Large-amplitude Electromagnetic Waves Research Articles

Stimulated scattering of electromagnetic waves in a two-electron plasma

The nonlinear interaction between large-amplitude electromagnetic waves and electron-acoustic (EA) waves in a two-electron-temperature plasma is considered, taking into account the combined effects of the radiation pressure and the thermal force involving the differential Joule heating of the electrons caused by the electromagnetic waves. By employing a fluid approach, we derive a system of coupled equations for the electromagnetic waves and the EA waves; the latter are nonlinearly driven by the radiation and thermal forces. We have carried out a normal mode analysis of our nonlinearly coupled equations, and have derived a general dispersion relation that is useful for studying different types of parametric instabilities. A new class of modulational instability in the collision-dominated regime is identified. The implications for space and laboratory plasmas are pointed out.

Enhanced electromagnetic emission from a dusty plasma

It is shown how enhanced electromagnetic emission (EEE) is induced when large amplitude electromagnetic waves scatter off acoustic-like modes in a dusty plasma. Following the formalism of standard parametric interactions and using the Rostoker superposition principle, a general expression for the ensemble average of the square of the vector potential of EEE is derived. The result should be useful in deducing the plasma parameters in-situ when intense radar beams are used for the diagnosis of the polar mesospheric summer echos in association with low-frequency irregularities.

Generation and Dynamics of Vortices. II.

It is shown that the interaction of large amplitude electromagnetic waves with a relativistic electron-positron plasma leads to a bunching of mass, energy, and angular momentum in stable, long-lived structures. This structure contains a quantized angular momentum, and is called Vortex soliton. The correspondence between the nonlinear Schrdinger equation and Euler equation is reviewed. Problem related with the creation of angular momentum is also addressed.

Absolute and convective nature of the modulational and Raman instabilities in the relativistic regime

The nature (convective or absolute) of the modulational and Raman instabilities is investigated in the relativistic regime, for a large amplitude electromagnetic wave propagating in a plasma of arbitrary density. The present paper extends previous results by Guérin et al. [Phys. Plasmas 2, 2807 (1995)]. Stability diagrams are obtained and compared to the stability properties found by a classical temporal stability analysis. It is shown that the modulational and the (forward and backward) Raman instabilities are individually convective. However, the Raman instability is absolute for large density and high intensity when the forward and backward Raman branches have merged into a hybrid Raman branch. Near quarter critical density, this hybrid branch is destabilized and is absolute from the threshold. The plasma equilibrium is convectively unstable between quarter critical and critical density at low intensity.

Modulational instabilities of electromagnetic waves in a collision-dominated dust-electron plasma

The nonlinear coupling between large amplitude electromagnetic waves and dust-acoustic waves (DAWs) in a dust-electron plasma is considered, taking into account the combined effects of the radiation pressure and the thermal force involving the differential Joule heating of the electrons caused by the electromagnetic waves. By employing a two-fluid approach, we derive a system of coupled equations for the electromagnetic waves and the DAWs; the latter are nonlinearly driven by the radiation and thermal forces. A normal mode analysis of our nonlinearly coupled equations is performed to obtain a general dispersion relation that is useful for studying modulational instabilities. Specific results for the latter are presented in the collision-dominated regime where the electron frequency is much larger than the DAW frequency. The implications to space plasmas are pointed out.

Vortex solitons: mass, energy, and angular momentum bunching in relativistic electron-positron plasmas.

It is shown that the interaction of large amplitude electromagnetic waves with a hot electron-positron (e-p) plasma (a principal constituent of the universe in the MeV epoch) leads to a bunching of mass, energy, and angular momentum in stable, long-lived structures. Electromagnetism in the MeV epoch, then, could provide a possible route for seeding the observed large-scale structure of the universe.

