Acceleration of protons and heavy ions to suprathermal energies during dipolarizations in the near-Earth magnetotail

In this work we present an analysis of the dynamics of suprathermal ions of different masses (H+, He+, O+) during prolonged dipolarizations in the near-Earth magnetotail (X > -17 R-E/according to Cluster/RAPID observations in 2001- 2005. All dipolarizations from our database were associated with fast flow braking and consisted of multiple dipolarization fronts (DFs). We found statistically that fluxes of suprathermal ions started to increase similar to 1 min before the dipolarization onset and continued to grow for similar to 1 min after the onset. The start of flux growth coincided with the beginning of a decrease in the spectral index . The decrease in gamma was observed for protons for similar to 1 min after the dipolarization onset, and for He+ and O+ ions for similar to 3 and similar to 5 min after the onset respectively. The negative variations of gamma for O+ ions were similar to 2.5 times larger than for light ions. This demonstrates more efficient acceleration for heavy ions. The strong negative variations of gamma were observed in finite energy ranges for all ion components. This indicates the possibility of nonadiabatic resonant acceleration of ions in the course of their interaction with multiple DFs during dipolarizations. Our analysis showed that some fraction of light ions can be accelerated up to energies >= 600 keV and some fraction of oxygen ions can be accelerated up to similar to 1.2 MeV. Such strong energy gains cannot be explained by acceleration at a single propagating DF and suggest the possibility of multistage ion acceleration in the course of their interaction with multiple DFs during the prolonged dipolarizations.