Response of Different Ion Species to Local Magnetic Dipolarization Inside Geosynchronous Orbit

This paper examines how hydrogen, helium, and oxygen (H, He, and O) ion fluxes at 1–1,000 keV typically respond to local magnetic dipolarization inside geosynchronous orbit. We extracted 144 dipolarizations that occurred at magnetic inclination >30° from the 2012–2016 tail seasons' observations of the Van Allen Probes spacecraft and then defined typical flux changes of these ion species by performing a superposed epoch analysis. On average, the dipolarization inside geosynchronous orbit is accompanied by a precursory transient decrease in the northward magnetic field component, transient impulsive enhancement in the westward electric field component, and decrease (increase) in the proton density (temperature). The coincident ion species experience an energy-dependent flux change, consisting of enhancement (depression) at energies above (below) ~50 keV. These properties morphologically resemble those around dipolarization fronts (or fast flows) in the near-Earth tail. A distinction among the ion species is the average energy of the flux ratio peak, being at 200–400 keV (100–200 keV) for He (H and O) ions. The flux ratio peaks at different energies likely reflect the different charge states of injected ionospheric and/or solar wind origin ion species. The ion spectra become harder for sharp dipolarizations, suggesting the importance of accompanying electric field in transporting and/or energizing the ions efficiently. Interestingly, the average flux ratio peak does not differ significantly among the ion species for ~2 min after onset, which implies that mass-dependent acceleration process is less important in the initial stage of dipolarization.