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Title: Simulations of ion acceleration at non-relativistic shocks. II. Magnetic field amplification

Abstract

We use large hybrid simulations to study ion acceleration and generation of magnetic turbulence due to the streaming of particles that are self-consistently accelerated at non-relativistic shocks. When acceleration is efficient, we find that the upstream magnetic field is significantly amplified. The total amplification factor is larger than 10 for shocks with Alfvénic Mach number M = 100, and scales with the square root of M. The spectral energy density of excited magnetic turbulence is determined by the energy distribution of accelerated particles, and for moderately strong shocks (M ≲ 30) agrees well with the prediction of resonant streaming instability, in the framework of quasilinear theory of diffusive shock acceleration. For M ≳ 30, instead, Bell's non-resonant hybrid (NRH) instability is predicted and found to grow faster than resonant instability. NRH modes are excited far upstream by escaping particles, and initially grow without disrupting the current, their typical wavelengths being much shorter than the current ions' gyroradii. Then, in the nonlinear stage, most unstable modes migrate to larger and larger wavelengths, eventually becoming resonant in wavelength with the driving ions, which start diffuse. Ahead of strong shocks we distinguish two regions, separated by the free-escape boundary: the far upstream, wheremore » field amplification is provided by the current of escaping ions via NRH instability, and the shock precursor, where energetic particles are effectively magnetized, and field amplification is provided by the current in diffusing ions. The presented scalings of magnetic field amplification enable the inclusion of self-consistent microphysics into phenomenological models of ion acceleration at non-relativistic shocks.« less

Authors:
;  [1]
  1. Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544 (United States)
Publication Date:
OSTI Identifier:
22370482
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 794; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; ALFVEN WAVES; AMPLIFICATION; ENERGY DENSITY; ENERGY SPECTRA; INSTABILITY; IONS; MACH NUMBER; MAGNETIC FIELDS; NONLINEAR PROBLEMS; QUASILINEAR PROBLEMS; RELATIVISTIC RANGE; SHOCK WAVES; SIMULATION; SUPERNOVA REMNANTS; TURBULENCE; WAVELENGTHS

Citation Formats

Caprioli, D., and Spitkovsky, A., E-mail: caprioli@astro.princeton.edu. Simulations of ion acceleration at non-relativistic shocks. II. Magnetic field amplification. United States: N. p., 2014. Web. doi:10.1088/0004-637X/794/1/46.
Caprioli, D., & Spitkovsky, A., E-mail: caprioli@astro.princeton.edu. Simulations of ion acceleration at non-relativistic shocks. II. Magnetic field amplification. United States. doi:10.1088/0004-637X/794/1/46.
Caprioli, D., and Spitkovsky, A., E-mail: caprioli@astro.princeton.edu. Fri . "Simulations of ion acceleration at non-relativistic shocks. II. Magnetic field amplification". United States. doi:10.1088/0004-637X/794/1/46.
@article{osti_22370482,
title = {Simulations of ion acceleration at non-relativistic shocks. II. Magnetic field amplification},
author = {Caprioli, D. and Spitkovsky, A., E-mail: caprioli@astro.princeton.edu},
abstractNote = {We use large hybrid simulations to study ion acceleration and generation of magnetic turbulence due to the streaming of particles that are self-consistently accelerated at non-relativistic shocks. When acceleration is efficient, we find that the upstream magnetic field is significantly amplified. The total amplification factor is larger than 10 for shocks with Alfvénic Mach number M = 100, and scales with the square root of M. The spectral energy density of excited magnetic turbulence is determined by the energy distribution of accelerated particles, and for moderately strong shocks (M ≲ 30) agrees well with the prediction of resonant streaming instability, in the framework of quasilinear theory of diffusive shock acceleration. For M ≳ 30, instead, Bell's non-resonant hybrid (NRH) instability is predicted and found to grow faster than resonant instability. NRH modes are excited far upstream by escaping particles, and initially grow without disrupting the current, their typical wavelengths being much shorter than the current ions' gyroradii. Then, in the nonlinear stage, most unstable modes migrate to larger and larger wavelengths, eventually becoming resonant in wavelength with the driving ions, which start diffuse. Ahead of strong shocks we distinguish two regions, separated by the free-escape boundary: the far upstream, where field amplification is provided by the current of escaping ions via NRH instability, and the shock precursor, where energetic particles are effectively magnetized, and field amplification is provided by the current in diffusing ions. The presented scalings of magnetic field amplification enable the inclusion of self-consistent microphysics into phenomenological models of ion acceleration at non-relativistic shocks.},
doi = {10.1088/0004-637X/794/1/46},
journal = {Astrophysical Journal},
number = 1,
volume = 794,
place = {United States},
year = {Fri Oct 10 00:00:00 EDT 2014},
month = {Fri Oct 10 00:00:00 EDT 2014}
}
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