Refractive Index, N, as a function of the frequency f (with ω = 2πf ) for a low-density plasma (i.e., f_ce > f_pe, required for the ECM to occur). The red areas corresponds to the + branch of the Appleton-Hartree expression, while the blue area correspond to the - branch. The oblique propagation dispersion curves for different θ are shown as thin coloured lines (with δθ = 5°), while dispersion curves for parallel propagation (θ = 0°) are shown as thick black lines, and those for perpendicular propagation (θ = 90°) as thick black dashed lines. The frequencies of the L-mode cutoff f_L, plasma cutoff f_pe , cyclotron resonance f_ce , upper hybrid resonance f_UH and R-mode cutoff f_R , are indicated on the x-axis. In this example, f_ce/ f_pe = 0.3. In reality, it is lower than that, but it has been enlarged here for the sake of readability.