Relativistic Electron-positron Plasma Research Articles

Laboratory realization of relativistic pair-plasma beams

Relativistic electron-positron plasmas are ubiquitous in extreme astrophysical environments such as black-hole and neutron-star magnetospheres, where accretion-powered jets and pulsar winds are expected to be enriched with electron-positron pairs. Their role in the dynamics of such environments is in many cases believed to be fundamental, but their behavior differs significantly from typical electron-ion plasmas due to the matter-antimatter symmetry of the charged components. So far, our experimental inability to produce large yields of positrons in quasi-neutral beams has restricted the understanding of electron-positron pair plasmas to simple numerical and analytical studies, which are rather limited. We present the first experimental results confirming the generation of high-density, quasi-neutral, relativistic electron-positron pair beams using the 440 GeV/c beam at CERN’s Super Proton Synchrotron (SPS) accelerator. Monte Carlo simulations agree well with the experimental data and show that the characteristic scales necessary for collective plasma behavior, such as the Debye length and the collisionless skin depth, are exceeded by the measured size of the produced pair beams. Our work opens up the possibility of directly probing the microphysics of pair plasmas beyond quasi-linear evolution into regimes that are challenging to simulate or measure via astronomical observations.

Accelerating self-modulated nonlinear waves in weakly and strongly magnetized relativistic plasmas

It is known that a nonlinear Schrödinger equation describes the self-modulation of a large amplitude circularly polarized wave in relativistic electron–positron plasmas in the weakly and strongly magnetized limits. Here, we show that such an equation can be written as a modified second Painlevé equation, producing accelerated propagating wave solutions for those nonlinear plasmas. This solution even allows the plasma wave to reverse its direction of propagation. The acceleration parameter depends on the plasma magnetization. This accelerating solution is different to the usual soliton solution propagating at constant speed.

Quasi-parallel propagating solitons in magnetised relativistic electron–positron plasmas

In this article, we study nonlinear waves propagating along the background magnetic field in relativistic electron–positron plasmas. Using the reductive perturbation method, we derive a three-dimensional equation describing these waves. When the perturbations do not vary in the directions orthogonal to the background magnetic field this equation reduces to the vector modified Kortewed–de Vries equation. We present solutions of the obtained equation in the form of planar solitary waves and describe the results of study of their stability with respect to transverse perturbations. We also study numerically non-planar solitary waves.

Fermi acceleration in relativistic collisionless plasma shocks correlates with anisotropic energy gains

Collisionless shocks generated by two colliding relativistic electron-positron plasma shells are studied using particle-in-cell simulations. Shocks are mediated by the Weibel instability (WI), and the kinetic energy of the fastest accelerated particles is found to be anisotropically modified by WI-induced electric fields. Specifically, we show that all particles interacting with the shock bifurcate into two groups based on their final relativistic Lorentz factor γ: slow (γ<γbf) and fast (γ>γbf), where γbf is the bifurcation Lorentz factor that was found to be approximately twice the initial (upstream) Lorentz factor γ0. We have found that the kinetic energies of the slow particles are equally affected by the longitudinal and transverse components of the shock electric field, whereas the fast particles are primarily accelerated by the transverse field component.

Dispersion relation of modulational instability for one-dimensional standing solitary waves in hot ultrarelativistic electron–positron plasmas

In this paper, the non-linear dispersion relation of the system for interactions between high intensity laser and hot relativistic electron–positron plasma is derived. We restrict the problem to the standing solitons in ultrarelativistic plasmas and apply the quasineutrality condition. The modulational instability growth rate for different values of plasma temperatures, velocities and wave numbers are illustrated numerically. It is shown that, by increasing the unperturbed plasma enthalpy, the modulational instability growth rate decreases for all the values of plasma fluid velocities. It is also found that the growth rate shows an increasing trend with plasma fluid velocity. Furthermore, the impact of wave amplitude on the modulational instability growth rate is investigated explicitly. The growth rate increases with wave number as well as wave amplitude.

