Electron-positron Pair Plasma Research Articles

Normal mode analysis of pair plasma with drifting species

Pair-ion plasmas consisting of C60+ and C60- ions have been generated in laboratory using fullerene as an ion source1,2. In addition, electron-positron plasmas are believed to be abundant in many astrophysical environments from pulsars to quasars3,4. In laboratory also, electron-positron pair plasma has been generated with the help of ultraintense lasers5 and by trapping of positron in a magnetic mirror configuration by electron cyclotron resonance heating6. Since plasmas are nonlinear and dispersive media, it would be very much interesting to examine whether such pair-plasmas support the excitation of solitary waves and solitons. This has motivated us to do analytical calculations for studying the modes of solitary wave propagation. One-dimensional two-fluid plasma having streaming positrons and electrons is studied through normal mode analysis. We analyze how many types of modes propagate in these plasmas and also compare our results with previously explored models by other researchers. We obtained the dispersion relation and solved for the phase velocity of the acoustic modes. Although the dispersion relation predicts four types of the modes, only two types of the modes (namely fast and slow mode) are possible in the present plasma model. We find all the roots to be real. Therefore, it may be inferred that the instability may be possible in the present plasma model.

Fast Reconnection in Low-density Hydrogen and Pair Plasmas

Magnetic reconnection without a guide field in low-background-density plasmas has been studied by means of two-dimensional particle-in-cell simulations, and results in hydrogen and electron-positron (pair) plasmas have been compared. Reconnection is impulsive, and maximum reconnection rates of the order of one (measured in units of the Alfven speed) have been observed in both types of plasmas when the background density in the Harris sheets is 1% of the current sheet density. This impulsive, strong reconnection electric field is important for particle acceleration. As the system evolves in time, the electron diffusion region extends in both inflow and outflow directions. Because of the broadening of the diffusion region, the aspect ratio of the diffusion region remains small, so that fast reconnection is sustained. In pair plasmas, the inertial term in the generalized Ohm's law becomes the most dominant term to balance the reconnection electric field before the maximum reconnection rate is attained, which contrasts with hydrogen plasmas where the most dominant term is the pressure tensor term.

Dispersion relations for Bernstein waves in a relativistic pair plasma

A fully relativistic treatment of Bernstein waves in an electron-positron pair plasma has remained too formidable a task, owing to the very complex nature of the problem. In this paper, we perform contour integration of the dielectric-response function and numerically compute the dispersion curves for a uniform magnetized relativistic electron-positron pair plasma. The behavior of the dispersion solution for several cases with different plasma temperatures is highlighted. In particular, we find two wave modes that exist only for large wavelengths and frequencies similar to the cyclotron frequency in a moderately relativistic pair plasma. The results presented here have important implications for the study of those objects where a hot magnetized electron-positron plasma plays a fundamental role in generating the observed radiation.

TWO-FLUID MAGNETOHYDRODYNAMIC SIMULATIONS OF RELATIVISTIC MAGNETIC RECONNECTION

We investigate the large-scale evolution of a relativistic magnetic reconnection in an electron-positron pair plasma by a relativistic two-fluid magnetohydrodynamic (MHD) code. We introduce an interspecies friction force as an effective resistivity to dissipate magnetic fields. We demonstrate that magnetic reconnection successfully occurs in our two-fluid system, and that it involves Petschek-type bifurcated current layers in a later stage. We further observe a quasi-steady evolution thanks to an open boundary condition, and find that the Petschek-type structure is stable over the long time period. Simulation results and theoretical analyses exhibit that the Petschek outflow channel becomes narrower when the reconnection inflow contains more magnetic energy, as previously claimed. Meanwhile, we find that the reconnection rate goes up to ~1 in extreme cases, which is faster than previously thought. The role of the resistivity, implications for reconnection models in the magnetically dominated limit, and relevance to kinetic reconnection works are discussed.

Electrostatic drift modes in quantum pair plasmas

Electrostatic drift waves in a nonuniform quantum magnetized electron-positron (pair) plasma are investigated. An explicit and straightforward analytical expression of the fluctuation frequency is presented. The effects induced by quantum fluctuations, density gradients, and magnetic field inhomogeneity on the wave frequencies are discussed and a purely quantum drift mode appears. The present analytical investigations are relevant to dense astrophysical objects as well as laboratory ultracold plasmas.

