Large-amplitude Electromagnetic Waves Research Articles

Specific features of electromagnetic waves in degenerate two stream electron-ion plasma: self-induced magnetic field

Abstract Generation and amplification of magnetic fields is a subject of interest to both laboratory and astrophysical plasmas. The counter-streaming instability or Weibel instability is a mechanism responsible for self-generating magnetic fields in plasmas. In this paper, we investigate that the non-stationary ponderomotive force of a large amplitude electromagnetic wave (EMWs) propagating through dense two stream quantum electron ion plasma, may lead to the generation of d.c. magnetic fields. It is shown that degeneracy parameters, specifically the exchange correlation potential and Fermi pressure can reduce/control the self-induced magnetic field. Next the linear propagation of EMWs through plasma under consideration has also been considered in the presence of exchange correlation potential along with other quantum effects. Linear dispersion relation, the instability conditions and growth rate have been derived. It is found that the degeneracy parameters stabilize the propagation of instability and lead to reduction of the exponentially growing magnetic field. Threshold scale length of EMWs is also obtained for the stable propagation. The present result may be account helpful to understand the seed magnetic fields in dense plasmas.

Transparency of fast radio burst waves in magnetar magnetospheres

ABSTRACT At least some fast radio bursts (FRBs) are produced by magnetars. Even though mounting observational evidence points towards a magnetospheric origin of FRB emission, the question of the location for FRB generation continues to be debated. One argument suggested against the magnetospheric origin of bright FRBs is that the radio waves associated with an FRB may lose most of their energy before escaping the magnetosphere because the cross-section for e± to scatter large-amplitude electromagnetic waves in the presence of a strong magnetic field is much larger than the Thompson cross-section. We have investigated this suggestion and find that FRB radiation travelling through the open field line region of a magnetar’s magnetosphere does not suffer much loss due to two previously ignored factors. First, the plasma in the outer magnetosphere ($r \gtrsim 10^9$ cm), where the losses are potentially most severe, is likely to be flowing outwards at a high Lorentz factor γp ≥ 103. Secondly, the angle between the wave vector and the magnetic field vector, θB, in the outer magnetosphere is likely of the order of 0.1 radian or smaller due in part to the intense FRB pulse that tilts open magnetic field lines so that they get aligned with the pulse propagation direction. Both these effects reduce the interaction between the FRB pulse and the plasma substantially. We find that a bright FRB with an isotropic luminosity $L_{\rm frb} \gtrsim 10^{42} \, {\rm erg \ s^{-1}}$ can escape the magnetosphere unscathed for a large section of the γp − θB parameter space, and therefore conclude that the generation of FRBs in magnetar magnetosphere passes this test.

ON EFFECTIVE ACCELERATION OF CHARGED PARTICLES IN VACUUM

The results of studying the dynamics of particles in the fields of large-amplitude transverse electromagnetic waves are presented. The main attention is paid to the description of the found conditions, under which the effective transfer of wave energy to charged particles in vacuum is possible.

Direct Multipoint Observations Capturing the Reformation of a Supercritical Fast Magnetosonic Shock

Abstract Using multipoint Magnetospheric Multiscale (MMS) observations in an unusual string-of-pearls configuration, we examine in detail observations of the reformation of a fast magnetosonic shock observed on the upstream edge of a foreshock transient structure upstream of Earth's bow shock. The four MMS spacecraft were separated by several hundred kilometers, comparable to suprathermal ion gyroradius scales or several ion inertial lengths. At least half of the shock reformation cycle was observed, with a new shock ramp rising up out of the “foot” region of the original shock ramp. Using the multipoint observations, we convert the observed time-series data into distance along the shock normal in the shock's rest frame. That conversion allows for a unique study of the relative spatial scales of the shock's various features, including the shock's growth rate, and how they evolve during the reformation cycle. Analysis indicates that the growth rate increases during reformation, electron-scale physics play an important role in the shock reformation, and energy conversion processes also undergo the same cyclical periodicity as reformation. Strong, thin electron-kinetic-scale current sheets and large-amplitude electrostatic and electromagnetic waves are reported. Results highlight the critical cross-scale coupling between electron-kinetic- and ion-kinetic-scale processes and details of the nature of nonstationarity, shock-front reformation at collisionless, fast magnetosonic shocks.

