Turbulence-induced magnetic fields and the structure of Cosmic Ray
  modified shocks
  We propose a model for Diffusive Shock Acceleration (DSA) in which stochastic
magnetic fields in the shock precursor are generated through purely fluid
mechanisms of a so-called small-scale dynamo. This contrasts with previous DSA
models that considered magnetic fields amplified through cosmic ray streaming
instabilities; i.e., either by way of individual particles resonant scattering
in the magnetic fields, or by macroscopic electric currents associated with
large-scale cosmic ray streaming. Instead, in our picture, the solenoidal
velocity perturbations that are required for the dynamo to work are produced
through the interactions of the pressure gradient of the cosmic ray precursor
and density perturbations in the inflowing fluid. Our estimates show that this
mechanism provides fast growth of magnetic field and is very generic. We argue
that for supernovae shocks the mechanism is capable of generating upstream
magnetic fields that are sufficiently strong for accelerating cosmic rays up to
around 10^16 eV. No action of any other mechanism is necessary.
