Two roots of the cold-plasma dispersion relation An⁴ − Bn² + C = 0, plotted as refractive index n² = c²k²/ω² against frequency. Red and blue are the fast and slow branches; green lines are cutoffs, amber are resonances, and grey shading marks n² < 0 — evanescent, where the wave cannot propagate at all. Set θ = 0° for parallel propagation (R and L waves: whistlers, Faraday rotation), θ = 90° for perpendicular (the O and X modes a tokamak interferometer actually uses).