in the outer EDR. It is shown that the EDR two-scale structure (inner and outer parts) is formed due to superposition of the nongyrotropic meandering electron population and gyrotropic electron population with large anisotropy parallel to the magnetic field upstream of the EDR. Inside the inner EDR the influence of the pressure anisotropy can largely be ignored. In the outer EDR, a thin electron layer with electron flow speed exceeding the E x B drift velocity is supported by large-momentum flux produced by the electron pressure anisotropy upstream of the EDR. We find that this fast electron exhaust flow with |V(e)xB|>|E| is in fact a constituent part of the EDR, a finding which will steer the interpretation of the Magnetospheric Multiscale Mission (MMS) data.

  • 23. Eriksson, S.
    et al.
    Lapenta, G.
    Newman, D. L.
    Phan, T. D.
    Gosling, J. T.
    Lavraud, B.
    Khotyaintsev, Y. V.
    Carr, C. M.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Goldman, M. V.
    On Multiple Reconnection X-Lines and Tripolar Perturbations of Strong Guide Magnetic Fields2015Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 805, nr 1, artikkel-id 43Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report new multi-spacecraft Cluster observations of tripolar guide magnetic field perturbations at a solar wind reconnection exhaust in the presence of a guide field B-M. which is almost four times as strong as the reversing field B-L. The novel tripolar field consists of two narrow regions of depressed B-M, with an observed 7%-14% Delta B-M magnitude relative to the external field, which are found adjacent to a wide region of enhanced BM within the exhaust. A stronger reversing field is associated with each B-M depression. A kinetic reconnection simulation for realistic solar wind conditions and the observed strong guide field reveals that tripolar magnetic fields preferentially form across current sheets in the presence of multiple X-lines as magnetic islands approach one another and merge into fewer and larger islands. The simulated Delta B-M/Delta X-N over the normal width Delta X-N between a B-M minimum and the edge of the external region agree with the normalized values observed by Cluster. We propose that a tripolar guide field perturbation may be used to identify candidate regions containing multiple X-lines and interacting magnetic islands at individual solar wind current sheets with a strong guide field.

  • 24. Goldman, M. V.
    et al.
    Newman, D. L.
    Lapenta, G.
    Andersson, L.
    Gosling, J. T.
    Eriksson, S.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Eastwood, J. P.
    Ergun, R.
    Cerenkov Emission of Quasiparallel Whistlers by Fast Electron Phase-Space Holes during Magnetic Reconnection2014Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, nr 14, s. 145002-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Kinetic simulations of magnetotail reconnection have revealed electromagnetic whistlers originating near the exhaust boundary and propagating into the inflow region. The whistler production mechanism is not a linear instability, but rather is Cerenkov emission of almost parallel whistlers from localized moving clumps of charge (finite-size quasiparticles) associated with nonlinear coherent electron phase space holes. Whistlers are strongly excited by holes without ever growing exponentially. In the simulation the whistlers are emitted in the source region from holes that accelerate down the magnetic separatrix towards the x line. The phase velocity of the whistlers upsilon(phi) in the source region is everywhere well matched to the hole velocity upsilon(H) as required by the Cerenkov condition. The simulation shows emission is most efficient near the theoretical maximum upsilon(phi) = half the electron Alfven speed, consistent with the new theoretical prediction that faster holes radiate more efficiently. While transferring energy to whistlers the holes lose coherence and dissipate over a few local ion inertial lengths. The whistlers, however, propagate to the x line and out over many 10's of ion inertial lengths into the inflow region of reconnection. As the whistlers pass near the x line they modulate the rate at which magnetic field lines reconnect.

  • 25.
    Gong, Jing
    et al.
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hart, Alistair
    Cray Inc..
    Henty, David
    University of Edinburgh.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Schliephake, Michael
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Fischer, Paul
    Argonne National Laboratory.
    Heisey, Katherine
    Argonne National Laboratory.
    OpenACC Acceleration of Nek5000: a Spectral Element Code2013Konferansepaper (Annet vitenskapelig)
  • 26.
    Gong, Jing
    et al.
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Laure, Erwin
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Otten, Matthew
    Fischer, Paul
    Min, Misun
    Nekbone performance on GPUs with OpenACC and CUDA Fortran implementations2016Inngår i: Journal of Supercomputing, ISSN 0920-8542, E-ISSN 1573-0484, Vol. 72, nr 11, s. 4160-4180Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a hybrid GPU implementation and performance analysis of Nekbone, which represents one of the core kernels of the incompressible Navier-Stokes solver Nek5000. The implementation is based on OpenACC and CUDA Fortran for local parallelization of the compute-intensive matrix-matrix multiplication part, which significantly minimizes the modification of the existing CPU code while extending the simulation capability of the code to GPU architectures. Our discussion includes the GPU results of OpenACC interoperating with CUDA Fortran and the gather-scatter operations with GPUDirect communication. We demonstrate performance of up to 552 Tflops on 16, 384 GPUs of the OLCF Cray XK7 Titan.

