TY - JOUR A1 - Stroman, Thomas A1 - Pohl, Martin A1 - Niemiec, Jacek A1 - Bret, Antoine T1 - Could cosmic rays affect instabilities in the Transition layer of nonrealativistic collisionless shocks? JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - There is an observational correlation between astrophysical shocks and nonthermal particle distributions extending to high energies. As a first step toward investigating the possible feedback of these particles on the shock at the microscopic level, we perform particle-in-cell (PIC) simulations of a simplified environment consisting of uniform, interpenetrating plasmas, both with and without an additional population of cosmic rays. We vary the relative density of the counterstreaming plasmas, the strength of a homogeneous parallel magnetic field, and the energy density in cosmic rays. We compare the early development of the unstable spectrum for selected configurations without cosmic rays to the growth rates predicted from linear theory, for assurance that the system is well represented by the PIC technique. Within the parameter space explored, we do not detect an unambiguous signature of any cosmic-ray-induced effects on the microscopic instabilities that govern the formation of a shock. We demonstrate that an overly coarse distribution of energetic particles can artificially alter the statistical noise that produces the perturbative seeds of instabilities, and that such effects can be mitigated by increasing the density of computational particles. KW - cosmic rays KW - instabilities KW - plasmas KW - shock waves Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/746/1/24 SN - 0004-637X VL - 746 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Bohdan, Artem A1 - Niemiec, Jacek A1 - Kobzar, Oleh A1 - Pohl, Martin T1 - Electron Pre-acceleration at Nonrelativistic High-Mach-number Perpendicular Shocks JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We perform particle-in-cell simulations of perpendicular nonrelativistic collisionless shocks to study electron heating and pre-acceleration for parameters that permit the extrapolation to the conditions at young supernova remnants. Our high-resolution large-scale numerical experiments sample a representative portion of the shock surface and demonstrate that the efficiency of electron injection is strongly modulated with the phase of the shock reformation. For plasmas with low and moderate temperature (plasma beta beta p =5.10(-4) and 0.5 beta p =), we explore the nonlinear shock structure and electron pre-acceleration for various orientations of the large-scale magnetic field with respect to the simulation plane, while keeping it at 90 degrees to the shock normal. Ion reflection off of the shock leads to the formation of magnetic filaments in the shock ramp, resulting from Weibel-type instabilities, and electrostatic Buneman modes in the shock foot. In all of the cases under study, the latter provides first-stage electron energization through the shock-surfing acceleration mechanism. The subsequent energization strongly depends on the field orientation and proceeds through adiabatic or second-order Fermi acceleration processes for configurations with the out-of-plane and in-plane field components, respectively. For strictly out-of-plane field, the fraction of suprathermal electrons is much higher than for other configurations, because only in this case are the Buneman modes fully captured by the 2D simulation grid. Shocks in plasma with moderate bp provide more efficient pre-acceleration. The relevance of our results to the physics of fully 3D systems is discussed. KW - acceleration of particles KW - instabilities KW - ISM: supernova remnants KW - methods: numerical KW - plasmas KW - shock Y1 - 2017 U6 - https://doi.org/10.3847/1538-4357/aa872a SN - 0004-637X SN - 1538-4357 VL - 847 PB - IOP Publ. Ltd. CY - Bristol ER - TY - GEN A1 - Bohdan, Artem A1 - Niemiec, Jacek A1 - Kobzar, Oleh A1 - Pohl, Martin T1 - Erratum: Electron Pre-acceleration at Nonrelativistic High-Mach-number Perpendicular Shocks (The astrophysical journal : an international review of spectroscopy and astronomical physics. - Vol 847, 2017, 71) T2 - The astrophysical journal : an international review of spectroscopy and astronomical physics Y1 - 2019 U6 - https://doi.org/10.3847/1538-4357/ab2f89 SN - 0004-637X SN - 1538-4357 VL - 880 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Bohdan, Artem A1 - Niemiec, Jacek A1 - Pohl, Martin A1 - Matsumoto, Yosuke A1 - Amano, Takanobu A1 - Hoshino, Masahiro T1 - Kinetic Simulations of Nonrelativistic Perpendicular Shocks of Young Supernova Remnants BT - I. Electron Shock-surfing Acceleration JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Electron injection at high Mach number nonrelativistic perpendicular shocks is studied here for parameters that are applicable to young SNR shocks. Using high-resolution large-scale two-dimensional fully kinetic particle-in-cell simulations and tracing individual particles, we in detail analyze the shock-surfing acceleration (SSA) of electrons at the leading edge of the shock foot. The central question is to what degree the process can be captured in 2D3V simulations. We find that the energy gain in SSA always arises from the electrostatic field of a Buneman wave. Electron energization is more efficient in the out-of-plane orientation of the large-scale magnetic field because both the phase speed and the amplitude of the waves are higher than for the in-plane scenario. Also, a larger number of electrons is trapped by the waves compared