Charge Proportional and Weakly Mass-Dependent Acceleration of Different Ion Species in the Earth's Magnetotail

Energetic particles with energies from tens of keV to a few hundred keV are frequently observed in the Earth's magnetotail. Here we study, by means of a test particle numerical simulation, the acceleration of different ion species (H⁺, He⁺, He⁺⁺, and Oⁿ⁺ with n = 1–6) in the presence of transient electromagnetic perturbations. All the considered ions develop power law tails at high energies, except for O⁺ ions. This is strongly correlated to the time that the particle spends in the current sheet. Ion acceleration is found to be proportional to the charge state, while it grows in a weaker way with the ion mass. We find that O^5+/6+ can reach energies higher than 500 keV. These results may explain the strong oxygen acceleration observed in the magnetotail.