  • 27.
    Gong, Jing
    et al.
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schliephake, Michael
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Laure, Erwin
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Cebamanos, Luis
    Hart, Alistair
    Min, Misun
    Fischer, Paul
    NekBone with Optimizaed OpenACC directives2015Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Accelerators and, in particular, Graphics Processing Units (GPUs) have emerged as promising computing technologies which may be suitable for the future Exascale systems. Here, we present performance results of NekBone, a benchmark of the Nek5000 code, implemented with optimized OpenACC directives and GPUDirect communications. Nek5000 is a computational fluid dynamics code based on the spectral element method used for the simulation of incompressible flow. Results of an optimized NekBone version lead to 78 Gflops performance on a single node. In addition, a performance result of 609 Tflops has been reached on 16, 384 GPUs of the Titan supercomputer at Oak Ridge National Laboratory.

     

  • 28.
    Gong, Jing
    et al.
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schliephake, Michael
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Laure, Erwin
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Peplinski, Adam
    Hart, Alistair
    Doleschal, Jens
    Henty, David
    Fischer, Paul
    Nek5000 with OpenACC2015Inngår i: Solving software challenges for exascale, 2015, s. 57-68Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Nek5000 is a computational fluid dynamics code based on the spectral element method used for the simulation of incompressible flows. We follow up on an earlier study which ported the simplified version of Nek5000 to a GPU-accelerated system by presenting the hybrid CPU/GPU implementation of the full Nek5000 code using OpenACC. The matrix-matrix multiplication, the Nek5000 gather-scatter operator and a preconditioned Conjugate Gradient solver have implemented using OpenACC for multi-GPU systems. We report an speed-up of 1.3 on single node of a Cray XK6 when using OpenACC directives in Nek5000. On 512 nodes of the Titan supercomputer, the speed-up can be approached to 1.4. A performance analysis of the Nek5000 code using Score-P and Vampir performance monitoring tools shows that overlapping of GPU kernels with host-accelerator memory transfers would considerably increase the performance of the OpenACC version of Nek5000 code.

  • 29. Henri, P.
    et al.
    Cerri, S. S.
    Califano, F.
    Pegoraro, F.
    Rossi, C.
    Faganello, M.
    Sebek, O.
    Travnicek, P. M.
    Hellinger, P.
    Frederiksen, J. T.
    Nordlund, A.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Keppens, R.
    Lapenta, G.
    Nonlinear evolution of the magnetized Kelvin-Helmholtz instability: From fluid to kinetic modeling2013Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, nr 10, s. 102118-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The nonlinear evolution of collisionless plasmas is typically a multi-scale process, where the energy is injected at large, fluid scales and dissipated at small, kinetic scales. Accurately modelling the global evolution requires to take into account the main micro-scale physical processes of interest. This is why comparison of different plasma models is today an imperative task aiming at understanding cross-scale processes in plasmas. We report here the first comparative study of the evolution of a magnetized shear flow, through a variety of different plasma models by using magnetohydrodynamic (MHD), Hall-MHD, two-fluid, hybrid kinetic, and full kinetic codes. Kinetic relaxation effects are discussed to emphasize the need for kinetic equilibriums to study the dynamics of collisionless plasmas in non trivial configurations. Discrepancies between models are studied both in the linear and in the nonlinear regime of the magnetized Kelvin-Helmholtz instability, to highlight the effects of small scale processes on the nonlinear evolution of collisionless plasmas. We illustrate how the evolution of a magnetized shear flow depends on the relative orientation of the fluid vorticity with respect to the magnetic field direction during the linear evolution when kinetic effects are taken into account. Even if we found that small scale processes differ between the different models, we show that the feedback from small, kinetic scales to large, fluid scales is negligible in the nonlinear regime. This study shows that the kinetic modeling validates the use of a fluid approach at large scales, which encourages the development and use of fluid codes to study the nonlinear evolution of magnetized fluid flows, even in the collisionless regime.

  • 30.
    Iakymchuk, Roman
    et al.
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST). KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Jordan, Herbert
    University of Innsbruck, Institute of Computer Science.
    Bo Peng, Ivy
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST). KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST). KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Laure, Erwin
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST). KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    A Particle-in-Cell Method for Automatic Load-Balancing with the AllScale Environment2016Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    We present an initial design and implementation of a Particle-in-Cell (PIC) method based on the work carried out in the European Exascale AllScale project. AllScale provides a unified programming system for the effective development of highly scalable, resilient and performance-portable parallel applications for Exascale systems. The AllScale approach is based on task-based nested recursive parallelism and it provides mechanisms for automatic load-balancing in the PIC simulations. We provide the preliminary results of the AllScale-based PIC implementation and draw directions for its future development. 