Particle acceleration and radiation reaction in strong spherical electromagnetic waves

ABSTRACT Strongly magnetized and fast-rotating neutron stars are known to be efficient particle accelerators within their magnetosphere and wind. They are suspected to accelerate leptons, protons, and maybe ions to extreme relativistic regimes where the radiation reaction significantly feeds back to their motion. In the vicinity of neutron stars, magnetic field strengths are close to the critical value of Bc ∼ 4.4 · 109 T and particle Lorentz factors of the order γ ∼ 109 are expected. In this paper, we investigate the acceleration and radiation reaction feedback in the pulsar wind zone where a large-amplitude low-frequency electromagnetic wave is launched starting from the light cylinder. We design a semi-analytical code solving exactly the particle equation of motion including radiation reaction in the Landau–Lifshits approximation for a null-like electromagnetic wave of arbitrary strength parameter and elliptical polarization. Under conventional pulsar conditions, asymptotic Lorentz factor as high as 108−109 is reached at large distances from the neutron star. However, we demonstrate that in the wind zone, within the spherical wave approximation, radiation reaction feedback remains negligible.

Large amplitude electromagnetic solitons in a fully relativistic magnetized electron-positron-pair plasma

Nonlinear propagation of purely stationary large amplitude electromagnetic (EM) solitary waves in a magnetized electron–positron (EP) plasma is studied using a fully relativistic two-fluid hydrodynamic model which accounts for physical regimes of both weakly relativistic (P≪nmc2) and ultrarelativistic (P≫nmc2) random thermal energies. Here, P is the thermal pressure, n the number density and m the mass of a particle, and c is the speed of light in vacuum. While both the sub-Alfvénic and super-Alfvénic solitons coexist in the weakly relativistic regime, the ultrarelativistic EP plasmas in contrast support only the sub-Alfvénic solitons. Different limits of the Mach numbers and soliton amplitudes are also examined in these two physical regimes.

Large-Amplitude Shock Electromagnetic Wave in a Nonlinear Transmission Line Based on a Distributed Semiconductor Diode

The problem of the formation and propagation of a large-amplitude shock electromagnetic wave in a strip transmission line (TL) based on a distributed semiconductor diode is analytically solved. The solution correctly considers its nonlinearity, dissipation, and time dispersion. The results of the theory are used to estimate the parameters of the TL as a sharpener of the pulse front voltage applied to the line input.

Ударная электромагнитная волна большой амплитуды в нелинейной линии передачи на основе распределенного полупроводникового диода

AbstractThe problem of the formation and propagation of a large-amplitude shock electromagnetic wave in a strip transmission line (TL) based on a distributed semiconductor diode is analytically solved. The solution correctly considers its nonlinearity, dissipation, and time dispersion. The results of the theory are used to estimate the parameters of the TL as a sharpener of the pulse front voltage applied to the line input.

Modulation of Ion and Electron Pitch Angle in the Presence of Large-amplitude, Low-frequency, Left-hand Circularly Polarized Electromagnetic Waves Observed by MMS

Abstract Most studies on low-frequency electromagnetic cyclotron waves have assumed a small wave amplitude, which ensures the reasonable application of linear and quasi-linear theories. However, the topic of large-amplitude electromagnetic cyclotron waves has not received much attention. Using Magnetospheric Multiscale measurements, this study observes low-frequency, left-hand circularly polarized electromagnetic waves with magnetic fluctuation ∼1–2 nT in the dusk flank side of the Earth’s magnetosheath. Considering the ambient magnetic field ∼15 nT therein, the relative wave amplitude is of the order of 0.1. These large magnetic field fluctuations result in a periodic variation of the ion pitch angle. The electron pitch angle exhibits a localized distribution feature with a timescale approximating the wave period. Moreover, some electrons are trapped at a pitch angle ∼90°, and the trapping is more remarkable as strong waves arise. These two features of the electron pitch angle distribution imply that the trapping of electrons (partly) results from large-amplitude electromagnetic cyclotron fluctuations. Our results illustrate the important role of large-amplitude electromagnetic cyclotron waves on the dynamics of charged particles.

