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

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 ~9–13 mV/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 >100 keV in ~10 min. 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 ~10 keV 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.