Dispersion relation (a, d) and growth rates (b, e) of the extraordinary wave modes according to the kinetic linear theory, with n_e/n₀ = 24% (upper panels) and 100% (lower panels). The blue and red lines refer to the X and Z modes; the purple and yellow lines refer to the two branches of the relativistic modes (R1 and R2). Panels (c) and (f) present the corresponding intensity ratio of E_x/iE_y for each mode.