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Collisionless shocks are commonly argued to be the sites of cosmic ray (CR)acceleration. We study the influence of CRs on weakly magnetized relativisticcollisionless shocks and apply our results to external shocks in gamma-rayburst (GRB) afterglows. The common view is that the transverse Weibelinstability (TWI) generates a small-scale magnetic field that facilitatescollisional coupling and thermalization in the shock transition. The TWI fieldis expected to decay rapidly, over a finite number of proton plasma skin depthsfrom the transition. However, the synchrotron emission in GRB afterglowssuggests that a strong and persistent magnetic field is present in the plasmathat crosses the shock; the origin of this field is a key open question. Herewe suggest that the common picture involving TWI demands revision. Namely, theCRs drive turbulence in the shock upstream on scales much larger than the skindepth. This turbulence generates a large-scale magnetic field that quenches TWIand produces a magnetized shock. The new field efficiently confines CRs andenhances the acceleration efficiency. The CRs modify the shocks in GRBafterglows at least while they remain relativistic. The origin of the magneticfield that gives rise to the synchrotron emission is plausibly in the CR-driventurbulence. We do not expect ultrahigh energy cosmic ray production in externalGRB shocks.Comment: 6 pages, 1 figur

The Cosmic‐Ray Precursor of Relativistic Collisionless Shocks: A Missing Link in Gamma‐Ray Burst Afterglows