We report that the optical polarization in the afterglow of GRB 091208B is measured at t = 149 - 706 s after the burst trigger , and the polarization degree is P = 10.4 \% \pm 2.5 \% .
The optical light curve at this time shows a power-law decay with index -0.75 \pm 0.02 , which is interpreted as the forward shock synchrotron emission , and thus this is the first detection of the early-time optical polarization in the forward shock ( rather than that in the reverse shock reported by ( 30 ) ) .
This detection disfavors the afterglow model in which the magnetic fields in the emission region are random on the plasma skin depth scales , such as amplified by the plasma instabilities , e.g. , Weibel instability .
We suggest that the fields are amplified by the magnetohydrodynamic instabilities , which would be tested by future observations of the temporal changes of the polarization degrees and angles for other bursts .