Hiroki Hasegawa

Persistent acceleration of positrons in a nonstationary shock wave

Long-time evolution of positrons accelerated in an oblique shock wave in an electron-positron-ion plasma is studied with relativistic, electromagnetic, particle simulations. In the early stage, some positrons move nearly parallel to the external magnetic field in the shock transition region and gain energy from the parallel electric field. The acceleration can become stagnant owing to the deformation of the wave profile. After the recovery of the shock profile, however, the acceleration can start again. By the end of simulation runs, ωpet=5000, positron Lorentz factors reached values ∼2000. In this second stage, three different types of acceleration are found. In the first type, the acceleration process is the same as that in the early stage. In the second type, positrons make gyromotions in the wave frame and gain energy mainly from the perpendicular electric field. In the third type, particle orbits are similar to curtate cycloids. Theoretical estimate for this energy increase is given.

Particle acceleration by a large-amplitude wave associated with an ion beam in a magnetized plasma

Large-amplitude waves associated with a relativistic ion beam in a magnetized plasma are investigated by means of relativistie electromagnetic particle simulations. In the simulations, it is shown that electromagnetic fields are induced by an ion bunch which has Gaussian density distribution and that their profiles are similar to those of magnetosonic solitons. Further, when an ion beam propagates obliquely to the external magnetic field, it is found that an induced electric field lias a parallel component along the magnetic field. Then. as the next step. giving another positron bunch, it is observed that some particles in a positron bunch are accelerated by the parallel electric field.

Study of self-consistent particle flows in a plasma blob with particle-in-cell simulations

The self-consistent particle flows in a filamentary coherent structure along the magnetic field line in scrape-off layer (SOL) plasma (plasma blob) have been investigated by means of a three-dimensional electrostatic particle-in-cell simulation code. The presence of the spiral current system composed of the diamagnetic and parallel currents in a blob is confirmed by the particle simulation without any assumed sheath boundary models. Furthermore, the observation of the electron and ion parallel velocity distributions in a blob shows that those distributions are far from Maxwellian due to modification with the sheath formation and that the electron temperature on the higher potential side in a blob is higher than that on the lower potential side. Also, it is found that the ions on the higher potential side are accelerated more intensively along the magnetic field line than those on the lower potential side near the edge. This study indicates that particle simulations are able to provide an exact current clo...

Images of Aurora Light Based on Macro–Micro-Interlocked Simulation

To understand the aurora arc formation, a simulation code using a macro-micro-interlocked algorithm has been developed and implemented on the original Earth Simulator. Based on the results, aurora light has been rendered taking the emission from oxygen atoms excited by precipitating energetic electrons into consideration using an original parallel-visualization software.