@article{BohdanNiemiecKobzaretal.2017,
  author    = {Bohdan, Artem and Niemiec, Jacek and Kobzar, Oleh and Pohl, Martin},
  title     = {Electron Pre-acceleration at Nonrelativistic High-Mach-number Perpendicular Shocks},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {847},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.3847/1538-4357/aa872a},
  pages     = {17},
  year      = {2017},
  abstract  = {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.},
  language  = {en}
}
@article{KobzarNiemiecPohletal.2017,
  author    = {Kobzar, Oleh and Niemiec, Jacek and Pohl, Martin and Bohdan, Artem},
  title     = {Spatio-temporal evolution of the non-resonant instability in shock precursors of young supernova remnants},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {469},
  journal   = {Monthly notices of the Royal Astronomical Society},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  organization    = {ANTARES Collaboration;H E S S Collaboration},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stx1201},
  pages     = {4985 -- 4998},
  year      = {2017},
  abstract  = {A non-resonant cosmic ray (CR) current-driven instability may operate in the shock precursors of young supernova remnants and be responsible for magnetic-field amplification, plasma heating and turbulence. Earlier simulations demonstrated magnetic-field amplification, and in kinetic studies a reduction of the relative drift between CRs and thermal plasma was observed as backreaction. However, all published simulations used periodic boundary conditions, which do not account for mass conservation in decelerating flows and only allow the temporal development to be studied. Here we report results of fully kinetic particle-in-cell simulations with open boundaries that permit inflow of plasma on one side of the simulation box and outflow at the other end, hence allowing an investigation of both the temporal and the spatial development of the instability. Magnetic-field amplification proceeds as in studies with periodic boundaries and, observed here for the first time, the reduction of relative drifts causes the formation of a shock-like compression structure at which a fraction of the plasma ions are reflected. Turbulent electric field generated by the non-resonant instability inelastically scatters CRs, modifying and anisotropizing their energy distribution. Spatial CR scattering is compatible with Bohm diffusion. Electromagnetic turbulence leads to significant non-adiabatic heating of the background plasma maintaining bulk equipartition between ions and electrons. The highest temperatures are reached at sites of large-amplitude electrostatic fields. Ion spectra show supra-thermal tails resulting from stochastic scattering in the turbulent electric field. Together, these modifications in the plasma flow will affect the properties of the shock and particle acceleration there.},
  language  = {en}
}