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Radial Transport of Higher‐Energy Oxygen Ions Into the Deep Inner Magnetosphere Observed by Van Allen Probes
Journal article   Open access   Peer reviewed

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

K Mitani, K Seki, K Keika, M Gkioulidou, L. J Lanzerotti, D. G Mitchell and C. A Kletzing
Geophysical research letters, Vol.45(10), pp.4534-4541
05/28/2018
DOI: 10.1029/2018GL077500
url
https://doi.org/10.1029/2018GL077500View
Published (Version of record) Open Access

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. Plain Language Summary Ion transport from the outer magnetosphere (≳36,000 km altitude) to the inner magnetosphere (≲36,000 km altitude) builds up the ring current in the inner magnetosphere. The ring current is the cause of a magnetic storm, that is the one of the largest scale turbulence in the Earth's magnetosphere. It is well known that protons and oxygen ions are the main components of the ring current. The transport mechanism of the ring current ions differs among ion energies. Lower‐energy (≲150 keV) ions are well known to be transported by convection electric field applying to magnetosphere. Higher‐energy (≳150 keV) protons are reported to be transported by diffusion due to nonuniform plasma distributions, while there are few reports about transport of higher‐energy oxygen ions. We found the radial selective transport of higher‐energy oxygen ions leaving behind protons in the inner magnetosphere during a magnetic storm on 23–25 April 2013 observed by the Van Allen Probes spacecraft. An enhancement of magnetic fluctuations, which can resonate with the drift and bounce motions of higher‐energy oxygen ions, is simultaneously observed. We suggest that the higher‐energy oxygen ions are transported to the inner magnetosphere selectively by the combination of two resonances: drift resonance and drift‐bounce resonance. Key Points High‐energy (≳150 keV) ring current oxygen ions are transported closer to Earth than the high‐energy protons during a storm main phase Both satellite and ground‐based observations show enhancements of 1–100 mHz magnetic field fluctuations in the inner magnetosphere Examination of resonance conditions suggests that the transport is caused by the combination of drift and drift‐bounce resonances
 magnetic storm oxygen ion ring current Van Allen Probes

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