Generation of suprathermal protons via parallel electron fire-hose instability: Particle-in-cell simulations

Abstract:

In situ observations have shown that the measured electron temperature anisotropy in the expanding solar wind is regulated by the electron fire-hose instabilities (EFI), which could be excited by excessive parallel temperature anisotropy. It is known that for parallel propagation mode the enhanced transverse fluctuations driven by the parallel EFI are resonant with the ions. In the present study, nonlinear properties of the parallel EFI are investigated using one-dimensional particle-in-cell simulations with various initial proton plasma betas. It is found that the protons in resonance with the left-hand polarized EFI modes are anisotropically heated and subsequently their resonant interactions give rise to the excitation of the ion-acoustic waves (IAW). It is shown that the intensity of the excited IAW is proportional to the values of the electron to proton temperature ratio. In addition, the presence of the unusual electrostatic modes driven by nonlinear behavior of the protons, especially for the lower proton beta simulations, leads to the formation of the suprathermal component in the proton parallel velocity distribution, although the parallel proton temperature does not practically change throughout the simulation period.