Top observation: the same recording as in Fig. 2 after application of the cross-talk correction and of the linear polarization basis rotation (see text). The measurement was done around μ = 0.94. The second observation (middle) reports measurements of October 15 (around μ = 0.95) in the region shown in Fig. 5. The third observation was done on November 5 (bottom) very close to disk center (μ = 1) in a very quiet region. Here we note the absence of linear polarization signature. For all recordings, the grey scale cuts are the same as in Fig. 2.