Gamma-ray bursts from unstable Poynting-dominated outflows

Poynting-flux driven outflows from magnetized rotators are a plausible explanation for gamma-ray burst engines. We suggest a new possibility for how such outflows might transfer energy into radiating particles. We argue that, in a region near the rotation axis, the Poynting flux drives non-linearly unstable large-amplitude electromagnetic waves (LAEMW) that ‘break’ at radii where the MHD approximation becomes inapplicable. In the ‘foaming’ (relativistically reconnecting) regions formed during the wave breaks, the random electric fields stochastically accelerate particles to ultrarelativistic energies which then radiate in turbulent electromagnetic fields. The typical energy of the emitted photons is a fraction of the fundamental Compton energy with plus additional boosting due to the bulk motion of the medium. The emission properties are similar to synchrotron radiation, with a typical cooling time ∼10−3 s. During the wave break, the plasma is also bulk accelerated in the outward radial direction and at larger radii can produce afterglows due to interactions with the external medium. The near equipartition fields required by afterglow models may be due to magnetic field regeneration in the outflowing plasma (similar to field generation by LAEMW in laser–plasma interactions) and mixing with the upstream plasma.

Asymmetric Supernovae, Pulsars, Magnetars, and Gamma‐Ray Bursts

We outline the possible physical processes, associated timescales, and energetics that could lead to the production of pulsars, jets, asymmetric supernovae, and weak gamma-ray bursts in routine circumstances and to a magnetar and perhaps stronger gamma-ray burst in more extreme circumstances in the collapse of the bare core of a massive star. The production of a LeBlanc-Wilson MHD jet could provide an asymmetric supernova and result in a weak gamma-ray burst when the jet accelerates down the stellar density gradient of a hydrogen-poor photosphere. The matter-dominated jet would be formed promptly, but requires 5 to 10 s to reach the surface of the progenitor of a Type Ib/c supernova. During this time, the newly-born neutron star could contract, spin up, and wind up field lines or turn on an alpha-Omega dynamo. In addition, the light cylinder will contract from a radius large compared to the Alfven radius to a size comparable to that of the neutron star. This will disrupt the structure of any organized dipole field and promote the generation of ultrarelativistic MHD waves (UMHDW) at high density and Large Amplitude Electromagnetic Waves (LAEMW) at low density. The generation of the these waves would be delayed by the cooling time of the neutron star about 5 to 10 seconds, but the propagation time is short so the UMHDW could arrive at the surface at about the same time as the matter jet. In the density gradient of the star and the matter jet, the intense flux of UMHDW and LAEMW could drive shocks, generate pions by proton-proton collision, or create electron/positron pairs depending on the circumstances. The UMHDW and LAEMW could influence the dynamics of the explosion and might also tend to flow out the rotation axis to produce a collimated gamma-ray burst.

Stimulated scattering of radiation off quasi-modes in an electron–positron plasma

Stimulated electron Compton scattering (SECS) as well as kinetic modulational instabilities of a large amplitude electromagnetic wave in an unmagnetized electron–positron plasma are considered when the radiation is scattered off a quasi-mode whose phase velocity equals the electron or positron thermal velocity. The growth rates of the instabilities are presented.

Nonthermal Radiation of Cosmological Gamma‐Ray Bursters

We use 1.5 dimensional particle-in-cell plasma simulations to study the interaction of a relativistic, strongly magnetized wind with an ambient medium. Such an interaction is a plausible mechanism that leads to generation of cosmological γ-ray bursts. We confirm the idea of Meszaros & Rees that an essential part (about 20%) of the energy that is lost by the wind in the process of its deceleration may be transferred to high-energy electrons and then to high-frequency (X-ray and γ-ray) emission. We show that in the wind frame the spectrum of electrons that are accelerated at the wind front and move ahead of the front is nearly a two-dimensional relativistic Maxwellian with a relativistic temperature T = mec2ΓT/k 6 × 109ΓT K, where ΓT is equal to 200Γ0, with the accuracy of ~20%, and Γ0 is the Lorentz factor of the wind, Γ0 102 for winds outflowing from cosmological γ-ray bursters. Our simulations point to an existence of a high-energy tail of accelerated electrons with a Lorentz factor of more than ~700Γ0. Large-amplitude electromagnetic waves are generated by the oscillating currents at the wind front. The mean field of these waves ahead of the wind front is an order of magnitude less than the magnetic field of the wind. High-energy electrons that are accelerated at the wind front and injected into the region ahead of the front generate synchro-Compton radiation in the fields of large-amplitude electromagnetic waves. This radiation closely resembles synchrotron radiation and can reproduce the nonthermal radiation of γ-ray bursts observed in the Ginga and BATSE ranges (from a few keV to a few MeV). Synchrotron photons that are generated in the vicinity of the wind front may be responsible for the radiation of γ-ray bursts in the EGRET energy range above a few ten MeV. The spectrum of γ-ray bursts in high-energy γ-rays may extend, in principle, up to the maximum energy of the accelerated electrons, which is about 1013(Γ0/102)2 eV in the frame of the γ-ray burster.

