Solar type III radiation as antenna problem - Electromagnetic wave generation by the beam-driven electron current

An one-dimensional fluid model is presented which describes the generation of  type III radiation as an antenna problem at which the triggering current pulse imitates the temporal evolution of the beam instability. The mechanism works without the involvement of the classical plasma emission via the parametric processes and the coalescence of waves. After linearization of the Maxwell-fluid equations and Fourier transform in space, the system of nine differential equations describing the temporal evolution of the fluid and electromagnetic quantities is solved numerically. It is shown that the commonly observed beating structure of the electromagnetic radiation in form of a double-peak in their spectra, commonly explained by parametric decay of the beam-excited Langmuir wave, is caused by the superposition of two wave modes of mixed polarisation (Langmuir/z wave) which belong the wave number of optimum mode coupling. Within in the same formalism the generation of the second harmonic of the electromagnetic radiation is calculated by taking into account the nonlinear currents as product of the first-order terms. Satellite observations of beam-excited Langmuir waves and solar type III radiation are discussed in the light of the presented antenna model.