Energy Spectra of Energetic Ions Around Quasi‐Parallel Shocks

We have performed a number of simulations of quasi-parallel shocks to investigate how the energy spectra of non-thermal components are controlled by the presence of alpha (He 2+ ) particles. The simulations are done for the Earth's bow shock-like situation by an one-dimensional hybrid code that treats ions as particles but electrons as a massless fluid. The upstream conditions are modified by varying the He 2+ ions content while keeping the proton content unchanged. The range of the He 2+ ion density ratio (R) variation relative to proton is from 0.1% to 30% including the typical ratio of 4 ∼ 5% for the solar wind. As observed in the upstream of the bow shock, the differential flux spectra of the two ion species obtained in the simulations from the downstream region are found to be well represented by exponential shapes. When the energy scale is presented in the energy-per-charge (E/Q) unit, the two spectra have the same characteristic energy (E C ) that increases in time. While E C is a function of the density ratio R and increases with increasing R, the equality between the two species holds throughout. Detailed analyses show that larger magnetic fluctuations brought about by the presence of He 2+ particles enable more efficient acceleration at the shock front. Thus it is via exciting, stronger turbulence that the additional upstream ram energy carried by He 2+ ions is poured into the process of hardening the energy spectrum of the non-thermal particles of both species.