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Universal behaviour of shock precursors in the presence of efficient cosmic-ray acceleration

The self-consistent interaction between energetic particles and self-generated hydromagnetic waves in a cosmic-ray pressure dominated plasma is considered. Using a three-dimensional hybrid MHD-kinetic code, which utilises a spherical harmonic expansion of the Vlasov-Fokker-Planck equation, high resolution simulations of the magnetic field growth including feedback on the cosmic rays are carried out. It is found that for shocks with high cosmic-ray acceleration efficiency, the magnetic fields become highly disorganised, resulting in near isotropic diffusion, independent of the initial orientation of the ambient magnetic field. The possibility of sub-Bohm diffusion is demonstrated for parallel shocks, while the diffusion coefficient approaches the Bohm limit from below for oblique shocks. This universal behaviour suggests that Bohm diffusion in the root mean squared field inferred from observation may provide a realistic estimate for the maximum energy acceleration timescale in young supernova remnants. Although disordered, the magnetic field is not self-similar suggesting non-uniform energy dependent behaviour of the energetic particle transport in the precursor. Possible indirect radiative signatures of cosmic-ray driven magnetic field amplification are discussed.