Bottom left (panel a): total flux I/S2 (blue, relative to S2) and Q/I flux ratio as a function of time for the May 27 flare. Top left (panel b): as in (a) but for the July 22 flare. Top right (panel c): I/S2 (blue), Q/(Q2 + U2)1/2 (red) and U/(Q2 + U2)1/2 (green) evolution during the July 28 flare. Bottom right (panel d): evolution of the July 28 flare in the plane of normalized Stokes parameters Q/(Q2 + U2)1/2 (horizontal) and U/(Q2 + U2)1/2 (vertical). The red arrows denote the polarization directions on sky. The blue cross in the lower right denotes a typical error bar.