Polarization Ratios of Turbulent Langmuir/ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Z</mml:mi> </mml:math> -mode Waves Generated by Electron Beams in Magnetized Solar Wind Plasmas

Abstract The polarization ratios F = ∣ E ⊥ ∣ 2 /∣ E ∣ 2 of beam-generated turbulent Langmuir/ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> -mode ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">L</mml:mi> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> ) waves and electromagnetic emissions radiated at plasma frequency ω p from such sources are studied in weakly magnetized and randomly inhomogeneous plasmas. Large-scale and long-term 2D/3V particle-in-cell simulations with parameters relevant to type III solar radio bursts are performed. Statistical studies using waveforms recorded by virtual satellites are carried out 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 δn turn out to be the key factor responsible for the increase in polarization ratios up to F ∼ 1. Indeed, it is demonstrated that linear mode conversion at constant frequency near ω p of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">L</mml:mi> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> waves scattering on δn is the most efficient and fastest process to produce large polarization ratios in randomly inhomogeneous plasmas. This is due to electromagnetic slow extraordinary <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> -mode wave emission by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">L</mml:mi> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">Z</mml:mi> </mml:math> wave turbulence. The results provide guidance to theoretical studies and useful support to estimate the average level of density fluctuations in solar wind plasmas.