Raman and Brillouin scattering instabilities of transverse electromagnetic waves in degenerate electron-ion plasmas

By applying the Maxwell and quantum hydrodynamic equations, we have studied the parametric instabilities of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) in an unmagnetized electron-ion quantum plasma. In this context, we have derived the nonlinear dispersion relations of the large-amplitude electromagnetic (EM) waves, the electrostatic electron plasma waves, and the ion-acoustic waves. The nonlinear evolution equations are then solved by utilizing the Fourier transform to obtain expressions for the three-wave decay and modulational instabilities with quantum corrections. It is found that the growth rate of the instabilities is a strong function of large-amplitude EM waves, and quantum effects due to Fermi pressure and quantum correlations stabilize both SRS and SBS instabilities. Expressions for the maximum growth rates attributed to SRS and SBS instabilities are also derived by ignoring the nonlinear correction shift on the frequency of EM waves. The relevance of nonlinear interaction of EM waves with a quantum dense astrophysical plasma is highlighted in the perspective of electron Fermi pressure and exchange-correlation effects, where the plasma density is high enough.

Persistence of Precursor Waves in Two-dimensional Relativistic Shocks

Abstract We investigated the efficiency of coherent upstream large-amplitude electromagnetic wave emission via synchrotron maser instability in relativistic magnetized shocks using two-dimensional particle-in-cell simulations. We considered a purely perpendicular shock in an electron–positron plasma. The coherent wave emission efficiency was measured as a function of the magnetization parameter σ, which is defined as the ratio of the Poynting flux to the kinetic energy flux. The wave amplitude was systematically smaller than that observed in one-dimensional simulations. However, it continued to persist, even at a considerably low magnetization rate, where the Weibel instability dominated the shock transition. The emitted electromagnetic waves were sufficiently strong to disturb the upstream medium, and transverse filamentary density structures of substantial amplitude were produced. Based on this result, we discuss the possibility of the wakefield acceleration model to produce nonthermal electrons in a relativistic magnetized ion–electron shock.

Observations of large-amplitude electromagnetic waves and associated wave–particle interactions at the dipolarization front in the Earth's magnetotail: A case study

Broadband frequency waves around the dipolarization front (DF) are believed to play a crucial role in the particle dynamics. Using the Cluster observations, we report in this study large-amplitude electromagnetic waves with frequencies just above the ion cyclotron frequency at the DF in the near-Earth magnetotail region. The waves have very large amplitudes of magnetic and electric field fluctuations, up to ~2nT and ~10mV/m, respectively. The magnetic fluctuations are predominately along the ambient magnetic field (B0), while the electric fluctuations are primarily perpendicular to B0. The observed waves are highly oblique with a propagation angle of ~100° with respect to the ambient magnetic field, and are also linearly polarized. These features are consistent with the properties of the ion Bernstein wave mode in the high plasma β region, and also with the properties of current-driven ion cyclotron waves driven by the electromagnetic current-driven Alfven instability. We also discuss the possibility of wave–particle interactions at the DF.

An experimental study of the interaction between a pulsed electron beam and a large-amplitude electromagnetic wave

We experimentally investigate the interaction between an electron beam with a periodically varying diameter and a large-amplitude electromagnetic wave. The effect of different factors on the pulsed beam formation and current density in bunches is established. Compared with the electron beam deceleration circuits (low-voltage vircator systems), the generators based on pulsed turbulent beams have a broader band due to the formation of a large number of space charge bunches and an integral power efficiency that is higher by a factor of 2–2.5.