  • 31. Innocenti, M. E.
    et al.
    Beck, A.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Lapenta, G.
    Momentum conservation in Multi-Level Multi-Domain (MLMD) simulations2016Inngår i: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 312, s. 14-18Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Momentum conservation and self-forces reduction are challenges for all Particle-In-Cell (PIC) codes using spatial discretization schemes which do not fulfill the requirement of translational invariance of the grid Green's function. We comment here on the topic applied to the recently developed Multi-Level Multi-Domain (MLMD) method. The MLMD is a semi-implicit method for PIC plasma simulations. The multi-scale nature of plasma processes is addressed by using grids with different spatial resolutions in different parts of the domain.

  • 32. Innocenti, M. E.
    et al.
    Beck, A.
    Ponweiser, T.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Lapenta, G.
    Introduction of temporal sub-stepping in the Multi-Level Multi-Domain semi-implicit Particle-In-Cell code Parsek2D-MLMD2015Inngår i: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 189, s. 47-59Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, the introduction of temporal sub-stepping in Multi-Level Multi-Domain (MLMD) simulations of plasmas is discussed. The MLMD method addresses the multi-scale nature of space plasmas by simulating a problem at different levels of resolution. A large-domain "coarse grid" is simulated with low resolution to capture large-scale, slow processes. Smaller scale, local processes are obtained through a "refined grid" which uses higher resolution. Very high jumps in the resolution used at the different levels can be achieved thanks to the Implicit Moment Method and appropriate grid interlocking operations. Up to now, the same time step was used at all the levels. Now, with temporal sub-stepping, the different levels can also benefit from the use of different temporal resolutions. This saves further resources with respect to "traditional" simulations done using the same spatial and temporal stepping on the entire domain. It also prevents the levels from working at the limits of the stability condition of the Implicit Moment Method. The temporal sub-stepping is tested with simulations of magnetic reconnection in space. It is shown that, thanks to the reduced costs of MLMD simulations with respect to single-level simulations, it becomes possible to verify with realistic mass ratios scaling laws previously verified only for reduced mass ratios. Performance considerations are also provided.

  • 33. Innocenti, M. E.
    et al.
    Cazzola, E.
    Mistry, R.
    Eastwood, J. P.
    Goldman, M. V.
    Newman, D. L.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Lapenta, G.
    Switch-off slow shock/rotational discontinuity structures in collisionless magnetic reconnection: What to look for in satellite observations2017Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, nr 8, s. 3447-3455Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In Innocenti et al. (2015) we have observed and characterized for the first time Petschek-like switch-off slow shock/rotational discontinuity (SO-SS/RD) compound structures in a 2-D fully kinetic simulation of collisionless magnetic reconnection. Observing these structures in the solar wind or in the magnetotail would corroborate the possibility that Petschek exhausts develop in collisionless media as a result of single X point collisionless reconnection. Here we highlight their signatures in simulations with the aim of easing their identification in observations. The most notable signatures include a four-peaked ion current profile in the out-of-plane direction, associated ion distribution functions, increased electron and ion anisotropy downstream the SS, and increased electron agyrotropy downstream the RDs.

  • 34. Innocenti, M. E.
    et al.
    Johnson, A.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Amaya, J.
    Deca, J.
    Olshevsky, V.
    Lapenta, G.
    Progress towards physics-based space weather forecasting with exascale computing2017Inngår i: Advances in Engineering Software, ISSN 0965-9978, E-ISSN 1873-5339, Vol. 111, s. 3-17Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Space weather is a rapidly growing field of science which studies processes occurring in the area of space between the Sun and the Earth. The development of space weather forecasting capabilities is a task of great societal relevance: space weather effects may damage a number of technological assets, among which power and communication lines, transformers, pipelines and the telecommunication infrastructure. Exascale computing is a fundamental ingredient for space weather forecasting tools based on physical, rather than statistical, models. We describe here our recent progresses towards a physics-based space weather forecasting tool with exascale computing. We select the semi-implicit, Particle In Cell, Implicit Moment Method implemented in the parallel, object-oriented, C++ iPic3D code as a promising starting point. We analyze the structure and the performances of the current version of the iPic3D code. We describe three algorithmic developments, the fully implicit method, the Multi-Level Multi-Domain method, and the fluid-kinetic method, which can help addressing the multiple spatial and temporal scales present in space weather simulations. We then examine, in a co-design approach, which requirements - vectorization, extreme parallelism and reduced communication - an application has to satisfy to fully exploit architectures such as GPUs and Xeon Phi's. We address how to modify the iPic3D code to better satisfy these requirements. We then describe how to port the iPic3D code to the DEEP architecture currently under construction. The FP7 project DEEP (www.deep-project.eu) aims at building an exascale-ready machine composed of a cluster of Xeon nodes and of a collection of Xeon Phi coprocessors, used as boosters. The aim of the DEEP project is to enable exascale performance for codes, such as iPic3D, composed of parts which exhibit different potential for extreme scalability. Finally, we provide examples of simulations of space weather processes done with the current version of the iPic3D code. © 2016 Elsevier Ltd.

