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Abstract: We propose a model for Diffusive Shock Acceleration DSA in which stochasticmagnetic fields in the shock precursor are generated through purely fluidmechanisms of a so-called small-scale dynamo. This contrasts with previous DSAmodels that considered magnetic fields amplified through cosmic ray streaminginstabilities; i.e., either by way of individual particles resonant scatteringin the magnetic fields, or by macroscopic electric currents associated withlarge-scale cosmic ray streaming. Instead, in our picture, the solenoidalvelocity perturbations that are required for the dynamo to work are producedthrough the interactions of the pressure gradient of the cosmic ray precursorand density perturbations in the inflowing fluid. Our estimates show that thismechanism provides fast growth of magnetic field and is very generic. We arguethat for supernovae shocks the mechanism is capable of generating upstreammagnetic fields that are sufficiently strong for accelerating cosmic rays up toaround 10^16 eV. No action of any other mechanism is necessary.



Author: A. Beresnyak, T. W. Jones, A. Lazarian

Source: https://arxiv.org/







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