Extinct radio pulsars as a source of subrelativistic positrons

ABSTRACT Extinct radio pulsars, in which stationary, self-sustaining generation of a relativistic electron–positron plasma becomes impossible when rotation brakes down, can be sources of a subrelativistic flux of positrons and electrons. We assume that the observed excess of positrons in the bulge and the disc of the Galaxy is associated with these old neutron stars. The production of pairs in their magnetospheres occurs due to one-photon absorption of gamma quanta of the Galactic and extragalactic backgrounds. The cascade process of plasma production leads to the flux of positrons escaping from the open magnetosphere ≃3 × 1034 s−1. The total flux of positrons from all old Galactic neutron stars with rotational periods 1.5 < P < 35 s is ≃3 × 1043 s−1. The energy of positrons is less than ≃10 MeV. The estimated characteristics satisfy the requirements for the positron source responsible for the 511-keV Galactic annihilation line.

Graviton-to-photon conversion effect in magnetized relativistic plasma

The graviton-to-photon conversion effect in a magnetized relativistic lepton plasma is considered. This effect can be important for the possible generation of electromagnetic radiation accompanying coalescence of relativistic compact neutron star – black hole binaries. The relativistic electron-positron plasma can be generated near the surface of a rotating magnetized neutron star (a radio pulsar). The formation of a relativistic compact binary containing a pulsar and a black hole is predicted by the evolution of massive binary stars. Prior to the coalescence of such a binary due to gravitational wave (GW) emission, a fraction of the GW power can be converted in the plasma outflow into a low-frequency electromagnetic (EM) waves, which can lead to additional radio power prior to the coalescence. Using the graviton- to-photon conversion mechanism in an external magnetic field, we calculate the fraction of GW power converted into the EM radiation in the relativistic plasma. The result is found to depend on the neutron star spin period P, plasma Lorentz factor γ and the cascade multiplicity λ, but independent of the neutron star magnetic field: K ≃ 10−35(P/1 s)2(γ/105)2(λ/105)−2. The possibility of the detection of the non-thermal EM counterparts from neutron star – black hole coalescences in the forthcoming GW observations by aLIGO/Virgo detectors is briefly discussed.

Energy accumulation mechanism in pulsar magnetospheric plasma eigen-waves and formation of Giant Radio Pulses

In the present work we consider energy accumulation mechanism in relativistic electron-positron plasma in the magnetosphere of pulsars. Waves propagating almost across the magnetic field lines are generated that accumulate the energy of plasma particles, as the waves stay significantly long in the resonance region. The accumulated energy is transmitted in the parallel direction of the magnetic field lines as soon as the non-linear plasma processes start to operate. It is suggested that exit of the accumulated energy via waves propagating along the magnetic field lines matching the viewing angle of the observer produces giant pulses. It is shown that these waves come in the radio domain, explaining the Giant Radio Pulse phenomenon.

Polarization and stratification of axionically active plasma in a dyon magnetosphere

The state of a static spherically symmetric relativistic axionically active multi-component plasma in the gravitational, magnetic and electric fields of an axionic dyon is studied in the framework of the Einstein - Maxwell - Boltzmann - axion theory. We assume that the equations of axion electrodynamics, the covariant relativistic kinetic equations, and the equation for the axion field with modified Higgs-type potential are nonlinearly coupled; the gravitational field in the dyon exterior is assumed to be fixed and to be of the Reissner-Nordstr\"om type. We introduce the extended Lorentz force, which acts on the particles in the axionically active plasma, and analyze the consequences of this generalization. The analysis of exact solutions, obtained in the framework of this model for the relativistic Boltzmann electron-ion and electron-positron plasmas, as well as, for degenerated zero-temperature electron gas, shows that the phenomena of polarization and stratification can appear in plasma, attracting attention to the axionic analog of the known Pannekoek-Rosseland effect.

Magnetic structures propagating in non-equilibrium relativistic plasma of pulsar wind nebulae

The kinetic model of highly non-equilibrium relativistic electron-positron plasma is used to study dynamical magnetic structures in pulsar wind nebulae (PWNe). The evolution equation which describes a propagation of a long-wavelength magnetosonic type perturbation of small but finite amplitude is derived. The wavelength is assumed to be longer than the scattering length of the background positrons and electrons. The rates of scattering of electrons and positrons by the stochastic magnetic field fluctuations are distinguished but the difference is supposed to be small compared with the gyrofrequencies of particles. The phase velocity, the dissipation rate and the dispersion length of the magnetic pulse are studied as the functions of plasma parameters and the scattering rates of electrons and positrons. The model being confronted to observations can help to determine the pulsar wind composition, particle distribution and non-thermal pressure in PWNe.

