Van Allen Probes observations of magnetic field dipolarization and its associated O + flux variations in the inner magnetosphere at L < 6.6

M Nosé; K Keika; C. A Kletzing; H. E Spence; C. W Smith; R. J MacDowall; G. D Reeves; B. A Larsen; D. G Mitchell

doi:10.1002/2016JA022549

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Van Allen Probes observations of magnetic field dipolarization and its associated O + flux variations in the inner magnetosphere at L < 6.6

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Van Allen Probes observations of magnetic field dipolarization and its associated O + flux variations in the inner magnetosphere at L < 6.6

M Nosé, K Keika, C. A Kletzing, H. E Spence, C. W Smith, R. J MacDowall, G. D Reeves, B. A Larsen and D. G Mitchell

Journal of geophysical research. Space physics, Vol.121(8), pp.7572-7589

08/2016

DOI: 10.1002/2016JA022549

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Abstract

We investigate the magnetic field dipolarization in the inner magnetosphere and its associated ion flux variations, using the magnetic field and energetic ion flux data acquired by the Van Allen Probes. From a study of 74 events that appeared at L = 4.5–6.6 between 1 October 2012 and 31 October 2013, we reveal the following characteristics of the dipolarization in the inner magnetosphere: (1) its time scale is approximately 5 min; (2) it is accompanied by strong magnetic fluctuations that have a dominant frequency close to the O+ gyrofrequency; (3) ion fluxes at 20–50 keV are simultaneously enhanced with larger magnitudes for O+ than for H+; (4) after a few minutes of the dipolarization, the flux enhancement at 0.1–5 keV appears with a clear energy-dispersion signature only for O+; and (5) the energy-dispersed O+ flux enhancement appears in directions parallel or antiparallel to the magnetic field. From these characteristics, we discuss possible mechanisms that can provide selective acceleration to O+ ions at >20 keV. We conclude that O+ ions at L = 5.4–6.6 undergo nonadiabatic local acceleration caused by oscillating electric field associated with the magnetic fluctuations and/or adiabatic convective transport from the plasma sheet to the inner magnetosphere by the impulsive electric field. At L = 4.5–5.4, however, only the former acceleration is plausible. We also conclude that the field-aligned energy-dispersed O+ ions at 0.1–5 keV originate from the ionosphere and are extracted nearly simultaneously to the onset of the dipolarization.

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Title: Subtitle: Van Allen Probes observations of magnetic field dipolarization and its associated O + flux variations in the inner magnetosphere at L < 6.6
Creators: M Nosé - Kyoto University
K Keika - Nagoya University
C. A Kletzing - University of Iowa
H. E Spence - University of New Hampshire
C. W Smith - University of New Hampshire
R. J MacDowall - Goddard Space Flight Center
G. D Reeves - Los Alamos National Laboratory
B. A Larsen - Los Alamos National Laboratory
D. G Mitchell - Johns Hopkins University Applied Physics Laboratory
Resource Type: Journal article
Publication Details: Journal of geophysical research. Space physics, Vol.121(8), pp.7572-7589
DOI: 10.1002/2016JA022549
ISSN: 2169-9380
eISSN: 2169-9402
Grant note: DOI: 10.13039/100000104, name: National Aeronautics and Space Administration, award: NAS5‐01072, NAS5‐01072, NAS5‐01072
Language: English
Date published: 08/2016
Academic Unit: Physics and Astronomy
Record Identifier: 9984199707002771

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