Nonlinear radiation by an electron beam in the whistler range: A tentative theoretical model

During injection into the ionospheric plasma of an electron beam, very low-frequency electrostatic noise has been detected far away from that beam. Owing to the characteristics of this emission a direct generation mechanism based, for instance, on a linear instability seems irrelevant. A different interpretation is proposed where the observed very low-frequency noise is related to a nonlinear antenna mechanism. Large-amplitude plasma waves which are strongly confined inside the beam, at frequencies close to the plasma frequency, are nonlinearly coupled and the corresponding beating produces a lower-frequency wave which can freely escape from the beam. A detailed comparison with observations is made to support this model.