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 - Niemiec, Jacek A1 - Pohl, Martin A1 - Bret, Antoine A1 - Wieland, Volkmar T1 - Nonrelativistic parallel shocks in unmagnetized and weakly magnetized plasmas JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present results of 2D3V particle-in-cell simulations of nonrelativistic plasma collisions with absent or parallel large-scale magnetic field for parameters applicable to the conditions at young supernova remnants. We study the collision of plasma slabs of different density, leading to two different shocks and a contact discontinuity. Electron dynamics play an important role in the development of the system. While nonrelativistic shocks in both unmagnetized and magnetized plasmas can be mediated by Weibel-type instabilities, the efficiency of shock-formation processes is higher when a large-scale magnetic field is present. The electron distributions downstream of the forward and reverse shocks are generally isotropic, whereas that is not always the case for the ions. We do not see any significant evidence of pre-acceleration, neither in the electron population nor in the ion distribution. KW - acceleration of particles KW - instabilities KW - ISM: supernova remnants KW - methods: numerical KW - plasmas KW - shock waves Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/759/1/73 SN - 0004-637X SN - 1538-4357 VL - 759 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 - JOUR A1 - Vafin, Sergei A1 - Rafighi, Iman A1 - Pohl, Martin A1 - Niemiec, Jacek T1 - The Electrostatic Instability for Realistic Pair Distributions in Blazar/EBL Cascades JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - This work revisits the electrostatic instability for blazar-induced pair beams propagating through the intergalactic medium (IGM) using linear analysis and PIC simulations. We study the impact of the realistic distribution function of pairs resulting from the interaction of high-energy gamma-rays with the extragalactic background light. We present analytical and numerical calculations of the linear growth rate of the instability for the arbitrary orientation of wave vectors. Our results explicitly demonstrate that the finite angular spread of the beam dramatically affects the growth rate of the waves, leading to the fastest growth for wave vectors quasi-parallel to the beam direction and a growth rate at oblique directions that is only a factor of 2-4 smaller compared to the maximum. To study the nonlinear beam relaxation, we performed PIC simulations that take into account a realistic wide-energy distribution of beam particles. The parameters of the simulated beam-plasma system provide an adequate physical picture that can be extrapolated to realistic blazar-induced pairs. In our simulations, the beam looses only 1% of its energy, and we analytically estimate that the beam would lose its total energy over about 100 simulation times. An analytical scaling is then used to extrapolate the parameters of realistic blazar-induced pair beams. We find that they can dissipate their energy slightly faster by the electrostatic instability than through inverse-Compton scattering. The uncertainties arising from, e.g., details of the primary gamma-ray spectrum are too large to make firm statements for individual blazars, and an analysis based on their specific properties is required. KW - gamma rays: general KW - instabilities KW - magnetic fields KW - relativistic processes KW - waves Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aab552 SN - 0004-637X SN - 1538-4357 VL - 857 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 -