Formation of He 2+ shell‐like distributions downstream of the Earth's bow shock

When the incident solar wind He 2+ and H + distributions cross the electrostatic potential at the shock, they are decelerated differentially due to their different charge‐ to‐mass ratios. This differential slowing will produce a He 2+ ring‐beam distribution immediately downstream from a quasi‐perpendicular shock. In this letter, we perform one‐dimensional (1D) hybrid simulations and investigate the evolution of the He 2+ ring‐beam distribution in magnetized plasma. The plasma is composed of three components: H + , He 2+ and electrons, where H + has a velocity distribution with large perpendicular temperature anisotropy. It is shown that both the He 2+ ring‐beam distribution and H + distribution with large perpendicular temperature anisotropy can excite ion cyclotron waves with propagation direction parallel to the ambient magnetic field, and then the waves pitch‐angle scatter the He 2+ ions. However, only the ion cyclotron waves excited by the He 2+ ring‐beam distribution can transform He 2+ into shell‐like distribution. The results can explain the He 2+ shell‐like distributions downstream of the Earth's bow shock, which have already been observed with the AMPTE/CCE and ISEE spacecraft.