Development of a Turbulent Outflow During Electron‐Positron Magnetic Reconnection

The mass symmetry between the two species in electron-positron (pair) plasmas has interesting consequences for collisionless magnetic reconnection because the Hall term, which plays a crucial role in supporting fast reconnection in electron-proton plasmas, vanishes. We perform kinetic simulations of pair reconnection in systems of various sizes, show that it remains fast, and identify the reason why this occurs. For sufficiently large systems a Weibel-like temperature anisotropy instability develops in the outflow from the X-point that causes the current layer to broaden and form a Petschek-like open outflow. We discuss the parameter regimes in which pair reconnection should be fast and the implications for astrophysical pair plasmas.

Wave kinetic description of quantum pair plasmas

AbstractThe dispersion relation for a quantum pair plasma is derived, by using a wave kinetic description. A general form of the kinetic dispersion relation for electrostatic waves in a two-component quantum plasma is established. The particular case of an electron–positron pair plasma is considered in detail. Exact expressions for Landau damping are derived, and the quasi-classical limit is discussed.

An asymmetric distribution of positrons in the Galactic disk revealed by γ-rays

Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis, accreting compact objects, and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk ( approximately 10-50 degrees from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10(41) positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.

Fast collisionless reconnection in electron-positron plasmas

Magnetic reconnection in electron-positron (pair) plasmas is studied by two-dimensional, electromagnetic, particle-in-cell simulations. In pair plasmas, the Hall current does not appear because there is no scale separation between ion and electron motion. Simulations show that fast reconnection is realized in both nonrelativistic and relativistic regimes. It is demonstrated that the divergence of the electron and positron pressure tensors essentially plays the role of an effective collisionless resistivity in realizing fast reconnection. Since there is no Hall current in pair plasmas, there is no quadrupolar structure in the out-of-plane magnetic field. Particle acceleration by reconnection is investigated. Major sites of particle acceleration are the regions in the vicinity of X lines, where ultrarelativistic particles are generated by the reconnection electric field.

Quantum-Electrodynamical Photon Splitting in Magnetized Nonlinear Pair Plasmas

We present for the first time the nonlinear dynamics of quantum electrodynamic (QED) photon splitting in a strongly magnetized electron-positron (pair) plasma. By using a QED corrected Maxwell equation, we derive a set of equations that exhibit nonlinear couplings between electromagnetic (EM) waves due to nonlinear plasma currents and QED polarization and magnetization effects. Numerical analyses of our coupled nonlinear EM wave equations reveal the possibility of a more efficient decay channel, as well as new features of energy exchange among the three EM modes that are nonlinearly interacting in magnetized pair plasmas. Possible applications of our investigation to astrophysical settings, such as magnetars, are pointed out.

Existence of bulk acoustic modes in pair plasmas

In view of applications to electron-positron and fullerene pair plasmas, a thorough discussion is given of the dispersion of linear electrostatic waves, before finding the parameter ranges in which stationary nonlinear electrostatic modes can exist, when there is a thermodynamic asymmetry between both constituents. Arguments for such an asymmetry can be found in the observed dispersion in fullerene pair plasma experiments, but point to small asymmetries. The existence of solitary modes is first discussed in general terms for various polytropic pressure-density relations, showing that the solitons are always compressive in both pair components. For large thermal asymmetries, there is at most a doubling of the densities, but small asymmetries lead to weakly nonlinear amplitudes, described by familiar Korteweg-de Vries solitons. Observations of an intermediate wave in the theoretical gap between the acoustic and Langmuir branches of the linear dispersion diagram might be accounted for, at least partially, by a train of weak solitons, as the difference between weak solitary modes and linear harmonic waves is rather difficult to ascertain experimentally.

