arXiv subjects

Masanori Iwamoto

Publications and source records attributed to Masanori Iwamoto.

17 recordsLinked to original sources

arXiv2026

Induced Scattering of Strong Waves in Pair Plasmas

We study induced (stimulated) scattering of linearly polarized, strong electromagnetic waves in pair plasmas, which is crucial for understanding the propagation of fast radio bursts (FRBs). Magnetars are the most promising progenitors of FRBs, and FRBs propagate through the magnetar wind and successfully escape before being significantly scattered. We revisit the steady-state solution of linearly polarized electromagnetic waves in pair plasmas with arbitrary amplitude, and demonstrate that the nonlinearity is characterized by the nonlinearity parameter $a_0ω_{pe}/ω_0$ rather than the dimensionless amplitude $a_0$, where $ω_{pe}$ is the electron plasma frequency and $ω_0$ is the wave frequency. We follow the time evolution of the steady-state solution for the linear regime $a_0ω_{pe}/ω_0 \ll 1$ by performing one-dimensional particle-in-cell simulations, and show that the conventional linear analysis of induced scattering assuming $a_0 \ll 1$ is applicable even for $a_0 > 1$ when the Lorentz boost due to the plasma motion in the incident wave is considered. The saturation level is controlled by $a_0ω_0/ω_{pe}$, which corresponds to the ratio of the wave energy to the plasma energy, and the incident wave is hardly scattered for $a_0ω_0/ω_{pe} \gg 1$. We discuss the application of our results to FRBs.

astro-ph.HE↗

arXiv2026

Interaction of Strong Electromagnetic Waves with Unmagnetized Pair Plasmas

We investigate analytically and numerically the interaction of strong electromagnetic waves with unmagnetized pair plasmas. We show that the interaction is governed by a single nonlinearity parameter, $\varepsilon_{\rm p}$, defined as the ratio of the wave strength parameter to the wave frequency in units of the plasma frequency (with both frequencies measured in the plasma rest frame prior to the interaction). When $\varepsilon_{\rm p}<1$, the number of wavelengths that propagate through the plasma without attenuation from induced Compton scattering is approximately $\varepsilon_{\rm p}^{-2/3}$. This attenuation can imprint sub-structures as narrow as a few wavelengths on the pulse profile. When $\varepsilon_{\rm p}>1$, the electromagnetic pulse acts as a relativistic piston and drives a shock into the plasma. Our results establish a framework for the interaction of strong electromagnetic waves with pair plasmas, a process relevant for intense radio pulses from neutron stars and for next-generation pair plasma experiments at multi-petawatt laser facilities.

physics.plasm-ph↗

arXiv2026

One-dimensional PIC Simulation of Induced Compton Scattering in Magnetized Electron-Positron Pair Plasma

We investigate induced Compton scattering of a circularly polarized Alfvén wave propagating in a magnetized electron-positron pair plasma using one-dimensional Particle-in-Cell (PIC) simulations. In this system, two distinct modes of density fluctuations, referred to as the charged mode and the neutral mode, are theoretically expected to arise through parametric instabilities. Our simulations confirm these predictions: in the charged mode, the electron and positron densities fluctuate oppositely (Langmuir-like), while in the neutral mode, the charge is Debye-screened and both species fluctuate in phase (acoustic-like). The linear growth rates obtained from the simulations are in good agreement with analytical estimates for both modes. We also find that, in some cases, the linear growth saturates before full scattering occurs, allowing the incident wave to propagate without significant attenuation. Our results allow us to determine whether induced Compton scattering grows linearly in magnetized pair plasmas, offering a foundation for studies of fast radio bursts and laser-plasma experiments.

astro-ph.HE↗

arXiv2025

Filamentation of the electromagnetic precursor in relativistic quasi-perpendicular electron-positron shocks

We present a scenario that could explain non-thermal particle acceleration in relativistic quasi-perpendicular electron-positron shocks, such as the termination shock of pulsar wind nebulae. The shock produces a strong electromagnetic precursor that propagates into the upstream plasma, which is initially threaded by a uniform background magnetic field. We show that the filamentation instability breaks the precursor into radiation filaments parallel to the shock normal. The transverse scale of the filaments is of the order of a few plasma skin depths. In the shock frame, the bulk Lorentz factor of the upstream plasma is significantly reduced inside the radiation filaments. Then, the instability produces a relativistic shear flow with strong velocity gradients on kinetic scales. The velocity gradients distort the background magnetic field lines, and generate a magnetic field component parallel to the shock normal that reverses across each radiation filament, a configuration that could trigger magnetic reconnection in the upstream plasma. These effects may accelerate particles before the plasma enters the shock.

