Fundamental Electromagnetic Emissions by a Weak Electron Beam in Solar Wind Plasmas with Density Fluctuations

C Krafft,P Savoini

doi:10.3847/2041-8213/ac46a7

Abstract

Abstract The generation of Langmuir wave turbulence by a weak electron beam in a randomly inhomogeneous plasma and its subsequent electromagnetic radiation are studied owing to two-dimensional particle-in-cell simulations in conditions relevant to type III solar radio bursts. The essential impact of random density fluctuations of average levels of a few percents of the background plasma on the characteristics of the electromagnetic radiation at the fundamental plasma frequency ω p is shown. Not only wave nonlinear interactions but also processes of Langmuir waves’ transformations on the density fluctuations contribute to the generation of such emissions. During the beam relaxation, the amount of electromagnetic energy radiated at ω p in a plasma with density fluctuations strongly exceeds that observed when the plasma is homogeneous. The fraction of Langmuir wave energy involved in the generation of electromagnetic emissions at ω p saturates around 10−4, i.e., one order of magnitude above that reached when the plasma is uniform. Moreover, whereas harmonic emission at 2ω p dominates over fundamental emission during the time evolution in a homogeneous plasma, fundamental emission is strongly dominant when the plasma contains density fluctuations, at least during several thousands of plasma periods before being overcome by harmonic emission when the total electromagnetic energy begins to saturate.

Highlights

Type III solar radio bursts have been observed for decades in interplanetary space (Reid & Ratcliffe 2014)
This study shows the essential impact of plasma density fluctuations on the intensity, the rate of growth, and the spectral distributions of the fundamental electromagnetic emissions
The results presented below are provided by three simulations using two different spatial boxes and two values of average levels DN = á(dn n0)2ñ1 2 of density fluctuations (n0 is the density of the background ions), i.e., ΔN = 0 for a homogeneous plasma and ΔN = 0.05 for a plasma involving random fluctuations δn Figure 1 shows the initial distribution of the background plasma density fluctuations δn/n0 in the plane (x,y) used for the “large” simulation box mentioned above, for ΔN ; 0.05

Summary

Introduction

Type III solar radio bursts have been observed for decades in interplanetary space (Reid & Ratcliffe 2014). The Letter presents 2D PIC simulations showing for the first time the radiation in randomly inhomogeneous plasmas of electromagnetic waves emitted at ωp by Langmuir wave turbulence generated by a weak electron beam These simulations use a panel of physical and numerical parameters that were not reached in previous works and take into account the presence of applied and self-consistently varying random plasma density fluctuations (Celnikier et al 1983; Krupar et al 2018) in an exceptionally large and well-resolved simulation box. This study shows the essential impact of plasma density fluctuations on the intensity, the rate of growth, and the spectral distributions of the fundamental electromagnetic emissions It demonstrates that three-waves’ nonlinear interactions contribute to their generation in solar wind plasmas, and processes involving Langmuir waves’ transformations on the background plasma inhomogeneities

Simulation Parameters

Electromagnetic Emissions

Conclusion