Evidence of strong energetic ion acceleration in the near‐Earth magnetotail

Until now it is still questionable whether ions are accelerated to energies above 100 keV in the near‐Earth current sheet (CS), in the vicinity of a possible near‐Earth neutral line. By using 11 years of 3‐D energetic ion flux data for protons, helium, and oxygen (~150 keV–1 MeV) from the RAPID instrument on board Cluster 4, we statistically study the energetic ion acceleration by investigating ion anisotropies in the near‐Earth magnetotail (−20 RE< X<−16 RE). It is found that the earthward (tailward) anisotropy of the energetic (>150 keV) ions (protons, He+, and O+) tend to become higher as the earthward (tailward) plasma bulk flows (measured by Cluster Ion Spectrometry experiment) become stronger. During such periods the presence of a strong acceleration source tailward (earthward) of Cluster spacecraft (S/C) is confirmed by the hardening energy spectra of the earthward (tailward) energetic ion flows. A good statistical correlation between tailward bulk flow, negative Bz, and the tailward anisotropy of energetic ions indicates that the strong ion acceleration might be related to a near‐Earth reconnection, which occurred earthward of the Cluster S/C. The energetic ion anisotropies do not show a clear dependence on the AEindex, which may indicate that the acceleration source(s) for the energetic ions could be spatially localized. We present the observational evidence of ion acceleration in the plasma sheetThe acceleration of ions to energies > 150 keV may occur in the near‐Earth CSThe acceleration is a spatially localized process