Complex eigenvalue plane for a homogeneous bounded medium with physical parameters taken as in Table 1. The blue, red, and green crosses denote the fast, slow, and thermal mode solutions, respectively, for k_⊥ = nπ/L. The number of nodes n takes on 25 equally spaced values in the interval [0, 250]. Left inset: zoom-in near the thermal mode sequence. The growth rates become more unstable when n increases. Right inset: zoom-in near the slow mode sequence, for positive real values of ω, becoming more damped for increasing n. The solid dots denote the accumulation points.