Evolutions of equatorial ring current ions during a magnetic storm

In this paper, we present evolutions of the phase space density (PSD) spectra of ring current (RC) ions based on observations made by Van Allen Probe B during a geomagnetic storm on 23–24 August 2016. By analyzing PSD spectra ratios from the initial phase to the main phase of the storm, we find that during the main phase, RC ions with low magnetic moment μ values can penetrate deeper into the magnetosphere than can those with high μ values, and that the μ range of PSD enhancement meets the relationship: S (O + ) > S (He + ) > S (H + ). Based on simultaneously observed ULF waves, theoretical calculation suggests that the radial transport of RC ions into the deep inner magnetosphere is caused by drift‐bounce resonance interactions, and the efficiency of these resonance interactions satisfies the relationship: η (O + ) > η (He + ) > η (H + ), leading to the differences in μ range of PSD enhancement for different RC ions. In the recovery phase, the observed decay rates for different RC ions meet the relationship: R (O + ) > R (He + ) > R (H + ), in accordance with previous theoretical calculations, i.e., the charge exchange lifetime of O + is shorter than those of H + and He + .