Generation mechanism of electrostatic waves in the upstream and shock transition regions of quasi-parallel shocks

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Abstract

[1] Primarily, one-dimensional full-particle electromagnetic simulations supplemented by a two-dimensional simulation are performed to investigate the excitation processes of electrostatic waves in quasi-parallel shocks. The characteristic waves are ion acoustic, ion beam-mode, and upper-hybrid waves. In the upstream region, only ion beam-mode electrostatic waves are excited by the reflected ion beam running almost parallel to the ambient magnetic field. We find that the ion beam-mode waves evolve into electrostatic solitary waves (ESW) for an appropriate ion beam velocity. The evolution of the ion beam-mode to ESW is most clearly observed in the simulation when the ambient magnetic field is taken to be parallel to the ion beam direction. We confirm that the ion beam-mode waves are excited only in the direction parallel to the ambient magnetic field. In the shock transition region (STR) the simulation results show that upper-hybrid waves are excited on the upstream side of the STR by the reflected ion beam running almost perpendicular to the ambient magnetic field. In the STR region a large-amplitude and quasi-monochromatic oblique whistler wave is also excited. On the downstream side of the STR, ion acoustic waves are excited. The mechanism of the excitation of the ion acoustic wave is the current-driven instability. This instability is generated by the velocity difference between ions and electrons due to electron acceleration in the shock potential. Of these three types of waves, the ion acoustic waves scatter electrons most effectively and contribute to the electron heating processes in the shock region.

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