arXiv subjects

Masahiro Hoshino

Publications and source records attributed to Masahiro Hoshino.

At least 37 records · Page 2Linked to original sources

arXiv2021

Anisotropic heating and magnetic field generation due to Raman scattering in laser-plasma interactions

We identify a mechanism for magnetic field generation in the interaction of intense electromagnetic waves and underdense plasmas. We show that Raman scattered plasma waves trap and heat the electrons preferentially in their propagation direction, resulting in a temperature anisotropy. In the trail of the laser pulse, we observe magnetic field growth which matches the Weibel mechanism due to the temperature anisotropy. We discuss the role of the initial electron temperature in our results. The predictions are confirmed with multi-dimensional particle-in-cell simulations. We show how this configuration is an experimental platform to study the long-time evolution of the Weibel instability.

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

Nonthermal electron and ion acceleration by magnetic reconnection in large laser-driven plasmas

Magnetic reconnection is a fundamental plasma process that is thought to play a key role in the production of nonthermal particles associated with explosive phenomena in space physics and astrophysics. Experiments at high-energy-density facilities are starting to probe the microphysics of reconnection at high Lundquist numbers and large system sizes. We have performed particle-in-cell (PIC) simulations to explore particle acceleration for parameters relevant to laser-driven reconnection experiments. We study particle acceleration in large system sizes that may be produced soon with the most energetic laser drivers available, such as at the National Ignition Facility. In these conditions, we show the possibility of reaching the multi-plasmoid regime, where plasmoid acceleration becomes dominant. Our results show the transition from \textit{X} point to plasmoid-dominated acceleration associated with the merging and contraction of plasmoids that further extend the maximum energy of the power-law tail of the particle distribution for electrons. We also find for the first time a system-size-dependent emergence of nonthermal ion acceleration in driven reconnection, where the magnetization of ions at sufficiently large sizes allows them to be contained by the magnetic field and energized by direct \textit{X} point acceleration. For feasible experimental conditions, electrons and ions can attain energies of $ε_{max,e} / k_{B} T_{e} > 100$ and $ε_{max,i} / k_{B} T_{i} > 1000$. Using PIC simulations with binary Monte Carlo Coulomb collisions we study the impact of collisionality on plasmoid formation and particle acceleration. The implications of these results for understanding the role reconnection plays in accelerating particles in space physics and astrophysics are discussed.

physics.plasm-ph↗

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

Stabilization of Magnetic Reconnection in Relativistic Current Sheet

Recently, magnetic reconnection, where a relativistically hot plasma is confined by a strong magnetic field, has received great attention in relation to astrophysical objects (e.g., pulsar magnetosphere and magnetar). However, reconnection with relativistic high-speed drift current in the plasma sheet has not been investigated yet. Thus, from both the theoretical and computational points of view, we studied the growth rate of relativistic reconnection for the high-speed drift current in a relativistically hot plasma sheet. Consequently, we argue that, contrary to the conventional understanding of the fast energy dissipation by the relativistic reconnection, the growth of magnetic reconnection is suppressed.

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↗

arXiv2019

PIC Simulation Methods for Cosmic Radiation and Plasma Instabilities

Particle acceleration in collisionless plasma systems is a central question in astroplasma and astroparticle physics. The structure of the acceleration regions, electron-ion energy equilibration, preacceleration of particles at shocks to permit further energization by diffusive shock acceleration, require knowledge of the distribution function of particles besides the structure and dynamic of electromagnetic fields, and hence a kinetic description is desirable. Particle-in-cell simulations offer an appropriate, if computationally expensive method of essentially conducting numerical experiments that explore kinetic phenomena in collisionless plasma. We review recent results of PIC simulations of astrophysical plasma systems, particle acceleration, and the instabilities that shape them.

astro-ph.HE↗

arXiv2019

Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. II. Influence of shock-surfing acceleration on downstream electron spectra

We explore electron pre-acceleration at high Mach-number nonrelativistic perpendicular shocks at, e.g., young supernova remnants, which are a prerequisite of further acceleration to very high energies via diffusive shock acceleration. Using fully kinetic particle-in-cell simulations of shocks and electron dynamics in them, we investigate the influence of shock-surfing acceleration at the shock foot on the nonthermal population of electrons downstream of the shock. The shock-surfing acceleration is followed by further energization at the shock ramp where the Weibel instability spawns a type of second-order Fermi acceleration. The combination of these two processes leads to the formation of a nonthermal electron population, but the importance of shock-surfing acceleration becomes smaller for larger ion-to-electron mass ratio in the simulation. We discuss the resulting electron spectra and the relevance of our results to the physics of systems with real ion-to-electron mass ratio and fully three-dimensional behavior.