On Fueling Gamma-Ray Bursts and Their Afterglows with Pulsars

Gamma-ray bursts (GRBs) and their afterglows seem to result from the dissipation of bulk energy in relativistic outflows, but their engine has not been identified. The engine could be a young pulsar formed from accretion-induced collapse with a dynamo-amplified magnetic field. Elsewhere, we suggest that such strong field (cosmological) pulsars may help to explain the bimodal GRB duration distribution. Here we discuss the possible roles of a pulsar for the afterglow. We derive the expected bolometric luminosity decay and emphasize that the extracted rotational energy could dissipate by shocks and/or by large-amplitude electromagnetic waves (LAEMWs). The simplest LAEMW approach predicts a slower decay in the observed afterglow peak frequency and a faster decay in the flux than the simplest blast wave, although more complicated models of both can produce different dependences. LAEMWs do not require the rapid magnetic field amplification demanded of the blast-wave approach because the emission originates from a nearly fixed radius. Different time evolution of GRB and post-GRB emission is also predicted. Evidence for a pulsar in a GRB would make some GRB engine models, such as neutron star mergers and black holes, unlikely. Thus, the question of a pulsar presence is important even if it could drive a canonical fireball.

Stimulated Brillouin scattering of an electromagnetic wave in a strongly magnetized plasma

In a strongly magnetized nonisothermal plasma, a large amplitude electromagnetic wave propagating at an angle θ0 to a strong magnetic field undergoes stimulated Brillouin scattering off the ion mode. In an isothermal plasma the process goes over to stimulated Compton scattering with considerably lower growth rate. In both cases the growth rate is sensitive to the angle θ=(θ1−θ0), where θ0 is the angle of the pump wave vector k0 with the dc magnetic field, Bsẑ, and θ1 is the angle of the sideband wave vector k1 with Bsẑ.

Particle acceleration by large-amplitude waves revisited

We reconsider the problem of particle acceleration by large-amplitude electromagnetic waves. We make use of a fully relativistic Hamiltonian formalism to show that, as opposed to nonelativistic results obtained by Kuo and Lee, acceleration in unmagnetized systems is severely arrested when the phase velocity of the electromagnetic mode approaches the speed of light. For subluminal waves, however, acceleration is shown to be still effective.

Relativistic Precessing Jets and Cosmological Gamma-Ray Bursts

We discuss the possibility that gamma-ray bursts may result from cosmological relativistic blob-emitting neutron star jets that precess past the line of sight. Beaming reduces the energy requirements, so that the jet emission can last longer than the observed burst duration. One precession mode maintains a short-duration timescale while a second keeps the beam from returning to the line of sight, consistent with the paucity of repeaters. The long life of these objects reduces the number required for production as compared to short-lived jets. Blobs can account for the time structure of the bursts. Here we focus largely on kinematic and timescale considerations of beaming, precession, and blobs—issues that are reasonably independent of the acceleration and jet collimation mechanisms. We do suggest that large-amplitude electromagnetic waves could be a source of blob acceleration.

Propagation of ultraintense laser pulses through overdense plasma layers

Due to relativistic effects, a large amplitude electromagnetic wave can propagate in a classically overdense plasma with ω2p≳ω2≳ω2p/γ, where ωp is the plasma frequency, ω the laser frequency, and γ the relativistic factor of an electron in the laser field. Particle-in-cell simulations are used to study the interaction of an ultrahigh intensity laser pulse in normal incidence on a one-dimensional preformed plasma layer. Both electrons and ions dynamics are included. The width of the layer is 10 to 30 μm and the plasma is characterized by (ωp/ω)2=1.5. During the penetration of the electromagnetic wave a large longitudinal electric field is generated. It results in a strong longitudinal heating of electrons which reach relativistic temperatures. This heating further lowers the effective plasma frequency ωp/γ so the layer becomes almost transparent after the plasma crossing by the wave front. Velocity of the wave front, reflection and transmission rates are studied as functions of the incident energy flux, the plasma thickness, and the pulse duration.

