Reconstruction Of Magnetospheric Ring Current Ion Distribution From ENA Images Onboard Multi-spacecrafts

Magnetic storm is one of the central topics of space weather and space physics research, and the enhancement of magnetospheric ring current is a characteristic of magnetic storm. The dynamic process in the storm-time ring current (RC) is related with the coupling of solar wind, magnetosphere and ionosphere/thermosphere. The RC particles are considered as originating from solar wind and ionosphere, but the contributions of the two sources are not completely clear. A lot of satellite observations have confirmed that the oxygen ions could become the dominant species of RC during some great magnetic storms. The oxygen ions O+ can only be from the ionosphere, so the O+ tracking is important for working out the role of the ionosphere for the storm-time RC dynamics. The energetic neutral atom (ENA) produced through the resonant charge exchange between the RC energetic ion with background cold neutral atom is not controlled by electric and magnetic field, which makes it possible to sense remotely the global distribution of RC outside the target region by ENA imaging technique. The TWINS (Two Wide-angle Imaging Neutral-atom Spectrometer) mission makes stereoscopic ENA imaging of the Earth magnetospheric from a twain of spacecrafts flying on two widely- separated Molniya orbits, which provides us opportunity to reconstruct the 3-D RC ion distribution from 2-D images onboard multi-satellites.This paper presents our built method of ENA emission voxel CT for inner magnetosphere, followed by a description of differential ENA voxel CT that we developed to overcome the large error and divergence of inversion iteration caused by instrument bias. Then a numerical simulation of the inversion from calculated ENA flux images is made to validate the reliability and advantage of the differential ENA voxel CT technique. In the simulation an empirical model of RC ion flux distribution is used, along with a real observational configuration of two TWINS satellites. After then, the differential ENA voxel CT method is implemented for TWINS ENA images obtained simultaneously by the two satellites at different energy levels ranging from 4 to 50 keV during the main phase of a great magnetic storm of July 15,2012. 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. The 3-D distributions of RC ion fluxes with L, MLT and latitudes are successfully retrieved. A comparison is given of the reconstructed RC energetic ion flux spectrum with THEMIS in-situ measurements, which shows clearly that the satellite in-situ measured ion flux spectra and the reconstructed one are consistent with each other very well. It demonstrates that the differential ENA voxel CT method developed in this paper is effective and reliable to a certain extent for reconstructing RC ions distribution from ENA images observed onboard multi-satellites. The reconstructed storm-time enhanced RC ion distribution shows (1) remarkable asymmetry of day to night favoring the post-midnight at 3.5-6.5 L in the lower and equatorial latitudes; (2) multiple (two or more) peaks structure in RC ion fluxes versus L shell in some MLT sections; (3) the magnetic local time (MLT) of the major equatorial ion flux peak is energy dependent, varying from about 02:00-03:00 MLT at lower energy of 4-8 keV gradually to about 23:00-00:00 MLT at higher energy of 50 keV. The results from analyzing the great magnetic storm of Oct.2011 present the evolution of ion flux and energy spectrum in RC part region during there typical period of quiet time, halfway of substorm expansion phase and maximum of substorm expansion phase.A method for separating H and O ENAs has been developed for medium energy neutral atom to obtain the information of RC Oxygen ion distribution. Based on TWINS satellite instrument structure and detecting principle, we develop a method for calculating the ENA differential flux from the level-0 raw data, which makes us extract and calculate ENA differential flux in any time period and any energy range. We use the pulse height distributions of the MicroChannel Plate (MCP) generated by the secondary electrons emitted from the carbon foil when ENAs pass through it to determine the composition of the ENA signal by a statistical fitting method. The results for different phase in two magnetic storm event show that the ratio of O flux enhance greatly during the main phase. The H to O ratio is complex with the spatial distribution, which may contain the transfers and energization information of O+in RC. This part of work is significant for researching the roles of O in the RC dynamics during magnetic storms.The characteristics and innovations in this paper:1. In this paper the method of voxel computerized tomography is applied for the first time in the inversion of ENA emission images measured onboard multi-spacecrafts to reconstruct the 3-D distribution of energetic ion fluxes in the Earth's magnetospheric RC region. To overcome the problem of iteration divergence caused by discordance in multi-satellite measurements, a differential ENA voxel CT method is introduced which increases the reliability of the result.2. According to TWINS satellite instrument parameters and detecting principle, we develop a method for calculating the ENA differential flux from the level-0 raw data, which makes us extract and calculate freely ENA differential flux along any direction in any time period and any energy range.3. A method for separating H and O EN As has been developed for medium energy neutral atom imaging, which has successfully separated H and O ENAs of the TWINS ENA data..