Radio pulsar drifting sub-pulses and diocotron instability

The potential role of a diocotron instability in causing drifting sub-pulses in radio pulsar emission is investigated for aligned magnetic rotators. It is assumed that the out-flowing plasma above a pulsar polar cap consists of an initially axially symmetric, hollow beam of relativistic electron positron pair plasma which carries an electric charge as well as a current. The occurrence of instability depends on shear in the angular velocity distribution of the beam as a function of axial distance. Instability occurs under typical pulsar conditions at mode numbers l 40. It destroys the symmetry of the equilibrium configuration and leads to a carousel of density columns which rotates at fixed angular pattern speed. The process is applied to two pulsars with observed carousels of drifting sub-pulses, and the diocotron instability at corresponding mode number and axial distance is used as a diagnostic for the charge and current density of the polar flow.

On the nonexistence of large amplitude stationary solitary waves in symmetric unmagnetized pair plasmas

Abstract. Waves in pair plasmas have a fundamentally different dispersion due to the equal charge-to-mass ratios between negative and positive charges. In view of possible applications e.g. to electron-positron and fullerene pair plasmas, it is shown that there are no stationary large amplitude nonlinear structures in symmetric unmagnetized pair plasmas.

Oblique propagation of large amplitude electromagnetic solitons in pair plasmas

Waves in pair plasmas have a fundamentally different dispersion due to the equal charge-to-mass ratios between negative and positive charges, which mix different time scales. In view of possible applications, e.g., to electron-positron and fullerene pair plasmas, stationary nonlinear structures are investigated for oblique or perpendicular propagation with respect to the static magnetic field. A generalized large amplitude extraordinary mode is found, having a linearly polarized electric field that is orthogonal to both the directions of wave propagation and of the static magnetic field. When the Alfvénic Mach number is in a suitably defined range, the pseudoenergy integral admits solutions with a negative wave electric field, in a cone around parallel propagation, and solutions with a positive wave electric field, at all angles of propagation. The exact analytical solution describing these solitary waves has also been obtained. At weakly nonlinear amplitudes, the solutions reduce to familiar solutions of the Korteweg–de Vries or modified Korteweg–de Vries equations.

Constraints on the energetics and plasma composition of relativistic jets in FR II sources

We explore the energetics and plasma composition in FR II sources using a new simple method of combining shock dynamics and radiation spectrum. The hot spots are identified with the reverse shocked region of jets. With the one-dimensional shock jump conditions taking account of the finite pressure of hot ICM, we estimate the rest mass and energy densities of the sum of thermal and non-thermal particles in hot spots. Independently, based on the Synchrotron Self-Compton (SSC) model, we estimate the number and energy densities of {\it non-thermal} electrons using the multi-frequency radiation spectrum of hot spots. We impose the condition that the obtained rest mass, internal energy, and number densities of non-thermal electrons should be lower than those of the total particles determined by shock dynamics. We apply this method to Cygnus A. We examine three extreme cases of pure electron-positron pair plasma (Case I), pure electron-proton plasma with separate thermalization (Case II), and pure electron-proton plasma in thermal-equilibrium (Case III). By detailed SSC analysis for Cygnus A and 3C123, we find that the energy density of non-thermal electrons is about 10 times larger than that of magnetic field. We find that the Case III is not acceptable because predicted photon spectra do not give a good fit to the observed one. We find that Case II can also be ruled out since the number density of non-thermal electrons exceeds that of the total number density. Hence, we find that only pure $e^{\pm}$ plasma (Case I) is acceptable among the three cases. Total kinetic power of jet and electron acceleration efficiency are also constrained by internal energy densities of non-thermal and total particles.

Wien Fireball Model of Relativistic Outflows in Active Galactic Nuclei

We study steady and spherically symmetric outflows of pure electron-positron pair plasma as a possible acceleration mechanism of relativistic jets up to the bulk Lorentz factor of greater than 10. These outflows are initiated by the ``Wien fireball'', which is optically thick to Compton scattering but thin to absorption and in a Wien equilibrium state between pairs and photons at a relativistic temperature.

