The radiation emission at the harmonics of the cyclotron frequency ΩF of the fusion reaction products has been observed consistently, in a variety of experiments, to correspond to the values of the magnetic field near the outer edge of a toroidal plasma column. This is explained in terms of the excitation of toroidal plasma modes that are well confined (localized) within a narrow shell close to the periphery of the plasma column. The intersection of this shell with the equatorial plane corresponds to a distance from the symmetry axis that agrees closely with that evaluated from the cyclotron frequency in the experiments carried out by the Joint European Torus. The structure and the localization of these modes (that are of the fast Alfvén type) do not depend on the fusion product population but on the parameters of the thermalizedplasma in which they are produced. Well above the first harmonic ΩF(ω ≃ 8ΩF), the observed spectrum becomes continuous, and this is explained (the numerical factor is consistent) as corresponding to the transition toward whistler-type modes, which occurs when the effects of magnetic shear become important. In this case, the radial distance over which these modes can be excited is broadened, and since the cyclotron frequency is a function of position, successive harmonics of the local cyclotron frequency can overlap. If the plasma current is increased, the fraction of fusion product whose orbits can reach the excitation shell, for the lower harmonics, at the periphery of the plasma column is also decreased. Then the intensity of the discrete part of the spectrum can be expected to decrease.