Polarization ratios of turbulent Langmuir/$\mathcal{Z}$-mode waves generated by electron beams in magnetized solar wind plasmas

Abstract

The polarization ratios $F=|E_\perp|^2/|E|^2$ of beam-generated turbulent Langmuir/$\mathcal{Z}$-mode ($\mathcal{LZ}$) waves and electromagnetic emissions radiated at plasma frequency $\omega_p$ by such sources are studied in weakly magnetized and randomly inhomogeneous plasmas owing to large-scale and long-term 2D/3V Particle-In-Cell simulations with parameters relevant to type III solar radio bursts. Statistical studies using waveforms recorded by virtual satellites are performed to determine the distributions of polarization ratios as a function of beam and plasma parameters. This efficient method, which mimics waveform recording by spacecraft in the solar wind, leads to results consistent with observations. Moreover, plasma random density fluctuations $\delta n$ turn out to be the key factor responsible for the increase in polarization ratios up to $F\simeq 1$. Indeed, it is demonstrated that linear mode conversion at constant frequency of $\mathcal{LZ}$ waves scattering on $\delta n$ is the most efficient and fast process to produce large polarization ratios in randomly inhomogeneous plasmas, due to electromagnetic slow extraordinary $\mathcal Z$-mode wave emission by $\mathcal{LZ}$ wave turbulence. Results provide guidance to theoretical studies and useful support to estimate the average level of density fluctuations $\Delta N$ in solar wind plasmas.