arXiv subjects

Takanobu Amano

Publications and source records attributed to Takanobu Amano.

At least 19 recordsLinked to original sources

arXiv2026

PIC simulations of nonrelativistic high-Mach-number oblique shocks propagating in a turbulent medium

Collisionless shocks are common in astrophysical systems and stand as sites of particle acceleration. While particles at perpendicular shocks may not return to the upstream region, at oblique shocks a fraction of energetic electrons manage to escape the shock and travel upstream. An extended region known as the electron foreshock is formed, where these reflected particles drive various instabilities that may promote electron acceleration. Here we present the first 2D3V particle-in-cell (PIC) simulations of electron-ion non-relativistic oblique shocks that explore the interaction of the foreshock with pre-existing compressive turbulence with relative amplitude of 15% based on interstellar medium estimates. We find that pre-existing turbulence influences the emergence and behavior of the whistler-wave instability, as it enhances the amplitudes of the magnetic-field fluctuations and leads to larger nonlinear structures. This impacts the dynamics of the reflected electrons, resulting in a shorter and hotter electron foreshock. At the end of our simulations, with pre-existing upstream turbulence we observe non-thermal electrons that are more numerous, reach higher energies, and carry a larger portion of the total energy.

physics.plasm-ph↗

arXiv2026

Generation of Whistler Waves by Reflected Electrons and Their Self-Confinement at Quasi-Perpendicular Shocks

We investigate the mechanism of whistler-mode wave generation by shock-reflected electrons at quasi-perpendicular collisionless shocks. By employing Liouville mapping to construct the electron velocity distribution function in the shock and performing linear instability analysis, we explore whistler wave generation by the mirror-reflected electrons near the upstream edge of the shock transition layer. We find that the reflected electrons can excite two distinct instabilities with different propagation directions when both the upstream electron beta $β_e$ and Alfven Mach number in the de Hoffmann-Teller frame $M_A/\cosθ_{bn}$ are sufficiently large, where $M_A$ is \Alfven Mach number and $θ_{bn}$ is the angle between the upstream magnetic field and the shock normal. In the parameter regime of Earth's bow shock, the instability threshold condition is roughly given by $M_A/\cosθ_{bn}\gtrsim50$. Since such shocks are super-critical with respect to the whistler critical Mach number, the generated waves cannot propagate upstream and will accumulate in the transition layer. Furthermore, we find that the pitch-angle scattering by the generated waves may trigger secondary instabilities on the same branch. We suggest that the sequence of instabilities likely happening within the shock transition layer can efficiently scatter the reflected electrons over a broad range of pitch angles. Consequently, the reflected electrons may be confined within the shock by the waves generated by themselves. The self-confinement provides the necessary ingredient of stochastic shock drift acceleration, which then offers a plausible mechanism for the electron injection into diffusive shock acceleration.

physics.plasm-ph↗

arXiv2024

Saturation Level of Ion Weibel Instability and Isotropization Length Scale in Electron-Ion Weibel-Mediated Shocks

Ion Weibel instability is considered to be the dominant physics for the dissipation in high-Mach number astrophysical shocks such as supernova remnant shocks and gamma-ray burst shocks. We study the instability dependence on various parameters using theory and particle-in-cell simulations. We demonstrate that electron physics determines the saturation level of the Weibel-generated magnetic field, even though the instability is driven by the ions. We discuss the application to astrophysical and laboratory laser experiment environments to clarify the roles of the ion Weibel instability. We develop a model for the isotropization length scale in Weibel-mediated shocks and compare its value to other characteristic length scales of each system. We find that electron heating to near equipartition is crucial for the formation of ultra-relativistic Weibel-mediated shocks. On the other hand, our results imply that non-relativistic shocks in typical interstellar medium are not purely mediated by the Weibel instability.

astro-ph.HE↗

arXiv2024

Statistical Analysis of High-frequency Whistler Waves at Earth's Bow Shock: Further Support for Stochastic Shock Drift Acceleration