  • 35. Innocenti, M. E.
    et al.
    Lapenta, G.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Beck, A.
    Vapirev, A.
    A Multi Level Multi Domain Method for Particle In Cell plasma simulations2013Inngår i: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 238, s. 115-140Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A novel adaptive technique for electromagnetic Particle In Cell (PIC) plasma simulations is presented here. Two main issues are identified as regards the development of the algorithm. First, the choice of the size of the particle shape function in progressively refined grids, with the decision to avoid both time-dependent shape functions and cumbersome particle-to-grid interpolation techniques, and, second, the necessity to comply with the strict stability constraints of the explicit PIC algorithm. The adaptive implementation presented responds to these demands with the introduction of a Multi Level Multi Domain (MLMD) system, where a cloud of self-similar domains is fully simulated with both fields and particles, and the use of an Implicit Moment PIC method as baseline algorithm for the adaptive evolution. Information is exchanged between the levels with the projection of the field information from the refined to the coarser levels and the interpolation of the boundary conditions for the refined levels from the coarser level fields. Particles are bound to their level of origin and are prevented from transitioning to coarser levels, but are repopulated at the refined grid boundaries with a splitting technique. The presented algorithm is tested against a series of simulation challenges.

  • 36. Innocenti, M. E.
    et al.
    Norgren, C.
    Newman, D.
    Goldman, M.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Lapenta, G.
    Study of electric and magnetic field fluctuations from lower hybrid drift instability waves in the terrestrial magnetotail with the fully kinetic, semi-implicit, adaptive multi level multi domain method2016Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 23, nr 5, artikkel-id 052902Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The newly developed fully kinetic, semi-implicit, adaptive multi-level multi-domain (MLMD) method is used to simulate, at realistic mass ratio, the development of the lower hybrid drift instability (LHDI) in the terrestrial magnetotail over a large wavenumber range and at a low computational cost. The power spectra of the perpendicular electric field and of the fluctuations of the parallel magnetic field are studied at wavenumbers and times that allow to appreciate the onset of the electrostatic and electromagnetic LHDI branches and of the kink instability. The coupling between electric and magnetic field fluctuations observed by Norgren et al. ["Lower hybrid drift waves: Space observations," Phys. Rev. Lett. 109, 055001 (2012)] for high wavenumber LHDI waves in the terrestrial magnetotail is verified. In the MLMD simulations presented, a domain ("coarse grid") is simulated with low resolution. A small fraction of the entire domain is then simulated with higher resolution also ("refined grid") to capture smaller scale, higher frequency processes. Initially, the MLMD method is validated for LHDI simulations. MLMD simulations with different levels of grid refinement are validated against the standard semi-implicit particle in cell simulations of domains corresponding to both the coarse and the refined grid. Precious information regarding the applicability of the MLMD method to turbulence simulations is derived. The power spectra of MLMD simulations done with different levels of refinements are then compared. They consistently show a break in the magnetic field spectra at k(perpendicular to)d(i) similar to 30, with d(i) the ion skin depth and k(perpendicular to) the perpendicular wavenumber. The break is observed at early simulated times, Omega(ci)t < 6, with Omega(ci) the ion cyclotron frequency. It is due to the initial decoupling of electric and magnetic field fluctuations at intermediate and low wavenumbers, before the development of the electromagnetic LHDI branch. Evidence of coupling between electric and magnetic field fluctuations in the wave-number range where the fast and slow LHDI branches develop is then provided for a cluster magnetotail crossing.

  • 37. Innocenti, M. E.
    et al.
    Tronci, C.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Lapenta, G.
    Grid coupling mechanism in the semi-implicit adaptive Multi-Level Multi-Domain method2016Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 719, nr 1, artikkel-id 12019Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Multi-Level Multi-Domain (MLMD) method is a semi-implicit adaptive method for Particle-In-Cell plasma simulations. It has been demonstrated in the past in simulations of Maxwellian plasmas, electrostatic and electromagnetic instabilities, plasma expansion in vacuum, magnetic reconnection [1, 2, 3]. In multiple occasions, it has been commented on the coupling between the coarse and the refined grid solutions. The coupling mechanism itself, however, has never been explored in depth. Here, we investigate the theoretical bases of grid coupling in the MLMD system. We obtain an evolution law for the electric field solution in the overlap area of the MLMD system which highlights a dependance on the densities and currents from both the coarse and the refined grid, rather than from the coarse grid alone: grid coupling is obtained via densities and currents.

