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

TL;DR

This study uses particle-in-cell simulations to analyze polarization ratios of Langmuir and Z-mode waves in the solar wind, highlighting the role of density fluctuations in polarization enhancement and providing insights relevant to solar radio burst observations.

Contribution

It introduces a simulation-based method to study polarization ratios in turbulent plasma waves, emphasizing the impact of density fluctuations on wave polarization in the solar wind.

Findings

Density fluctuations increase polarization ratios up to F≈1.

Linear mode conversion efficiently produces large polarization ratios.

Results align with spacecraft observations of solar radio bursts.

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.