Extended regimes of energetic (superthermal) electron beams at the origin of solar radio bursts

Often invoked to explain solar radio bursts is the standard model of plasma systems with electron beams and their Langmuir wave excitations, although the parameterization favorable to these excitations is very narrow. Here we use first-principle kinetic theory and numerical simulations to prove a direct as well as an indirect involvement of electrostatic electron-beam waves in the generation of radio emissions. At first sight, these primary excitations with frequency below the plasma frequency do not conform to the nonlinear wave decays in the standard model. However, at their origin are denser or cooler electron beams than in the standard model, which mostly fall within the typical parameterization of plasma sources of type II and type III solar radio bursts. These radio bursts are associated with energetic solar events, such as coronal mass ejections and coronal eruptions, and can be exploited in forecasting these events, provided we understand their origin and propagation. Moreover, broadbands of downshifted excitations are confirmed by in situ observations in association with interplanetary shocks and electron beams, and by contrast with narrowband Langmuir waves.