Self-guiding of electromagnetic beams in degenerate relativistic electron-positron plasma

The possibility of self-trapped propagation of electromagnetic beams in the fully degenerate relativistic electron-positron plasma has been studied applying the Fluid-Maxwell model; it is shown that dynamics of such beams can be described by the generalized nonlinear Schrödinger equation with specific type of saturating nonlinearity. Existence of radially symmetric localized solitary structures is demonstrated. It is found that stable solitary structures exist for the arbitrary level of degeneracy.

Low-lying resonances and relativistic screening in Big Bang nucleosynthesis

We explore effects of the screening due to the relativistic electron-positron plasma and presence of resonances in the secondary reactions leading to A=7 nuclei during the Big Bang Nucleosynthesis. In particular, we investigate and examine possible low-lying resonances in the $^7$Be($^3$He, $\gamma$)$^{10}$C reaction and examine the resultant destruction of $^7$Be for various resonance locations and strengths. While a resonance in the $^{10}$C compound nucleus is thought to have negligible effects we explore the possibility of an enhancement from plasma screening that may adjust the final $^7$Be abundance. We find the effects of relativistic screening and possible low-lying resonances to be relatively small in the standard Early Universe models.

Propagation and Generation of Electromagnetic Waves at Proton Gyrofrequencies in a Relativistic Electron–Positron Plasma. II. Excitation of Electromagnetic Waves

In part I of this work [1], we study the dispersion characteristics of low-frequency waves in a relativistic electron–positron plasma. In part II, we examine the electromagnetic wave instability in this plasma caused by an admixture of nonrelativistic protons with energy comparable with the energy of relativistic low-mass particles. The instability occurs in the frequency band between the fundamental harmonic of proton gyrofrequency and the fundamental harmonic of relativistic electron gyrofrequency. The results can be used for the interpretation of known observations of the pulsar emissions obtained with a high time and frequency resolution. The considered instability can probably be the initial stage of the microwave radio emission nanoshots typical of the pulsar in the Crab Nebula.

The stationary non-equilibrium plasma of cosmic-ray electrons and positrons

The statistical properties of the two-component plasma of cosmic-ray electrons and positrons measured by the AMS-02 experiment on the International Space Station and the HESS array of imaging atmospheric Cherenkov telescopes are analyzed. Stationary non-equilibrium distributions defining the relativistic electron–positron plasma are derived semi-empirically by performing spectral fits to the flux data and reconstructing the spectral number densities of the electronic and positronic components in phase space. These distributions are relativistic power-law densities with exponential cutoff, admitting an extensive entropy variable and converging to the Maxwell–Boltzmann or Fermi–Dirac distributions in the non-relativistic limit. Cosmic-ray electrons and positrons constitute a classical (low-density high-temperature) plasma due to the low fugacity in the quantized partition function. The positron fraction is assembled from the flux densities inferred from least-squares fits to the electron and positron spectra and is subjected to test by comparing with the AMS-02 flux ratio measured in the GeV interval. The calculated positron fraction extends to TeV energies, predicting a broad spectral peak at about 1 TeV followed by exponential decay.

Propagation of localized structures in relativistic magnetized electron-positron plasmas using particle-in-cell simulations

We use a particle-in-cell simulation to study the propagation of localized structures in a magnetized electron-positron plasma with relativistic finite temperature. We use as initial condition for the simulation an envelope soliton solution of the nonlinear Schrödinger equation, derived from the relativistic two fluid equations in the strongly magnetized limit. This envelope soliton turns out not to be a stable solution for the simulation and splits in two localized structures propagating in opposite directions. However, these two localized structures exhibit a soliton-like behavior, as they keep their profile after they collide with each other due to the periodic boundary conditions. We also observe the formation of localized structures in the evolution of a spatially uniform circularly polarized Alfvén wave. In both cases, the localized structures propagate with an amplitude independent velocity.

