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Electrostatic solitary waves (ESW) and solitons are widely observed nonlinear plasma phenomena in various space environments, which may be generated by the electron streaming instability as shown in many particle simulations. The predicted electron holes associated with the ESW, however, are not observed by the recent high resolution spacecraft. This raises a possibility for the ion acoustic solitons being the potential candidate, which are described by the Sagdeev potential theory with hot electrons and cold ions being treated by the kinetic equilibrium and fluid models, respectively. The assumption of [Formula: see text] adopted in the theoretical models for ion acoustic solitons, however, imposes a great constraint for the space applications considering that [Formula: see text] may vary in a wide range of 0.1–10 in the Earth's space environments. This paper examines the effect of [Formula: see text] on ion acoustic solitons by including a finite temperature in the fluid equations for the ions, which, however, can no longer be solved based on the standard Sagdeev potential method. It is shown based on the nonlinear theory that larger [Formula: see text] may result in larger propagation speeds and the critical flow velocity for the existence of steady solitons increases with increasing [Formula: see text] values. The nonlinear solutions for various [Formula: see text] values may be characterized by an effective Mach number. For [Formula: see text] ≫ 1 the hot ions and cold electrons shall be described by the kinetic and fluid models, respectively, which may result in negative electric potentials opposite to the standard ion acoustic solitons. Comparisons between the model calculations and observations are made.

In a magnetized uniform plasma, firehose instability may arise as a result of pressure anisotropy of P_{||}>P_{⊥}, where P_{||} and P_{⊥} are the thermal pressure parallel and perpendicular to the magnetic field, respectively. In this paper, we examine the parallel firehose instability in electron-positron plasmas based on the particle simulations along with the linear fluid theory, which may give rise to the dispersion relation, instability criteria, and growth rate, etc., for comparisons with those calculated from the kinetic simulations. As for the firehose instability in electron-proton plasmas, the magnetic field grows rapidly and then decreases with oscillations. The nonlinear saturated state complies with the linear stability criterion, α=μ_{0}(P_{||}-P_{⊥})/B^{2}=1, derived from the fluid theory only for relatively smaller values of initial α or ω_{p}/ω_{c}, where ω_{p} and ω_{c} are the plasma and cyclotron frequencies, respectively. For relatively larger values of initial α and ω_{p}/ω_{c}, the saturated α values are smaller than 1 as a result of kinetic resonant effects. The dominant wave numbers are kc/ω_{p}

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.

Physical review / E 98(1), 013203 (2018). doi:10.1103/PhysRevE.98.013203, Electron-positron plasmas may be present in laboratory experiments and certain astrophysical environments. Due to the inertia symmetry there have been some debates about whether the nonlinear electrostatic solitons generated easily in electron-proton plasmas may develop in electron-positron plasmas. In this study we show the formation of electrostatic solitons and electromagnetic Alfvén waves in extensive magnetized electron-positron plasma system generated by streaming instabilities based on electromagnetic particle simulations along with fluid theory analyses. In particular, the four-component beam-plasma system may lead to the formation of interlacing electron and positron solitons in the early evolution stage $(say, t < 20ω_{p}^{−1}$, where ωp is the plasma frequency) and large-amplitude Alfvén waves in the later phase $(say, t > 150ω^{−1}_{p})$. The magnetic-field perturbations are generated by the beam and firehose-type instabilities associated with temperature anisotropy of $T_{∥} > T_{⊥ }$resulting from the parallel plasma heating by the electrostatic instability. It is shown that the growth rates and the dominant wavelengths of both electrostatic and electromagnetic instabilities are consistent with the fluid theory. The coexistence of electrostatic solitons and electromagnetic Alfvén waves with significant magnetic field fluctuations in the same system is a unique feature of electron-positron plasmas and the unified theory behind the formation mechanisms is well addressed in the paper., Published by APS, Woodbury, NY

Electrostatic solitary waves (ESWs) and solitons are widely present in the solar system plasma environment. Many theoretical and numerical studies have been carried out to address the formation and structure of ESWs and solitons in electron-ion plasmas. Due to the inertia symmetry, the issue of whether solitons can exist in pair plasmas has been raised and has been discussed in a number of papers. Recently, we have shown that interlacing electron and positron holes in phase space associated with periodic positive and negative potentials may be generated by current-free electron and positron beams streaming in stationary electron-positron background plasmas [Jao and Hau, Phys. Rev. E 86, 056401 (2012)]. The question remains of whether pure electron or positron holes with positive or negative polarity may exist in neutral electron-positron plasmas. In this paper, we show the formation of electron (positron) holes associated with ESWs of positive (negative) potential based on the particle-in-cell simulations of bump-on-tail streaming instability in pair plasmas. The fluid theory shows that the coexistence of two unstable modes with different wavelengths is the essential condition for the generation of electrostatic solitons and hole structures in an electron-positron plasma.

In an electron-proton plasma, electrostatic solitary waves and hole structures can easily be generated by streaming instability due to the asymmetric inertia between ions and electrons. It has been argued theoretically whether electrostatic solitons and/or hole structures can form in a pair plasma. This paper presents results on the formation of pair electrostatic hole structure in an electron-positron plasma based on one-dimensional electrostatic particle-in-cell simulations. In particular, we show the feature of interlacing electron and positron holes in phase space generated by current-free electron and positron beams streaming in a stationary electron-positron background plasma. The coexistent electron and positron holes are associated with periodic interlacing of positive and negative potentials, respectively. Detailed comparisons between simulation results and linear theory of streaming instability in pair plasmas are made and the thermodynamic state is inferred.

Due to the inertia symmetry the issue of whether electrostatic solitons may possibly form in pair plasmas has been addressed in a number of papers. Recently we have shown that pair solitons with interlacing electron and positron holes in electron–positron plasmas may form by means of streaming instability based on one-dimensional particle-in-cell simulations Jao and Hau (2012 Phys. Rev. E 86 056401). In this paper we present the first simulation results of two-dimensional electrostatic solitons in pair plasmas imbedded in a stationary background magnetic field. It is shown that the features of electrostatic solitary structures may depend on the ratio of to where and denote the electron cyclotron and plasma frequency, respectively. In particular, for weakly magnetized or unmagnetized plasmas with both parallel and transverse electric field with the same order of magnitude may first develop and be dissipated in the nonlinear stage such that electrostatic solitons are unable to form by the streaming instability, while for pair solitons resembling those occurring in one-dimensional simulations may possibly form. Comparisons between linear fluid theory and particle-in-cell simulations are made.

Electrons streaming in a background electron and ion plasma may lead to the formation of electrostatic solitary wave (ESW) and hole structure which have been observed in various space plasma environments. Past studies on the formation of ESW are mostly based on the particle simulations due to the necessity of incorporating particle's trapping effects. In this study, the fluid aspects and thermodynamics of streaming instabilities in electron-ion plasmas including bi-streaming and bump-on-tail instabilities are addressed based on the comparison between fluid theory and the results from particle-in-cell simulations. The energy closure adopted in the fluid model is the polytropic law of d(pρ−γ)/dt=0 with γ being a free parameter. Two unstable modes are identified for the bump-on-tail instability and the growth rates as well as the dispersion relation of the streaming instabilities derived from the linear theory are found to be in good agreement with the particle simulations for both bi-streaming and bump-on-t...

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