We statistically investigate high-frequency whistler waves (with frequencies higher than $\sim 10$ % of the local elect ron cyclotron frequency) at Earth's bow shock using Magnetospheric Multi-Scale (MMS) spacecraft observations. We focus specifically on the wave power within the shock transition layer, where we expect electron acceleration via stochastic sh ock drift acceleration (SSDA) to occur associated with efficient pitch-angle scattering by whistler waves. We find that the wave power is positively correlated with both the Alfvén Mach number in the normal incidence frame $M_{\rm A}$ and in the de Hoffmann-Teller frame $M_{\rm A}/\cos θ_{Bn}$. The empirical relation with $M_{\rm A}/\cos θ_{Bn}$ is compared with the theory of SSDA that predicts a threshold wave power proportional to $(M_{\rm A}/\cos θ_{Bn})^{-2}$. The result suggests that the wave power exceeds the theoretical threshold for $M_{\rm A} / \cos θ_{Bn} \gtrsim 30-60$, beyond which efficient electron acceleration is expected. This aligns very well with previous statistical analysis of electron acceleration at Earth's bow shock (M. Oka, G eophys.~Res.~Lett., 33, 5, 2006). Therefore, we consider that this study provides further support for SSDA as the mechanism of electron acceleration at Earth's bow shock. At higher-Mach-number astrophysical shocks, SSDA will be able to inject electrons into the diffusive shock acceleration process for subsequent acceleration to cosmic-ray energies.

physics.space-ph↗

arXiv2023

Enhanced Magnetic Field Amplification by Ion-Beam Weibel Instability in Weakly Magnetized Astrophysical Shocks

We examine the evolution of ion-beam Weibel instability at strong collisionless shocks in weakly magnetized media. We find that a finite background magnetic field substantially affects both linear and nonlinear phases of the instability, depending on whether the background electrons behave magnetized or not. Particle-in-cell simulations for magnetized electrons identify a dynamo-like mechanism of magnetic field amplification, which eventually leads to spontaneous magnetic reconnection. We conclude that this scenario is applicable to typical young supernova remnant shocks.

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

Electron-Ion Temperature Ratio in Astrophysical Shocks

Collisionless shock waves in supernova remnants and the solar wind heat electrons less effectively than they heat ions, as is predicted by kinetic simulations. However, the values of T$_e$/T$_p$ inferred from the H alpha profiles of supernova remnant shocks behave differently as a function of Mach number or Alfvén Mach number than what is measured in the solar wind or predicted by simulations. Here we determine T$_e$/T$_p$ for supernova remnant shocks using H alpha profiles, shock speeds from proper motions, and electron temperatures from X-ray spectra. We also improve the estimates of sound speed and Alfvén speed used to determine Mach numbers. We find that the H alpha determinations are robust and that the discrepancies among supernova remnant shocks, solar wind shocks and computer-simulated shocks remain. We discuss some possible contributing factors, including shock precursors, turbulence and varying preshock conditions.

astro-ph.GA↗

arXiv2022

Theory of Electron Injection at Oblique Shock of Finite Thickness

A theory of electron injection into diffusive shock acceleration (DSA) for the generation of cosmic-ray electrons at collisionless shocks is presented. We consider a recently proposed particle acceleration mechanism called stochastic shock drift acceleration (SSDA). We find that SSDA may be understood as a diffusive particle acceleration mechanism at an oblique shock of finite thickness. More specifically, it is described by a solution to the diffusion-convection equation for particles with the characteristic diffusion length comparable to the shock thickness. On the other hand, the same equation yields the standard DSA if the diffusion length is much longer than the thickness. Although SSDA predicts, in general, a spectral index steeper than DSA, it is much more efficient for low-energy electron acceleration and is favorable for injection. The injection threshold energy corresponds to the transition energy between the two different regimes. It is of the order of $0.1\text{-}1$ MeV in typical interstellar and interplanetary conditions if the dissipation scale of turbulence around the shock is determined by the ion inertial length. The electron injection is more efficient at high $M_\textrm{A} / \cos θ_{Bn}$ where $M_\textrm{A}$ and $θ_{Bn}$ are the Alfvén Mach number and the shock obliquity. The theory suggests that efficient acceleration of electrons to ultra-relativistic energies will be more easily realized at high-Mach-number young supernova remnant shocks, but not at weak or moderate shocks in the heliosphere unless the upstream magnetic field is nearly perpendicular to the shock normal.

