Distribution of eigenvalues along the real axis. Top panel: Results for constant viscosity (top line) or constant Prandtl number (bottom line). Eigenvalues are computed with parameters n = 3, Q = 1, Ω = 0.15, η = 0.18, L_max = 100, and Nr = 50 using the QZ-algorithm. Parameters E and E_t are given in the legend. Bottom panel: Same as above, but with constant heat diffusion and varying viscosity. Eigenvalues have been rescaled by the ratio of viscosities.