Tomographic reconstruction of storm time RC ion distribution from ENA images on board multiple spacecraft

Abstract

AbstractA quantitative retrieval of 3‐D distribution of energetic ions as energetic neutral atoms (ENA) sources is a challenging task. In this paper the voxel computerized tomography (CT) method is initially applied to reconstruct the 3‐D distribution of energetic ions in the magnetospheric ring current (RC) region from ENA emission images on board multiple spacecraft. To weaken the influence of low‐altitude emission (LAE) on the reconstruction, the LAE‐associated ENA intensities are corrected by invoking the thick‐target approximation. To overcome the divergence in iteration due to discordant instrument biases, a differential ENA voxel CT method is developed. The method is proved reliable and advantageous by numerical simulation for the case of constant bias independent of viewing angle. Then this method is implemented with ENA data measured by the Two Wide‐angle Imaging Neutral‐atom Spectrometers mission which performs stereoscopic ENA imaging. The 3‐D spatial distributions and energy spectra of RC ion flux intensity are reconstructed for energies of 4–50 keV during the main phase of a major magnetic storm. The retrieved ion flux distributions seem to correspond to an asymmetric partial RC, located mainly around midnight favoring the postmidnight with L = 3.5–7.0 in the equatorial plane. The RC ion distributions with magnetic local time depend on energy, with major equatorial flux peak for lower energy located east of that for higher energy. In comparison with the ion energy spectra measured by Time History of Events and Macroscale Interactions during Substorms‐D satellite flying in the RC region, the retrieved spectrum from remotely sensed ENA images are well matched with the in situ measurements.