Multi-species Ion Acceleration in 3D Magnetic Reconnection with Hybrid-kinetic Simulations

Magnetic reconnection drives explosive particle acceleration in a wide range of space and astrophysical applications. The energized particles often include multiple species (electrons, protons, heavy ions), but the underlying acceleration mechanism is poorly understood. In-situ observations of these heavy ions offer a more stringent test of acceleration mechanisms, but the multi-scale nature of reconnection hinders studies on heavy-ion acceleration. Here we employ hybrid simulations (fluid electron, kinetic ions) to capture 3D reconnection over an unprecedented range of scales. For the first time, our simulations demonstrate nonthermal acceleration of all available ion species into power-law spectra. The reconnection layers consist of fragmented kinking flux ropes as part of the reconnection-driven turbulence, which produces field-line chaos critical for accelerating all species. The upstream ion velocities influence the first Fermi reflection for injection and then species with lower charge-to-mass ratio (Q/M) have delayed onsets of Fermi acceleration as they interact with growing flux ropes. The resulting spectra have similar power-law indices (p~4.5), but different maximum energy/nucleon proportional to (Q/M)^α, where α~0.6 for low plasma beta, and both p and α increase as beta approaches unity. These findings are consistent with observations at heliospheric current sheets and the magnetotail, strongly suggesting Fermi acceleration as the dominant multi-species-ion acceleration mechanism.