Panel a: sketch of the dispersion relation of the electromagnetic waves in a plasma, following the Appleton-Hartree equation. Only R−X and L−O modes can escape the plasma and become radio waves. Panel b: beaming angle of the oblique emissions as a function of the ratio of the emission angular frequency to the maximal electron cyclotron angular frequency, for different values of the electrons energy and N = 1. Panel c: beaming angle of the perpendicular emissions as a function of ω_p/ω_c outside of a cavity. Symbols correspond to terrestrial auroral kilometric radiation (AKR) beaming angle measurements by Louarn & Le Quéau (1996b). Lines are theoretical beaming angles computed using Eqs. (20) and (25), constrained by density measurements of Louarn & Le Quéau (1996b) for orbit 176 (solid line), orbit 1260 (dashed), and orbit 165 (dot-dashed).