  • 38. Innocenti, M.E.
    et al.
    Goldman, M.
    Newman, D.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Lapenta, G.
    Evidence of magnetic field switch-off in collisionless magnetic reconnection2015Inngår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 810, nr 2Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The long-term evolution of large domain particle-in-cell simulations of collisionless magnetic reconnection is investigated following observations that show two possible outcomes for collisionless reconnection: toward a Petschek-like configuration or toward multiple X points. In the present simulation, a mixed scenario develops. At earlier time, plasmoids are emitted, disrupting the formation of Petschek-like structures. Later, an almost stationary monster plasmoid forms, preventing the emission of other plasmoids. A situation reminiscent of Petschek’s switch-off then ensues. Switch-off is obtained through a slow shock/rotational discontinuity compound structure. Two external slow shocks (SS) located at the separatrices reduce the in-plane tangential component of the magnetic field, but not to zero. Two transitions reminiscent of rotational discontinuities (RD) in the internal part of the exhaust then perform the final switch-off. Both the SS and the RD are characterized through analysis of their Rankine-Hugoniot jump conditions. A moderate guide field is used to suppress the development of the firehose instability in the exhaust.

  • 39.
    Ivanov, Ilya
    et al.
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Gong, Jing
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Akhmetova, Dana
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Peng, Ivy Bo
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Laure, Erwin
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Machado, Rui
    Rahn, Mirko
    Bartsch, Valeria
    Hart, Alistair
    Fischer, Paul
    Evaluation of Parallel Communication Models in Nekbone, a Nek5000 mini-application2015Inngår i: 2015 IEEE International Conference on Cluster Computing, IEEE , 2015, s. 760-767Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Nekbone is a proxy application of Nek5000, a scalable Computational Fluid Dynamics (CFD) code used for modelling incompressible flows. The Nekbone mini-application is used by several international co-design centers to explore new concepts in computer science and to evaluate their performance. We present the design and implementation of a new communication kernel in the Nekbone mini-application with the goal of studying the performance of different parallel communication models. First, a new MPI blocking communication kernel has been developed to solve Nekbone problems in a three-dimensional Cartesian mesh and process topology. The new MPI implementation delivers a 13% performance improvement compared to the original implementation. The new MPI communication kernel consists of approximately 500 lines of code against the original 7,000 lines of code, allowing experimentation with new approaches in Nekbone parallel communication. Second, the MPI blocking communication in the new kernel was changed to the MPI non-blocking communication. Third, we developed a new Partitioned Global Address Space (PGAS) communication kernel, based on the GPI-2 library. This approach reduces the synchronization among neighbor processes and is on average 3% faster than the new MPI-based, non-blocking, approach. In our tests on 8,192 processes, the GPI-2 communication kernel is 3% faster than the new MPI non-blocking communication kernel. In addition, we have used the OpenMP in all the versions of the new communication kernel. Finally, we highlight the future steps for using the new communication kernel in the parent application Nek5000.

  • 40.
    Ivanov, Ilya
    et al.
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Machado, Rui
    Rahn, Mirko
    Akhmetova, Dana
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Laure, Erwin
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Gong, Jing
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Fischer, Paul
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Evaluating New Communication Models in the Nek5000 Code for Exascale2015Konferansepaper (Annet vitenskapelig)
  • 41. Johlander, A.
    et al.
    Schwartz, S. J.
    Vaivads, Andris
    Khotyaintsev, Yu. V.
    Gingell, I.
    Peng, Bo
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Lindqvist, Per-Arne
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Ergun, R. E.
    Marklund, G. T.
    Plaschke, F.
    Magnes, W.
    Strangeway, R. J.
    Russell, C. T.
    Wei, H.
    Torbert, R. B.
    Paterson, W. R.
    Gershman, D. J.
    Dorelli, J. C.
    Avanov, L. A.
    Lavraud, B.
    Saito, Y.
    Giles, B. L.
    Pollock, C. J.
    Burch, J. L.
    Rippled Quasiperpendicular Shock Observed by the Magnetospheric Multiscale Spacecraft2016Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 117, nr 16, artikkel-id 165101Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Collisionless shock nonstationarity arising from microscale physics influences shock structure and particle acceleration mechanisms. Nonstationarity has been difficult to quantify due to the small spatial and temporal scales. We use the closely spaced (subgyroscale), high-time-resolution measurements from one rapid crossing of Earth's quasiperpendicular bow shock by the Magnetospheric Multiscale (MMS) spacecraft to compare competing nonstationarity processes. Using MMS's high-cadence kinetic plasma measurements, we show that the shock exhibits nonstationarity in the form of ripples.