astro-ph.HE↗

arXiv2021

Critical comparison of collisionless fluid models: Nonlinear simulations of parallel firehose instability

Two different fluid models for collisionless plasmas are compared. One is based on the classical Chew-Goldberger-Low (CGL) model that includes a finite Larmor radius (FLR) correction and the Landau closure for the longitudinal mode. Another one takes into account the effect of cyclotron resonance in addition to Landau resonance, which is referred to as the cyclotron resonance closure (CRC) model. While the linear property of the parallel firehose instability is better described by the CGL model, the electromagnetic ion cyclotron instability driven unstable by the cyclotron resonance is reproduced only by the CRC model. Nonlinear simulation results for the parallel firehose instability performed with the two models are also discussed. Although the linear and quasilinear isotropization phases are consistent with theory in both models, long-term behaviors may be substantially different. The final state obtained by the CRC model may be reasonably understood in terms of the marginal stability condition. In contrast, the lack of cyclotron damping in the CGL model makes it rather difficult to predict the long-term behavior with a simple physical argument. This suggests that incorporating the collisionless damping both for longitudinal and transverse modes is crucial for a nonlinear fluid simulation model of collisionless plasmas.

physics.plasm-ph↗

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

Mach Number Dependence of Ion-scale Kinetic Instability at Collisionless Perpendicular Shock: Condition for Weibel-dominated Shock

We investigate ion-scale kinetic plasma instabilities at the collisionless shock using linear theory and nonlinear Particle-in-Cell (PIC) simulations. We focus on the Alfvén-ion-cyclotron (AIC), mirror, and Weibel instabilities, which are all driven unstable by the effective temperature anisotropy induced by the shock-reflected ions within the transition layer of a strictly perpendicular shock. We conduct linear dispersion analysis with a homogeneous plasma model to mimic the shock transition layer by adopting a ring distribution with finite thermal spread to represent the velocity distribution of the reflected ions. We find that, for wave propagation parallel to the ambient magnetic field, the AIC instability at lower Alfvén Mach numbers tends to transition to the Weibel instability at higher Alfvén Mach numbers. The instability property is, however, also strongly affected by the sound Mach number. We conclude that the instability at a strong shock with Alfvén and sound Mach numbers both in excess of $\sim 20{\rm -}40$ may be considered as Weibel-like in the sense that the reflected ions behave essentially unmagnetized. Two-dimensional PIC simulations confirm the linear theory and find that, with typical parameters of young supernova remnant shocks, the ring distribution model produces magnetic fluctuations of the order of the background magnetic field, which is smaller than those observed in previous PIC simulations for Weibel-dominated shocks. This indicates that the assumption of the gyrotropic reflected ion distribution may not be adequate to quantitatively predict nonlinear behaviors of the dynamics in high Mach number shocks.

physics.plasm-ph↗

arXiv2021

Electron Acceleration at Rippled Low-Mach-number Shocks in High-beta Collisionless Cosmic Plasmas

Using large-scale fully-kinetic two-dimensional particle-in-cell simulations, we investigate the effects of shock rippling on electron acceleration at low-Mach-number shocks propagating in high-$β$ plasmas, in application to merger shocks in galaxy clusters. We find that the electron acceleration rate increases considerably when the rippling modes appear. The main acceleration mechanism is stochastic shock-drift acceleration, in which electrons are confined at the shock by pitch-angle scattering off turbulence and gain energy from the motional electric field. The presence of multi-scale magnetic turbulence at the shock transition and the region immediately behind the main shock overshoot is essential for electron energization. Wide-energy non-thermal electron distributions are formed both upstream and downstream of the shock. The maximum energy of the electrons is sufficient for their injection into diffusive shock acceleration. We show for the first time that the downstream electron spectrum has a~power-law form with index $p\approx 2.5$, in agreement with observations.