to the in-plane configuration. We conclude that significant modifications of the simulation parameters are needed to reach the same level of SSA efficiency as in simulations with out-of-plane magnetic field or 3D simulations. KW - acceleration of particles KW - instabilities KW - ISM: supernova remnants KW - methods: numerical KW - plasmas KW - shock waves Y1 - 2019 U6 - https://doi.org/10.3847/1538-4357/ab1b6d SN - 0004-637X SN - 1538-4357 VL - 878 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Bohdan, Artem A1 - Niemiec, Jacek A1 - Pohl, Martin A1 - Matsumoto, Yosuke A1 - Amano, Takanobu A1 - Hoshino, Masahiro T1 - Kinetic Simulations of Nonrelativistic Perpendicular Shocks of Young Supernova Remnants. II. Influence of Shock-surfing Acceleration on Downstream Electron Spectra JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We explore electron preacceleration at high-Mach-number nonrelativistic perpendicular shocks at, e.g., young supernova remnants, which are a prerequisite of further acceleration to very high energies via diffusive shock acceleration. Using fully kinetic particle-in-cell simulations of shocks and electron dynamics in them, we investigate the influence of shock-surfing acceleration (SSA) at the shock foot on the nonthermal population of electrons downstream of the shock. The SSA is followed by further energization at the shock ramp where the Weibel instability spawns a type of second-order Fermi acceleration. The combination of these two processes leads to the formation of a nonthermal electron population, but the importance of SSA becomes smaller for larger ion-to-electron mass ratios in the simulation. We discuss the resulting electron spectra and the relevance of our results to the physics of systems with real ion-to-electron mass ratios and fully three-dimensional behavior. KW - Shocks KW - Space plasmas KW - Supernova remnants KW - Interstellar medium Y1 - 2019 U6 - https://doi.org/10.3847/1538-4357/ab43cf SN - 0004-637X SN - 1538-4357 VL - 885 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Mizuno, Yosuke A1 - Pohl, Martin A1 - Niemiec, Jacek A1 - Zhang, Bing A1 - Nishikawa, Ken-Ichi A1 - Hardee, Philip E. T1 - Magnetic field amplification and saturation in turbulence behind a relativistic shock JF - Monthly notices of the Royal Astronomical Society N2 - We have investigated via 2D relativistic magnetohydrodynamic simulations the long-term evolution of turbulence created by a relativistic shock propagating through an inhomogeneous medium. In the post-shock region, magnetic field is strongly amplified by turbulent motions triggered by pre-shock density inhomogeneities. Using a long-simulation box we have followed the magnetic field amplification until it is fully developed and saturated. The turbulent velocity is subrelativistic even for a strong shock. Magnetic field amplification is controlled by the turbulent motion and saturation occurs when the magnetic energy is comparable to the turbulent kinetic energy. Magnetic field amplification and saturation depend on the initial strength and direction of the magnetic field in the pre-shock medium, and on the shock strength. If the initial magnetic field is perpendicular to the shock normal, the magnetic field is first compressed at the shock and then can be amplified by turbulent motion in the post-shock region. Saturation occurs when the magnetic energy becomes comparable to the turbulent kinetic energy in the post-shock region. If the initial magnetic field in the pre-shock medium is strong, the post-shock region becomes turbulent but significant field amplification does not occur. If the magnetic energy after shock compression is larger than the turbulent kinetic energy in the post-shock region, significant field amplification does not occur. We discuss possible applications of our results to gamma-ray bursts and active galactic nuclei. KW - MHD KW - relativistic processes KW - shock waves KW - turbulence KW - methods: numerical KW - gamma-ray burst: general Y1 - 2014 U6 - https://doi.org/10.1093/mnras/stu196 SN - 0035-8711 SN - 1365-2966 VL - 439 IS - 4 SP - 3490 EP - 3503 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Mizuno, Yosuke A1 - Pohl, Martin A1 - Niemiec, Jacek A1 - Zhang, Bing A1 - Nishikawa, Ken-Ichi A1 - Hardee, Philip E. T1 - Magnetic-field amplification by turbulence in a relativistic shockpropagating through an inhomogeneous medium JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We perform two-dimensional relativistic magnetohydrodynamic simulations of a mildly relativistic shock propagating through an inhomogeneous medium. We show that the postshock region becomes turbulent owing to preshock density inhomogeneity, and the magnetic field is strongly amplified due to the stretching and folding of field lines in the turbulent velocity field. The amplified magnetic field evolves into a filamentary structure in two-dimensional simulations. The magnetic energy spectrum is flatter than the Kolmogorov spectrum and indicates that a so-called small-scale dynamo is occurring in the postshock region. We also find that the amount of magnetic-field amplification depends on the direction of the mean preshock magnetic field, and the timescale of magnetic-field growth depends on the shock strength. KW - gamma-ray burst: general KW - magnetohydrodynamics (MHD) KW - methods: numerical KW - relativistic processes KW - shock waves KW - turbulence Y1 - 2011 U6 - https://doi.org/10.1088/0004-637X/726/2/62 SN - 0004-637X VL - 726 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Nishikawa, Ken-Ichi