Artificial excitation of ELF waves with frequency of Schumann resonance

We report results from the experiment aimed at the artificial excitation of extremely low-frequency (ELF) electromagnetic waves with frequencies corresponding to the frequency of Schumann resonance. Electromagnetic waves with these frequencies can form a standing pattern inside the spherical cavity formed by the surface of the Earth and the ionosphere. In the experiment the ELF waves were excited by heating the ionosphere with X-mode HF electromagnetic waves generated at the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. The experiment demonstrates that heating of the ionosphere can excite relatively large-amplitude electromagnetic waves with frequencies in the range 7.8–8.0Hz when the ionosphere has a strong F layer, the frequency of the HF radiation is in the range 3.20–4.57MHz, and the electric field greater than 5mV/m is present in the ionosphere.

Large-amplitude electromagnetic waves in magnetized relativistic plasmas with temperature

Abstract. Propagation of large-amplitude waves in plasmas is subject to several sources of nonlinearity due to relativistic effects, either when particle quiver velocities in the wave field are large, or when thermal velocities are large due to relativistic temperatures. Wave propagation in these conditions has been studied for decades, due to its interest in several contexts such as pulsar emission models, laser-plasma interaction, and extragalactic jets. For large-amplitude circularly polarized waves propagating along a constant magnetic field, an exact solution of the fluid equations can be found for relativistic temperatures. Relativistic thermal effects produce: (a) a decrease in the effective plasma frequency (thus, waves in the electromagnetic branch can propagate for lower frequencies than in the cold case); and (b) a decrease in the upper frequency cutoff for the Alfvén branch (thus, Alfvén waves are confined to a frequency range that is narrower than in the cold case). It is also found that the Alfvén speed decreases with temperature, being zero for infinite temperature. We have also studied the same system, but based on the relativistic Vlasov equation, to include thermal effects along the direction of propagation. It turns out that kinetic and fluid results are qualitatively consistent, with several quantitative differences. Regarding the electromagnetic branch, the effective plasma frequency is always larger in the kinetic model. Thus, kinetic effects reduce the transparency of the plasma. As to the Alfvén branch, there is a critical, nonzero value of the temperature at which the Alfvén speed is zero. For temperatures above this critical value, the Alfvén branch is suppressed; however, if the background magnetic field increases, then Alfvén waves can propagate for larger temperatures. There are at least two ways in which the above results can be improved. First, nonlinear decays of the electromagnetic wave have been neglected; second, the kinetic treatment considers thermal effects only along the direction of propagation. We have approached the first subject by studying the parametric decays of the exact wave solution found in the context of fluid theory. The dispersion relation of the decays has been solved, showing several resonant and nonresonant instabilities whose dependence on the wave amplitude and plasma temperature has been studied systematically. Regarding the second subject, we are currently performing numerical 1-D particle in cell simulations, a work that is still in progress, although preliminary results are consistent with the analytical ones.

THE ROLE OF SUPERLUMINAL ELECTROMAGNETIC WAVES IN PULSAR WIND TERMINATION SHOCKS

The dynamics of a standing shock front in a Poynting-flux dominated relativistic flow is investigated by using a one-dimensional, relativistic, two-fluid simulation. An upstream flow containing a circularly polarized, sinusoidal magnetic shear wave is considered, mimicking a wave driven by an obliquely rotating pulsar. It is demonstrated that this wave is converted into large amplitude electromagnetic waves with superluminal phase speeds by interacting with the shock when the shock-frame frequency of the wave exceeds the proper plasma frequency. The superluminal waves propagate in the upstream, modify the shock structure substantially, and form a well-developed precursor region ahead of a subshock. Dissipation of Poynting flux occurs in the precursor as well as in the downstream region through a parametric instability driven by the superluminal waves. The Poynting flux remaining in the downstream region is carried entirely by the superluminal waves. The downstream plasma is therefore an essentially unmagnetized, relativistically hot plasma with a non-relativistic flow speed, as suggested by observations of pulsar wind nebulae.

