Particle-in-cell Simulation of Current-driven Solar Type III Radiation and Whistler Waves in a Core–strahl Plasma: Relevance to PSP and Other Space Observations

The aim of this paper is to demonstrate that electron current oscillations may generate electromagnetic waves as type III radiation and whistler waves without the involvement of the classical plasma emission via the coalescence of waves. Particle-in-cell (PIC) simulation results of a core–strahl plasma without initial current compensation are presented, which describe the conversion of current-driven Langmuir oscillations/waves into type III radiation, whereby simultaneously whistler waves are excited. In contrast to the classical approach of V. L. Ginzburg & V. V. Zhelezniakov, after which beam-excited Langmuir waves in a two-step process are converted into electromagnetic radiation, any instability is suppressed by selecting a low strahl velocity. Rather, electric field oscillations at the electron plasma frequency are triggered by the initially noncompensated current of the strahl. The arising electromagnetic fields exhibit amplitude oscillations, which are caused by the superposition of the two wave modes of mixed polarization at the point of mode coupling. This basic mechanism of wave generation and transformation has already been described in earlier papers using simple fluid models. It is also the topic of the companion paper. Besides the fundamental electromagnetic radiation, the second harmonic of nearly the same intensity has been obtained, which is an indication of its generation by nonlinear currents. Measurements of Langmuir waves, type III radiation, and whistler waves on board various satellites in the solar wind, in particular Parker Solar Probe observations, are analyzed in the light of our results. Interpretations of earlier PIC simulations are critically reviewed.