Injection of He2+ Ions at a parallel shock Original Research Article

The very initial acceleration from thermal to supra-thermal energies at collisionless shocks is still under debate. The present work is an extension of the injection and acceleration model at quasi-parallel shocks (Sugiyama and Fujimoto, 1999), in which large-amplitude monochromatic upstream waves play the key role. Some upstream protons are selected out of the thermal population during their first encounter with the shock and are accelerated to several times the ram energy (E0) when they are injected into the upstream / downstream region. The injection and acceleration take place very quickly within a few times the inverse upstream proton gyro-frequency Ωi−1. Here, we have applied this injection model to alpha particle (He2+) and demonstrated that the same process works. Both in test particle calculation and hybrid simulation, it is observed that selected alpha particles are accelerated to a suprathermal energy level. In a test particle calculation, some of the selected alpha particles stay around the shock much longer (20 Ωi−1) and are accelerated to considerably higher energy (50 E0) than protons (∼10 E0). In a hybrid simulation, however, such long trapped alpha particles are not detected. This difference suggests that in a realistic self-consistent shock system, there is an/are additional process(es) to release the alpha particles from a shock region before sufficient acceleration occurs. Otherwise, the shock structure would be strongly modified by the presence of a significant energy flow into the highly energized alpha particles.