Nonlocal theory of electromagnetic wave decay into two electromagnetic waves in a rippled density plasma channel

Parametric decay of a large amplitude electromagnetic wave into two electromagnetic modes in a rippled density plasma channel is investigated. The channel is taken to possess step density profile besides a density ripple of axial wave vector. The density ripple accounts for the momentum mismatch between the interacting waves and facilitates nonlinear coupling. For a given pump wave frequency, the requisite ripple wave number varies only a little w.r.t. the frequency of the low frequency decay wave. The radial localization of electromagnetic wave reduces the growth rate of the parametric instability. The growth rate decreases with the frequency of low frequency electromagnetic wave.

Relativistic x-ray quantum free-electron lasers: a collective Klein–Gordon model

We present a quantum-relativistic model for the nonlinear interaction between large-amplitude electromagnetic (EM) waves and a quantum plasma. The model is based on a collective Klein–Gordon equation for the relativistic electrons, which is coupled with the Maxwell equations for the EM and electrostatic fields. The model is used to derive a nonlinear dispersion relation for the EM scattering instabilities in a relativistic quantum plasma. With application to the quantum free-electron laser (FEL), a relativistic electron beam is passing through an intense EM wave used as a wiggler to produce coherent tunable radiation. The frequency upshift of the radiation is tuned by the energy of the electron beam. The nonlinear dispersion relation reveals the importance of quantum recoil effects and oblique scattering of the radiation on the gain of the quantum FEL.

Relativistic electron scattering by large amplitude electromagnetic ion cyclotron waves: The role of phase bunching and trapping

Test particle computations are performed to study the interactions of relativistic electrons with electromagnetic ion cyclotron waves propagating parallel to the background magnetic field in a homogeneous plasma. In addition to their pitch angle scattering, electrons experience phase bunching and trapping and become clustered in gyrophase angle when the wave amplitude is sufficiently large. The concepts of electron phase bunching and trapping are defined and first illustrated for electrons interacting with a monochromatic wave. When the wave amplitude is sufficiently large, the electrons in or nearly in resonance with the wave are mainly phase trapped while the off‐resonance electrons are only phase bunched. Phase trapping of electrons leads to nonlinear advection of the electrons toward the resonant pitch angle, at which the electrons with a specified kinetic energy are in resonance with the wave. For a broadband spectrum of incoherent waves, these general trends of electron phase bunching and trapping remain qualitatively the same, but they are not as evident as in the monochromatic wave case due to phase mixing of the electrons. The pitch angle diffusion coefficient and advection rate are evaluated from test particle computations of relativistic electron scattering by broadband waves. The results agree well with quasi‐linear results when the waves are sufficiently weak, as expected. When the waves become strong, however, the diffusion coefficient and advection rate deviate from the predictions of quasi‐linear theory, due to electron phase bunching and trapping. The diffusion coefficient is a flatter function of pitch angle than that predicted by quasi‐linear theory. Electron phase bunching and trapping introduce nonlinear advection of the electrons, which is additional to the inherent advection included in the diffusion equation of quasi‐linear theory and shifts the electrons toward the pitch angle at which the electrons with a specified kinetic energy are in resonance with the wave at the spectral peak. Finally, our results suggest that electron phase bunching and trapping by large amplitude waves flatten the pitch angle distribution and reduce the loss rate for relativistic electrons trapped in the radiation belts, compared to the predictions of quasi‐linear theory. The present study focuses only on local electron scattering in a homogeneous background magnetic field and the results are therefore subject to possible modification after bounce averaging.

Thermal effects on the propagation of large-amplitude electromagnetic waves in magnetized relativistic electron-positron plasma

The propagation of circularly polarized electromagnetic waves along a constant background magnetic field in an electron-positron plasma is calculated by means of both a fluid and a kinetic theory treatment. In the fluid theory, relativistic effects are included in the particle motion, the wave field, and in the thermal motion by means of a function f, which depends only on the plasma temperature. In this work we analyze the consistency of the fluid results with those obtained from a kinetic treatment, based on the relativistic Vlasov equation. The corresponding kinetic dispersion relation is numerically studied for various temperatures, and results are compared with the fluid treatment. Analytic expressions for the Alfvén velocity are obtained for the fluid and kinetic models, and it is shown that, in the kinetic treatment, the Alfvén branch is suppressed for large temperatures.