Yutaka Ohira
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View article: Simulated Characteristics of On‐Ground Gamma‐Ray Glows
Simulated Characteristics of On‐Ground Gamma‐Ray Glows Open
High‐energy emissions from thunderclouds have been observed in the Hokuriku Japanese region during winter. The winter thunderclouds have lower altitudes that allow ground measurements due to the shorter distance between electrified regions…
View article: The Electron Temperature Distribution and the High Ionization Just behind the Shock in the Cygnus Loop
The Electron Temperature Distribution and the High Ionization Just behind the Shock in the Cygnus Loop Open
The physical processes behind astrophysical collisionless shocks, such as thermal relaxation and ionization after shock passage, remain poorly understood. To investigate these processes, we analyze the northeastern region of the Cygnus Loo…
View article: PIC Simulation of a Relativistic Shock Propagating to Electron, Proton, and Heavy Ion Plasmas
PIC Simulation of a Relativistic Shock Propagating to Electron, Proton, and Heavy Ion Plasmas Open
View article: The electron temperature distribution and the high ionization just behind the shock in the Cygnus Loop
The electron temperature distribution and the high ionization just behind the shock in the Cygnus Loop Open
The physical processes behind astrophysical collisionless shocks, such as thermal relaxation and ionization after shock passage, remain poorly understood. To investigate these processes, we analyze the northeastern region of the Cygnus Loo…
View article: Very-high-energy gamma-rays from cosmic rays escaping from Galactic black hole binaries
Very-high-energy gamma-rays from cosmic rays escaping from Galactic black hole binaries Open
We solve the cosmic ray diffusion around a Galactic black hole binary (microquasars) by considering the finite size of the escape region and the continuous cosmic ray injection. We find that the energy spectrum of escaping cosmic rays in t…
View article: Two-component jet model for the afterglow emission of GRB 201216C and GRB 221009A and implications for jet structure of very-high-energy gamma-ray bursts
Two-component jet model for the afterglow emission of GRB 201216C and GRB 221009A and implications for jet structure of very-high-energy gamma-ray bursts Open
View article: Collisionless Shocks Having Relativistic Velocities in Relativistically Hot Plasmas
Collisionless Shocks Having Relativistic Velocities in Relativistically Hot Plasmas Open
Shocks in relativistically hot plasmas are thought to exist in various high-energy astrophysical phenomena, but it is not clear how relativistic collisionless shocks are formed, and whether particles are accelerated by the shock as in the …
View article: Property of downstream turbulence driven by the special relativistic shock–clump interaction
Property of downstream turbulence driven by the special relativistic shock–clump interaction Open
Three-dimensional special relativistic magnetohydrodynamic simulations are performed to investigate properties of the downstream turbulence generated by the interaction between a relativistic shock wave and multiple clumps. We analyse the …
View article: Plasma Instability Evolution and Particle Heating in the Foot Region of Perpendicular Shocks in Young Supernova Remnants
Plasma Instability Evolution and Particle Heating in the Foot Region of Perpendicular Shocks in Young Supernova Remnants Open
Particle acceleration from collisionless shocks is a key mechanism in the generation of cosmic rays. In particular, shocks in young supernova remnants (SNRs) are considered to be a major source of galactic cosmic rays. This study investiga…
View article: Ideas and Requirements for the Global Cosmic-Ray Observatory (GCOS)
Ideas and Requirements for the Global Cosmic-Ray Observatory (GCOS) Open
After a successful kick-off meeting in 2021. two workshops in 2022 and 2023 on the future Global Cosmic-Ray Observatory (GCOS) focused mainly on a straw man design of the detector and science possibilities for astro- and particle physics. …
View article: Laser-Driven Proton-Only Acceleration in a Multicomponent Near-Critical-Density Plasma
Laser-Driven Proton-Only Acceleration in a Multicomponent Near-Critical-Density Plasma Open
An experimental investigation of collisionless shock ion acceleration is presented using a multicomponent plasma and a high-intensity picosecond duration laser pulse. Protons are the only accelerated ions when a near-critical-density plasm…
View article: Collisionless Shock in a Relativistically Hot Unmagnetized Electron–Positron Plasma
Collisionless Shock in a Relativistically Hot Unmagnetized Electron–Positron Plasma Open
In this work, we investigate collisionless shocks propagating in a relativistically hot unmagnetized electron–positron plasma. We estimate the dissipation fraction at shocks in the relativistically hot plasma, showing that it is sufficient…
View article: Evolution of the Secondary Electron Spectrum during Cosmic-Ray Discharge in the Universe
Evolution of the Secondary Electron Spectrum during Cosmic-Ray Discharge in the Universe Open
We recently found that streaming cosmic rays (CRs) induce a resistive electric field that can accelerate secondary electrons produced by CR ionization. In this work, we study the evolution of the energy spectrum of secondary electrons by n…
View article: Particle Acceleration and Magnetic Field Amplification by Relativistic Shocks in Inhomogeneous Media
Particle Acceleration and Magnetic Field Amplification by Relativistic Shocks in Inhomogeneous Media Open
Particle acceleration and magnetic field amplification in relativistic shocks propagating in inhomogeneous media are investigated by three-dimensional magnetohydrodynamical (MHD) simulations and test-particle simulations. The MHD simulatio…
View article: Research on improving the accuracy of PINNs for solid mechanics problems
Research on improving the accuracy of PINNs for solid mechanics problems Open
View article: Observational constraints on the maximum energies of accelerated particles in supernova remnants
Observational constraints on the maximum energies of accelerated particles in supernova remnants Open
