Acceleration of ions to suprathermal energies by turbulence in the plasmoid-like magnetic structures

We study energetic spectra of H+, He+, and O+ ion fluxes in the energy range ≥130 keV measured by Cluster/Research with Adaptive Particle Imaging Detectors (RAPID) instruments during 37 intervals of the tailward bulk flow propagation in the near‐Earth tail (at X ≤ −19 RE). In all events from our database, the plasmoid‐like magnetic structures with the superimposed low‐frequency magnetic and electric field fluctuations were observed along with the tailward bulk flows. The plasmoid‐like structures were associated with the enhancements of energetic ion fluxes and the hardening of energy spectra of H+ and He+ ion components in 80% of events and of O+ ion component in 64% of events. The hardening of energy spectra was more pronounced for heavy ions than for protons. The analysis of the magnetic structures and power spectral density (PSD) of the magnetic and electric field fluctuations from our database revealed the following factors favorable for the ion energization: (1) the spatial scale of a plasmoid should exceed the thermal gyroradius of a given ion component in the neutral plane inside the plasmoid; (2) the PSD of the magnetic fluctuations near the gyrofrequency of a particular ion component should exceed ~ 50.0 nT2/Hz for oxygen ions; while the energization of helium ions and protons takes place for much lower values of the PSD. The kinetic analysis of ion dynamics in the plasmoid‐like magnetic configuration similar to the observed one with the superimposed turbulence confirms the importance of ion resonant interactions with the low‐frequency electromagnetic fluctuations for ion energization inside plasmoids. Key Points Spectra of energetic H+, He+, O+ fluxes were studied tailward of X line Strong ion acceleration occurs in turbulent region inside plasmoids Ions are accelerated due to resonant interaction with EM fluctuations