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

Particle Energization in Space Plasmas: Towards a Multi-Point, Multi-Scale Plasma Observatory. A White Paper for the Voyage 2050 long-term plan in the ESA's Science Programme

This White Paper outlines the importance of addressing the fundamental science theme < > through a future ESA mission. The White Paper presents five compelling science questions related to particle energization by shocks, reconnection,waves and turbulence, jets and their combinations. Answering these questions requires resolving scale coupling, nonlinearity and nonstationarity, which cannot be done with existing multi-point observations. In situ measurements from a multi-point, multi-scale L-class plasma observatory consisting of at least 7 spacecraft covering fluid, ion and electron scales are needed. The plasma observatory will enable a paradigm shift in our comprehension of particle energization and space plasma physics in general, with very important impact on solar and astrophysical plasmas. It will be the next logical step following Cluster, THEMIS and MMS for the very large and active European space plasmas community. Being one of the cornerstone missions of the future ESA Voyage 2035-2050 science program, it would further strengthen the European scientific and technical leadership in this important field.

physics.space-ph↗

arXiv2019

Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. I. Electron shock-surfing acceleration

Electron injection at high Mach-number nonrelativistic perpendicular shocks is studied here for parameters that are applicable to young SNR shocks. Using high-resolution large-scale two-dimensional fully kinetic particle-in-cell (PIC) simulations and tracing individual particles we in detail analyze the shock surfing acceleration (SSA) of electrons at the leading edge of the shock foot. The central question is to what degree the process can be captured in 2D3V simulations. We find that the energy gain in SSA always arises from the electrostatic field of a Buneman wave. Electron energization is more efficient in the out-of-plane orientation of the large-scale magnetic field because both the phase speed and the amplitude of the waves are higher than for the in-plane scenario. Also, a larger number of electrons is trapped by the waves compared to the in-plane configuration. We conclude that significant modifications of the simulation parameters are needed to reach the same level of SSA efficiency as in simulations with out-of-plane magnetic field or 3D simulations.

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

Energy Partition between Ion and Electron of Collisionless Magnetic Reconnection

The plasma heating during collisionless magnetic reconnection is investigated using particle-in-cell simulations. We analyze the time evolution of the plasma temperature associated with the motion of the reconnecting flux tube, where the plasma temperature is defined as second-order moment of velocity distribution function in the simulation frame/in the center of the flux tube frame, and we show that the plasma heating during magnetic reconnection can be separated into two distinct stages: the nonadiabatic heating stage, in which the magnetic field lines are just reconnecting in the X-type diffusion region, and the adiabatic heating stage, in which the flux tube is shrinking after two flux tubes merge. During the adiabatic heating stage, the plasma temperature $T$ can be approximated by $TV^{γ-1}=const.$, where $γ=5/3$ is the specific heat, and $V$ is the volume of the flux tube. In the nonadiabatic heating stage, we found numerically that the ratio of the increment of the ion temperature to that of the electron temperature can be approximated by $ΔT_i/ΔT_e \approx (m_i/m_e )^{1/4}$, where $m_i$ and $m_e$ are the ion and electron mass, respectively. We also present a theoretical model based on a magnetic-diffusion-dominated reconnection to explain the simulation result.

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

Electron Surfing and Drift Accelerations in a Weibel-dominated High-Mach-number Shock

How electrons get accelerated to relativistic energies in a high-Mach-number quasi-perpendicular shock is presented by means of ab initio particle-in-cell simulations in three dimensions. We found that coherent electrostatic Buneman waves and ion-Weibel magnetic turbulence coexist in a strong-shock structure whereby particles gain energy during shock-surfing and subsequent stochastic drift accelerations. Energetic electrons that initially experienced the surfing acceleration undergo pitch-angle diffusion by interacting with magnetic turbulence and continuous acceleration during confinement in the shock transition region. The ion-Weibel turbulence is the key to the efficient nonthermal electron acceleration.

astro-ph.HE↗

arXiv2017

Stratified Simulations of Collisionless Accretion Disks

This paper presents a series of stratified shearing-box simulations of collisionless accretion disks in the recently developed framework of kinetic magnetohydrodynamics (MHD), which can handle finite non-gyrotropy of a pressure tensor. Although a fully kinetic simulation predicted a more efficient angular-momentum transport in collisionless disks than in the standard MHD regime, the enhanced transport has not been observed in past kinetic MHD approaches to gyrotropic pressure anisotropy. For the purpose of investigating this missing link between the fully kinetic and MHD treatments, this paper pays attention to the role of non-gyrotropic pressure, and makes a first attempt to incorporate certain collisionless effects into disk-scale, stratified disk simulations. When the timescale of gyrotropization was longer than, or comparable to, the disk rotation frequency of the orbit, we found that the finite non-gyrotropy selectively remaining in the vicinity of current sheets contributes to suppressing magnetic reconnection in the shearing-box system. This leads to increases both in the saturated amplitude of the MHD turbulence driven by magnetorotational instabilities and in the resultant efficiency of angular-momentum transport. Our results seem favorable for fast advection of magnetic fields toward the rotation axis of a central object, which is required to launch an ultra-relativistic jet from a black-hole accretion system in, for example, a magnetically arrested disk state.

astro-ph.HE↗