Stimulated scattering instabilities of electromagnetic waves in strongly coupled plasmas

The nonlinear interaction between large-amplitude electromagnetic waves and new collective low-frequency modes of dense strongly coupled two-component plasmas has been considered. We present specific results for growth rates and thresholds of stimulated Brillouin scattering as well as modulational instabilities.

Parametric decays of electromagnetic ion cyclotron waves in a H+ ‐He+ ‐O+ magnetosphericlike plasma

Parametric decays of large‐amplitude electromagnetic ion cyclotron waves (EICW) due to a minor O+ ion component in the magnetosphere are studied. It is shown that the presence of O+ ions leads to a number of new wave couplings which in turn lead to new instabilities. Some couplings involve sound waves carried mainly by the O+ ions, and a sideband EICW which has a resonance at the O+ ion gyrofrequency. These are decay instabilities which can lead to O+ heating through Landau damping and/or resonance absorption. There is also a modulational instability involving two sideband EICW, one propagating forward and the other propagating backward relative to the external magnetic field. These waves can also transfer energy to the O+ ions through resonance absorption. The other branches of the dispersion relation, namely, the He+ and proton branch, have additional decay instabilities (Gomberoff et al., 1995) due to the presence of a minor O+ ion component. It is also shown that in the fluid description, the decays to sound waves associated with the minority heavy ion species have growth rates comparable to, or even larger than, the decays to the acoustic branch corresponding to the majority proton species.

Modulational and Raman instabilities in the relativistic regime

A large amplitude electromagnetic wave propagating in a plasma is known to be subject to severe modulational and Raman instabilities. Previous works were devoted to the weakly relativistic limit and applied mainly to a cold underdense plasma. One extends these works to include the fully relativistic limit for a circularly polarized light for which one derives the dispersion relation in a one-dimensional plasma. The characteristics of the instabilities are also calculated in the case where the plasma is classically overdense, with 1<(ωp/ω0)2<γ, where ωp is the plasma frequency, ω0 is the laser frequency, and γ is the relativistic factor of an electron in the laser field. Particle-in-cell simulations confirm the results of the numerical solutions of the dispersion relation. For (ωp/ω0)2/γ=0.57 the growth rate can be as large as 0.52ω0. The nonlinear stage of the instability results in a strong heating of the electron distribution function. The theory is further extended to the case of an initially hot plasma, for which the dispersion relation of the instabilities is established. Its analytical solution is given in the case of a low density plasma. Particle-in-cell simulations are used to treat the general case. One observes a strong reduction of the growth rate of the instability, which tends to restore the possibility to propagate relativistic waves in plasmas.

Group velocity of large-amplitude electromagnetic waves in a plasma.

The nonlinear group velocity of a laser pulse propagating in a cold underdense unmagnetized plasma is examined. Analytical expressions for the group velocity are derived for various pulse length regimes. These expressions reduce to the usual \ensuremath{\partial}\ensuremath{\omega}/\ensuremath{\partial}k form for small amplitude and are verified for aribtrary amplitude using particle in cell simulations on a cyclic mesh. We find that the leading edge of a pulse moves at the linear group velocity and that the phase velocity of the excited wake is less than the group velocity of the pulse for symmetrically shaped pulses. The techniques used can be applied to other waves in a plasma.

Stimulated Brillouin scattering of electromagnetic waves in magnetized semiconductor plasmas.

The phenomenon of stimulated Brillouin scattering of a large-amplitude electromagnetic wave by the electron acoustic wave has been analytically investigated in a piezoelectric semiconductor plasma in the presence of an external uniform magnetic field. The Boltzmann equation with a Krook model for the collisional term has been solved in the guiding center coordinates for the nonlinear response of the plasma electrons. It is noticed that the magnetic field enhances the growth rate of the parametric instability in contrast with the fluid-model results.