Mechanism for generation of radio emissions from the planetary and heliospheric foreshocks

Abstract

The question of how low-frequency radio emissions from the foreshocks in the heliosphere might be generated is considered. It is argued that the free energy contained in an electron beam distribution is first transformed into electrostatic Langmuir waves. The non-linear interactions of these waves which can be responsible for the electromagnetic emission are then treated in the semi-classical formalism. Comparison of our results with electromagnetic radiation coming from upstream of the Earth's bow shock shows that the model adequately explains the generation of plasma waves at planetary shocks. By analogy, this model can provide a quantitative explanation of intensity of radio emissions at 2 to 3 kHz detected by the Voyager plasma wave instrument in the outer heliosphere provided that the electron beams generating Langmuir waves exist also in the postshock plasma due to secondary shocks in the compressed solar wind beyond the termination shock.