The electrostatic electron beam-plasma instabilities and nonlinear radio emissions. Theory vs. PIC simulations

Electrostatic wave instabilities induced by energetic electron beams are believed to be at the origin of radio emissions reported by the observations of interplanetary shocks and solar coronal sources. We revisit the electron beam-plasma configurations found susceptible to nonlinear radio (electromagnetic) emissions, but which led to contradictory outcomes both in the linear theory and especially in the numerical simulations. The results of a new dispersion and stability analysis are presented, in which the electron populations are modeled both with standard Maxwellian velocity distributions and with Kappa distributions revealed by in situ measurements. We thus describe not only the exact nature of these electrostatic fluctuations (e.g., electron beam modes, modified Langmuir waves, or electron acoustic waves), but also a series of characteristics that help to distinguish them in observations. The particle-in-cell simulations confirm the predictions of the linear theory, and show for the first time how the nonlinear radio emissions are modified due to the Kappa distributions of the electron populations.