  • 42.
    Jordanova, V. K.
    et al.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Delzanno, G. L.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Henderson, M. G.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Godinez, H. C.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Jeffery, C. A.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Lawrence, E. C.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Morley, S. K.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Moulton, J. D.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Vernon, L. J.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Woodroffe, J. R.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Brito, T. V.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Engel, M. A.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Meierbachtol, C. S.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Svyatsky, D.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Yu, Y.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA.;Beihang Univ, Beijing, Peoples R China..
    Toth, G.
    Univ Michigan, Ann Arbor, MI 48109 USA..
    Welling, D. T.
    Univ Michigan, Ann Arbor, MI 48109 USA..
    Chen, Y.
    Univ Michigan, Ann Arbor, MI 48109 USA..
    Haiducek, J.
    Univ Michigan, Ann Arbor, MI 48109 USA..
    Markidis, Stefano
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Albert, J. M.
    Air Force Res Lab, Kirtland AFB, NM USA..
    Birn, J.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA.;Space Sci Inst, Boulder, CO USA..
    Denton, M. H.
    Space Sci Inst, Boulder, CO USA.;New Mexico Consortium, Los Alamos, NM USA..
    Horne, R. B.
    British Antarctic Survey, Cambridge, England..
    Specification of the near-Earth space environment with SHIELDS2018Inngår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 177, s. 148-159Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Predicting variations in the near-Earth space environment that can lead to spacecraft damage and failure is one example of "space weather" and a big space physics challenge. A project recently funded through the Los Alamos National Laboratory (LANL) Directed Research and Development (LDRD) program aims at developing a new capability to understand, model, and predict Space Hazards Induced near Earth by Large Dynamic Storms, the SHIELDS framework. The project goals are to understand the dynamics of the surface charging environment (SCE), the hot (keV) electrons representing the source and seed populations for the radiation belts, on both macro and micro-scale. Important physics questions related to particle injection and acceleration associated with magnetospheric storms and substorms, as well as plasma waves, are investigated. These challenging problems are addressed using a team of world-class experts in the fields of space science and computational plasma physics, and state-of-the-art models and computational facilities. A full two-way coupling of physics-based models across multiple scales, including a global MHD (BATS-R-US) embedding a particle-in-cell (iPIC3D) and an inner magnetosphere (RAM-SCB) codes, is achieved. New data assimilation techniques employing in situ satellite data are developed; these provide an order of magnitude improvement in the accuracy in the simulation of the SCE. SHIELDS also includes a post-processing tool designed to calculate the surface charging for specific spacecraft geometry using the Curvilinear Particle-In-Cell (CPIC) code that can be used for reanalysis of satellite failures or for satellite design.

  • 43. Khotyaintsev, Yu. V.
    et al.
    Divin, A.
    Vaivads, A.
    Andre, M.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Energy conversion at dipolarization fronts2017Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, nr 3, s. 1234-1242Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We use multispacecraft observations by Cluster in the Earth's magnetotail and 3-D particle-in-cell simulations to investigate conversion of electromagnetic energy at the front of a fast plasma jet. We find that the major energy conversion is happening in the Earth (laboratory) frame, where the electromagnetic energy is being transferred from the electromagnetic field to particles. This process operates in a region with size of the order several ion inertial lengths across the jet front, and the primary contribution to E . j is coming from the motional electric field and the ion current. In the frame of the front we find fluctuating energy conversion with localized loads and generators at sub-ion scales which are primarily related to the lower hybrid drift instability excited at the front; however, these provide relatively small net energy conversion.

  • 44. Korovinskiy, D. B.
    et al.
    Divin, A.
    Erkaev, N. V.
    Ivanova, V. V.
    Ivanov, I. B.
    Semenov, V. S.
    Lapenta, G.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Biernat, H. K.
    Zellinger, M.
    MHD modeling of the double-gradient (kink) magnetic instability2013Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 118, nr 3, s. 1146-1158Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The paper presents the detailed numerical investigation of the "double-gradient mode," which is believed to be responsible for the magnetotail flapping oscillations-the fast vertical (normal to the layer) oscillations of the Earth's magnetotail plasma sheet with a quasiperiod similar to 100-200 s. The instability is studied using the magnetotail near-equilibrium configuration. For the first time, linear three-dimensional numerical analysis is complemented with full 3-D MHD simulations. It is known that the "double-gradient mode" has unstable solutions in the region of the tailward growth of the magnetic field component, normal to the current sheet. The unstable kink branch of the mode is the focus of our study. Linear MHD code results agree with the theory, and the growth rate is found to be close to the peak value, provided by the analytical estimates. Full 3-D simulations are initialized with the numerically relaxed magnetotail equilibrium, similar to the linear code initial condition. The calculations show that current layer with tailward gradient of the normal component of the magnetic field is unstable to wavelengths longer than the curvature radius of the field line. The segment of the current sheet with the earthward gradient of the normal component makes some stabilizing effect (the same effect is registered in the linearized MHD simulations) due to the minimum of the total pressure localized in the center of the sheet. The overall growth rate is close to the theoretical double-gradient estimate averaged over the computational domain.

