  Energetic particles with energies from tens of keV to a few hundreds 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 O$^{n+}$ 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 spend 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.
