He2+ dynamics and ion cyclotron waves in the downstream of quasi‐perpendicular shocks: 2‐D hybrid simulations

The free energy provided by the ion temperature anisotropy is considered to be the source of ion cyclotron waves in the downstream of a quasi-perpendicular shock. Besides the proton cyclotron waves excited by the proton temperature anisotropy, He2+ is decelerated differentially from the protons by the shock due to its different charge-to-mass ratio and forms a bunched ring-like distribution in the immediate downstream of the quasi-perpendicular shock. However, how the helium cyclotron waves associated with the anisotropic distribution of He2+ are excited is still in debate. In this paper, with two-dimensional (2-D) hybrid simulations, we investigate He2+ dynamics and its role in the ion cyclotron waves downstream of quasi-perpendicular shocks (the proton plasma beta in the upstream is 0.4). A bunched ring-like distribution of He2+ is formed in the immediate downstream of the quasi-perpendicular shocks; then it evolves into a shell-like distribution. At last, a bi-Maxwellian distribution of He2+ is generated in the far downstream. In the medium and low Mach number shocks, besides the proton cyclotron waves excited near the shock front, there is another enhancement of the magnetic fluctuations in the downstream. The results show that the helium cyclotron waves can be driven directly by the bunched ring-like distribution of He2+ in a low or medium Mach number quasi-perpendicular shock. The relevance of our simulation results to the satellite observations is also discussed in this paper.