In the Poynting Flux-dominated outflow ( the initial ratio of the electromagnetic energy flux to the particle energy flux \sigma _ { 0 } \gg 1 ) model for Gamma-ray bursts , particularly the \gamma - ray emission phase , nearly half of the internally dissipated magnetic energy is converted into the \gamma - ray energy emission and the rest is converted into the kinetic energy of the outflow .
Consequently , at the end of the \gamma - ray burst , \sigma decreases significantly ( \sigma \sim 1 or even smaller ) .
We numerically investigate the very early reverse shock emission powered by such mildly magnetized outflows interacting with medium—uniform interstellar medium ( ISM ) or stellar wind ( WIND ) .
We show that for \sigma \sim 0.05 - 1 and typical parameters of Gamma-ray bursts , both the ISM-ejecta interaction and the WIND-ejecta interaction can power very strong optical emission ( m _ { R } \sim 10 - 12 { th } magnitude or even brighter ) .
Similar to the very early afterglow powered by the non-magnetized ejecta interacting with the external medium , the main difference between the ISM-ejecta interaction case and the WIND-ejecta interaction case is that , before the reverse shock crosses the ejecta , the R-band emission flux increases rapidly for the former , but for the latter it increases only slightly .

At the very early stage , the ejecta are ultra-relativistic .
Due to the beaming effect , the random magnetic field generated in shocks contained in the viewing area is axisymmetric , unless the line of sight is very near the edge of ejecta .
The formula \Pi _ { net } \approx 0.60 b ^ { 2 } / ( 1 + b ^ { 2 } ) ( where b is the ratio of the ordered magnetic field strength to that of random one ) has been proposed to describe the net linear polarization of the synchrotron radiation coming from the viewing area .
For \sigma \sim 0.05 - 1 , the ordered magnetic field dominates over the random one generated in the reverse shock ( As usual , we assume that a fraction \epsilon _ { B } \sim 0.01 of the thermal energy of the reverse shock has been converted into the magnetic energy ) , the high linear polarization is expected .
We suggest that the linear polarization detection of the early multi-wavelength afterglow is required to see whether the outflows powering GRBs are magnetized or not .