Dust-driven streaming instability and magnetic field amplification downstream of supernova remnant shocks

Context. The acceleration of cosmic rays up to PeV energies at supernova remnant shocks requires an amplification of the ambient magnetic field. The amplification mechanism must operate upstream of the shock, to prevent the escape of particles from the system. Observational evidence of field amplification has indeed been obtained by means of X-ray observations. However, such observations only constrain the magnetic field strength downstream of the shock. Aims. Here we describe a mechanism of magnetic field amplification that operates downstream of the shock. It is based on a plasma instability triggered by the drift of charged interstellar dust grains overtaken by the shock. Methods. We computed the growth rate of the instability, we estimated the level of magnetic field amplification expected downstream of supernova remnant shocks, and we compared our results with observations. Results. In some cases (most notably Cas A) this mechanism might explain the presence of the X-ray filaments observed at supernova remnant shocks, without requiring any amplification of the magnetic field upstream of the shock and therefore any acceleration of cosmic rays to ultrahigh energies.