Propagation and Generation of Electromagnetic Waves at Proton Gyrofrequencies in a Relativistic Electron–Positron Plasma. I. Low-Frequency Weakly Damped Electromagnetic Waves

We consider the dispersion characteristics of electromagnetic waves in a plasma with strong magnetic field and equal content of relativistic electrons and positrons, whose synchrotron radiation can be the source of optical radiation of a pulsar. It is shown that when a small fraction of nonrelativistic protons with a nonequilibrium distribution function is present in the plasma, an effective instability can develop at frequencies below the first harmonic of the relativistic gyrofrequency of electrons, namely, at the harmonics of the proton gyrofrequency. This instability leads to the excitation of the O- and X-mode electromagnetic waves, which can, in principle, be related with the observed pulsar radiation. In part I of this paper, we study dispersion characteristics of low-frequency electromagnetic waves (with frequencies below the relativistic gyrofrequency of electrons) in an ultrarelativistic electron–positron plasma with an isotropic momentum distribution function of the particles. Instabilities of the O- and X-mode waves and the conditions of escape of the radiation from the region of strong magnetic field into a rarefied isotropic plasma will be considered in paper II. The results can be used in the interpretation of known experimental data on the dynamic pulsar radiation spectra obtained with high temporal and frequency resolution.

Kinetic transverse dispersion relation for relativistic magnetized electron-positron plasmas with Maxwell-Jüttner velocity distribution functions

We use a kinetic treatment to study the linear transverse dispersion relation for a magnetized isotropic relativistic electron-positron plasma with finite relativistic temperature. The explicit linear dispersion relation for electromagnetic waves propagating along a constant background magnetic field is presented, including an analytical continuation to the whole complex frequency plane for the case of Maxwell-Jüttner velocity distribution functions. This dispersion relation is studied numerically for various temperatures. For left-handed solutions, the system presents two branches, the electromagnetic ordinary mode and the Alfvén mode. In the low frequency regime, the Alfvén branch has two dispersive zones, the normal zone (where ∂ω/∂k > 0) and an anomalous zone (where ∂ω/∂k < 0). We find that in the anomalous zone of the Alfvén branch, the electromagnetic waves are damped, and there is a maximum wave number for which the Alfvén branch is suppressed. We also study the dependence of the Alfvén velocity and effective plasma frequency with the temperature. We complemented the analytical and numerical approaches with relativistic full particle simulations, which consistently agree with the analytical results.

Particle-in-cell simulation for parametric decays of a circularly polarized Alfvén wave in relativistic thermal electron-positron plasma

Parametric decays of a left-handed circularly polarized Alfvén wave propagating along a constant background magnetic field in a relativistic thermal electron-positron plasma are studied by means of a one dimensional relativistic particle-in-cell simulation. Relativistic effects are included in the Lorentz equation for the momentum of the particles and in their thermal motion, by considering a Maxwell-Jüttner velocity distribution function for the initial condition. In the linear stage of the simulation, we find many instabilities that match the predictions of relativistic fluid theory. In general, the growth rates of the instabilities increase as the pump wave amplitude is increased, and decrease with a raise in the plasma temperatures. We have confirmed that for very high temperatures the Alfvén branch is suppressed, consistent with analytical calculations.

Self-modulation of nonlinear Alfvén waves in a strongly magnetized relativistic electron-positron plasma

We study the self-modulation of a circularly polarized Alfvén wave in a strongly magnetized relativistic electron-positron plasma with finite temperature. This nonlinear wave corresponds to an exact solution of the equations, with a dispersion relation that has two branches. For a large magnetic field, the Alfvén branch has two different zones, which we call the normal dispersion zone (where dω/dk>0) and the anomalous dispersion zone (where dω/dk<0). A nonlinear Schrödinger equation is derived in the normal dispersion zone of the Alfvén wave, where the wave envelope can evolve as a periodic wave train or as a solitary wave, depending on the initial condition. The maximum growth rate of the modulational instability decreases as the temperature is increased. We also study the Alfvén wave propagation in the anomalous dispersion zone, where a nonlinear wave equation is obtained. However, in this zone the wave envelope can evolve only as a periodic wave train.

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