Fluid approach of current-driven Langmuir waves associated with type III radiation and whistler waves: Relevance to PSP and other solar wind observations
A theoretical model on the basis of fluid-Maxwell equations for an electron-ion plasma is presented which describes the conversion of current-driven Langmuir waves into type III radiation whereby simultaneously an excitation of whistler waves may occur. In contrast to the classical approach of Ginzburg and Zhelezniakov (1958) after which beam-excited Langmuir waves in a two-step process are converted in electromagnetic radiation, the presented mechanism works without any instability and wave coalescence. Rather the electric field oscillations at the electron plasma frequency can be triggered by different realisations of the driving current, e.g. by the (uncompensated) net current of the strahl at t=0 in a core-strahl plasma or by given current variations which may represent different situations in space, as shocks, magnetic switch-backs etc.. A linearized system of equations is used to describe the mode coupling occurring at oblique propagation between the mostly electrostatic Langmuir wave and the adjacent electromagnetic left-hand polarized (L) wave. The simplicity of the fluid model allows without great effort the parameters of the current profiles to be varied and thus to simulate a wide range of possible experimental conditions. Measurements of Langmuir waves, type III radiation and whistler waves on board various satellites in the solar wind, and in particular some of the recent results of the Parker Solar Probe are interpreted in the light of the theoretical model presented. For the case of the uncompensated strahl, the fluid approach is confirmed by fully kinetic PIC simulations. One comparison is shown in the Appendix.