Effects of Pre-Existing Upstream Turbulence on Magnetic Fields and Particle Acceleration at Astrophysical Shocks

We consider effects of pre-existing, large-scale turbulence upstream of a shock on the magnetic field and the acceleration of charged particles. Turbulent magnetic-field-line mixing plays a large role in particle transport. Also, turbulent {\it density} fluctuations upstream of the shock have a large effect on the magnetic field downstream (Giacalone and Jokipii, Ap. J., 633, L41, 2007). For high Alfv\`en-Mach-number shocks, the downstream magnetic field is amplified considerably above the value obtained from the shock jump conditions. These effects may provide a robust and natural understanding of recent observations at astrophysical shocks. The magnetic-field amplification implied by our simulations should exceed factors of $100$, consistent with observed X-rays from supernova remnants, which require magnetic fields of $100 \mu$G. These are much larger than expected from the shock jump conditions. In this case, the upstream field is not amplified, so cosmic-rays with energies approaching the ``knee'' in the spectrum require rapid acceleration, which can occur at the quasi-perpendicular part of the supernova blast wave, where the turbulent field-line mixing plays a large role. Further, recent observations by the Voyager 1 spacecraft downstream of the heliospheric termination shock show that the magnetic field has large magnitude fluctuations. We suggest that these and other effects of pre-existing turbulence play an important role in many astrophysical and heliospheric shocks.