Radial Transport of Higher‐Energy Oxygen Ions Into the Deep Inner Magnetosphere Observed by Van Allen Probes

Abstract

The transport mechanism of the ring current ions differs among ion energies. Lower‐energy (≲150 keV) ions are well known to be transported convectively. Higher‐energy (≳150 keV) protons are reported to be transported diffusively, while there are few reports about transport of higher‐energy oxygen ions. We report the radial transport of higher‐energy oxygen ions into the deep inner magnetosphere during the late main phase of the magnetic storm on 23–25 April 2013 observed by the Van Allen Probes spacecraft. An enhancement of 1–100 mHz magnetic fluctuations is simultaneously observed. Observations of 3 and 30 mHz geomagnetic pulsations indicate the azimuthal mode number is ≤10. The fluctuations can resonate with the drift and bounce motions of the oxygen ions. The results suggest that the combination of the drift and drift‐bounce resonances is responsible for the radial transport of higher‐energy oxygen ions.

Drift‐Bounce Resonance Between Pc5 Pulsations and Ions at Multiple Energies in the Nightside Magnetosphere: Arase and MMS Observations

“…It would be appropriate to consider that the solar wind generates the Pc5 wave and it feeds energy to the O + ions through the bounce resonance. Mitani et al () proposed that the drift‐bounce resonance contributes to the selective penetration of O + ions into the inner magnetosphere from the plasma sheet during the late main phase to early recovery phase, which is consistent with our scenario. The present study first demonstrates the selective acceleration of O + ions due to the bounce resonance in the nightside inner magnetosphere.…”

Statistical Study of Selective Oxygen Increase in High‐Energy Ring Current Ions During Magnetic Storms

“…In section , we summarize the results and discuss possible mechanisms for producing SOI events. We conclude that the selective transport of high‐energy oxygen ions is due to the combination of drift and drift‐bounce resonances as suggested by Mitani et al (). We also suggest other mechanisms for producing SOI events.…”

Ion Energies Dominating Energy Density in the Inner Magnetosphere: Spatial Distributions and Composition, Observed by Arase/MEP‐i

“…Mass-dependent acceleration may be possible in the plasma sheet with magnetic and electric field perturbations (e.g., Catapano et al, 2016), in the reconnection region and around the separatrix between closed and open magnetic fields (where the Hall electric field exists; Liang et al, 2017), around a dipolarization front (i.e., a reconnection jet front) where the magnetic field increases in less than seconds and a strong electric field is thus induced (Runov et al, 2015), near the transition region from the stretched magnetic field to the dipole-like field where a fast flow associated with reconnection slows down and the electric field is induced by the magnetic field pileup (Nakayama et al, 2016). Possible waves are dispersive Alfvén waves (e.g., Chaston et al, 2016), which can accelerate O+ that are simultaneously extracted from the topside ionosphere and/or preexist on the same field lines; waves/fluctuations with a frequency range of electron-ion cyclotron waves (e.g., Nosé et al, 2014); and ULF waves (e.g., Mitani et al, 2018;Oimatsu et al, 2018), which can resonate with drifting and bouncing ring current ions. However, as higher-energy O+ are more difficult to transport to the inner magnetosphere, it is still a controversial issue whether a large number of those accelerated O+ can penetrate to around/inside geosynchronous orbit.…”