astro-ph.HE↗

arXiv2025

Induced Compton scattering in magnetized electron and positron pair plasma

A formulation for the parametric instability of electromagnetic (EM) waves in magnetized pair plasma is developed. The linear growth rate of induced Compton scattering is derived analytically for frequencies below the cyclotron frequency for the first time. We identify three modes of density fluctuation: ordinary, charged, and neutral modes. In the charged mode, the ponderomotive force separates charges (electrons and positrons) longitudinally, in contrast to the nonmagnetized case. We also recognize two effects that significantly reduce the scattering rate for waves polarized perpendicular to the magnetic field: (1) the gyroradius effect due to the magnetic suppression of particle orbits, and (2) Debye screening for wavelengths larger than the Debye length. Applying this to fast radio bursts (FRBs), we find that these effects facilitate the escape of X-mode waves from the magnetosphere and outflow of a magnetar and neutron star, enabling 100\% polarization as observed. Our formulation provides a foundation for consistently addressing the nonlinear interaction of EM waves with magnetized plasma in astrophysics and laser physics.

astro-ph.HE↗

arXiv2024

Escape of fast radio bursts from magnetars

Fast radio bursts (FRBs) are bright extragalactic transients likely produced by magnetars. We study the propagation of FRBs in magnetar winds, assuming that the wind is strongly magnetized and composed of electron-positron pairs. We focus on the regime where the strength parameter of the radio wave, $a_0$, is larger than unity, and the wave frequency, $ω_0$, is larger than the Larmor frequency in the background magnetic field, $ω_{\rm L}$. We show that strong radio waves with $a_0>1$ are able to propagate when $ω_0 > a_0ω_{\rm L}$, as the plasma current is a linear function of the wave electric field. The dispersion relation is independent of the wave strength parameter when $ω_0 > a_0ω_{\rm L}$. Instead, radio waves could be damped when $ω_0 < a_0ω_{\rm L}$, as a significant fraction of the wave energy is used to compress the plasma and amplify the background magnetic field. Our results suggest that FRBs should be produced at large distances from the magnetar (i.e., $R>10^{12}{\rm\; cm}$, where the condition $ω_0 > a_0ω_{\rm L}$ is satisfied). Alternatively, the structure of the magnetar wind should be strongly modified during a flare to allow the escape of FRBs produced at radii $R<10^{12}{\rm\; cm}$.

astro-ph.HE↗

arXiv2024

Propagation of strong electromagnetic waves in tenuous plasmas

We study the propagation of electromagnetic waves in tenuous plasmas, where the wave frequency, $ω_0$, is much larger than the plasma frequency, $ω_{\rm P}$. We show that in pair plasmas nonlinear effects are weak for $a_0 \ll ω_0/ω_{\rm P}$, where $a_0$ is the wave strength parameter. In electron-proton plasmas a more restrictive condition must be satisfied, namely either $a_0\ll 1/ω_{\rm P}τ_0$, where $τ_0$ is the duration of the radiation pulse, or $a_0\ll 1$. We derive the equations that govern the evolution of the pulse in the weakly nonlinear regime. Our results have important implications for the modeling of fast radio bursts. We argue that: (i) Millisecond duration bursts with a smooth profile must be produced in a proton-free environment, where nonlinear effects are weaker. (ii) Propagation through an electron-proton plasma near the source can imprint a sub-microsecond variability on the burst profile.

astro-ph.HE↗

arXiv2023

Linearly-polarized Coherent Emission from Relativistic Magnetized Ion-electron Shocks

Fast radio bursts (FRBs) are millisecond transient astrophysical phenomena and bright at radio frequencies. The emission mechanism, however, remains unsolved yet. One scenario is a coherent emission associated with the magnetar flares and resulting relativistic shock waves. Here, we report unprecedentedly large-scale simulations of relativistic magnetized ion-electron shocks, showing that strongly linear-polarized electromagnetic waves are excited. The kinetic energy conversion to the emission is so efficient that the wave amplitude is responsible for the brightness. We also find a polarization angle swing reflecting shock front modulation, implicating the polarization property of some repeating FRBs. The results support the shock scenario as an origin of the FRBs.

