Electrostatic and electromagnetic waves driven by current free electron beam instability: Effects of positron, background composition, and ion-to-electron mass ratio

C.-S Jao,L.-N Hau

doi:10.1063/5.0016198

Abstract

Counterstreaming electron beams drifting along the magnetic field in electron–proton plasmas may lead to the generation of electrostatic (ES) waves and structures widely observed in planetary magnetospheres. In electron–positron plasmas, streaming electron/positron beams along the background magnetic field may first result in ES waves followed by the electromagnetic (EM) waves with substantial magnetic field perturbations, as shown by Jao and Hau [Phys. Rev. E 98, 013203 (2018)]. In this study, we examine the formation of ES and EM waves driven by current free counterstreaming electron beams in electron–ion background plasmas and current free electron–positron beams in electron–proton–positron background plasmas based on one-dimensional EM particle-in-cell simulations with the drift velocity being along the background magnetic field. For the former cases, the effects of ion-to-electron mass ratios are examined, which have implications for two- and three-dimensional particle in cell simulations with reduced mass ratios. For the latter cases, the effects of positrons are examined, which have applications to astrophysical plasma environments and laboratory experiments. It is shown that the presence of positrons or light ions may shorten the occurrence times between the ES and EM waves driven by current free electron beam instability. The EM waves have relatively longer wavelengths as compared to the ES waves and the magnetic field evolution resembles parallel proton firehose instability. The effects of positrons, background compositions, and ion-to-electron mass ratios on the occurrence of ES and EM waves are consistent with the linear fluid theory.

Highlights

Kinetic particle simulations are essential for the study of nonlinear plasma physics, in which the charged particles are governed by the equations of motions and the electric/magnetic fields are described by the Maxwell equations
Umeda et al.6 have studied the perpendicular shocks based on 2D particle in cell (PIC) simulations with various ion-to-electron mass ratios and found that two stream instability becomes more dominant for higher mass ratios
Due to the inertia symmetry, ES and EM waves may coexist in electron–positron plasma systems driven by the same streaming instability

Summary

Introduction

Kinetic particle simulations are essential for the study of nonlinear plasma physics, in which the charged particles are governed by the equations of motions and the electric/magnetic fields are described by the Maxwell equations. For electron–proton plasma systems, it is still impractical to use real mass ratios in the two- and three-dimensional PIC simulations to study the electromagnetic phenomena involving the physics of both electrons and massive ions with extensive spatial and temporal domains. Hong et al. have examined the ion beam driven electrostatic instability by varying the ion-to-electron scitation.org/journal/adv mass ratio in the 1D ES PIC simulations. A systematic study of electron beam instabilities, including Weibel and two-stream instabilities, is examined by Bret based on the linear fluid theory for relativistic cold plasmas, which shows that the dominant modes may depend on the mass ratio. Real mass ratios are feasible for some 2D PIC experiments in which the major physics arises from fast electrons with limited temporal and spatial scales.

Objectives

Conclusion