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

Here 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 0.1–5keV 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 >20keV. 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 tomore » the inner magnetosphere by the impulsive electric field. At L = 4.5–5.4, however, only the former acceleration is plausible. Finally, 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.« less
Authors:
ORCiD logo [1] ;  [2] ;  [3] ;  [4] ;  [4] ;  [5] ; ORCiD logo [6] ;  [6] ;  [7]
  1. Kyoto Univ. (Japan). Graduate School of Science
  2. Nagoya Univ. (Japan). Inst. for Space-Earth Environmental Research
  3. Univ. of Iowa, Iowa City, IA (United States). Dept. of Physics and Astronomy
  4. Univ. of New Hampshire, Durham, NH (United States). Inst. for the Study of Earth, Oceans, and Space
  5. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States). Solar System Exploration Division
  6. Los Alamos National Lab. (LANL), Los Alamos, NM (United States); New Mexico Consortium, Los Alamos, NM (United States). Space Sciences Division
  7. Johns Hopkins Univ., Laurel, MD (United States). Applied Physics Lab.
Publication Date:
Report Number(s):
LA-UR-16-22082
Journal ID: ISSN 2169-9380; TRN: US1703004
Grant/Contract Number:
AC52-06NA25396; 25287127; 16H04057; 26800257; 921648; 967399; 937836; NAS5-01072
Type:
Accepted Manuscript
Journal Name:
Journal of Geophysical Research. Space Physics
Additional Journal Information:
Journal Volume: 121; Journal Issue: 8; Journal ID: ISSN 2169-9380
Publisher:
American Geophysical Union
Research Org:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org:
USDOE; National Aeronautic and Space Administration (NASA); Ministry of Education, Culture, Sports, Science and Technology (MEXT)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; 58 GEOSCIENCES; Heliospheric and Magnetospheric Physics; dipolarization; inner magnetosphere; substorm; O+ acceleration; ionospheric outflow; magnetic fluctuation
OSTI Identifier:
1402606