  • 45. Korovinskiy, D. B.
    et al.
    Divin, A. V.
    Erkaev, N. V.
    Semenov, V. S.
    Artemyev, A. V.
    Ivanova, V. V.
    Ivanov, I. B.
    Lapenta, G.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Biernat, H. K.
    The double-gradient magnetic instability: Stabilizing effect of the guide field2015Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 22, nr 1, artikkel-id 012904Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The role of the dawn-dusk magnetic field component in stabilizing of the magnetotail flapping oscillations is investigated in the double-gradient model framework (Erkaev et al., Phys. Rev. Lett. 99, 235003 (2007)), extended for the magnetotail-like configurations with non-zero guide field By. Contribution of the guide field is examined both analytically and by means of linearized 2-dimensional (2D) and non-linear 3-dimensional (3D) MHD modeling. All three approaches demonstrate the same properties of the instability: stabilization of current sheet oscillations for short wavelength modes, appearing of the typical (fastest growing) wavelength lambda(peak) of the order of the current sheet width, decrease of the peak growth rate with increasing B-y value, and total decay of the mode for B-y similar to 0: 5 in the lobe magnetic field units. Analytical solution and 2D numerical simulations claim also the shift of lambda(peak) toward the longer wavelengths with increasing guide field. This result is barely visible in 3D simulations. It may be accounted for the specific background magnetic configuration, the pattern of tail-like equilibrium provided by approximated solution of the conventional Grad-Shafranov equation. The configuration demonstrates drastically changing radius of curvature of magnetic field lines, R-c. This, in turn, favors the "double-gradient" mode (lambda > R-c) in one part of the sheet and classical "ballooning" instability (lambda < R-c) in another part, which may result in generation of a "combined" unstable mode. (C) 2015 AIP Publishing LLC.

  • 46.
    Korovinskiy, D.
    et al.
    Austrian Acad Sci, Space Res Inst, Schmiedlstr 6, A-8042 Graz, Austria.;St Petersburg State Univ, St Petersburg 198504, Russia..
    Divin, A.
    Swedish Inst Space Phys, SE-75121 Uppsala, Sweden..
    Ivanova, V.
    St Petersburg State Univ, St Petersburg 198504, Russia..
    Erkaev, N.
    SB RAS, Inst Computat Modelling, Krasnoyarsk 660036, Russia.;Siberian Fed Univ, Krasnoyarsk 660041, Russia..
    Semenov, V.
    St Petersburg State Univ, St Petersburg 198504, Russia..
    Ivanov, I.
    St Petersburg State Univ, St Petersburg 198504, Russia..
    Biernat, H.
    Austrian Acad Sci, Space Res Inst, Schmiedlstr 6, A-8042 Graz, Austria..
    Lapenta, G.
    Katholieke Univ Leuven, Ctr Plasma Astrofys, Dept Wiskunde, B-3001 Leuven, Belgium..
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    MHD Modeling of the Kink "Double-gradient" Branch of the Ballooning Instability in the Magnetotail2014Inngår i: NUMERICAL MODELING OF SPACE PLASMA FLOWS: ASTRONUM-2013 / [ed] Pogorelov, NV Audit, E Zank, GP, ASTRONOMICAL SOC PACIFIC , 2014, Vol. 488, s. 149-154Konferansepaper (Fagfellevurdert)
    Abstract [en]

    We present a numerical investigation of the double-gradient mode, which is believed to be responsible for the magnetotail flapping oscillations the fast vertical oscillations of the Earth's magnetotail plasma sheet (quasiperiod similar to 100 - 200 s). It is known that this mode has an unstable solution in the region of the tailward-growing normal magnetic field component. The kink branch of the mode is the focus of our study. The instability is studied using the magnetotail near-equilibrium configuration, fixed by the approximate solution of the Grad-Shafranov equation. The linear three-dimensional numerical analysis is complemented with full 3-D MUD simulations. The results of our linearized MHD code agree with the theory, and the growth rate is found to be close to the peak value provided by an analytical estimate. Also, the eigenfunctions, calculated analytically, are very similar to the perturbations obtained numerically. The full 3D MHD simulations are initialized with the numerically relaxed magnetotail equilibrium, similar to the linear code initial condition. The calculations show that the double-gradient mode is excited in a region of small radii of the magnetic field lines curvature, which is in accordance with the analytical predictions. In contrast to the linearized MHD simulations, non-local interactions are involved; hence, the overall growth rate turns out to be close to the theoretical estimate averaged over the computational domain.

