The Acceleration of Thermal Ions at a Strong, Quasi-Parallel Interplanetary Shock: A Hybrid Simulation

Using a self-consistent hybrid simulation, with kinetic protons and fluid electrons, we investigate the acceleration of thermal protons and minor ions (alphas, 3H_e ⁺⁺, and C⁵⁺) by a quasi-parallel collisionless shock. The results are compared to spacecraft observations of a strong interplanetary shock seen by the Advanced Composition Explorer on DOY 94, 2001, which was associated with significant increases in the flux of > 50 keV/nuc ions. Our simulation uses similar plasma and shock parameters to those observed. The densities of minor ions for two of the species (alphas and C⁵⁺) were based on observations at thermal energies for this shock, and we used a nominal value for the density of 3H_e ⁺⁺, since no observations at thermal energies was available to us. Acceleration of the ions by the shock leads to a high-energy tail in the distribution in the post-shock plasma for all ion species. We find that by extrapolating the simulated tails to the higher energies measured by ACE/EPAM and ACE/ULEIS, the intensity matches well the observations for protons, alphas, and carbon. This suggests that thermal solar wind, accelerated directly at the shock, is a significant source of the observed high-energy protons and these minor ions.