3-D ion distribution and evolution in storm-time RC Retrieved from TWINS ENA by differential voxel CT technique

The quantitative retrieval of the 3-D spatial distribution of the parent energetic ions of ENA from a 2-D ENA image is a quite challenge task. The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission of NASA is the first constellation to perform stereoscopic magnetospheric imaging of energetic neutral atoms (ENA) from a pair of spacecraft flying on two widely-separated Molniya orbits. TWINS provides a unique opportunity to retrieve the 3-D distribution of ions in the ring current (RC) by using a volumetric pixel (voxel) CT inversion method. In this study the voxel CT method is implemented for a series of differential ENA fluxes averaged over about 6 to 7 sweeps (corresponding to a time period of about 9 min.) at different energy levels ranging from 5 to 100 keV, obtained simultaneously by the two satellites during the main phase of a great magnetic storm with minimum Sym-H of -156 nT on 24-25 October 2011. The data were selected to span a period about 50 minutes during which a large substorm was undergoing its expansion phase first and then recovery. The ENA species of O and H are distinguished for some time-segments by analyzing the signals of pulse heights of second electrons emitted from the carbon foil and impacted on the MCP detector in the TWINS sensors. In order to eliminate the possible influence on retrieval induced by instrument bias error, a differential voxel CT technique is applied. The flux intensity of the ENAs' parent ions in the RC has been obtained as a function of energy, L value, MLT sector and latitude, along with their time evolution during the storm-time substorm expansion phase. Forward calculations proved the reliability of the retrieved results. It shows that the RC is highly asymmetric, with a major concentration in the midnight to dawn sector for equatorial latitudes. Halfway through the substorm expansion there occurred a large enhancement of equatorial ion flux at lower energy (5 keV) in the dusk sector, with narrow extent (less than 2 MLT hours) near geosynchronous orbit of 5.5~7.0 Re, which is likely to be oxygen-rich. The ion flux spectra underwent dramatic changes. At the expansion maximum of the substorm, the spectra show a single peak at higher energy (~80 keV). For the midpoint of the substorm expansion there are two peaks at higher and lowest energies, showing a valley at about 20 keV. A preliminary composition analysis of ENA parent ions shows a prominent storm-time enhancement of oxygen flux and a complicated spatial pattern of O+/H+ flux intensity ratio. Acknowledgment: We thank greatly the TWINS mission leaders and staffs who make the Mission success and provide publically the valuable data. Thanks to Dr. Goldstein for his helpful suggestions.