Global 3-D Distributions of O⁺ and H⁺ Ions in the Inner Magnetosphere Reconstructed by Voxel Tomography From TWINS ENA Images During a Large Magnetic Storm

Using improved voxel tomography method for energetic neutral atom (ENA) emission, global 3-D distributions of O+ and H+ ions in the inner magnetosphere are reconstructed from ENA images remotely sensed by the Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission during 15 July 2012 magnetic storm. The compositions of ENA-O and -H are separated by statistical fitting based on start-pulse-height distributions. The errors in iterative inversions range from 0.12 to 0.28 at 16 and 32 keV. The reconstructed ion flux intensities at different storm phases match well with independent satellite in situ measurements. It is found that: (a) Responding to a pulse jump of solar wind dynamic pressure along with a northward reversal of IMF B-z during the initial phase, H+ ion fluxes enhanced greatly on outer dipole L-shell of 6.5-8.0 in pre-midnight off equator. Ts-05 model tracing and pitch angles distribution of line-of-sights for ENA imaging suggest these be likely attributed to nearly field-aligned earthward plasma flow from plasma-sheet boundary layer region. Meanwhile, the trapped ring current (RC) ion fluxes peak at L similar to 5 in the pre-midnight equatorial region, decreasing with increasing latitude. (b) During the storm maximum the equatorial O+ fluxes have two separated peaks. The main one appeared in dusk at L similar to 4, while the minor one in the pre-dawn appeared at very low L similar to 2.75 where the flux ratio O+/(O++H+) reached 60%. (c) The O+ flux fractions over equator exhibit prominent dusk-dawn asymmetry, reversing from dusk quadrant prevailing before the main phase to dawn ascendant during the storm maximum.