Ion two‐stream instabilities in the auroral acceleration zone

Abstract

Ion beams streaming upward out of the ionosphere are frequently observed in the auroral acceleration cavity. They mainly consist of H+ and O+ ions drifting along the magnetic field lines and are believed to have been accelerated upward by the same parallel potential drop at the transition from the topside ionosphere. Due to the mass difference, H+ and O+ form two distinct beams drifting at different speeds, whereby two‐stream instabilities may develop. Here we present 1D and 2D PIC simulations of the interaction between the H+ and O+ beams in the cavity. Our model describes an open system, where a population of hot plasma sheet protons and hot electrons is at rest while H+ and O+ are constantly injected at the boundary, emulating thus the ion influx from the bottom of the cavity. The study's results indicate two regimes according to the kinetic energy of the injected beams. For moderate energies the unstable waves are of the acoustic type and propagate parallel to the magnetic field. They can trap H+ and O+ ions, leading to phase‐space vortices and the accompanying signatures of ion solitary waves in the electric field. For larger energies, the parallel acoustic waves are replaced by oblique cyclotron waves. The latter waves crisscross, forming a herringbone pattern that energizes the O+ ions perpendicularly and leads to drifting conics. By contrast, the protons are hardly energized perpendicularly. The transition between the two regimes depends upon the electron temperature and the ion composition. In either case energy is transferred from the protons to the oxygens.