Shock vorticity generation from accelerated ion streaming in the precursor of ultrarelativistic gamma-ray burst external shocks

We investigate the interaction of nonthermal ions (protons and nuclei) accelerated in an ultrarelativistic blast wave with the preexisting magnetic field of the medium into which the blast wave propagates. While particle acceleration processes such as diffusive shock acceleration can accelerate ions and electrons, the accelerated electrons suffer larger radiative losses. Under certain conditions, the ions can attain higher energies and reach farther ahead of the shock than the electrons, and so the nonthermal particles can be partially charge separated. To compensate for the charge separation, the upstream plasma develops a return current, which, as it flows across the magnetic field, drives transverse acceleration of the upstream plasma and a growth of density contrast in the shock upstream. If the density contrast is strong by the time the fluid is shocked, vorticity is generated at the shock transition. The resulting turbulence can amplify the postshock magnetic field to the levels inferred from gamma-ray burst afterglow spectra and light curves. Therefore, since the upstream inhomogeneities are induced by the ions accelerated in the shock, they are generic even if the blast wave propagates into a medium of uniform density. We speculate about the global structure of the shock precursor and delineate several distinct physical regimes that are classified by an increasing distance from the shock and, correspondingly, a decreasing density of nonthermal particles that reach that distance.