Particle-in-cell simulations of the relaxation of electron beams in inhomogeneous solar wind plasmas

TL;DR

This study uses fully kinetic particle-in-cell simulations to investigate how background density irregularities in the solar wind affect electron beam relaxation and Langmuir wave dynamics, confirming theoretical predictions.

Contribution

First fully kinetic PIC simulations of electron beam relaxation in inhomogeneous plasmas, validating quasi-linear and Hamiltonian models with new insights into wave behavior.

Findings

In homogeneous plasma, Langmuir waves are generated at resonance and lead to wave packet growth.

In inhomogeneous plasma, high-energy electrons are produced, unlike in homogeneous cases.

Density fluctuations cause refraction of Langmuir waves, generating backward waves.

Abstract

Previous theoretical considerations of electron beam relaxation in inhomogeneous plasmas have indicated that the effects of the irregular solar wind may account for the poor agreement of homogeneous modelling with the observations. Quasi-linear theory and Hamiltonian models based on Zakharov's equations have indicated that when a level of density fluctuations is above a given threshold, density irregularities act to de-resonate the beam-plasma interaction, restricting Langmuir wave growth on the expense of beam energy. This work presents the first fully kinetic particle-in-cell (PIC) simulations of beam relaxation under the influence of density irregularities. We aim to independently determine the influence of background inhomogeneity on the beam-plasma system, and to test theoretical predictions and alternative models using a fully kinetic treatment. We carry out 1D PIC simulations of a bump-on-tail unstable electron beam in the presence of increasing levels of background inhomogeneity using the fully electromagnetic, relativistic EPOCH PIC code. We find that in the case of homogeneous background plasma density, Langmuir wave packets are generated at the resonant condition and then quasi-liear relaxation leads to a dynamic increase of wavenumbers generated. No electron acceleration is seen - unlike in the inhomogeneous experiments, all of which produce high-energy electrons. For the inhomogeneous experiments we also observe the generation of backwards propagating Langmuir waves, which is shown directly to be due to the refraction of the packets off the density gradients. Our fully kinetic PIC simulations broadly confirm the findings of quasi-linear theory and the Hamiltonian model based on Zakharov's equations. Strong density fluctuations modify properties of excited Langmuir waves altering their dispersion properties.