Steepening of Cosmic Ray Spectra in Shocks with Varying Magnetic Field Direction

Abstract

Cosmic ray (CR) spectra, both measured upon their arrival at the Earth's atmosphere and inferred from the emission in supernova remnants (SNRs), appear to be significantly steeper than the "standard" diffusive shock acceleration (DSA) theory predicts. Although the reconstruction of the primary spectra introduces an additional steepening due to propagation effects, there is a growing consensus in the CR community that these corrections fall short to explain the newest high-precision data. Using 2D hybrid simulations, we investigate a new mechanism that may steepen the spectrum during the acceleration in SNR shocks. Most of the DSA treatments are limited to homogeneous shock environments. To investigate whether inhomogeneity effects can produce the necessary extra steepening, we assume that the magnetic field changes its angle along the shock front. The rationale behind this approach is the strong dependence of the DSA efficiency upon the field angle, ${\theta }_{\mathrm{Bn}}$. Our results show that the variation of shock obliquity along its face results in a noticeable steepening of the DSA spectrum. Compared to simulations of quasi-parallel shocks, we observe an increase of the spectral index by $\Delta q=0.1-0.15$. Possible extrapolation of the limited simulation results to more realistic SNR conditions are briefly considered.

arXiv comment8 pages, 7 figures

References in corpus (15)

• Cosmic-ray propagation and interactions in the Galaxy
• PAMELA Measurements of Cosmic-ray Proton and Helium Spectra
• Simulations of Ion Acceleration at Non-relativistic Shocks. I. Acceleration Efficiency
• The origin of Galactic cosmic rays: challenges to the standard paradigm
• Direct Measurement of the Cosmic-Ray Proton Spectrum from 50 GeV to 10 TeV with the Calorimetric Electron Telescope on the International Space Station
• Acceleration Rates and Injection Efficiencies in Oblique Shocks
• Diffusive shock acceleration and magnetic field amplification
• Particle acceleration by collisionless shocks containing large-scale magnetic-field variations
• Cosmic ray acceleration by shocks: spectral steepening due to turbulent magnetic field amplification
• The effect of cosmic-ray acceleration on supernova blast wave dynamics
• Acceleration of Cosmic Rays in Supernova Shocks: elemental selectivity of the injection mechanism
• Particle-in-cell simulations of particle energization from low Mach number fast mode shocks
• Cosmic Ray Spectrum Steepening in Supernova Remnants -- I. Loss-Free Self-Similar Solution
• Expansion of a radially symmetric blast shell into a uniformly magnetized plasma
• Newly-Discovered Anomalies in Galactic Cosmic Rays: Time for Exotic Scenarios?

Cited by in corpus (9)

• Kinetic Simulations of Cosmic-Ray-Modified Shocks II: Particle Spectra
• Kinetic Simulations of Cosmic-Ray-Modified Shocks I: Hydrodynamics
• Steep Cosmic Ray Spectra with Revised Diffusive Shock Acceleration
• Dynamics of millicharged dark matter in supernova remnants
• Evolution and observational signatures of the cosmic ray electron spectrum in SN 1006
• Shock acceleration with oblique and turbulent magnetic fields
• Diffusive shock acceleration at oblique high Mach number shocks
• Electron Energization in Quasi-Parallel Shocks: Test-Particle-Electrons in a Proton Driven Turbulence
• Flat Spectra of Energetic Particles in Interplanetary Shock Precursors