Heat maps of real frequency, negative damping rate, and real polarization in logarithmic scale as functions of the plasma beta for fixed wavenumbers of k = 0.5 (left) and k = 1.0 (right). As in Fig. 3, the different rows correspond to the different oxygen ion concentrations, orange curves are contours of characteristic values of damping rate, and white curves are contours of Re(P) = 0. The grey-shaded region indicates a domain of linear polarization.