  • 47. Kumar, P.
    et al.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Lapenta, G.
    Meerbergen, K.
    Roose, D.
    High performance solvers for implicit particle in cell simulation2013Inngår i: 2013 International Conference on Computational Science, Elsevier, 2013, s. 2251-2258Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A three-dimensional implicit particle-in-cell (iPIC3D) method implemented by S. Markidis et. al. in ["Multiscale simulations of plasma with iPIC3D", Mathematics and Computers in Simulation, 80(2010), 1509-1519] allows time steps at magnetohydrodynamics time scale. The code requires the solution of two linear systems: A Poisson system related to divergence cleaning, and a system related to a second order formulation of Maxwell equation. In iPIC3D, the former is the most costly. To reduce the cost of solving the Poisson system, a parallel matrix assembly and partitioning method are implemented, and conjugate gradient and algebraic multigrid (AMG) solvers from the Hypre library are called. The scalability of AMG as a solver is studied for 1D and 3D partitionings and compared to that of CG.

  • 48. Lapenta, G.
    et al.
    Goldman, M.
    Newman, D.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST).
    Where should MMS look for electron diffusion regions?2016Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 719, nr 1, artikkel-id 12011Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A great possible achievement for the MMS mission would be crossing electron diffusion regions (EDR). EDR are regions in proximity of reconnection sites where electrons decouple from field lines, breaking the frozen in condition. Decades of research on reconnection have produced a widely shared map of where EDRs are. We expect reconnection to take place around a so called x-point formed by the intersection of the separatrices dividing inflowing from outflowing plasma. The EDR forms around this x-point as a small electron scale box nested inside a larger ion diffusion region. But this point of view is based on a 2D mentality. We have recently proposed that once the problem is considered in full 3D, secondary reconnection events can form [Lapenta et al., Nature Physics, 11, 690, 2015] in the outflow regions even far downstream from the primary reconnection site. We revisit here this new idea confirming that even using additional indicators of reconnection and even considering longer periods and wider distances the conclusion remains true: secondary reconnection sites form downstream of a reconnection outflow causing a sort of chain reaction of cascading reconnection sites. If we are right, MMS will have an interesting journey even when not crossing necessarily the primary site. The chances are greatly increased that even if missing a primary site during an orbit, MMS could stumble instead on one of these secondary sites.

  • 49. Lapenta, Giovanni
    et al.
    Goldman, Martin
    Newman, David
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Propagation speed of rotation signals for field lines undergoing magnetic reconnection2013Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, nr 10, s. 102113-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Reconnection is associated with two bending of the magnetic field lines. Considering the usual plane of a 2D reconnection simulation, the first bending is in-plane and produces the needed topological changes by bringing oppositely directed filed lines in proximity. The second is typical of fast reconnection and is out of plane, leading to the formation of the Hall magnetic field. This second rotation has recently been observed to proceed at superAlfvenic speeds and to carry substantial energy fluxes (Shay et al., Phys. Rev. Lett. 107, 065001 (2011)). We revisit these rotations with a new diagnostics based on dispersing a multitude of virtual probes into a kinetic simulation, akin the approach of multi spacecraft missions. The results of the new diagnostics are compared with the theory of characteristics applied to the two fluid model. The comparison of virtual probes and the method of characteristics confirm the findings relative to the out of plane rotation and uncover the existence of two families of characteristics. Both are observed in the simulation. The early stage of reconnection develops on the slower compressional branch and the later faster phase develops on the faster torsional branch. The superAlfvenic signal is only relevant in the second phase.

  • 50. Lapenta, Giovanni
    et al.
    Goldman, Martin
    Newman, David
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz).
    Divin, Andrey
    Electromagnetic energy conversion in downstream fronts from three dimensional kinetic reconnection2014Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 21, nr 5, s. 055702-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The electromagnetic energy equation is analyzed term by term in a 3D simulation of kinetic reconnection previously reported by Vapirev et al. [J. Geophys. Res.: Space Phys. 118, 1435 (2013)]. The evolution presents the usual 2D-like topological structures caused by an initial perturbation independent of the third dimension. However, downstream of the reconnection site, where the jetting plasma encounters the yet unperturbed pre-existing plasma, a downstream front is formed and made unstable by the strong density gradient and the unfavorable local acceleration field. The energy exchange between plasma and fields is most intense at the instability, reaching several pW/m(3), alternating between load (energy going from fields to particles) and generator (energy going from particles to fields) regions. Energy exchange is instead purely that of a load at the reconnection site itself in a region focused around the x-line and elongated along the separatrix surfaces. Poynting fluxes are generated at all energy exchange regions and travel away from the reconnection site transporting an energy signal of the order of about S approximate to 10(-3)W/m(2). (C) 2014 AIP Publishing LLC.

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