Electromagnetic Proton/Proton Instability and Its Implications for Ion Heating in the Extended Fast Solar Wind

Two-dimensional hybrid simulations are performed in this paper to investigate the proton/proton instability in low beta plasma. The obliquely propagating Alfvén waves are found to be unstable to the proton/proton instability. At first, the Alfvén waves have a nearly linear polarization, and both the ambient protons and minor ions O6+ can be resonantly heated. The heating is primarily in the direction perpendicular to the background magnetic field. With the evolution of the instability, the obliquely propagating Alfvén waves gradually become left-hand polarized, and then cannot resonantly heat the ambient protons or minor ions O6+. The effects of the plasma beta and temperature anisotropy of the ambient protons on the evolution of the instability are also studied in this paper. Finally, the implications of our simulation results for ion heating in the extended fast solar wind are discussed.