Radiative Collimation of Electron–Positron Jets II

Abstract We consider a collimation and acceleration mechanism for relativistic jets emanating from an inner region of a relativistic accretion disk surrounding a Kerr black hole, with Keplerian angular momentum under disk radiation fields, taking into account the effect of radiation drag. When the jets consist of electron–positron pair plasmas and the disk luminosity is of the order of the Eddington luminosity, we found that the terminal speed is about $0.9 \,c$ for a Schwarzschild case, while it becomes $\sim 0.96 \,c$ for an extreme Kerr case. In such a case, the opening angle of jets is of the order of $5^\circ$. We also examined the opening angle and the listing degree when the disk radiation is not axisymmetric, but has a one-armed pattern. For the case of 0.3 amplitude, the opening angle is of the order of a few degrees, and the jet direction is bent by several degrees to $10^\circ$.

Relativistic parsec–scale jets: II. Synchrotron emission

We calculate the optically thin synchrotron emission of fast electrons and positrons in a spiral stationary magnetic field and a radial electric field of a rotating relativistic strongly magnetized force–free jet consisting of electron-positron pair plasma. The magnetic field has a helical structure with a uniform axial component and a toroidal component that is maximal inside the jet and decreasing to zero towards the boundary of the jet. Doppler boosting and swing of the polarization angle of synchrotron emission due to the relativistic motion of the emitting volume are calculated. The distribution of the plasma velocity in the jet is consistent with the electromagnetic field structure. Two spatial distributions of fast particles are considered: uniform, and concentrated in the vicinity of the Alfvén resonance surface. The latter distribution corresponds to the regular acceleration by an electromagnetic wave in the vicinity of its Alfvén resonance surface inside the jet. The polarization properties of the radiation have been obtained and compared with the existing VLBI polarization measurements of parsec–scale jets in BL Lac sources and quasars. Our results give a natural explanation of the observed bimodality in the alignment between the electric field vector of the polarized radiation and the projection of the jet axis on the plane of the sky. We interpret the motion of bright knots as a phase velocity of standing spiral eigenmodes of electromagnetic perturbations in a cylindrical jet. The degree of polarization and the velocity of the observed proper motion of bright knots depend upon the angular rotational velocity of the jet. The observed polarizations and velocities of knots indicate that the magnetic field lines are bent in the direction opposite to the direction of the jet rotation.

Pulsar emissions

The standard model for radio pulsars implies that the `pulsar plasma' in the source region is a highly relativistic, strongly magnetized, one-dimensional, electron–positron pair plasma. Relativistic quantum effects lead to a correction of first order in ℏ to the linear response of a pulsar plasma due to its spin dependence, but this is small for highly relativistic particles. Wave dispersion in the radio range involves the X-mode, Alfvén mode and L–O mode. It is argued that the most plausible radio emission mechanism involves a nonresonant beam instability due to a beam mode that couples to the L–O mode. It is suggested that `orthogonal modes' in the observed radio emission arise from mode coupling at sharp gradients at the edges of ducts, and that the observed circular polarization is associated with gyrotropy (a difference in the electron and positron distributions) and it develops as a propagation effect.

Radio Pulses through a Magnetized Relativistic Pair Plasma Flow

Radio pulses emitted from a pulsar magnetosphere travel across the light cylinder and through a magnetized relativistic pulsar wind composed mainly of an electron-positron (e) pair plasma embedded in a spiral magnetic field. Besides the Doppler-shifted cyclotron resonance and wind advections of hybrid oscillations or low-frequency waves, we identify two cutoff frequencies at ωpe and ~(ω + ω/γ2)1/2 for radio waves of two orthogonal polarizations perpendicular and parallel to the projected spin axis in the sky plane, where ωpe is the proper e pair plasma frequency, ωc is the relativistic Larmor frequency, and γ is the Lorentz factor of e bulk flow. Radio pulse spectra of most pulsars show low-frequency cutoffs at νcut 100 MHz. For (ω + ω/γ2)1/2 < 2πνcut, one may bracket the e particle loss rate of a pulsar wind. If for some cases, alternative to the wisdom that all such spectral cutoffs are intrinsic to the emission mechanism(s) near a pulsar, the magnetized e pair plasma of a pulsar wind actually causes the low-frequency cutoffs of radio pulse spectra, then spectral polarization studies of the two different frequency cutoffs can yield ωpe and ωc/γ. Together with the observed rates of pulsar spin and spin-down, one may derive key properties of a pulsar wind, including the e particle loss rate.