astro-ph.HE↗

arXiv2021

A non-local fluid closure for modeling cyclotron resonance in collisionless magnetized plasmas

A fluid description for collisionless magnetized plasmas that takes into account the effect of cyclotron resonance has been developed. Following the same approach as the Landau fluid closure, the heat flux components associated with transverse electromagnetic fluctuations are approximated by a linear combination of lower-order moments in wavenumber space. The closure successfully reproduces the linear cyclotron resonance for electromagnetic waves propagating parallel to the ambient magnetic field. In the presence of finite temperature anisotropy, the model gives approximately correct prediction for an instability destabilized via the cyclotron resonance. A nonlinear simulation demonstrates the wave growth consistent with the linear theory followed by the reduction of initial anisotropy, and finally, the saturation of the instability. The isotropization may be understood in terms of quasi-linear theory, which is developed within the framework of the fluid model but very similar to its fully kinetic counterpart. The result indicates that both linear and nonlinear collisionless plasma responses are approximately incorporated in the fluid model.

physics.plasm-ph↗

arXiv2021

Magnetic field amplification by the Weibel instability at planetary and astrophysical high-Mach-number shocks

Collisionless shocks are ubiquitous in the Universe and often associated with strong magnetic field. Here we use large-scale particle-in-cell simulations of non-relativistic perpendicular shocks in the high-Mach-number regime to study the amplification of magnetic field within shocks. The magnetic field is amplified at the shock transition due to the ion-ion two-stream Weibel instability. The normalized magnetic-field strength strongly correlates with the Alfvénic Mach number. Mock spacecraft measurements derived from PIC simulations are fully consistent with those taken in-situ at Saturn's bow shock by the Cassini spacecraft.

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↗

arXiv2020

Kinetic simulation of nonrelativistic perpendicular shocks of young supernova remnants. IV. Electron heating

High Mach number collisionless shocks are found in planetary systems and supernova remnants (SNRs). Electrons are heated at these shocks to the temperature well above the Rankine-Hugoniot prediction. However processes responsible for electron heating are still not well understood. We use a set of large-scale Particle-In-Cell simulations of non-relativistic shocks in high Mach number regime to clarify the electron heating processes. The physics of these shocks is defined by ion reflection at the shock ramp. Further interaction of the reflected ions and the upstream plasma excites electrostatic Buneman and two-stream ion-ion Weibel instabilities. Electrons are heated via shock surfing acceleration, the shock potential, magnetic reconnection, stochastic Fermi scattering and the shock compression. The main contributor is the shock potential. Magnetic field lines are tangled due to the Weibel instability, which allows the parallel electron heating by the shock potential. The constrained model of the electron heating predicts the ion-to-electron temperature ratio within observed values at SNR shocks and in Saturn's bow shock.

astro-ph.HE↗

arXiv2020

Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. III. Magnetic reconnection

Fully kinetic two-dimensional particle-in-cell simulations are used to study electron acceleration at high-Mach-number nonrelativistic perpendicular shocks. SNR shocks are mediated by the Weibel instability which is excited because of an interaction between shock-reflected and upstream ions. Nonlinear evolution of the Weibel instability leads to the formation of current sheets. At the turbulent shock ramp the current sheets decay through magnetic reconnection. The number of reconnection sites strongly depends on the ion-to-electron mass ratio and the Alfvénic Mach number of the simulated shock. Electron acceleration is observed at locations where magnetic reconnection operates. For the highest mass ratios almost all electrons are involved in magnetic reconnection, which makes the magnetic reconnection the dominant acceleration process for electrons at these shocks. We discuss the relevance of our results for 3D systems with realistic ion-to-electron mass ratio.

astro-ph.HE↗