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Title: Generation of large-amplitude electric field and subsequent enhancement of O⁺ ion flux in the inner magnetosphere during substorms
Authors: Nakayama, Y.
Ebihara, Y.
Tanaka, T.
Author's alias: 中山, 洋平
海老原, 祐輔
Keywords: substorm
inner magnetosphere
heavy ions
acceleration
transport
Issue Date: Jun-2015
Publisher: American Geophysical Union (AGU)
Journal title: Journal of Geophysical Research A: Space Physics
Volume: 120
Issue: 6
Start page: 4825
End page: 4840
Abstract: Energetic O⁺ ions are rapidly enhanced in the inner magnetosphere because of abrupt intensification of the dawn-to-dusk electric field and significantly contribute to the ring current during substorms. Here we examine the generation mechanism of the dawn-to-dusk electric field that accelerates the O⁺ ions and the spatial and temporal evolution of the differential flux of the O⁺ ions by using a test particle simulation in the electric and magnetic fields that are provided by a global magnetohydrodynamics (MHD) simulation. In the MHD simulation, strong dawn-to-dusk electric field appears in the near-Earth tail region by a joint action of the earthward tension force and pileup of magnetic flux near an onset of substorm expansion. The peak of the electric field is ~9-13mV/m and is located ~1-2 RE earthward of the peak of the plasma bulk speed because of the pileup. O⁺ ions coming from the lobe are accelerated from ~eV to >100keV in ~10min. The reconstructed flux of the O⁺ ions shows that at ~7 RE near midnight, the flux has a peak near a few tens of keV and the flux below ~10keV is small. This structure, called a "void" structure, is consistent with the Polar observation and can be regarded as a manifestation of the acceleration of unmagnetized ions perpendicular to the magnetic field. In the inner magnetosphere (at 6.0 RE), reconstructed energy-time spectrograms show the nose dispersion structure that is also consistent with satellite observations.
Rights: An edited version of this paper was published by AGU. Copyright 2015 American Geophysical Union.
URI: http://hdl.handle.net/2433/237232
DOI(Published Version): 10.1002/2015JA021240
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