  We show that the relativistic two-stream instability can naturally generate
strong magnetic fields with 1e-5 - 1e-1 of the equipartition energy density, in
the collisionless shocks of Gamma-Ray-Burst (GRB) sources. The generated fields
are parallel to the shock front and fluctuate on the very short scale of the
plasma skin depth. The synchrotron radiation emitted from the limb-brightened
source image is linearly polarized in the radial direction relative to the
source center. Although the net polarization vanishes under circular symmetry,
GRB sources should exhibit polarization scintillations as their radio afterglow
radiation gets scattered by the Galactic interstellar medium. Detection of
polarization scintillations could therefore test the above mechanism for
magnetic field generation.
