Particle Energization at Shock Fronts Due to Turbulence and Inhomogeneities in Hybrid Simulations.

Suprathermal ions exist throughout the universe but their origin remains poorly understood. Quantifying the origin of suprathermal ions will shed new light on existing observations, such as by Voyager, IBEX, and STEREO; and predict future observations, such as by Solar Orbiter, Parker Solar Probe, and IMAP. They will also help the heliophysics community better understand fundamental plasma processes. Suprathermal ions have energies in the 2-100 keV range and should exist in any collisionless plasma, though a variety of mechanisms may be responsible for their creation. The suprathermal tail of the solar-wind ion distribution connects the bulk (Maxwellian) ion distribution to the energetic (MeVs) population. It tends to follow a power law, though observations have reported a wide range of power-law indices (from -2 to -8), depending on local plasma parameters. Observations at 1 AU indicate that suprathermal particles are locally accelerated at compression regions while Voyager observations at the termination shock support a model of local shock acceleration. Suprathermal ions may represent the seed population for solar energetic particles, but the precise energization mechanism is unknown. This work presents 3-D hybrid (ion-scale) simulations of quasi-perpendicular shocks with a range of Mach numbers, shock-normal angles, and plasma betas. The parameter ranges mimic various scenarios in the solar wind in order to elucidate the dependence of particle energy distribution and trajectories on local plasma conditions. The results show that Mach number exerts a greater influence on the presence of a suprathermal component than do shock-normal angle and plasma beta. This work also shows that transient density structures can affect the overall particle distribution and increase diffusion along the shock normal. Transient structures also modify the shock front, thereby causing local shock properties to deviate from global properties. This has implications for conclusions about the relationship between shock properties and particle distributions drawn from in situ spacecraft data.