astro-ph.HE↗

arXiv2023

Kinetic Simulations of the Filamentation Instability in Pair Plasmas

The nonlinear interaction between electromagnetic waves and plasmas attracts significant attention in astrophysics because it can affect the propagation of Fast Radio Bursts (FRBs) -- luminous millisecond-duration pulses detected at radio frequency. The filamentation instability (FI) -- a type of nonlinear wave-plasma interaction -- is considered to be dominant near FRB sources, and its nonlinear development may also affect the inferred dispersion measure of FRBs. In this paper, we carry out fully kinetic particle-in-cell simulations of the FI in unmagnetized pair plasmas. Our simulations show that the FI generates transverse density filaments, and that the electromagnetic wave propagates in near vacuum between them, as in a waveguide. The density filaments keep merging until force balance between the wave ponderomotive force and the plasma pressure gradient is established. We estimate the merging timescale and discuss the implications of filament merging for FRB observations.

astro-ph.HE↗

arXiv2023

Saturation of the filamentation instability and dispersion measure of Fast Radio Bursts

Nonlinear effects are crucial for the propagation of Fast Radio Bursts (FRBs) near the source. We study the filamentation of FRBs in the relativistic winds of magnetars, which are commonly invoked as the most natural FRB progenitors. As a result of filamentation, the particle number density and the radiation intensity develop strong gradients along the direction of the wind magnetic field. A steady state is reached when the plasma pressure balances the ponderomotive force. In such a steady state, particles are confined into periodically spaced thin sheets, and electromagnetic waves propagate between them as in a waveguide. We show that: (i) The dispersion relation resembles that in the initial homogeneous plasma, but the effective plasma frequency is determined by the separation of the sheets, not directly by the mean particle density. (ii) The contribution of relativistic magnetar winds to the dispersion measure of FRBs could be several orders of magnitude larger than previously thought. The dispersion measure of the wind depends on the properties of individual bursts (e.g. the luminosity), and therefore can change significantly among different bursts from repeating FRBs. (iii) Induced Compton scattering is suppressed because most of the radiation propagates in near vacuum regions.

astro-ph.HE↗

arXiv2021

Particle Acceleration by Pickup Process Upstream of Relativistic Shocks

Particle acceleration at magnetized purely perpendicular relativistic shocks in electron-ion plasmas are studied by means of two-dimensional particle-in-cell simulations. Magnetized shocks with the upstream bulk Lorentz factor $γ_1 \gg 1$ are known to emit intense electromagnetic waves from the shock front, which induce electrostatic plasma waves (wakefield) and transverse filamentary structures in the upstream region via the stimulated/induced Raman scattering and the filamentation instability, respectively. The wakefield and filaments inject a fraction of incoming particles into a particle acceleration process, in which particles are once decoupled from the upstream bulk flow by the wakefield, and are piked up again by the flow. The picked-up particles are accelerated by the motional electric field. The maximum attainable Lorentz factor is estimated as $γ_{max,e} \sim αγ_1^3$ for electrons and $γ_{max,i} \sim (1+m_eγ_1/m_i)γ_1^2$ for ions, where $α\sim 10$ is determined from our simulation results. $α$ can increase up to $γ_1$ for weakly magnetized shock if $γ_1$ is sufficiently large. This result indicates that highly relativistic astrophysical shocks such as external shocks of gamma-ray bursts can be an efficient particle accelerator.

astro-ph.HE↗

arXiv2021

Mildly relativistic magnetized shocks in electron-ion plasmas -- II. Particle acceleration and heating

Particle acceleration and heating at mildly relativistic magnetized shocks in electron-ion plasma are investigated with unprecedentedly high-resolution two-dimensional particle-in-cell simulations that include ion-scale shock rippling. Electrons are super-adiabatically heated at the shock, and most of the energy transfer from protons to electrons takes place at or downstream of the shock. We are the first to demonstrate that shock rippling is crucial for the energization of electrons at the shock. They remain well below equipartition with the protons. The downstream electron spectra are approximately thermal with a limited supra-thermal power-law component. Our results are discussed in the context of wakefield acceleration and the modelling of electromagnetic radiation from blazar cores.

