Generation of large-amplitude electric field and subsequent enhancement of O⁺ ion flux in the inner magnetosphere during substorms

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.