Charge proportional and weakly mass-dependent acceleration of different ion species in the Earth's magnetotail

TL;DR

This study uses numerical simulations to show that ion acceleration in Earth's magnetotail depends on charge and mass, with higher charge states reaching energies over 500 keV, explaining observed oxygen energization.

Contribution

It demonstrates that ion acceleration is proportional to charge and weakly dependent on mass, revealing charge state effects on energetic particle acceleration in the magnetotail.

Findings

Ion energy spectra develop power-law tails, except for O+ ions.

Ion acceleration scales with charge state, weakly with mass.

O5+/6+ ions can exceed 500 keV energy.

Abstract

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.