astro-ph.HE↗

arXiv2020

Mildly relativistic magnetized shocks in electron-ion plasmas I. Electromagnetic shock structure

Mildly relativistic shocks in magnetized electron-ion plasmas are investigated with 2D kinetic particle-in-cell simulations of unprecedentedly high resolution and large scale for conditions that may be found at internal shocks in blazar cores. Ion-scale effects cause corrugations along the shock surface whose properties somewhat depend on the configuration of the mean perpendicular magnetic field, that is either in or out of the simulation plane. We show that the synchrotron maser instability persists to operate in mildly relativistic shocks in agreement with theoretical predictions and produces coherent emission of upstream-propagating electromagnetic waves. Shock front ripples are excited in both mean-field configurations and they engender effective wave amplification. The interaction of these waves with upstream plasma generates electrostatic wakefields.

astro-ph.HE↗

arXiv2019

Precursor Wave Amplification by Ion-Electron Coupling through Wakefield in Relativistic Shocks

We investigated electromagnetic precursor wave emission in relativistic shocks by using two-dimensional particle-in-cell simulations. We found that the wave amplitude is significantly enhanced by a positive feedback process associated with ion-electron coupling through the wakefields for high magnetization. The wakefields collapse during the nonlinear process of the parametric decay instability in the near-upstream region, where nonthermal electrons and ions are generated. The intense coherent emission and the particle acceleration may opperate in high-energy astrophysical objects.

astro-ph.HE↗

arXiv2019

The Efficiency of Coherent Radiation from Relativistic Shocks

We discuss a mechanism for intense electromagnetic wave emission at an astrophysical relativistic shock in a magnetized collisionless plasma. At the magnetized shock, the particle reflection by a compressed magnetic field of the shock produces a ring-like distribution in momentum, which gives rise to plasma instabilities. Intense and coherent high-frequency electromagnetic waves will be emitted if the synchrotron maser instability (SMI) is excited, whereas non-propagating magnetic fluctuations will be generated when the Weibel instability (WI) is the dominant mode. The problem is of great astrophysical interest because if intense radiation is emitted, the interaction with the upstream medium induces a large-amplitude electrostatic field (or Wakefield), which may play a role for the acceleration of ultra-high-energy cosmic rays. We review our recent effort to measure the efficiency of the electromagnetic wave emission using fully self-consistent, two-dimensional (2D) particle-in-cell (PIC) simulations for pair plasmas. We found that the emission efficiency in 2D was systematically lower than one dimensional (1D) PIC simulation results. However, the power remains finite even when the WI is active to generate large-amplitude magnetic fluctuations. Astrophysical implications of the present results are briefly discussed.

astro-ph.HE↗

arXiv2018

Precursor Wave Emission Enhanced by Weibel Instability in Relativistic Shocks

We investigated the precursor wave emission efficiency in magnetized purely perpendicular relativistic shocks in pair plasmas. We extended our previous study to include the dependence of upstream magnetic field orientations. We performed two-dimensional particle-in-cell simulations and focused on two magnetic field orientations: the magnetic field to be in the simulation plane (i.e., in-plane configuration) and perpendicular to the simulation plane (i.e., out-of-plane configuration). Our simulations in the in-plane configuration demonstrated that not only extraordinary but also ordinary mode waves are excited. We quantified the emission efficiency as a function of the magnetization parameter $σ_e$ and found that the large-amplitude precursor waves are emitted for a wide range of $σ_e$. We found that especially at low $σ_e$, the magnetic field generated by Weibel instability amplifies the ordinary mode wave power. The amplitude is large enough to perturb the upstream plasma, and transverse density filaments are generated as in the case of the out-of-plane configuration investigated in the previous study. We confirmed that our previous conclusion holds regardless of upstream magnetic field orientations with respect to the two-dimensional simulation plane. We discuss the precursor wave emission in three dimensions and the feasibility of wakefield acceleration in relativistic shocks based on our results.

astro-ph.HE↗

arXiv2017

Persistence of precursor waves in two-dimensional relativistic shocks

We investigated the efficiency of coherent upstream large-amplitude electromagnetic wave emission via synchrotron maser instability at relativistic magnetized shocks by using two-dimensional particle-in-cell simulations. We considered the 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 by 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 for the production of non-thermal electrons in a relativistic magnetized ion-electron shock.

astro-ph.HE↗