A1 - Mizuno, Yosuke A1 - Niemiec, Jacek A1 - Kobzar, Oleh A1 - Pohl, Martin A1 - Gomez, Jose L. A1 - Dutan, Ioana A1 - Frederiksen, Jacob Trier A1 - Nordlund, Ake A1 - Meli, Athina A1 - Sol, Helene A1 - Hardee, Philip E. A1 - Hartmann, Dieter H. T1 - Microscopic Processes in Global Relativistic Jets Containing Helical Magnetic Fields JF - Galaxies : open access journal N2 - In the study of relativistic jets one of the key open questions is their interaction with the environment on the microscopic level. Here, we study the initial evolution of both electron-proton (e(-)-p(+)) and electron-positron (e(+/-)) relativistic jets containing helical magnetic fields, focusing on their interaction with an ambient plasma. We have performed simulations of "global" jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI). In our initial simulation study these kinetic instabilities are suppressed and new types of instabilities can grow. In the e(-)-p(+) jet simulation a recollimation-like instability occurs and jet electrons are strongly perturbed. In the e(+/-) jet simulation a recollimation-like instability occurs at early times followed by a kinetic instability and the general structure is similar to a simulation without helical magnetic field. Simulations using much larger systems are required in order to thoroughly follow the evolution of global jets containing helical magnetic fields. KW - relativistic jets KW - particle-in-cell simulations KW - global jets KW - helical magnetic fields KW - kinetic instabilities KW - kink instability Y1 - 2016 U6 - https://doi.org/10.3390/galaxies4040038 SN - 2075-4434 VL - 4 PB - MDPI CY - Basel ER - TY - GEN A1 - Nishikawa, Ken-Ichi A1 - Mizuno, Yosuke A1 - Niemiec, Jacek A1 - Kobzar, Oleh A1 - Pohl, Martin A1 - Gómez, Jose L. A1 - Duţan, Ioana A1 - Pe’er, Asaf A1 - Frederiksen, Jacob Trier A1 - Nordlund, Åke A1 - Meli, Athina A1 - Sol, Helene A1 - Hardee, Philip E. A1 - Hartmann, Dieter H. T1 - Microscopic processes in global relativistic jets containing helical magnetic fields T2 - Galaxies N2 - In the study of relativistic jets one of the key open questions is their interaction with the environment on the microscopic level. Here, we study the initial evolution of both electron-proton (e(-)-p(+)) and electron-positron (e(+/-)) relativistic jets containing helical magnetic fields, focusing on their interaction with an ambient plasma. We have performed simulations of "global" jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI). In our initial simulation study these kinetic instabilities are suppressed and new types of instabilities can grow. In the e(-)-p(+) jet simulation a recollimation-like instability occurs and jet electrons are strongly perturbed. In the e(+/-) jet simulation a recollimation-like instability occurs at early times followed by a kinetic instability and the general structure is similar to a simulation without helical magnetic field. Simulations using much larger systems are required in order to thoroughly follow the evolution of global jets containing helical magnetic fields. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 446 KW - relativistic jets KW - particle-in-cell simulations KW - global jets KW - helical magnetic fields KW - kinetic instabilities KW - kink instability Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-407604 ER - TY - JOUR A1 - Nishikawa, Ken-Ichi A1 - Mizuno, Yosuke A1 - Gomez, Jose L. A1 - Dutan, Ioana A1 - Meli, Athina A1 - White, Charley A1 - Niemiec, Jacek A1 - Kobzar, Oleh A1 - Pohl, Martin A1 - Frederiksen, Jacob Trier A1 - Nordlund, Ake A1 - Sol, Helene A1 - Hardee, Philip E. A1 - Hartmann, Dieter H. T1 - Microscopic Processes in Global Relativistic Jets Containing Helical Magnetic Fields: Dependence on Jet Radius JF - Galaxies : open access journal N2 - In this study, we investigate the interaction of jets with their environment at a microscopic level, which is a key open question in the study of relativistic jets. Using small simulation systems during past research, we initially studied the evolution of both electron-proton and electron-positron relativistic jets containing helical magnetic fields, by focusing on their interactions with an ambient plasma. Here, using larger jet radii, we have performed simulations of global jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities, such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the mushroom instability (MI). We found that the evolution of global jets strongly depends on the size of the jet radius. For example, phase bunching of jet electrons, in particular in the electron-proton jet, is mixed with a larger jet radius as a result of the more complicated structures of magnetic fields with excited kinetic instabilities. In our simulation, these kinetic instabilities led to new types of instabilities in global jets. In the electron-proton jet simulation, a modified recollimation occurred, and jet electrons were strongly perturbed. In the electron-positron jet simulation, mixed kinetic instabilities occurred early, followed by a turbulence-like structure. Simulations using much larger (and longer) systems are required in order to further thoroughly investigate the evolution of global jets containing helical magnetic fields. KW - relativistic jets KW - particle-in-cell simulations KW - global jets KW - helical magnetic fields KW - kinetic instabilities KW - kink-like instability KW - recollimation shocks KW - polarized radiation Y1 - 2017 U6 - https://doi.org/10.3390/galaxies5040058 SN - 2075-4434 VL - 5 PB - MDPI CY - Basel ER -