Supernova remnants (SNRs) are thought to be the most plausible sources of\nGalactic cosmic rays. One of the principal questions is whether they are\naccelerating particles up to the maximum energy of Galactic cosmic rays\n($\\sim$PeV). In …
View article: Relativistic Runaway Electron Avalanche Development Near the Electric Field Threshold in Inhomogeneous Air
Relativistic Runaway Electron Avalanche Development Near the Electric Field Threshold in Inhomogeneous Air Open
Relativistic Runaway Electrons Avalanches (RREAs) development depends on the applied electric field and the environment's air density. This dependency controls the RREA exponential growth length scale. The RREA development affects the brem…
View article: Systematic trends and variety of particle acceleration processes in supernova remnants as promising Galactic cosmic-ray accelerators
Systematic trends and variety of particle acceleration processes in supernova remnants as promising Galactic cosmic-ray accelerators Open
Supernova remnants are believed to be promising sources of Galactic cosmic rays. One of the principal questions is whether they are accelerating particles up to the maximum energy of Galactic cosmic rays ($\\sim$PeV). In this work, we syst…
View article: Particle acceleration in a relativistic shock in inhomogeneous media
Particle acceleration in a relativistic shock in inhomogeneous media Open
It is known that the maximum energy of Cosmic Rays (CRs) reaches to around $10^{20}$ eV. One of the candidates of the acceleration mechanism of these highest-energy CRs is shock acceleration in a relativistic shock. However, it is also kno…
View article: Particle-in-cell Simulation of a Relativistic Shock Propagating in an Electron-Proton-Helium Plasma
Particle-in-cell Simulation of a Relativistic Shock Propagating in an Electron-Proton-Helium Plasma Open
Chemical abundances in cosmic rays are crucial information about their origin and acceleration mechanisms. The observed chemical composition of the cosmic rays shows enhancements to the solar abundances. The cause of the chemical enhanceme…
View article: Acceleration of secondary electrons by cosmic-ray discharge in the universe
Acceleration of secondary electrons by cosmic-ray discharge in the universe Open
We recently proposed a new discharge process caused by streaming cosmic rays (CRs) in the universe. The CRs current drives a return current of thermal electrons to compensate for the CR current. Then, a resistive electric field is induced …
View article: Ambient conditions to reproduce gamma-ray glow energy spectra assuming cosmic ray as source
Ambient conditions to reproduce gamma-ray glow energy spectra assuming cosmic ray as source Open
The electric field of thunderclouds modifies components and energy spectra of the cosmic-ray air shower resulting in an enhancement of gamma-ray fluxes on the ground, known as a gamma-ray glow. The Gamma-Ray Observation of Winter THundercl…
View article: Intergalactic Heating Induced by Streaming Cosmic Rays in the Early Universe
Intergalactic Heating Induced by Streaming Cosmic Rays in the Early Universe Open
Our Universe has undergone a transition from a cold and neutral state to a hot and ionized state. In the standard picture, this transition is caused by UV and X-ray photons emitted by stars and galaxies. However, cosmic rays (CRs) can also…
View article: Relativistic runaway electron avalanche development near the electric field threshold in inhomogeneous air
Relativistic runaway electron avalanche development near the electric field threshold in inhomogeneous air Open
Relativistic Runaway Electrons Avalanches (RREAs) development depends on the applied electric field and the environment air density. This dependency controls the RREA exponential growth length scale. The RREA development affects the bremss…
View article: Resistive heating induced by streaming cosmic rays around a galaxy in the early Universe
Resistive heating induced by streaming cosmic rays around a galaxy in the early Universe Open
It is expected that cosmic rays (CRs) escape from high-redshift galaxies at redshift z ∼ 10–20 because CRs are accelerated by supernova remnants of the first stars. Although ultraviolet and X-ray photons are widely considered the main sour…
View article: An Off-axis Jet Model for Multi-wavelength Afterglow Emission of GRB 190829A detected by H.E.S.S.
An Off-axis Jet Model for Multi-wavelength Afterglow Emission of GRB 190829A detected by H.E.S.S. Open
In the past three years, ground-based Imaging Atmospheric Cherenkov Telescopes, such as the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the High Energy Stereoscopic System (H.E.S.S.), have reported the detection of the…
View article: Off-axis jet scenario for early afterglow emission of low-luminosity Gamma-Ray Burst GRB 190829A
Off-axis jet scenario for early afterglow emission of low-luminosity Gamma-Ray Burst GRB 190829A Open
View article: Interaction of a Relativistic Magnetized Collisionless Shock with a Dense Clump
Interaction of a Relativistic Magnetized Collisionless Shock with a Dense Clump Open
The interactions between a relativistic magnetized collisionless shock and dense clumps have been expected to play a crucial role in magnetic field amplification and cosmic-ray acceleration. We investigate this process using two-dimensiona…
View article: X-ray line diagnostics of ion temperature at cosmic ray accelerating collisionless shocks
X-ray line diagnostics of ion temperature at cosmic ray accelerating collisionless shocks Open
A novel collisionless shock jump condition is suggested by modeling the entropy production at the shock transition region. We also calculate downstream developments of the atomic ionization balance and the ion temperature relaxation in sup…
View article: The Gamma-Ray Emission from the Supernova Remnant RX J1713.7-3946 Interacting with Two-phase Medium
The Gamma-Ray Emission from the Supernova Remnant RX J1713.7-3946 Interacting with Two-phase Medium Open
We study the origin of gamma-rays from the supernova remnant (SNR) RX J1713.7-3946. Using an analytical model, we calculate the distribution of cosmic rays (CRs) around the SNRs. Motivated by the results of previous studies, we assume that…