Eigenvalues of the polarization block of the and matrices. As with temperature, the spectra of the two matrices are very similar. In this case we also have tens of poorly constrained modes. We also expect to have two nearly degenerate modes, one for each Stokes parameter (cf. Fig. 7). Though they are treated as singular because of numerical reasons, their degeneracy is partially broken, as explained in Sect. 4.2.